413
6
Assessing Transformation
Pathways
Coordinating Lead Authors:
Leon Clarke (USA), Kejun Jiang (China)
Lead Authors:
Keigo Akimoto (Japan), Mustafa Babiker (Sudan / Saudi Arabia), Geoffrey Blanford (USA / Germany),
Karen Fisher-Vanden (USA), Jean-Charles Hourcade (France), Volker Krey (IIASA / Germany), Elmar
Kriegler (Germany), Andreas Löschel (Germany), David McCollum (IIASA / USA), Sergey Paltsev
(Belarus / USA), Steven Rose (USA), Priyadarshi R. Shukla (India), Massimo Tavoni (Italy), Bob van
der Zwaan (Netherlands), Detlef P. van Vuuren (Netherlands)
Contributing Authors:
Hannes Böttcher (Austria / Germany), Katherine Calvin (USA), Katie Daenzer (USA), Michel
den Elzen (Netherlands), Subash Dhar (India / Denmark), Jiyong Eom (Republic of Korea),
Samuel Hoeller (Germany), Niklas Höhne (Germany), Nathan Hultman (USA), Peter Irvine
(UK / Germany), Jessica Jewell (IIASA / USA), Nils Johnson (IIASA / USA), Amit Kanudia (India),
Agnes Kelemen (Hungary), Klaus Keller (Germany / USA), Peter Kolp (IIASA / Austria), Mark
Lawrence (USA / Germany), Thomas Longden (Australia / Italy), Jason Lowe (UK), André Frossard
Pereira de Lucena (Brazil), Gunnar Luderer (Germany), Giacomo Marangoni (Italy), Nigel Moore
(Canada / Germany), Ionna Mouratiadou (Greece / Germany), Nils Petermann (Germany), Philip
Rasch (USA), Keywan Riahi (IIASA / Austria), Joeri Rogelj (Switzerland / Belgium), Michiel Schaeffer
(Netherlands / USA), Stefan Schäfer (Germany), Jan Sedlacek (Switzerland), Laura Sokka (Finland),
Christoph von Stechow (Germany), Ian Sue Wing (Trinidad and Tobago / USA), Naomi Vaughan
(UK), Thilo Wiertz (Germany), Timm Zwickel (Germany)
Review Editors:
Wenying Chen (China), John Weyant (USA)
Chapter Science Assistant:
Laura Sokka (Finland)
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Chapter 6
This chapter should be cited as:
Clarke L., K. Jiang, K. Akimoto, M. Babiker, G. Blanford, K. Fisher-Vanden, J.-C. Hourcade, V. Krey, E. Kriegler, A. Löschel,
D. McCollum, S. Paltsev, S. Rose, P. R. Shukla, M. Tavoni, B. C. C. van der Zwaan, and D.P. van Vuuren, 2014: Assessing
Transformation Pathways. In: Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III
to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Edenhofer, O., R. Pichs-Madruga,
Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. Savolainen, S.
Schlömer, C. von Stechow, T. Zwickel and J.C. Minx (eds.)]. Cambridge University Press, Cambridge, United Kingdom and
New York, NY, USA.
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Chapter 6
Contents
Executive Summary � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 418
6�1 Introduction � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 420
6�1�1 Framing and evaluating transformation pathways
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 420
6�1�2 New mitigation scenarios since AR4
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 420
6.1.2.1 Non-idealized international implementation scenarios
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421
6.1.2.2 Limited technology scenarios
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421
6�2 Tools of analysis � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 422
6�2�1 Overview of integrated modelling tools
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 422
6�2�2 Overview of the scenario ensemble for this assessment
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 423
6�2�3 Uncertainty and the interpretation of large scenario ensembles
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 423
6�2�4 Interpretation of model inability to produce particular scenarios
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 424
6�3 Climate stabilization: Concepts, costs and implications for the macro economy,
sectors and technology port folios, taking into account differences across regions
� � � � � � � � � � � � � 424
6�3�1 Baseline scenarios
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 424
6.3.1.1 Introduction to baseline scenarios
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
6.3.1.2 The drivers of baseline energy-related emissions
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
6.3.1.3 Baseline emissions projections from fossil fuels and industry
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425
6.3.1.4 Baseline CO
2
emissions from land use and emissions of non-CO
2
gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426
6.3.1.5 Baseline radiative forcing and cumulative carbon emissions
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428
6�3�2 Emissions trajectories, concentrations, and temperature in transformation pathways
� � � � � � � � � � � � � � � � � � � 428
6.3.2.1 Linking between different types of scenarios
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428
6.3.2.2 The timing of emissions reductions: The influence of technology, policy, and overshoot
. . . . . . . . . . . . 433
6.3.2.3 Regional roles in emissions reductions
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434
6.3.2.4 Projected CO
2
emissions from land use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435
6.3.2.5 Projected emissions of other radiatively important substances
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436
6.3.2.6 The link between concentrations, radiative forcing, and temperature
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438
6�3�3 Treatment of impacts and adaptation in transformation pathways
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 441
6�3�4 Energy sector in transformation pathways
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 443
6�3�5 Land and bioenergy in transformation pathways
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 445
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6�3�6 The aggregate economic implications of transformation pathways � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 448
6.3.6.1 Overview of the aggregate economic implications of mitigation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 448
6.3.6.2 Global aggregate costs of mitigation in idealized implementation scenarios
. . . . . . . . . . . . . . . . . . . . . . . . 449
6.3.6.3 The implications of technology portfolios for aggregate global economic costs
. . . . . . . . . . . . . . . . . . . . . 453
6.3.6.4 Economic implications of non-idealized international mitigation policy implementation
. . . . . . . . . . . . 459
6.3.6.5 The interactions between policy tools and their implementation, pre-existing taxes,
market failures, and other distortions
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455
6.3.6.6 Regional mitigation costs and effort-sharing regimes
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456
6�4 Integrating long- and short-term perspectives � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 462
6�4�1 Near-term actions in a long-term perspective
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 462
6�4�2 Near-term emissions and long-term transformation pathways
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 462
6�4�3 The importance of near-term technological investments and development of institutional capacity
� � � � 464
6�5 Integrating technological and societal change � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 466
6�5�1 Technological change
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 466
6�5�2 Integrating societal change
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 467
6�6 Sustainable development and transformation pathways, taking into
account differences across regions
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 468
6�6�1 Co-benefits and adverse side-effects of mitigation measures:
Synthesis of sectoral information and linkages to transformation pathways
� � � � � � � � � � � � � � � � � � � � � � � � � � � � 472
6�6�2 Transformation pathways studies with links to other policy objectives
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 472
6.6.2.1 Air pollution and health
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473
6.6.2.2 Energy security
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475
6.6.2.3 Energy access
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476
6.6.2.4 Employment
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476
6.6.2.5 Biodiversity conservation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476
6.6.2.6 Water use
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477
6.6.2.7 Integrated studies of multiple objectives
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477
6�7 Risks of transformation pathways � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 478
6�8 Integrating sector analyses and transformation scenarios � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 478
6�8�1 The sectoral composition of GHG emissions along transformation pathways
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � 478
6�8�2 Mitigation from a cross-sectoral perspective: Insights from integrated models
� � � � � � � � � � � � � � � � � � � � � � � � � � 479
6�8�3 Decarbonizing energy supply
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 480
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Chapter 6
6�8�4 Energy demand reductions and fuel switching in end-use sectors � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 480
6�8�5 Options for bioenergy production, reducing land-use change emissions,
and creating land-use GHG sinks
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 484
6�9 Carbon and radiation management and other geo- engineering options
including environmental risks
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 484
6�9�1 Carbon dioxide removal
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 485
6.9.1.1 Proposed carbon dioxide removal methods and characteristics
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485
6.9.1.2 Role of carbon dioxide removal in the context of transformation pathways
. . . . . . . . . . . . . . . . . . . . . . . . . 486
6�9�2 Solar radiation management
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 486
6.9.2.1 Proposed solar radiation management methods and characteristics
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486
6.9.2.2 The relation of solar radiation management to climate policy and transformation pathways
. . . . . . . 487
6�9�3 Summary
� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 489
6�10 Gaps in knowledge and data � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 489
6�11 Frequently Asked Questions � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 490
References � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 491
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Executive Summary
Stabilizing greenhouse gas (GHG) concentrations will require large-
scale transformations in human societies, from the way that we pro-
duce and consume energy to how we use the land surface. A natural
question in this context is what will be the ‘transformation pathway’
towards stabilization; that is, how do we get from here to there? The
topic of this chapter is transformation pathways. The chapter is pri-
marily motivated by three questions. First, what are the near-term and
future choices that define transformation pathways, including the goal
itself, the emissions pathway to the goal, technologies used for and
sectors contributing to mitigation, the nature of international coordi-
nation, and mitigation policies? Second, what are the key characteris-
tics of different transformation pathways, including the rates of emis-
sions reductions and deployment of low-carbon energy, the magnitude
and timing of aggregate economic costs, and the implications for other
policy objectives such as those generally associated with sustainable
development? Third, how will actions taken today influence the options
that might be available in the future? As part of the assessment in this
chapter, data from over 1000 new scenarios published since the IPCC
Fourth Assessment Report (AR4) were collected from integrated mod-
elling research groups, many from large-scale model intercomparison
studies. In comparison to AR4, new scenarios, both in this AR5 dataset
and more broadly in the literature assessed in this chapter, consider
more ambitious concentration goals, a wider range of assumptions
about technology, and more possibilities for delays in additional global
mitigation beyond that of today and fragmented international action.
Atmospheric concentrations in baseline scenarios collected for
this assessment (scenarios without additional efforts to con-
strain emissions beyond those of today) all exceed 450 parts
per million (ppm) carbon dioxide-equivalent (CO
2
eq) by 2030
and lie above the RCP6�0 representative concentration path-
way in 2100 (770 ppm CO
2
eq in 2100); the majority lie below
the RCP8�5 concentration pathway in 2100 (1330 ppm CO
2
eq
in 2100) (high confidence). The scenario literature does not system-
atically explore the full range of uncertainty surrounding development
pathways and the possible evolution of key drivers such as popula-
tion, technology, and resources. However, the baseline scenarios do
nonetheless strongly suggest that absent explicit efforts at mitigation,
cumulative CO
2
emissions since 2010 will exceed 700 GtCO
2
by 2030,
exceed 1500 GtCO
2
by 2050, and potentially be well over 4000 GtCO
2
by 2100. [Section 6.3.1]
Scenarios can be distinguished by the long-term concentration
level they reach by 2100; however, the degree to which concen-
trations exceed (overshoot) this level before 2100 is also impor-
tant (high confidence). The large majority of scenarios produced in the
literature that reach about 450 ppm CO
2
eq by 2100 are characterized
by concentration overshoot facilitated by the deployment of carbon
dioxide removal (CDR) technologies. Many scenarios have been con-
structed to reach about 550 ppm CO
2
eq by 2100 without overshoot.
Scenarios with more overshoot exhibit less mitigation today, but they
often rest on the assumption that future decision makers deploy CDR
technologies at large scale. An assessment in this chapter of geophysi-
cal climate uncertainties consistent with the dynamics of Earth System
Models assessed in Working Group I (WG I) provides estimates of the
temperature implications of different emissions pathways. This assess-
ment found that the likelihood of exceeding temperature goals this
century increases with peak concentration levels, which are higher in
overshoot scenarios. [6.3.2]
All major-emitting regions make substantial reductions from
their baseline CO
2
eq emissions over the century in scenarios
that bring atmospheric concentrations to about 550 ppm CO
2
eq
or below by 2100 (high confidence). In most scenarios collected for
this assessment that reach concentrations of about 550 ppm CO
2
eq by
2100, global CO
2
eq emissions are reduced by more than 50 %, and in
some cases by more than 100 %, by the end of the century relative to
2010 levels. The CO
2
eq emissions are brought to near or below zero
by 2100 in the majority of the scenarios reaching concentrations of
about 450 ppm CO
2
eq by 2100. In large part because baseline emis-
sions from the countries not part of the Organisation for Economic Co-
operation and Development (OECD) in 1990 are projected to outstrip
those from the OECD-1990 countries, the total CO
2
eq reductions from
baseline occurring in the non-OECD-1990 countries are larger than in
the OECD-1990 countries, particularly in scenarios that cost-effectively
allocate emissions reductions across countries. Emissions peak earlier
in the OECD-1990 countries than in the non-OECD-1990 countries in
these cost-effective scenarios. [6.3.2]
Bringing concentrations to about 550 ppm CO
2
eq or below by
2100 will require large-scale changes to global and national
energy systems, and potentially to the use of land; these
changes are inconsistent with both long- and short-term trends
(high confidence). Accelerated electrification of energy end use, cou-
pled with decarbonization of the majority of electricity generation by
2050 and an associated phaseout of freely emitting coal generation,
is a common feature of scenarios reaching about 550 ppm CO
2
eq or
less by 2100. Scenarios suggest that sectors currently using liquid fuel
are more costly to decarbonize than electricity and may be among the
last sectors to be decarbonized for deep CO
2
emissions reductions.
Scenarios articulate very different changes in the land surface, reflect-
ing different assumptions about the potential for bioenergy produc-
tion, afforestation, and reduced deforestation. Studies indicate a large
potential for energy use reductions, but also demonstrate that these
reductions will not be sufficient by themselves to constrain GHG emis-
sions. [6.3.4, 6.3.5, 6.8]
Estimates of the aggregate economic costs of mitigation vary
widely, but increase with stringency of mitigation (high confi-
dence). Most scenario studies collected for this assessment that are
based on the idealized assumptions that all countries of the world begin
mitigation immediately, there is a single global carbon price applied to
well-functioning markets, and key technologies are available, estimate
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that reaching about 450 ppm CO
2
eq by 2100 would entail global con-
sumption losses of 1 – 4 % in 2030 (median of 1.7 %), 2 – 6 % in 2050
(median of 3.4 %), and 3 – 11 % in 2100 (median of 4.8 %) relative to
what would happen without mitigation. These consumption losses cor-
respond to an annual average reduction of consumption growth of 0.06
to 0.20 percentage points from 2010 to 2030 (median of 0.09), 0.06
to 0.17 percentage points through 2050 (median of 0.09), and 0.04 to
0.14 percentage points over the century (median of 0.06). To put these
losses in context, studies assume annual average consumption growth
rates without mitigation between 1.9 % and 3.8 % per year until 2050
and between 1.6 % and 3.0 % per year over the century. These growth
rates correspond to increases in total consumption from roughly four-
fold to over ten-fold over the century. Costs for maintaining concen-
trations at around 550 ppmCO
2
eq are estimated to be roughly one-
third to two-thirds lower. Substantially higher and lower cost estimates
have been obtained based on assumptions about less idealized policy
implementations, interactions with pre-existing distortions, non-climate
market failures, or complementary policies. (Limits on technology and
delayed mitigation are discussed below.) [6.3.6]
Effort-sharing frameworks could help address distributional
issues and decouple regional mitigation investments from
financial burdens, but could be associated with significant inter-
national financial flows (medium confidence). In the absence of
effort-sharing frameworks, cost-effectively allocating emissions across
countries would yield an uneven distribution of mitigation costs. Sce-
narios indicate that this would lead to higher relative costs in develop-
ing economies as well as for many fossil fuel exporters. Studies explor-
ing effort-sharing frameworks in the context of a global carbon market
estimate that the financial flows to ameliorate this asymmetry could
be on the order of hundreds of billions of USD per year before mid-cen-
tury to bring concentrations to about 450 ppm CO
2
eq in 2100. [6.3.6]
Emissions through 2030 will have strong implications for the
challenges of, and options for, bringing concentrations to about
450 to about 500 ppm CO
2
eq by the end of the twenty-first cen-
tury (high confidence). The vast majority of cost-effective scenarios
leading to 2100 concentrations of about 450 to about 500 ppm CO
2
eq
are characterized by 2030 emissions roughly between 30 GtCO
2
eq
and 50 GtCO
2
eq. Scenarios with emissions above 55 GtCO
2
eq in 2030
are predominantly driven by delays in additional mitigation relative
to what would be most cost-effective. These scenarios are character-
ized by substantially higher rates of emissions reductions from 2030
to 2050, a larger reliance on CDR technologies in the long term, and
higher transitional and long-term economic impacts. Due to these
challenges, many models with 2030 emissions in this range could not
produce scenarios reaching about 450 ppm CO
2
eq in 2100. Studies
confirm that delaying additional mitigation through 2030 has substan-
tially larger influence on the subsequent challenges of mitigation than
delaying only through 2020. [6.3.2, 6.4]
The availability of key technologies and improvements in the
cost and performance of these technologies will have important
implications for the challenge of achieving concentration goals
(high confidence). Many models in recent multi-model comparisons
could not produce scenarios reaching approximately 450 ppm CO
2
eq
by 2100 with broadly pessimistic assumptions about key mitigation
technologies. Large-scale deployment of CDR technologies in particular
is relied upon in many of these scenarios in the second-half of the cen-
tury. For those models that could produce such scenarios, pessimistic
assumptions about important technologies for decarbonizing non-elec-
tric energy supply significantly increased the discounted global mitiga-
tion costs of reaching about 450 ppm and about 550 ppm CO
2
eq by
the end of the century, with the effect being larger for more stringent
goals. These studies also showed that reducing energy demand can
potentially decrease mitigation costs significantly. [6.3.2, 6.3.4, 6.3.6,
6.4]
Mitigation efforts will influence the costs of meeting other
policy objectives� Recent studies indicate that climate policies
significantly reduce the costs of reaching energy security and
air quality objectives (medium evidence, high agreement). The asso-
ciated economic implications for these objectives are not taken into
account in most scenario studies. Sectoral studies suggest that the
potential for co-benefits of energy end-use mitigation measures out-
weighs the potential for adverse side-effects, whereas the evidence
suggests this may not be the case for all supply-side and AFOLU mea-
sures. The overall welfare implications associated with these additional
objectives have not been assessed thoroughly in the literature. [6.6]
There is uncertainty about the potential of geoengineering by
CDR or solar radiation management (SRM) to counteract climate
change, and all techniques carry risks and uncertainties (high
confidence). A range of different SRM and CDR techniques has been
proposed, but no currently existing technique could fully replace miti-
gation or adaptation efforts. Nevertheless, many low-GHG concentra-
tion scenarios rely on two CDR techniques, afforestation and biomass
energy with carbon dioxide capture and storage (BECCS), which some
studies consider to be comparable with conventional mitigation meth-
ods. Solar radiation management could reduce global mean tempera-
tures, but with uneven regional effects, for example on temperature and
precipitation, and it would not address all of the impacts of increased
CO
2
concentrations, such as ocean acidification. Techniques requiring
large-scale interventions in the earth system, such as ocean fertilization
or stratospheric aerosol injections, carry significant risks. Although pro-
posed geoengineering techniques differ substantially from each other,
all raise complex questions about costs, risks, governance, and ethical
implications of research and potential implementation. [6.9]
Despite the advances in our understanding of transformation path-
ways since AR4, many avenues of inquiry remain unanswered. Impor-
tant future research directions include the following: development of
a broader set of socioeconomic and technological storylines to sup-
port development of scenarios; scenarios explicitly pursuing a wider
set of climate goals, including those related to temperature change;
more mitigation scenarios that include impacts from, and adaptations
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Chapter 6
to, a changing climate, including energy and land use systems critical
for mitigation; expanded treatment of the benefits and risks of CDR
and SRM options; expanded treatment of co-benefits and adverse
side-effects of mitigation pathways; improvements in the treatment
and understanding of mitigation options and responses in end-use sec-
tors in transformation pathways; and more sophisticated treatments
of land use and land use-based mitigation options in mitigation sce-
narios. [6.10]
6.1 Introduction
6�1�1 Framing and evaluating transformation
pathways
Stabilizing greenhouse gas (GHG) concentrations at any level will
require deep reductions in GHG emissions. Net global CO
2
emissions,
in particular, must eventually be brought to or below zero. Emissions
reductions of this magnitude will require large-scale transformations in
human societies, from the way that we produce and consume energy
to how we use the land surface. The more ambitious the stabilization
goal, the more rapid this transformation must occur. A natural question
in this context is what will be the transformation pathway toward sta-
bilization; that is, how do we get from here to there?
The topic of this chapter is transformation pathways. The chapter is
motivated primarily by three questions. First, what are the near-term
and future choices that define transformation pathways including, for
example, the goal itself, the emissions pathway to the goal, the tech-
nologies used for and sectors contributing to mitigation, the nature
of international coordination, and mitigation policies? Second, what
are the key decision making outcomes of different transformation
pathways, including the magnitude and international distribution of
economic costs and the implications for other policy objectives such
as those associated with sustainable development? Third, how will
actions taken today influence the options that might be available in
the future?
Two concepts are particularly important for framing any answers to
these questions. The first is that there is no single pathway to stabiliza-
tion of GHG concentrations at any level. Instead, the literature eluci-
dates a wide range of transformation pathways. Choices will govern
which pathway is followed. These choices include, among other things,
the long-term stabilization goal, the emissions pathway to meet that
goal, the degree to which concentrations might temporarily overshoot
the goal, the technologies that will be deployed to reduce emissions,
the degree to which mitigation is coordinated across countries, the
policy approaches used to achieve these goals within and across coun-
tries, the treatment of land use, and the manner in which mitigation is
meshed with other policy objectives such as sustainable development.
The second concept is that transformation pathways can be distin-
guished from one another in important ways. Weighing the character-
istics of different pathways is the way in which deliberative decisions
about transformation pathways would be made. Although measures of
aggregate economic implications have often been put forward as key
deliberative decision making factors, these are far from the only char-
acteristics that matter for making good decisions. Transformation path-
ways inherently involve a range of tradeoffs that link to other national
and policy objectives such as energy and food security, the distribu-
tion of economic costs, local air pollution, other environmental factors
associated with different technology solutions (e. g., nuclear power,
coal-fired carbon dioxide capture and storage (CCS)), and economic
competitiveness. Many of these fall under the umbrella of sustainable
development.
A question that is often raised about particular stabilization goals
and transformation pathways to those goals is whether the goals or
pathways are ‘feasible’. In many circumstances, there are clear physi-
cal constraints that can render particular long-term goals physically
impossible. For example, if additinional mitigation beyond that of
today is delayed to a large enough degree and carbon dioxide removal
(CDR) options are not available (see Section 6.9), a goal of reaching
450 ppm CO
2
eq by the end of the 21st century can be physically impos-
sible. However, in many cases, statements about feasibility are bound
up in subjective assessments of the degree to which other character-
istics of particular transformation pathways might influence the ability
or desire of human societies to follow them. Important characteristics
include economic implications, social acceptance of new technolo-
gies that underpin particular transformation pathways, the rapidity
at which social and technological systems would need to change to
follow particular pathways, political feasibility, and linkages to other
national objectives. A primary goal of this chapter is to illuminate these
characteristics of transformation pathways.
6�1�2 New mitigation scenarios since
AR4
Since the IPCC Fourth Assessment Report (AR4), the integrated mod-
elling community has produced a range of new transformation path-
way scenarios. Major advances include an increase in the number of
scenarios exploring the following: low-concentration goals such as
450 ppm CO
2
eq; overshoot emissions trajectories with and without
CDR technologies; a variety of international mitigation policy configu-
rations, including fragmented action and delays in additional mitiga-
tion beyond that of today; and the implications of variations in tech-
nology cost, performance, and availability. The literature also includes
a small but growing set of scenarios and research exploring the link-
age between mitigation and other policy objectives, an increasingly
sophisticated treatment of the role of land use in mitigation, and sce-
narios exploring non-market approaches to mitigation. Two particularly
important categories for the discussion in this chapter are non-ideal-
ized international implementation scenarios and scenarios with limits
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on technology cost, performance, or availability. These categories of
scenarios are discussed in more detail below.
6�1�2�1 Non-idealized international implementation
scenarios
At the time of AR4, the majority of mitigation scenarios were based on
the idealized assumption that mitigation is undertaken where and
when it is least expensive. Such ‘idealized implementation’ scenarios
assume the imposition of a global price on carbon that reaches across
countries, permeates all economic sectors within countries, and rises
over time in a way that will minimize discounted economic costs over
a long period of time, typically through 2100. These are often referred
to as ‘cost-effective’ scenarios, because they lead to the lowest aggre-
gate global mitigation costs under idealized assumptions about the
functioning of markets and economies (see Section 6.3.6). However,
the reality of international strategies for mitigation is one of different
countries taking on mitigation at different times and using different
and independent implementation approaches. Responding to this real-
ity, the research community has produced a large set of ‘non-idealized’
international implementation scenarios for reaching long-term concen-
tration goals. Often, but not always, non-idealized implementation is
focused on the coming decades, with a transition toward idealized
implementation in the long run. In addition to individual papers (for
example, Richels et al., 2007; Edmonds et al., 2008; Luderer et al.,
2014b; Rogelj etal., 2013a), there have been a number of multi-model
projects exploring non-idealized implementation scenarios (Table 6.1).
This chapter relies heavily on those multi-model studies.
There are a number of ways that scenarios may deviate from the ideal-
ized implementation, but two are most prominent in the new litera-
ture. One set of scenarios includes those in which near-term mitigation
is inconsistent with — typically less than — what would be called for to
minimize the discounted, century-long costs of meeting a long-term
goal such as 450 ppm CO
2
eq by 2100. These scenarios are intended to
capture the implications of ‘delayed action’ or ‘delayed mitigation’ or
‘constrained near-term ambition’. Mitigation is not undertaken ‘when’
it would be least expensive. The other set of scenarios includes those
in which the price on carbon is not consistent across countries. Some
countries reduce emissions more aggressively than others, particularly
in the near-term, so that mitigation is not undertaken ‘where’ it is least
expensive. These scenarios are intended to capture the implications
of ‘fragmented action’ or ‘delayed participation’. Non-idealized inter-
national implementation scenarios may include one or both of these
deviations.
6�1�2�2 Limited technology scenarios
Scenario research prior to AR4 emphasized the importance of tech-
nology in constraining the costs of mitigation. A range of individual
papers had made initial explorations of this space for more than a
decade before AR4. Since AR4, however, a range of new studies have
emerged including large model intercomparison studies, that have
focused on the implications of limitations on technology cost, per-
formance, availability on the cost and other characteristics of meet-
ing concentration goals such as 450 ppm CO
2
eq by 2100. The large
model intercomparison studies include Energy Modeling Forum (EMF)
27 (Krey etal., 2014; Kriegler etal., 2014a), ADAM (Adaptation and
Mitigation Strategies: Supporting European Climate Policy) (Edenhofer
etal., 2010), RECIPE (Report on Energy and Climate Policy in Europe)
(Luderer etal., 2012a; Tavoni etal., 2012), and AMPERE (Assessment
of Climate Change Mitigation Pathways and Evaluation of the Robust-
ness of Mitigation Cost Estimates) (Riahi et al., 2014). In addition
to the large model intercomparison studies, a number of individual
Table 6�1 | Multi-model studies exploring non-idealized international implementation
Multi-Model Study Description
EMF 22 (Clarke etal., 2009) Delayed participation (fragmented action) scenarios in which Organisation for Economic Co-operation and Development (OECD)
countries begin mitigation immediately; Brazil, Russia, India, and China begin after 2030; remaining countries begin after
2050. Scenarios meet various 2100 concentration goals, with and without overshooting the concentration goal.
EMF 27 (Blanford etal., 2014;
Kriegler etal., 2014a)
Delayed and limited participation scenario with Annex I adopting 80 % emissions reductions until 2050, non-Annex I adopting a global
50 % emissions reduction by 2050 after 2020, and resource exporting countries not undertaking emissions reductions.
AMPERE (Kriegler etal.,
2014c; Riahi etal., 2014)
Two studies: AMPERE WP2 focused on delayed mitigation scenarios with the world following moderate
mitgation until 2030, and adopting long-term concentration goals thereafter.
AMPERE WP3 focused on delayed participation scenarios with EU27 or EU27 and China acting immediately and the remaining countries
transitioning from moderate policies to a global carbon pricing regime (without mitigation goal) between 2030 and 2050.
LIMITS (Kriegler etal., 2013b;
Tavoni etal., 2013)
Delayed mitgation scenarios with the world following two levels of moderate fragmented action through 2020 or 2030, and
adopting two long-term concentration goals thereafter. Three different effort-sharing schemes are considered.
RoSE (Luderer etal., 2014a) Delayed mitgation scenarios with the world following moderate fragmented action in the near
term and adopting a long-term concentration goal after 2020 or 2030.
Note: The Energy Modeling Forum (EMF) 27, AMPERE (Assessment of Climate Change Mitigation Pathways and Evaluation of the Robustness of Mitigation Cost Estimates), LIMITS
(Low Climate Impact Scenarios and the Implications of Reguired Tight Emission Control Strategies), and RoSE (Roadmaps Towards Sustainable Energy Futures) studies also included
scenarios of moderate fragmented action throughout the 21st century without the goal of meeting any specific long-term concentration.
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Chapter 6
research papers and reports have explored this space since AR4, typi-
cally constrained to a single model (Richels etal., 2007; Calvin etal.,
2009a; Krey and Riahi, 2009; van Vliet etal., 2009; Riahi etal., 2012;
Luderer etal., 2013; Rogelj etal., 2013b). In many cases, these stud-
ies have simply assumed that particular technologies, such as CCS
or nuclear power, may not be available. In others, studies have put
constraints on resource supplies, for example, the supply of bioenergy.
In others, they have called for variations in cost and performance of
different technologies. Many have also explored the implications of
energy end-use improvements.
6.2 Tools of analysis
6�2�1 Overview of integrated modelling tools
The long-term scenarios assessed in this chapter were generated
primarily by large-scale, integrated models that can project key char-
acteristics of transformation pathways to mid-century and beyond.
These models represent many of the most relevant interactions among
important human systems (e. g., energy, agriculture, the economic
system), and often represent important physical processes associated
with climate change (e. g., the carbon cycle). Other approaches to
explore transformation pathways include qualitative scenario methods
and highly aggregated modelling tools, such as those used for cost-
benefit analysis (see Box 6.1 on cost-benefit analysis, p.394). These
other approaches provide a different level of quantitative information
about transformation pathways than scenarios from large-scale inte-
grated models.
All integrated models share some common traits. Most fundamentally,
integrated models are simplified, stylized, numerical approaches to
represent enormously complex physical and social systems. They take
in a set of input assumptions and produce outputs such as energy
system transitions, land-use transitions, economic effects of mitiga-
tion, and emissions trajectories. Important input assumptions include
population growth, baseline economic growth, resources, technologi-
cal change, and the mitigation policy environment. The models do not
structurally represent many social and political forces that can influ-
ence the way the world evolves (e. g., shocks such as the oil crisis of
the 1970s). Instead, the implications of these forces enter the model
through assumptions about, for example, economic growth and
resource supplies. The models use economics as the basis for decision
making. This may be implemented in a variety of ways, but it funda-
mentally implies that the models tend toward the goal of minimizing
the aggregate economic costs of achieving mitigation outcomes, unless
they are specifically constrained to behave otherwise. In this sense, the
scenarios tend towards normative, economics-focused descriptions of
the future. The models typically assume fully functioning markets and
competitive market behavior, meaning that factors such as non-market
transactions, information asymmetries, and market power influencing
decisions are not effectively represented. Maintaining a long-term,
integrated, and often global perspective involves tradeoffs in terms
of the detail at which key processes can be represented in integrated
models. Hence, the models do not generally represent the behaviour
of certain important system dynamics, such as economic cycles or the
operation of electric power systems important for the integration of
solar and wind power, at the level of detail that would be afforded by
analyses that the focus exclusively on those dynamics.
Beyond these and other similarities, integrated modelling approaches
can be very different, and these differences can have important impli-
cations for the variation among scenarios that emerge from different
models. The following paragraphs highlight a number of key differ-
ences in model structure. To provide insight into the implications of
these tradeoffs, potential implications for aggregate economic costs
are provided as examples, when appropriate.
Economic coverage and interactions� Models differ in terms of the
degree of detail with which they represent the economic system and
the degree of interaction they represent across economic sectors. Full-
economy models (e. g., general equilibrium models) represent inter-
actions across all sectors of the economy, allowing them to explore
and understand ripple effects from, for example, the imposition of
a mitigation policy, including impacts on overall economic growth.
Partial-economy models, on the other hand, take economic activ-
ity as an input that is unresponsive to policies or other changes such
as those associated with improvements in technology. These models
tend to focus more on detailed representations of key systems such
as the energy system. All else equal, aggregate economic costs would
tend to be higher in full-economy models than in partial-economy
models because full-economy models include feedbacks to the entire
economy. On the other hand, full-economy models may include more
possibilities for substitution in sectors outside of those represented in
partial-economy models, and this would tend to reduce aggregate eco-
nomic costs.
Foresight� Perfect-foresight models (e. g., intertemporal optimization
models) optimize over time, so that all future decisions are taken into
account in today’s decisions. In contrast, recursive-dynamic models
make decisions at each point in time based only on the information in
that time period. In general, perfect-foresight models would be likely to
allocate emissions reductions more efficiently over time than recursive-
dynamic models, which should lead to lower aggregate costs.
Representation of trade� Models differ in terms of how easy it is
for goods to flow across regions. On one end of the spectrum are
models assuming goods are homogeneous and traded easily at one
world price (Heckscher-Ohlin) or that there is one global producer
(quasi-trade). On the other end of the spectrum are models assuming
a preference for domestic goods over imported goods (Armington) or
models without explicit trade across regions (e. g., models with import
supply functions). In general, greater flexibility to trade will result in
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lower-aggregate mitigation costs because the global economy is more
flexible to undertake mitigation where it is least expensive. More gen-
erally, many partial-equilibrium models include trade only in carbon
permits and basic energy commodities. These models are not capable
of exploring the full nature of carbon leakage that might emerge from
mitigation policies, and particularly those associated with fragmented
international action.
Model flexibility� The flexibility of models describes the degree
to which they can change course. Model flexibility is not a single,
explicit choice for model structure. Instead, it is the result of a range
of choices that influence, for example, how easily capital can be reallo-
cated across sectors including the allowance for premature retirement
of capital stock, how easily the economy is able to substitute across
energy technologies, whether fossil fuel and renewable resource con-
straints exist, and how easily the economy can extract resources. The
complexity of the different factors influencing model flexibility makes
clear delineations of which models are more or less flexible difficult.
Evaluation and characterization of model flexibility is an area of cur-
rent research (see Kriegler etal., 2014b). Greater flexibility will tend to
lower mitigation costs.
Sectoral, regional, technology, and GHG detail� Models differ dra-
matically in terms of the detail at which they represent key sectors and
systems. These differences influence not only the way that the models
operate, but also the information they can provide about transforma-
tion pathways. Key choices include the number of regions, the degree
of technological detail in each sector, which GHGs are represented and
how, whether land use is explicitly represented, and the sophistica-
tion of the model of earth system process such as the carbon cycle.
Some models include only CO
2
emissions, many do not treat land-use
change (LUC) and associated emissions, and many do not have sub-
models of the carbon cycle necessary to calculate CO
2
concentrations.
In addition, although the scenarios in this chapter were generated
from global models that allow for the implications of mitigation for
international markets to be measured, regional models can provide
finer detail on the implications for a specific region’s economy and dis-
tributional effects. The effects of detail on aggregate mitigation costs
are ambiguous
Representation of technological change� Models can be catego-
rized into two groups with respect to technological change. On one
end of the spectrum, models with exogenous technological change
take technology as an input that evolves independently of policy mea-
sures or investment decisions. These models provide no insight on
how policies may induce advancements in technology. On the other
end of the spectrum, models with endogenous technological change
(also known as induced technological change) allow for some por-
tion of technological change to be influenced by deployment rates
or investments in research and development (R&D). Models featuring
endogenous technological change are valuable for understanding how
the pace of technological change might be influenced by mitigation
policies.
6�2�2 Overview of the scenario ensemble for
this assessment
The synthesis in this chapter is based on a large set of new scenarios
produced since AR4. The number of models has increased and model
functionality has significantly improved since AR4, allowing for a
broader set of scenarios in the AR5 ensemble. The majority of these
scenarios were produced as part of multi-model comparisons. Most
model intercomparison studies produce publicly available databases
that include many of the key outputs from the studies. Although crucial
for our understanding of transformation pathways, these intercompari-
son exercises are not the only source of information on transformation
pathways. A range of individual studies has been produced since AR4,
largely assessing transformation pathways in ways not addressed in
the model intercomparison exercises. For the purposes of this assess-
ment, an open call was put forward for modellers to submit scenarios
not included in the large model intercomparison databases. These
scenarios, along with those from many of the model intercomparison
studies, have been collected in a database that is used extensively in
this chapter. A summary of the models and model intercomparison
exercises that generated the scenarios referenced in this chapter can
be found in Annex II.10.
6�2�3 Uncertainty and the interpretation of
large scenario ensembles
The interpretation of large ensembles of scenarios from different mod-
els, different studies, and different versions of individual models is a
core component of the assessment of transformation pathways in this
chapter. Indeed, many of the tables and figures represent ranges of
results across all these dimensions.
There is an unavoidable ambiguity in interpreting ensemble results in
the context of uncertainty. On the one hand, the scenarios assessed in
this chapter do not represent a random sample that can be used for
formal uncertainty analysis. Each scenario was developed for a specific
purpose. Hence, the collection of scenarios included in this chapter does
not necessarily comprise a set of ‘best guesses.’ In addition, many of
these scenarios represent sensitivities, particularly along the dimensions
of future technology availability and the timing of international action
on climate change, and are therefore highly correlated. Indeed, most of
the scenarios assessed in this chapter were generated as part of model
intercomparison exercises that impose specific assumptions, often
regarding long-term policy approaches to mitigation, but also in some
cases regarding fundamental drivers like technology, population growth,
and economic growth. In addition, some modelling groups have gener-
ated substantially more scenarios than others, introducing a weighting
of scenarios that can be difficult to interpret. At the same time, however,
with the exception of pure sensitivity studies, the scenarios were gen-
erated by experts making informed judgements about how key forces
might evolve in the future and how important systems interact. Hence,
although they are not explicitly representative of uncertainty, they do
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provide real and often clear insights about our lack of knowledge about
key forces that might shape the future (Fischedick etal., 2011; Krey and
Clarke, 2011). The synthesis in this chapter does not attempt to resolve
the ambiguity associated with ranges of scenarios, and instead focuses
simply on articulating the most robust and valuable insights that can
be extracted given this ambiguity. However, wherever possible, scenario
samples are chosen in such a way as to reduce bias, and these choices
are made clear in the discussion and figure legends.
6�2�4 Interpretation of model inability to
produce particular scenarios
A question that is often raised about particular stabilization goals and
transformation pathways is whether the goals or pathways are ‘fea-
sible’ (see Section 6.1). Integrated models can be helpful in informing
this question by providing information about key elements of transfor-
mation pathways that might go into assessments of feasibility, such
as rates of deployment of energy technologies, rates of reductions
in global and regional emissions, aggregate economic costs, finan-
cial flows among regions, and links to other policy objectives such as
energy security or energy prices. However, beyond cases where physi-
cal laws might be violated to achieve a particular scenario (for exam-
ple, a 2100 carbon budget is exceeded prior to 2100 with no option for
negative emissions), these integrated models cannot determine feasi-
bility in an absolute sense.
This is an important consideration when encountering situations in
which models are incapable of producing scenarios. Many models
have been unable to achieve particularly aggressive concentration
goals such as reaching 450 ppm CO
2
eq by 2100, particularly under
challenging technological or policy constraints. In some cases, this
may be due to the violation of real physical laws, the most common of
which is when the cumulative carbon budget associated with meeting
a long-term goal is exceeded without options to remove carbon from
the atmosphere. Frequently, however, instances of model infeasibility
arise from pushing models beyond the boundaries of what they were
built to explore, for example, rates of change in the energy system that
exceed what the model can represent, or carbon prices sufficiently
high that they conflict with the underlying computational structure.
Indeed, in many cases, one model may be able to produce scenarios
while another will not, and model improvements over time may result
in feasible scenarios that previously were infeasible. Hence, although
these model infeasibilities cannot generally be taken as an indicator of
feasibility in an absolute sense, they are nonetheless valuable indica-
tors of the challenge associated with achieving particular scenarios.
For this reason, whenever possible, this chapter highlights those situa-
tions where models were unable to produce scenarios.
Unfortunately, this type of result can be difficult to fully represent in
an assessment because, outside of model intercomparison studies
intended explicitly to identify these circumstances, only scenarios that
could actually be produced (as opposed that could not be produced)
are generally published. Whether certain circumstances are under-
represented because they have been under-examined or because they
have been examined and the scenarios failed is a crucial distinction,
yet one that it is currently not possible to fully report. Model infeasibili-
ties can bias results in important ways, for example, the costs of miti-
gation, because only those models producing scenarios can provide
estimated costs (Tavoni and Tol, 2010).
6.3 Climate stabilization:
Concepts, costs and
implications for the macro
economy, sectors and
technology port folios,
taking into account
differences across regions
6�3�1 Baseline scenarios
6�3�1�1 Introduction to baseline scenarios
Baseline scenarios are projections of GHG emissions and their key driv-
ers as they might evolve in a future in which no explicit actions are
taken to reduce GHG emissions. Baseline scenarios play the important
role of establishing the projected scale and composition of the future
energy, economic, and land-use systems as a reference point for mea-
suring the extent and nature of required mitigation for a given climate
goal. Accordingly, the resulting estimates of mitigation effort and costs
in a particular mitigation scenario are always conditional upon the
associated baseline.
Although the range of emissions pathways across baseline scenarios in
the literature is broad, it may not represent the full potential range of
possibilities. There has been comparatively little research formally con-
structing or eliciting subjective probabilities for comprehensive ranges
of the key drivers of baseline emissions in a country-specific context,
and this remains an important research need for scenario develop-
ment. As discussed in Section 6.2, although the range of assumptions
used in the literature conveys some information regarding modellers’
expectations about how key drivers might evolve and the associated
implications, several important factors limit its interpretation as a true
uncertainty range. An important distinction between scenarios in this
regard is between those that are based on modellers’ ‘default’ assump-
tions and those that are harmonized across models within specific
studies. The former can be considered a better, although still imperfect,
representation of modellers’ expectations about the future, while, as is
discussed below, the latter consider specific alternative views that in
some cases span a larger range of possible outcomes.
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Chapter 6
6�3�1�2 The drivers of baseline energy-related emissions
As discussed in Chapter 5, the drivers of the future evolution of energy-
related emissions in the baseline can be summarized by the terms of
the Kaya identity: population, per capita income, energy intensity of
economic output, and carbon intensity of energy. At the global level,
baseline projections from integrated models are typically characterized
by modest population growth stabilizing by the end of the century, fast
but decelerating growth in income, decline in energy intensity, and
modest changes in carbon intensity with ambiguous sign (Figure 6.1).
There is comparatively little variation across model scenarios in pro-
jected population growth, with virtually all modelling studies relying
on central estimates (UN, 2012). One exception is the RoSE project
(Bauer etal., 2014b; Calvin etal., 2014b; De Cian etal., 2014), which
explicitly considers high population scenarios, as well as the storyline
beneath the representative concentration pathways (RCP) 8.5 scenario.
Among the majority of default population projections, there are some
minor differences across models, for example, the extent to which
declining rates for certain regions in coming decades are incorporated.
On the other hand, there is substantially more variation in model pro-
jections of per capita income, with a few scenarios harmonized at both
the low and high ends of the range, and energy intensity, for which
two studies (AMPERE and EMF27) specified alternative ‘fast’ decline
baselines. Still, the interquartile range of default assumptions for both
indicators is narrow, suggesting that many scenarios are based on a
Figure 6�1 | Global baseline projection ranges for Kaya factors. Scenarios harmonized with respect to a particular factor are depicted with individual lines. Other scenarios are
depicted as a range with median emboldened; shading reflects interquartile range (darkest), 5th — 95th percentile range (lighter), and full range (lightest), excluding one indi-
cated outlier in panel a) Scenarios are filtered by model and study for each indicator to include only unique projections. Model projections and historic data are normalized to
1 in 2010. Gross domestic product (GDP) is aggregated using base-year market exchange rates. Energy and carbon intensity are measured with respect to total primary energy.
Sources: UN (2012), WG III AR5 Scenario Database (Annex II.10). Historic data: JRC / PBL (2013), IEA (2012a), see Annex II.9; Heston etal. (2012), World Bank (2013), BP (2013).
Index (2010=1)
Index (2010=1)
Index (2010=1)
Index (2010=1)
1 Outlier
History
History
History
History
Default
Fast
1970 1990 2010 2030 2050 2070 2090 1970 1990 2010 2030 2050 2070 2090
0
0.5
1.0
2.0
1.5
0
2
4
8
6
10
0
1.0
2.0
1.5
2.5
0
0.5
1.0
2.0
1.5
1970 1990 2010 2030 2050 2070 2090 1970 1990 2010 2030 2050 2070 2090
Historic Trend:
Average Rate
of Growth
1970-2010 =
1.4%
Historic Trend:
Average Rate
of Decline
1970-2010 =
0.8%
a) Population b) GDP Per Capita
c) Energy Intensity of GDP d) Carbon Intensity of Energy
0.5
Harmonized Default
UN Variants
(High, Medium, Low)
Harmonized High
Harmonized Low
0-100
th
5-95
th
25-75
th
Percentile
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Assessing Transformation Pathways
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similar underlying narrative. Models project a faster global average
growth rate in the future as dynamic emerging economies constitute
an increasing share of global output. Energy intensity declines more
rapidly than in the past, with an especially marked departure from the
historical trend for ‘fast’ energy intensity decline scenarios. Carbon
intensity, typically viewed as a model outcome driven by resource and
technology cost assumptions, is projected in most baseline scenarios to
change relatively little over time, but there are exceptions in both
directions. Declining carbon intensity could result from rapid improve-
ments in renewable technologies combined with rising fossil fuel
prices. Conversely, the fossil share in energy could rise with favourable
resource discoveries, or the fossil mix could become more carbon
intensive, for example, due to replacement of conventional petroleum
with heavier oil sands or coal-to-liquids.
While all models assume increasing per capita income and declin-
ing energy intensity, broad ranges are projected and high uncer-
tainty remains as to what rates might prevail. Most models describe
income growth as the result of exogenous improvement over time in
labour productivity. The processes of technological advance by which
such improvement occurs are only partially understood. Changes
in aggregate energy intensity over time are the net result of several
trends, including both improvements in the efficiency of energy end-
use technology and structural changes in the composition of energy
demand. Structural changes can work in both directions: there may be
increased demand for energy-intensive services such as air-condition-
ing as incomes rise, while on the production side of the economy, there
may be shifts to less energy-intensive industries as countries become
wealthier. Although increasing energy intensity has been observed for
some countries during the industrialization stage, the net effect is usu-
ally negative, and in general energy intensity has declined consistently
over time. Both efficiency improvements and structural change can be
driven by changes in energy prices, but to a significant extent both are
driven by other factors such as technological progress and changing
preferences with rising incomes. Most integrated models are able to
project structural and technological change only at an aggregate level,
although some include explicit assumptions for certain sectors (Sugi-
yama etal., 2014).
Because of limited variation in population and carbon-intensity projec-
tions, the relative strength of the opposing effects of income growth
and energy intensity decline (summarized by changes in per capita
energy), plays the most important role in determining the growth of
emissions in the baseline scenario literature (see Blanford etal., 2012).
Assumptions about the evolution of these factors vary strongly across
regions. In general, rates of change in population, income, energy
intensity, and per capita energy are all expected to be greater in devel-
oping countries than in currently developed countries in coming
decades, although this pattern has not necessarily prevailed in the
past 40 years, as non-OECD-1990 countries had slower energy inten-
sity decline than OECD-1990 countries (Figure 6.2). Among default
energy-intensity scenarios, assumed rates of change appear to be pos-
itively correlated between income and energy intensity, so that equiv-
alent per capita energy outcomes are realized through varying combi-
nations of these two indicators. The harmonized shift in the energy
intensity decline rate leads to very low per capita energy rates, with
global per capita energy use declining in a few cases (Figure 6.2). Pro-
jected emissions are essentially the product of per capita energy and
carbon intensity projections, with most variation in future emissions
scenarios explained by variation in per capita energy; the highest
emissions projections arise from instances with high levels in both
indicators (Figure 6.3).
6�3�1�3 Baseline emissions projections from fossil fuels
and industry
Based on the combination of growing population, growing per capita
energy demand, and a lack of significant reductions in carbon intensity
of energy summarized in the previous section, global baseline emis-
sions of CO
2
from fossil fuel and industrial (FF&I) sources are projected
to continue to increase throughout the 21st century (Figure 6.4, left
panel). Although most baseline scenarios project a deceleration in
emissions growth, especially compared to the rapid rate observed
in the past decade, none is consistent in the long run with the path-
ways in the two most stringent RCP scenarios (Sections 2.6 and 4.5),
with the majority falling between the 6.0 and 8.5 pathways (see IPCC
(2013), Chapter 12 for a discussion of the RCP study). The RCP 8.5
pathway has higher emissions than all but a few published baseline
scenarios. Projections for baseline FF&I CO
2
emissions in 2050 range
from only slightly higher than current levels (in scenarios with explicit
assumptions about fast energy intensity decline) to nearly triple cur-
rent levels.
Figure 6�2 | Average rates of change between 2010 and 2050 in baseline scenarios
for GDP per capita and energy intensity of GDP in OECD-1990 and Non-OECD-1990.
There are 62 of 77 unique default intensity scenarios and 22 of 24 unique fast inten-
sity scenarios plotted. Omitted are scenarios without OECD-1990 break-out. Sources:
UN (2012), WG III AR5 Scenario Database (Annex II.10). Historic data: JRC / PBL
(2013), IEA (2012a), see Annex II.9; Heston etal. (2012), World Bank (2013), BP
(2013).
-4
-3
-2
-1
0
0 1 2 3 4 5
Average Rate ofChangein EnergyIntensity [%]
AverageRateofChangeinGDP perCapita [%]
0%
-1% per Capita Energy
+1%
+2%
OECD-1990 History(1970-2010)
OECD-1990 Projections(2010-2050)
Non-OECD-1990History (1970-2010)
Non-OECD-1990 Projections(2010-2050)
FastEnergy IntensityDeclineScenarios
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Chapter 6
Figure 6�4 | Global FF&I CO
2
emissions in baseline scenarios with default growth assumptions (grey range) and fast energy intensity decline (gold range) (left panel), and for OECD-
1990 vs. non-OECD-1990 (right panel) from 1970 to 2100. RCP scenarios are shown for comparison with the global baseline ranges. Scenarios are depicted as ranges with median
emboldened; shading reflects interquartile range (darkest), 5th – 95th percentile range (lighter), and full extremes (lightest). Absolute projections are subject to variation in reported
base-year emissions arising from different data sources and calibration approaches (Chaturvedi etal., 2012). Some of the range of variation in reported 2010 emissions reflects dif-
ferences in regional definitions. Sources: WG III AR5 Scenario Database (Annex II.10), van Vuuren etal. (2011a). Historic data: JRC / PBL (2013), IEA (2012a), see Annex II.9.
RCP 8.5
RCP 6.0
RCP 4.5
RCP 2.6
0
20
40
60
80
100
120
140
1970 1990 2010 2030 2050 2070 2090
History
-20
20
60
100
140
180
1970 1990 2010 2030 2050 2070 2090
Annual Fossil Fuel and Industrial CO
2
Emissions [GtCO
2
/yr]
Annual Fossil Fuel and Industrial CO
2
Emissions [GtCO
2
/yr]
OECD-1990
Non-OECD-1990
History
Default
Fast
5
th
Percentile
Max
Min
75
th
Percentile
95
th
Percentile
Median
25
th
Percentile
Figure 6�3 | Indexed change through 2050 in carbon intensity of energy and per capita energy use in baseline scenarios. Color reflects indexed 2050 global fossil fuel and indus-
trial (FF&I) CO
2
emissions according to key in right panel showing histogram of plotted scenarios. For default population projections, emissions are correlated with chart position;
exceptions with high population are noted. Source: UN (2012), WG III AR5 Scenario Database (Annex II.10). Historic data: JRC / PBL (2013), IEA (2012a), see Annex II.9; BP (2013).
2050 Carbon Intensity Relative to 2010
2050 Per Capita Energy Relative to 2010
RCP 8.5
1.1-1.3 1.3-1.5 1.5-1.7 1.7-1.9 1.9-2.1 2.1-2.3 2.3-2.5 2.5-2.7 2.7-2.9 2.9-3.1
Number of Scenarios
2050 Fossil Fuel and Industrial CO
2
Emissions Relative to 2010
0.6
0.8
1
1.2
1.4
0
20
40
60
80
100
0.6 0.8 1 1.2 1.4 1.6 1.8 2
Population Outlier
RoSE High Population
A common characteristic of all baseline scenarios is that the major-
ity of emissions over the next century occur among non-OECD-1990
countries (Figure 6.4, right panel). Because of its large and growing
population and projected rates of economic growth relatively faster
than the industrialized OECD-1990 countries, this region is projected to
have the dominant share of world energy demand over the course of
the next century. While the range of emissions projected in the OECD-
1990 region remains roughly constant (a few models have higher
growth projections), nearly all growth in future baseline emissions is
projected to occur in the non-OECD-1990 countries. It is important to
note that while a baseline by construction excludes explicit climate
policies, management of non-climate challenges, particularly in the
context of sustainable development, will likely impact baseline GHG
pathways. Many of these policy objectives (but likely not all) are taken
into account in baseline scenarios, such as reductions in local air pol-
lution and traditional biomass use and fuel switching more generally
away from solids towards refined liquids and electricity. Section 6.6
provides more details on this issue.
6�3�1�4 Baseline CO
2
emissions from land use and
emissions of non-CO
2
gases
Baseline projections for global land-use related carbon emissions and
sequestration (also referred to as net Agriculture, Forestry and Other
Land Use (AFOLU) CO
2
emissions) are made by a smaller subset of
models. Net AFOLU CO
2
emissions have greater historical uncertainty
than FF&I emissions as discussed in Section 11.2 (Pan et al., 2011;
428428
Assessing Transformation Pathways
6
Chapter 6
Houghton etal., 2012). Baseline projections for land-use related CO
2
emissions reflect base-year uncertainty and suggest declining annual
net CO
2
emissions in the long run (Figure 6.5, left panel). In part,
projections are driven by technological change, as well as projected
declining rates of agriculture area expansion, a byproduct of decelerat-
ing population growth. Though uncertain, the estimated contribution
of land-use related carbon over the coming century is small relative
to emissions from fossil fuels and industry, with some models project-
ing a net sink late in the century. For non-CO
2
GHGs, the contribu-
tion in CO
2
eq terms is larger than land-use CO
2
with projected emis-
sions increasing over time (Figure 6.5, left panel). Along with fugitive
methane and a few industrial sources, land-use related activities are
projected to be a major driver of non-CO
2
emissions, accounting for
roughly 50 % of total methane (CH
4
) emissions and 90 % of nitrous
oxide (N
2
O) emissions. Total CO
2
eq emissions are projected as the
sum of FF&I CO
2
, land-use related CO
2
, and non-CO
2
(Figure 6.5, right
panel), with FF&I CO
2
constituting around 80 %.
6�3�1�5 Baseline radiative forcing and cumulative carbon
emissions
The emissions pathways for all of the emissions from the scenarios col-
lected for this assessment were run through a common version of the
MAGICC model to obtain estimates of CO
2
eq concentrations (Section
6.3.2). As a result of projected increasing emissions in the scenarios,
radiative forcing from all sources continues to grow throughout the
century in all baseline scenarios, exceeding 550 CO
2
eq (3.7 W / m
2
)
between 2040 and 2050, while 450 CO
2
eq (2.6 W / m
2
) is surpassed
between 2020 and 2030 (Figure 6.6, left panel). Again, the majority of
baseline forcing scenarios fall below the RCP 8.5 path but above RCP
6.0. Total forcing projections include the highly uncertain contribution
of aerosols and other non-gas agents, which are based on the MAGICC
model’s median estimates of forcing as a function of aerosol emissions
(for scenarios that do not project emissions of these substances, emis-
sions were prescribed from other sources; see Annex II.10). Due to
variation in driver assumptions, which may not reflect true uncertainty,
baseline scenarios could lead to a range of long-term climate out-
comes, with cumulative carbon emissions from 1751 to 2100 reaching
between 1.5 and 3 TtC (Figure 6.6, right panel). Noting that all of the
baseline scenarios reviewed here include improvements to technology
throughout the economy, there is strong evidence that, conditional on
rates of growth assumed in the literature, technological change in the
absence of explicit mitigation policies is not sufficient to bring about
stabilization of GHG concentrations.
6�3�2 Emissions trajectories, concentrations,
and temperature in transformation
pathways
6�3�2�1 Linking between different types of scenarios
There are important differences among long-term scenarios that compli-
cate comparison between them. One difference is the nature of the goal
itself. The majority of long-term scenarios focus on reaching long-term
radiative forcing or GHG concentration goals. However, scenarios based
on other long-term goals have also been explored in the literature. This
includes scenarios focused on specific policy formulations (e. g., goal of
50 % emission reduction in 2050 (G8, 2009) or the pledges made in the
context of United Nations Framework Convention on Climate Change
(UNFCCC) (UNFCCC, 2011a; b)), those based on cumulative emissions
goals over a given period, those based on prescribed carbon prices, and
those resulting from cost-benefit analysis (see Box 6.1 for a discussion
of cost-benefit analysis scenarios). A second important difference is that
some scenarios include all relevant forcing agents, while others only
cover a subset of gases or focus only on CO
2
. Finally, some scenarios
Box 6�1 | Cost benefit analysis scenarios
Cost-benefit studies (e. g. Tol, 1997; Nordhaus and Boyer, 2000;
Hope, 2008) monetize the impacts of climate change and then
balance the economic implications of mitigation and climate
damages to identify the optimal trajectory of emissions reductions
that will maximize total welfare. There are other frameworks of
analysis for considering impacts as well (Bradford, 1999; Barrett,
2008; Keller etal., 2008b). For example, risk assessment is also
often used to determine overall goals. A theoretical discussion of
cost-benefit analysis, including models that have conducted these
analyses, can be found in both Chapters 2 and 3. One important
characteristic of cost-benefit analyses is that the bulk of research
in this domain has been conducted using highly-aggregate models
that do not have the structural detail necessary to explore the
nature of energy system or agricultural and land-use transitions
that are the focus of this chapter. For this reason, they are not
assessed in this chapter. In contrast, the scenarios explored here
rely on more detailed integrated models and have been imple-
mented in a cost-effectiveness framework, meaning that they are
designed to find a least-cost approach to meeting a particular
goal, such as a concentration goal in 2100. Additionally, the
scenarios and models described in this chapter typically examine
mitigation independent from potential feedbacks from climate
impacts and adaptation responses. A discussion of studies that
do incorporate impacts into their assessment of transformation
pathways, and a characterization of how these feedbacks might
affect mitigation strategies, is provided in Section 6.3.3.
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6
Chapter 6
Figure 6�5 | Global CO
2
-equivalent emissions in baseline scenarios by component (left panel) and total (right panel) for baseline scenarios. Net AFOLU CO
2
and total non-CO
2
(CH
4
,
N
2
O, and F-gases) projections are shown for individual models from EMF27. The FF&I CO
2
projections are depicted in detail above (see Fig.6.4); the range is truncated here. FF&I
CO
2
includes CO
2
from AFOLU fossil fuel use. Total CO
2
eq emissions* are shown for all baseline scenarios with full coverage, depicted as a range with median emboldened; shad-
ing reflects interquartile range (darkest), 5th – 95th percentile range (lighter), and full range (lightest). Sources: WG III AR5 Scenario Database (Annex II.10); historic data: JRC / PBL
(2013), IEA (2012a), see Annex II.9.
Note: In this chapter, CO
2
eq emissions are constructed using Global Warming Potentials (GWPs) over a 100-year time horizon derived from the IPCC Second Assessment Report
(see Annex II.9.1 for the GWP values of the different GHGs). A discussion about different GHG metrics can be found in Sections 1.2.5 and 3.9.6.
0-100
th
5-95
th
25-75
th
Percentile
History
Total GHG
-5
5
15
25
35
Net AFOLU CO
2
Non-CO
2
FF&I CO
2
0
40
80
120
160
200
1970 1990 2010 2030 2050 2070 2090 1970 1990 2010 2030 2050 2070 2090
Annual GHG Emissions [GtCO
2
eq/yr]
Annual GHG Emissions [GtCO
2
eq/yr]
Figure 6�6 | Total radiative forcing (left panel) and cumulative carbon emissions since 1751 (right panel) in baseline scenario literature compared to RCP scenarios. Forcing was
estimated ex-post from models with full coverage the median output from the MAGICC results. Secondary axis in the left panel expresses forcing in CO
2
eq concentrations. Scenarios
are depicted as ranges with median emboldened; shading reflects interquartile range (darkest), 5th – 95th percentile range (lighter), and full range (lightest). Sources: WG III AR5
Scenario Database (Annex II.10); Boden etal. (2013); Houghton (2008); van Vuuren etal. (2011a).
0
1
2
3
4
5
6
7
8
9
10
Total Radiative Forcing [W/m
2
]
1600
1200
900
700
550
450
0
1
2
3
2010 2030 2050 2070 2090 2010 2030 2050 2070 2090
Cumulative Carbon Emissions [TtC]
0.55 TtC (1751-2010)
CO
2
-Equivalent Concentration
[ppm CO
2
eq]
Percentile0-100
th
5-95
th
25-75
th
RCP 8.5
RCP 6.0
RCP 4.5
RCP 2.6
430430
Assessing Transformation Pathways
6
Chapter 6
allow concentrations to temporarily exceed long-term goals (overshoot
scenarios), while others are formulated so that concentrations never
exceed the long-term goal (‘not-to-exceed scenarios’).
Despite these differences, it is necessary for the purposes of assess-
ment to establish comparability across scenarios. To this end, scenarios
assessed here have been grouped according to several key param-
eters (Table 6.2) (for more detail on this process, see Annex II.10). The
main criterion for grouping is the full radiative forcing level in 2100,
expressed in CO
2
eq concentrations. (Full radiative forcing here includes
GHGs, halogenated gases, tropospheric ozone, aerosols, and land-use
related albedo change). Radiative-forcing levels are often used as goal
in scenarios, and the RCPs have been formulated in terms of this indica-
tor (Moss etal., 2010; van Vuuren etal., 2011a). The scenario catego-
ries were chosen to relate explicitly to the four RCPs. A similar table in
AR4 (Table 3.5) presented equilibrium values rather than 2100 values.
Equilibrium values (as presented in AR4) and 2100 concentration and
temperature values (as presented in this report) cannot easily be com-
pared given the wide range of possible post-2100 trajectories and the
lags in the physical processes that govern both. In particular, equilib-
rium values assume that concentrations stay constant after 2100, while
many scenarios in the literature since AR5 show increasing or decreas-
ing concentrations in 2100. Thus, it is more appropriate to focus on 21st
century values to avoid relying on additional assumptions about post-
2100 dynamics.
Another issue that complicates comparison across scenarios reported
in the literature is that the Earth-System components (e. g., the carbon
cycle and climate system) of integrated models can vary substantially
(van Vuuren et al., 2009b). Hence, similar emissions pathways may
arrive at different 2100CO
2
eq concentration levels and climate out-
comes in different models. To provide consistency in this regard across
the scenarios assessed in the scenario database for AR5 (Annex II.10),
and to facilitate the comparison with the assessment in Working Group
I (WGI), the variation originating from the use of different models was
removed by running all the scenarios in the database with at least
information on Kyoto gas emissions through a standard reduced-form
climate model called MAGICC (see Meinshausen etal., 2011a; b; c;
Rogelj etal., 2012). For each scenario, MAGICC was run multiple times
using a distribution of Earth-System parameters, creating an ensemble
of MAGICC runs. The resulting median concentration from this distribu-
tion was used to classify each scenario (see Section 6.3.2.6 for more on
this process and a discussion of temperature outcomes). This means
that the median concentration information reported here does not
reflect uncertainty by Earth-System components, unless mentioned
otherwise, and it also means that the concentrations may differ from
those that were originally reported in the literature for the individual
models and scenarios.
The consistency of the MAGICC model version used here and the more
comprehensive general circulation models used in the WGI report
(IPCC, 2013) is discussed in Section 6.3.2.6, where MAGICC is also
used to produce probabilistic temperature estimates. The CO
2
eq con-
centration in 2010 based on the parameters used in this version of
MAGICC is roughly consistent with the 2011 radiative forcing estimate
from WGI.
Table 6�2 | Definition of CO
2
eq concentration categories used in this assessment, the mapping used to allocate scenarios based on different metrics to those categories, and the
number of scenarios that extend through 2100 in each category. [Note: This table shows the mapping of scenarios to the categories; Table 6.3. shows the resulting characteristics
of the categories using this mapping. The table only covers the scenarios with information for the full 21st century. The mapping of scenarios based on 2011 – 2050 cumulative total
CO
2
eq emissions is described in the Methods and Metrics Annex.
CO
2
-equivalent concentration in 2100 (ppm
CO
2
eq) (based on full radiative forcin g)
1
Secondary categorization criter ia
2
Corresponding RC P
3
No of scenarios extending through 2100
CO
2
eq concentration
(ppm)
Radiative forcing
(W / m
2
)
Kyoto gas only
CO
2
eq concentration
in 2100 (ppm)
Cumulative total
CO
2
emissions
2011 – 2100 (GtCO
2
)
Tota l
4
With Overshoot
Greater than
0�4W / m
2
430 – 480 2.3 – 2.9 450 – 500 < 950 RCP 2.6 114 (114) 72 (72)
480 – 530 2.9 – 3.45 500 – 550 950 – 1500 251 (257) 77 (77)
530 – 580 3.45 – 3.9 550 – 600 1500 – 1950 198 (222) 22 (22)
580 – 650 3.9 – 4.5 600 – 670 1950 – 2600
RCP 4.5
102 (109) 8 (8)
650 – 720 4.5 – 5.1 670 – 750 2600 – 3250 27 (27) 0 (0)
720 – 1000 5.1 – 6.8 750 – 1030 3250 – 5250 RCP .6 111 (120) 0 (0)
>1000 > 6.8 >1030 > 5250 RCP 8.5 160 (166) 0 (0)
1
Scenarios with information for the full 21st century were categorized in different categories based on their 2100 full radiative forcing / CO
2
eq concentration level (including
GHGs and other radiatively active substances).
2
If insufficient information was available to calculate full forcing, scenarios were categorized, in order of preference, by 2100 Kyoto gas forcing or cumulative CO
2
emissions
in the 2011 – 2100 period. Scenarios extending only through 2050 were categorized based on cumulative CO
2
emissions in the 2011 – 2050 period. Those scenarios are not
included in this table. (See the Methods and Metrics Annex for more information.)
3
The column indicates the corresponding RCP falling within the scenario category based on 2100 CO
2
equivalent concentration.
4
Number of scenarios in the respective category, which report at least total CO
2
emissions (and potentially other GHGs and other radiatively active substances) to 2100. Numbers
in parentheses denote all scenarios in the respective category, including those scenarios that report only CO
2
emissions from fossil fuels and industry (but not land-use CO
2
).
431431
Assessing Transformation Pathways
6
Chapter 6
Table 6�3 | Key characteristics of the scenarios categories introduced in Table 6.2. For all parameters, the 10th to 90th percentile of the scenarios are shown.
1
Source: WG III AR5 Scenario Database (Annex II.10).
CO
2
-
equivalent
concentration
in 2100
(ppm CO
2
eq )
2
Subcategories
Cumulative CO
2
emiss ions
3
(GtCO
2
)
CO
2
eq�
emissions in
2050 relative
to 2010 (% )
4
CO
2
eq
emissions in
2100 relative
to 2010 (%)
Concentration (ppm )
5
Temperature (relative to 1850 – 1900 )
6, 7
2011 – 2050 2011 – 2100 CO
2
in 2100 Peak CO
2
eq�
2100 Temperature
(°C)
Probability of
Exceeding
1�5 °C (%)
Probability of
Exceeding
2 °C (%)
Probability of
Exceeding
3 °C (%)
Probability of
Exceeding
4 °C (%)
430 – 480 Total range 550 – 1300 630 – 1180 – 72 to – 41 – 118 to – 78 390 – 435 465 – 530 1.5 – 1.7 (1.0 – 2.8) 49 – 86 12 – 37 1 – 3 0 – 1
Overshoot <0.4 W / m
2
550 – 1030 630 – 1180 – 72 to – 49 – 94 to – 78 390 – 435 465 – 500 1.5 – 1.7 (1.0 – 2.6) 49 – 72 12 – 22 1 – 2 0 – 0
Overshoot >0.4 W / m
2
920 – 1300 670 – 1180 – 66 to – 41 – 118 to – 103 400 – 435 505 – 530 1.6 – 1.7 (1.1 – 2.8) 76 – 86 22 – 37 1 – 3 0 – 1
480 – 530 Total range 860 – 1600 960 – 1550 – 57 to 4
8
– 179 to – 127 425 – 460 505 – 575 1.7 – 2.1 (1.2 – 3.3) 80 – 96 32 – 61 3 – 10 0 – 2
Overshoot <0.4 W / m
2
870 – 1240 960 – 1490 – 57 to – 42 – 103 to – 76 425 – 460 505 – 560 1.8 – 2.0 (1.2 – 3.2) 81 – 94 32 – 56 3 – 10 0 – 2
Overshoot >0.4 W / m
2
1060 – 1600 1020 – 1500 – 54 to 4
8
– 179 to – 98 425 – 460 530 – 575 1.8 – 2.1 (1.2 – 3.3) 86 – 96 38 – 61 3 – 10 1 – 2
No exceedance of 530 ppm CO
2
eq 860 – 1180 960 – 1430 – 57 to – 42 – 107 to – 73 425 – 455 505 – 530 1.7 – 1.9 (1.2 – 2.9) 80 – 87 32 – 40 3 – 4 0 – 1
Exceedance of 530 ppm CO
2
eq 1130 – 1530 990 – 1550 – 55 to – 25 – 114 to – 90 425 – 460 535 – 575 1.8 – 2.0 (1.2 – 3.3) 88 – 96 39 – 61 4 – 10 1 – 2
530 – 580 Total range 1070 – 1780 1170 – 2240 – 47 to 7 – 184 to – 59 425 – 520 540 – 640 2.0 – 2.3 (1.4 – 3.6) 93 – 99 54 – 84 8 – 19 1 – 3
Overshoot <0.4 W / m
2
1090 – 1490 1400 – 2190 – 47 to – 12 – 86 to – 60 465 – 520 545 – 585 2.0 – 2.2 (1.4 – 3.6) 93 – 96 55 – 71 8 – 14 1 – 2
Overshoot >0.4 W / m
2
1540 – 1780 1170 – 2080 – 7 to 7 – 184 to – 98 425 – 505 590 – 640 2.1 – 2.2 (1.4 – 3.6) 95 – 99 63 – 84 8 – 19 1 – 3
No exceedance of 580 ppm CO
2
eq 1070 – 1460 1240 – 2240 – 47 to – 19 – 81 to – 59 450 – 520 540 – 575 2.0 – 2.2 (1.4 – 3.6) 93 – 95 54 – 70 8 – 13 1 – 2
Exceedance of 580 ppm CO
2
eq 1420 – 1750 1170 – 2100 – 16 to 7 – 183 to – 86 425 – 510 585 – 640 2.1 – 2.3 (1.4 – 3.6) 95 – 99 66 – 84 8 – 19 1 – 3
580 – 650 Total range 1260 – 1640 1870 – 2440 – 38 to 24 – 134 to – 50 500 – 545 585 – 690 2.3 – 2.6 (1.5 – 4.2) 96 – 100 74 – 93 14 – 35 2 – 8
650 – 720 Total range 1310 – 1750 2570 – 3340 – 11 to 17 – 54 to – 21 565 – 615 645 – 710 2.6 – 2.9 (1.8 – 4.5) 99 – 100 88 – 95 26 – 43 4 – 10
720 – 1000 Total range 1570 – 1940 3620 – 4990 18 to 54 – 7 to 72 645 – 780 765 – 935 3.1 – 3.7 (2.1 – 5.8) 100 – 100 97 – 100 55 – 83 14 – 39
>1000 Total range 1840 – 2310 5350 – 7010 52 to 95 74 to 178 810 – 975 1075 – 1285 4.1 – 4.8 (2.8 – 7.8) 100 – 100 100 – 100 92 – 98 53 – 78
1
Italicized text in blue shows results of the subset of the scenarios from column one. One subcategory distinguishes scenarios that have a large overshoot (i. e., a maximum forcing during the 21st century that is >0.4 W / m
2
higher than
its 2100 forcing) from those that do not have a large overshoot. The second set of subcategories shows whether a scenario exceeds the maximum equivalent concentration level of its category somewhere before 2100. For categories
above 580 ppm CO
2
eq, the information in the row ‘total range’ refers to the 10th to 90th percentiles for the total set of scenarios in the category. For the categories below 580 ppm CO
2
eq, the total range is based on the 10th to 90th
percentiles of the subcategories (the lowest and highest values from the subcategories).
2
The CO
2
eq concentration includes the forcing of all GHGs including halogenated gases and tropospheric ozone, as well as aerosols and albedo change (calculated on the basis of the total forcing from a simple carbon cycle / climate
model MAGICC).
3
For comparison of the cumulative CO
2
emissions estimates assessed here with those presented in WGI AR5, an amount of 515 [445 to 585] GtC (1890 [1630 to 2150] GtCO
2
), was already emitted by 2011 since 1870 (WGI Sec-
tion 12.5). Note that cumulative CO
2
emissions are presented here for different periods of time (2011 – 2050 and 2011 – 2100) while cumulative CO
2
emissions in WGI AR5 are presented as total compatible emissions for the RCPs
(2012 – 2100) or for total compatible emissions for remaining below a given temperature target with a given likelihood. (WGI Table SPM.3, WGI SPM.E.8)
4
The global 2010 emissions are 31 % above the 1990 emissions (consistent with the historic GHG emission estimates presented in this report). CO
2
eq emissions include the basket of Kyoto gases (CO
2
, CH4, N2O as well as F-gases).
5
The assessment in WGIII AR5 involves a large number of scenarios published in the scientific literature and is thus not limited to the RCPs. To evaluate the CO
2
eq concentration and climate implications of these scenarios, the MAGICC
model was used in a probabilistic mode (see Annex II). For a comparison between MAGICC model results and the outcomes of the models used in WGI AR5, see WGI Sections 12.4.1.2, 12.4.8 and Section 6.3.2.6 of this report.
Reasons for differences with WGI AR5 SPM Table.2 include the difference in reference year (1986 – 2005 vs. 1850 – 1900 here), difference in reporting year (2081 – 2100 vs 2100 here), set-up of simulation (CMIP5 concentration-driven
versus MAGICC emission-driven here), and the wider set of scenarios (RCPs versus the full set of scenarios in the WGIII AR5 scenario database here).
6
Temperature change in 2100 is provided for a median estimate of the MAGICC calculations, which illustrates differences between the emissions pathways of the scenarios in each category. The range of temperature change in the
parentheses includes in addition also the carbon cycle and climate system uncertainties as represented by the MAGICC model (see 6.3.2.6 for further details). The temperature data compared to the 1850 – 1900 reference year was
calculated by taking all projected warming relative to 1986 – 2005, and adding 0.61 °C for 1986 – 2005 compared to 1850 – 1900, based on HadCRUT4, as also applied in WGI Table SPM.2.
7
Temperature change is reported for the year 2100, which is not directly comparable to the equilibrium warming reported in WGIII AR4 (see Table 3.5; see also Section 6.3.2). For the 2100 temperature estimates, the transient climate
response (TCR) is the most relevant system property. The assumed 90 % range of the TCR for MAGICC is 1.2 – 2.6 °C (median 1.8 °C). This compares to the 90 % range of TCR between 1.2 – 2.4 °C for CMIP5 (WGI Section 9.7) and an
assessed likely range of 1 – 2.5 °C from multiple lines of evidence reported in the WGI AR5 (Box 12.2 in Section 12.5).
8
The high estimate is influenced by multiple scenarios from the same model in this category with very large net negative CO
2
eq emissions of about 40 GtCO
2
eq / yr in the long term. The higher bound CO
2
eq emissions estimate, exclud-
ing extreme net negative emissions scenarios and thus comparable to the estimates from the other rows in the table, is about – 19 % in 2050 relative to 2010.
432432
Assessing Transformation Pathways
6
Chapter 6
To compare scenarios with different coverage of relevant substances or
goals, a set of relationships was developed to map scenarios with only
sufficient information to assess Kyoto gas forcing or with information
only on cumulative CO
2
budgets to the full-forcing CO
2
eq concentra-
tion categories (Table 6.2 and Method and Metrics Annex). Scenarios
without full forcing information and that extend to the end of the cen-
tury were mapped, in order of preference, by Kyoto gas forcing in 2100
or by cumulative CO
2
budgets from 2011 to 2100. In addition, scenar-
ios that only extend to mid-century were mapped according to cumu-
lative CO
2
budgets from 2011 to 2050. These mappings allow for a
practical, though still imperfect, means to compare between scenarios
with different constructions.
The categories leading to CO
2
eq concentration above 720 ppm con-
tain mostly baseline scenarios and some scenarios with very modest
mitigation policies (Figure 6.7). The categories from 580 – 720 ppm
CO
2
eq contain a small number of baseline scenarios at the upper end
of the range, some scenarios based on meeting long-term concentra-
tion goals such as 650 ppm CO
2
eq by 2100, and a number of scenarios
without long-term concentration goals but based instead on emissions
goals. There has been a substantial increase in the number of scenarios
in the two lowest categories since AR4 (Fisher etal., 2007). The RCP2.6
falls in the 430 – 480 ppm CO
2
eq category based on its forcing level by
2100. A limited number of studies (Rogelj et al 2013a,b; Luderer et
al, 2013) have explored emissions scenarios leading to concentrations
below 430 ppm CO
2
eq by 2100. These scenarios were not submitted to
the AR5 database.
This mapping between different types of scenarios allows for roughly
comparable assessments of characteristics of scenarios, grouped by
2100 full-forcing CO
2
eq concentration, across the full database of sce-
narios collected for AR5 (Table 6.3.). The cumulative CO
2
budgets from
2011 to 2100 in each category in Table 6.3 span a considerable range.
This variation in CO
2
budgets results from the range of concentration
levels assigned to each category, the timing of emission reductions, and
variation in non-CO
2
emissions, including aerosols. Although this leads
Figure 6�7 | Emissions pathways for total CO
2
and Kyoto gases for the various categories defined in Table 6.2. The bands indicate the 10
th
to 90
th
percentile of the scenarios
included in the database. The grey bars to the right of the top panels indicate the 10
th
to 90
th
percentile for baseline scenarios (see Section6.3.1). The bottom panels show for
the combined categories 430 – 530 ppm and 530 – 650 ppm CO
2
eq the scenarios with and without net negative emissions larger than 20 GtCO
2
eq / yr. Source: WG III AR5 Scenario
Database (Annex II.10).
Annual CO
2
Emissions [GtCO
2
/yr]
Total GHG Emissions in all AR5 Scenarios Total CO
2
Emissions in all AR5 Scenarios
GHG Emissions with Different Assumptions for Negative Emissions
Annual GHG Emissions [GtCO
2
eq/yr]
Annual GHG Emissions [GtCO
2
eq/yr]
GHG Emissions with Different Assumptions for Negative Emissions
> 1000
720 - 1000
580 - 720
530 - 580
480 - 530
430 - 480
Full AR5 Database Range
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
RCP2.6
2000 2020 2040 2060 2080 2100
-20
0
20
40
60
80
100
120
140
Baseline Range (2100)
RCP8.5
RCP6.0
RCP4.5
RCP2.6
430-530 ppm CO
2
eq 530-650 ppm CO
2
eq
Baseline Range (2100)
> 20 GtCO
2
/yr
All AR5 Scenarios
< 20 GtCO
2
/yr
Net Negative Emissions
> 20 GtCO
2
/yr
All AR5 Scenarios
< 20 GtCO
2
/yr
Net Negative Emissions
RCP2.6
2000 2020 2040 2060 2080 2100
-40
-20
0
20
40
60
80
2000 2020 2040 2060 2080 2100
-40
-20
0
20
40
60
80
2000 2020 2040 2060 2080 2100
-20
0
20
40
60
80
100
120
140
RCP8.5
RCP6.0
RCP4.5
RCP2.6
Annual GHG Emissions [GtCO
2
eq/yr]
90
th
percentile
Median
10
th
percentile
> 1000
720 - 1000
580 - 720
530 - 580
480 - 530
430 - 480
Full AR5 Database Range
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
433433
Assessing Transformation Pathways
6
Chapter 6
to a wider range of CO
2
budgets than for the scenarios used in WGI
(SPM Figure 10), the central estimates for the period 2011 – 2100 are
very consistent. (Temperature results are discussed in Section 6.3.2.6).
An important distinction between scenarios is the degree to which con-
centrations exceed the 2100 goal before decreasing to reach it. Table
6.3 includes subcategories for scenarios in which concentrations exceed
their 2100 level by more than 0.4 W / m
2
and scenarios that sometime
during the century overshoot the upper-bound concentration level of
the category. Both subcategories result in different emission profiles
and temperature outcomes compared to those that do not meet these
criteria (see Section 6.3.2.6 regarding temperature outcomes).
6�3�2�2 The timing of emissions reductions: The
influence of technology, policy, and overshoot
There are many different emissions pathways associated with meet-
ing 2100 CO
2
eq concentrations (Figure 6.7). For all categories below
a 2100 CO
2
eq concentration of 720 ppm CO
2
eq, emissions are reduced
in the long-run relative to current levels. The decision on timing of
emission reductions is a complex one. Model scenarios are typically
designed to find the least-cost pathway to meet a long-term goal, in
some cases under specific constraints, such as the availability of cer-
tain technologies or the timing and extent of international participa-
tion. Because models differ in, among other things, technology rep-
resentations and baseline assumptions, there are clear differences
among scenarios with regards to the timing of emissions reductions
and the allocation of reductions across gases.
Three interrelated factors are particularly important determinants of
emissions profiles in the modelling literature: (1) the degree of over-
shoot, (2) technology options and associated deployment decisions,
and (3) policy assumptions. Overshoot scenarios scenarios entail less
mitigation today in exchange for greater reductions later (Wigley,
2005; Meinshausen etal., 2006; den Elzen and van Vuuren, 2007; Nus-
baumer and Matsumoto, 2008). Overshooting a long-term concentra-
tion goal, however, may lead to higher transient temperature change
than if the goal is never exceeded (Section 6.3.2.6). Overshoot is par-
ticularly important for concentration goals that are close to today’s
levels. The majority of scenarios reaching 480 ppm CO
2
eq or below
by 2100, for instance, rely on overshoot pathways. Those that do not
include overshoot, need faster emissions reductions (and associated
energy system changes) during the next 1 – 2 decades (Calvin etal.,
2009b).
The second consideration is technology. The most critical set of tech-
nologies in the context of the timing of emission reductions is CDR
technologies, which can be used to generate negative emissions (van
Vuuren et al., 2007; Edenhofer etal., 2010; Azar et al., 2010, 2013;
van Vuuren and Riahi, 2011; Tavoni and Socolow, 2013). In most model
studies in the literature, negative emissions are generated via the use
of biomass energy with carbon dioxide capture and storage (BECCS),
and to a lesser extent, afforestation, though in principle other options
could potentially result in negative emissions as well (see Section
6.9). CDR technologies have not been applied yet at large scale. The
potential of afforestation is limited, and the use of BECCS is ultimately
constrained by the potential for CCS and biomass supply (van Vuuren
etal., 2013). CDR technologies have two key implications for transfor-
mation pathways. One is that by removing emissions from the atmo-
sphere, CDR technologies can compensate for residual emissions from
technologies and sectors with more expensive abatement. The second
is that CDR technologies can create net negative emissions flows,
which allow faster declines in concentrations in the second half of
the century and thus facilitate higher near-term emissions, effectively
expanding the potential scope for overshoot. In model comparison
studies, many of the models that could not produce scenarios lead-
ing to concentrations of about 450 ppm CO
2
eq by 2100, particularly
in combination with delayed or fragmented policy approaches, did
not include CDR techniques (Clarke et al., 2009). The vast majority
of scenarios with overshoot of greater than 0.4 W / m
2
(greater than
20 ppm CO
2
eq) deploy CDR technologies to an extent that net global
CO
2
emissions become negative. Evidence is still mixed whether CDR
technologies are essential for achieving very low GHG concentration
goals (Rose etal., 2013). A limited number of studies have explored
scenarios with net negative emissions as large as 20 GtCO
2
per year or
more (lower panels Figure 6.7), which allow for very substantial delays
in emission reductions. However, the majority of studies have explored
futures with smaller, but often still quite substantial, contributions of
CDR technologies. Technology portfolio assumptions other than CDR
technologies (e. g., regarding renewables, CCS, efficiency, and nuclear
power) can also have implications for emissions trajectories, although
these are often less pronounced and may in fact shift mitigation earlier
or later (Rogelj etal., 2012; Eom etal., 2014; Krey etal., 2014; Kriegler
etal., 2014a; Riahi etal., 2014).
The third consideration is policy structure. Since AR4, scenario studies
have increasingly focused on the outcomes of fragmented international
action and global delays in emission reduction (Clarke etal., 2009; van
Vliet etal., 2012; Kriegler etal., 2013b; Tavoni etal., 2013; Rogelj etal.,
2013a; see Riahi etal., 2014). Considering both idealized implementa-
tion and non-idealized implementation scenarios, a considerable range
of 2020 and 2030 emissions can be consistent with specific long-term
goals. Although studies show that low long-term concentration goals
could still be met with near-term emissions above those in idealized
scenarios, initial periods of delay are typically followed by periods
rapid reductions in subsequent decades (Kriegler etal., 2014c; Riahi
etal., 2014). This has important implications for costs and technology
transitions, among other things (see Section 6.3.5). In general, delays
in mitigation decrease the options for meeting long-term goals and
increase the risk of foreclosing on certain long-term goals (Riahi etal.,
2014).
The intersection of these three factors — overshoot, CDR technologies,
and delayed mitigation — can be viewed in the context of emissions
pathways over the next several decades, for example, the emissions
434434
Assessing Transformation Pathways
6
Chapter 6
level in 2030 (Figure 6.8). For a given range of forcing at the end of the
century, pathways with the lowest levels in 2030 have higher emis-
sions in the long run and slower rates of decline in the middle of the
century. On the other hand, high emissions in 2030 leads to more rapid
declines in the medium term and lower or eventually net negative
emissions in the long-run, with the pattern exaggerated in a few
extreme scenarios exploring deployment of CDR of 20 GtCO
2
/ yr or
more. (See Section 6.4 for a more thorough discussion of the relation-
ship between near-term actions and long-term goals.) Deeper long-
term goals also interact with these factors. For example, scenarios
leading to concentrations below 430 ppm CO
2
eq by 2100 (Rogelj etal.,
2013a,b; Luderer etal., 2013) feature large-scale application of CDR
technologies in the long-term, and most of them have deep emission
reductions in the near term.
A final observation is that the characteristics of emissions profiles dis-
cussed here are, in many cases, driven by the cost-effectiveness fram-
ing of the scenarios. A more comprehensive consideration of timing
would also include, among other things, considerations of the tradeoff
between the risks related to both transient and long-term climate
change, the risks associated with deployment of specific technologies
and expectation of the future developments of these technologies,
short-term costs and transitional challenges, flexibility in achieving
climate goals, and the linkages between emissions reductions and a
wide range of other policy objectives (van Vuuren and Riahi, 2011; Krey
etal., 2014; Riahi etal., 2014).
6�3�2�3 Regional roles in emissions reductions
The contribution of different regions to mitigation is directly related to
the formulation of international climate policies. In idealized imple-
mentation scenarios, which assume a uniform global carbon price, the
extent of mitigation in each region depends most heavily on relative
baseline emissions, regional mitigation potentials, and terms of trade
effects. All of these can vary significantly across regions (van Vuuren
etal., 2009a; Clarke etal., 2012; Tavoni etal., 2013; Chen etal., 2014;
van Sluisveld etal., 2013). In this idealized implementation environ-
ment, the carbon budgets associated with bringing concentrations to
between 430 and 530 ppm CO
2
eq in 2100 are generally highest in Asia,
smaller in the OECD-1990, and lowest for other regions (Figure 6.9, left
panel). However, the ranges for each of these vary substantially across
scenarios. Mitigation in terms of relative reductions from baseline
emissions is distributed more similarly between OECD-1990, ASIA, and
Economies in Transition (EIT) across scenarios (Figure 6.9, right panel).
The Middle East and Africa (MAF) region and especially Latin America
(LAM) have the largest mitigation potential. In absolute terms, the
remaining emissions in the mitigation scenarios are largest in Asia
(Figure 6.9, left panel) as are the absolute emissions reductions (Figure
6.9, right panel), due to the size of this region. It is important to note
that the mitigation costs borne by different regions and countries do
not need to translate directly from the degree of emissions reductions,
because the use of effort-sharing schemes can reallocate economic
costs (see Section6.3.6.6).
Figure 6�8 | Emissions pathways from three model comparison exercises with explicit 2030 emissions goals. Mitigation scenarios are shown for scenarios reaching 430 – 530 ppm
CO
2
eq in 2100 (left panel) and 530 – 650 ppm CO
2
eq in 2100 (right panel). Scenarios are distinguished by their 2030 emissions: <50 GtCO
2
eq, 50 – 55 GtCO
2
eq, and >55 GtCO
2
eq.
Individual emissions pathways with net negative emissions of > 20 GtCO
2
/ yr in the second-half of the century are shown as solid black lines. The full range of the scenarios in the
AR5 database is given as dashed black lines. (Source: Scenarios from intermodelling comparisons with explicit interim goals (AMPERE: Riahi etal. (2014); LIMITS: Kriegler etal.
(2013b), ROSE: Luderer etal. (2014a), and WG III AR5 Scenario Database (Annex II.10)).
-40
2000 2020 2040 2060 2080 2100
-40
-20
0
20
40
60
80
2100
2050
2030
2000 2020 2040 2060 2080 2100
-20
0
20
40
60
80
2100
2050
2030
430 – 530 ppm CO
2
eq
530 – 650 ppm CO
2
eq
Emissions Pathways are Grouped
According 2030 Emissions:
> 55 GtCO
2
50-55 GtCO
2
< 50 GtCO
2
Full AR5 Database Range
> 20 GtCO
2
Net Negative
Emissions in 2100 Pathways
Annual GHG Emissions [GtCO
2
eq/yr]
Annual GHG Emissions [GtCO
2
eq/yr]
Figure 6�9 | Regional carbon budget (left panel) and relative mitigation effort (right panel) for mitigation scenarios reaching 430 – 530 ppm CO
2
eq in 2100, based on cumulative
CO
2
emissions from 2010 to 2100. Carbon budgets below 0 and relative mitigation above 100 % can be achieved via negative emissions. The number of scenarios is reported
below the regional acronyms. The number of scenarios outside the figure range is noted at the top. Source: WG III AR5 Scenario Database (Annex II.10), idealized implementation
and default technology cases.
# of Scenarios: 108 86 101103 7997103106102112
10
th
Percentile
Mean
Outlier
75
th
Percentile
90
th
Percentile
Median
25
th
Percentile
30
40
50
60
70
80
90
100
110
120
OECD-1990 ASIA LAM MAF EIT
Mitigation [% Reduction from Baseline]
-200
0
200
400
600
800
1000
Cumulative Emissions 2010-2100 [GtCO
2
]
OECD-1990 ASIA LAM MAF EIT
3
4
435435
Assessing Transformation Pathways
6
Chapter 6
6�3�2�3 Regional roles in emissions reductions
The contribution of different regions to mitigation is directly related to
the formulation of international climate policies. In idealized imple-
mentation scenarios, which assume a uniform global carbon price, the
extent of mitigation in each region depends most heavily on relative
baseline emissions, regional mitigation potentials, and terms of trade
effects. All of these can vary significantly across regions (van Vuuren
etal., 2009a; Clarke etal., 2012; Tavoni etal., 2013; Chen etal., 2014;
van Sluisveld etal., 2013). In this idealized implementation environ-
ment, the carbon budgets associated with bringing concentrations to
between 430 and 530 ppm CO
2
eq in 2100 are generally highest in Asia,
smaller in the OECD-1990, and lowest for other regions (Figure 6.9, left
panel). However, the ranges for each of these vary substantially across
scenarios. Mitigation in terms of relative reductions from baseline
emissions is distributed more similarly between OECD-1990, ASIA, and
Economies in Transition (EIT) across scenarios (Figure 6.9, right panel).
The Middle East and Africa (MAF) region and especially Latin America
(LAM) have the largest mitigation potential. In absolute terms, the
remaining emissions in the mitigation scenarios are largest in Asia
(Figure 6.9, left panel) as are the absolute emissions reductions (Figure
6.9, right panel), due to the size of this region. It is important to note
that the mitigation costs borne by different regions and countries do
not need to translate directly from the degree of emissions reductions,
because the use of effort-sharing schemes can reallocate economic
costs (see Section6.3.6.6).
Figure 6�9 | Regional carbon budget (left panel) and relative mitigation effort (right panel) for mitigation scenarios reaching 430 – 530 ppm CO
2
eq in 2100, based on cumulative
CO
2
emissions from 2010 to 2100. Carbon budgets below 0 and relative mitigation above 100 % can be achieved via negative emissions. The number of scenarios is reported
below the regional acronyms. The number of scenarios outside the figure range is noted at the top. Source: WG III AR5 Scenario Database (Annex II.10), idealized implementation
and default technology cases.
# of Scenarios: 108 86 101103 7997103106102112
10
th
Percentile
Mean
Outlier
75
th
Percentile
90
th
Percentile
Median
25
th
Percentile
30
40
50
60
70
80
90
100
110
120
OECD-1990 ASIA LAM MAF EIT
Mitigation [% Reduction from Baseline]
-200
0
200
400
600
800
1000
Cumulative Emissions 2010-2100 [GtCO
2
]
OECD-1990 ASIA LAM MAF EIT
3
4
The transient emission reductions implications also vary across regions
in idealized implementation scenarios (Table 6.4). In general, emissions
peak in the OECD-1990 sooner than in other countries with higher
baseline growth. Similarly, emissions are reduced in the OECD-1990
countries by 2030 relative to today, but they may increase in other
regions, particularly the fast-growing Asian and MAF regions.
Deviations from the idealized implementation, either through global
delays in mitigation or delays by particular countries or regions, will
lead to different regional contributions to emissions reductions. When
mitigation is undertaken by a subset of regions, it will have implications
on other non-participating countries through energy markets, terms of
trade, technology spillovers, and other leakage channels. Multi model
ensembles have shown leakage rates of energy-related emissions to
be relatively contained, often below 20 % (Arroyo-Curras etal., 2014;
Babiker, 2005; Bauer et al., 2014a; Blanford et al., 2014; Böhringer
etal., 2012; Bosetti and De Cian, 2013; Kriegler etal., 2014c). Policy
instruments such as border carbon adjustment can effectively reduce
these effects further (Böhringer etal., 2012). Leakage in land use, on
the other hand, could be substantial, though fewer studies have quan-
tified it (Calvin etal., 2009).
6�3�2�4 Projected CO
2
emissions from land use
Net AFOLU CO
2
emissions (see Figure 6.5) result from an interplay
between the use of land to produce food and other non-energy prod-
ucts, to produce bioenergy, and to store carbon in land. Land-manage-
ment practices can also influence CO
2
emissions (see Section 6.3.5).
Currently about 10 – 20 % of global CO
2
emissions originate from land
use and LUC. In general, most scenarios show declining CO
2
emissions
from land use as a result of declining deforestation rates, both with
and without mitigation (see also Section 6.3.1.4). In fact, many scenar-
ios project a net uptake of CO
2
as a result of reforestation after 2050
(Figure 6.10).
Scenarios provide a wide range of outcomes for the contribution of CO
2
emissions from land use (see Section 11.9 for a sample from a model
intercomparison study). However, one difficulty in interpreting this
range is that many scenarios were developed from models that do not
explicitly look at strategies to reduce net AFOLU CO
2
emissions. None-
theless, the spread in net AFOLU emissions still reflects the implications
of land-use related mitigation activities — bioenergy, avoided defores-
tation, and afforestation — in both models that explicitly represent land
use and those that do not (see Section 6.3.5 for a detailed discussion).
Some studies emphasize a potential increase in net AFOLU emissions
due to bioenergy production displacing forests (van Vuuren etal., 2007;
Searchinger etal., 2008; Wise etal., 2009; Melillo etal., 2009; Reilly
et al., 2012). Others show a decrease in net AFOLU emissions as a
result of decreased deforestation, forest protection, and / or net affor-
estation enacted as a mitigation measure (e. g. Wise etal., 2009; Popp
etal., 2011b; Riahi etal., 2011; Reilly etal., 2012). Wise etal. (2009)
show a range of results from a single model, first focusing mitigation
policy on the energy sector, thereby emphasizing the bioenergy produc-
tion effect, and then focusing policy more broadly to also encourage
afforestation and slow deforestation. Reilly etal. (2012) conduct a simi-
lar analysis, but with more policy design alternatives. However, policies
to induce large-scale land-related mitigation will be challenging and
actual implementation will affect costs and net benefits (Lubowski and
Rose, 2013) (see Section 6.3.5, Section 6.8 and Chapter 11).
436436
Assessing Transformation Pathways
6
Chapter 6
6�3�2�5 Projected emissions of other radiatively
important substances
Beyond CO
2
, the scenario literature has focused most heavily on the
mitigation opportunities for the gases covered by the Kyoto protocol,
including the two most important non-CO
2
gases, CH
4
and N
2
O. Atten-
tion is also increasingly being paid to the climate consequences of
other emissions such as aerosols and ozone precursors (e. g. Shindell
etal., 2012; Rose etal., 2014b). Although several models have pro-
duced projections of aerosol forcing and have incorporated these emis-
sions into the constraint on total forcing, most of them do not have
specific mitigation measures for these emissions.
For non-CO
2
Kyoto gases, the relative depth and timing of emissions
reductions are influenced by two primary factors: (1) the abatement
potential and costs for reducing emissions of different greenhouse forc-
ers, and (2) the strategies for making tradeoffs between them. With
respect to abatement potential and costs, studies indicate that in the
short run, there are many low-cost options to reduce non-CO
2
gases
relative to opportunities to reduce CO
2
emissions. Partially as a result,
studies indicate that short-term reduction strategies may rely more
heavily in the near term on non-CO
2
gases than in the long run (Wey-
ant etal., 2006; Lucas etal., 2007). In the longer run, emission reduc-
tions, particularly for CH
4
and N
2
O, are expected to be constrained by
several hard-to-mitigate sources such as livestock and the application
of fertilizers. This ultimately results in lower reduction rates than for
CO
2
for the lower concentration categories despite slower growth in
baseline projections (see Figure 6.11, and also discussed by Lucas etal.,
2007). For scenarios resulting in 430 – 480 CO
2
eq concentration in
2100, CH
4
reductions in 2100 are about 50 % compared to 2005. For
Figure 6�10 | Net AFOLU CO
2
emissions in mitigation scenarios. The left panel shows cumulative net CO
2
emission (2011 – 2100) from energy / industry (horizontal axis) and AFOLU
(land use) (vertical axis). The right panel shows net CO
2
emission from land use as function of time. FF&I CO
2
includes CO
2
from AFOLU fossil fuel use. Source: WG III AR5 Scenario
Database (Annex II.10).
25
th
Percentile
75
th
Percentile
Median
-1000
-750
-500
-250
0
250
500
-10
-8
-6
-4
-2
0
2
4
>1000
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
720-1000
580-720
530-580
480-530
430-480
Net AFOLU CO
2
Emissions (2011-2100) [GtCO
2
/yr]
Cumulative Net AFOLU CO
2
Emissions (2011-2100) [GtCO
2
]
Cumulative Fossil Fuel and Industrial CO
2
Emissions [GtCO
2
]
0 2000 2000 2020 2040 2060 2080 21004000 6000 8000
10-90
th
Percentile
Figure 6�11 | Emissions reductions for different GHGs in 2030, 2050, and 2100. The left panel shows 2010 historic emissions and the bars in the right panel indicate changes
compared to 2010 (AR5 Scenario Database). FF&I CO
2
includes CO
2
from AFOLU fossil fuel use. Source: WG III AR5 Scenario Database (AnnexII.10). Historic data: JRC / PBL (2013),
IEA (2012a), see AnnexII.9.
HFCs
PFCs
SF6
CO
2
CH
4
N
2
O
210020502030
2010
GHG Emissions [GtCO
2
eq/yr]
HFCs, PFCs, SF
6
N
2
O
CH
4
FF&I CO
2
Net AFOLU CO
2
3002001000-100-2003002001000-100-2003002001000-100-200
Change Compared to 2010 Emissions [%]Change Compared to 2010 Emissions [%]Change Compared to 2010 Emissions [%]
430-480 ppm CO
2
eq
480-530 ppm CO
2
eq
530-580 ppm CO
2
eq
580-650 ppm CO
2
eq
650-720 ppm CO
2
eq
720-1000 ppm CO
2
eq
>1000 ppm CO
2
eq
10
th
Percentile
90
th
Percentile
Mean
0
10
20
30
40
60
50
Table 6�4 | Regional peak year of CO
2
emission and emissions reductions in 2030 over 2010, for 430 – 530 and 530 – 650 ppm CO
2
eq scenarios. Negative values for emissions
reductions indicate that 2030 emissions are higher than in 2010. Figures are averages across models. The numbers in parenthesis show the interquartile range across scenarios.
The number of underlying scenarios is the same as in Figure 6.9. Source: WG III AR5 Scenario Database (Annex II.10), idealized implementation and default technology scenarios.
OECD-1990 ASIA LAM MAF EIT
Peak year of emissions 430 – 530 ppm CO
2
eq 2010
(2010 / 2010)
2020
(2015 / 2030)
2015
(2010 / 2020)
2020
(2010 / 2030)
2014
(2010 / 2015)
Peak year of emissions 530 – 650 ppm CO
2
eq 2014
(2010 / 2015)
2030
(2030 / 2030)
2020
(2010 / 2030)
2034
(2020 / 2040)
2016
(2010 / 2020)
2030 Emission
reductions w. r. t. 2010
430 – 530 ppm CO
2
eq 32 %
(23 / 40 %)
– 1 %
(– 15 / 14 %)
35 %
(16 – 59 %)
8 %
(– 7 / 18 %)
32 %
(18 / 40 %)
2030 Emission
reductions w.r.t. 2010
530 – 650 ppm CO
2
eq 14 %
(6 / 21 %)
– 34 %
(– 43 / – 26 %)
9 %
(– 17 / 41 %)
– 22 %
(– 41 / – 12 %)
8 %
(– 5 / 16 %)
437437
Assessing Transformation Pathways
6
Chapter 6
potential and costs for reducing emissions of different greenhouse forc-
ers, and (2) the strategies for making tradeoffs between them. With
respect to abatement potential and costs, studies indicate that in the
short run, there are many low-cost options to reduce non-CO
2
gases
relative to opportunities to reduce CO
2
emissions. Partially as a result,
studies indicate that short-term reduction strategies may rely more
heavily in the near term on non-CO
2
gases than in the long run (Wey-
ant etal., 2006; Lucas etal., 2007). In the longer run, emission reduc-
tions, particularly for CH
4
and N
2
O, are expected to be constrained by
several hard-to-mitigate sources such as livestock and the application
of fertilizers. This ultimately results in lower reduction rates than for
CO
2
for the lower concentration categories despite slower growth in
baseline projections (see Figure 6.11, and also discussed by Lucas etal.,
2007). For scenarios resulting in 430 – 480 CO
2
eq concentration in
2100, CH
4
reductions in 2100 are about 50 % compared to 2005. For
Figure 6�11 | Emissions reductions for different GHGs in 2030, 2050, and 2100. The left panel shows 2010 historic emissions and the bars in the right panel indicate changes
compared to 2010 (AR5 Scenario Database). FF&I CO
2
includes CO
2
from AFOLU fossil fuel use. Source: WG III AR5 Scenario Database (AnnexII.10). Historic data: JRC / PBL (2013),
IEA (2012a), see AnnexII.9.
HFCs
PFCs
SF6
CO
2
CH
4
N
2
O
210020502030
2010
GHG Emissions [GtCO
2
eq/yr]
HFCs, PFCs, SF
6
N
2
O
CH
4
FF&I CO
2
Net AFOLU CO
2
3002001000-100-2003002001000-100-2003002001000-100-200
Change Compared to 2010 Emissions [%]Change Compared to 2010 Emissions [%]Change Compared to 2010 Emissions [%]
430-480 ppm CO
2
eq
480-530 ppm CO
2
eq
530-580 ppm CO
2
eq
580-650 ppm CO
2
eq
650-720 ppm CO
2
eq
720-1000 ppm CO
2
eq
>1000 ppm CO
2
eq
10
th
Percentile
90
th
Percentile
Mean
0
10
20
30
40
60
50
N
2
O, the most stringent scenarios result in emission levels just below
today’s level. For halogenated gases, emission growth is significantly
reduced for the lower concentration categories, but variation among
models is large, ranging from a 90 % reduction to a 100 % increase
compared to 2005.
Strategies for making tradeoffs across greenhouse forcers must account
for differences in both radiative effectiveness and atmospheric lifetime
and the associated impacts on near-term and long-term climate change.
They must also consider relationships between gases in terms of com-
mon sources and non-climate impacts such as air pollution control.
Models handle these tradeoffs differently, but there are essentially two
classes of approaches. Most models rely on exogenous metrics such as
Global Warming Potentials (GWPs) (discussed further below) and trade
off abatement among gases based on metric-weighted prices. Other
models make the tradeoff on the basis of economic optimization over
time and the physical characterization of the gases within the model
with respect to a specified goal such as total forcing (e. g. Manne and
Richels, 2001). Differences both within these classes of approaches
and among them lead to very different results, especially with respect
to the timing of mitigation for short-lived substances. Several stud-
ies have looked into the role of these substances in mitigation (Shine
etal., 2007; Berntsen etal., 2010; UNEP and WMO, 2011; Myhre etal.,
2011; McCollum etal., 2013a; Rose etal., 2014a). Studies can be found
that provide argument for early emission reduction as well as a more
delayed response of short-lived forcers. Arguments for early reductions
emphasize the near-term benefits for climate and air pollution asso-
ciated with ozone and particulate matter. An argument for a delayed
response is that, in the context of long-term climate goals, reducing
short-lived forcers now has only a very limited long-term effect (Smith
and Mizrahi, 2013).
Model analysis has also looked into the impact of using different sub-
stitution metrics (see Section3.9.6 for a theoretical discussion the
implication of various substitution metrics and Section8.7 of the WGI
report for the physical aspects of substitution metrics). In most cur-
rent climate policies, emission reductions are allocated on the basis
of GWPs for a time of horizon of 100 years. Several papers have
explored the use of metrics other than 100-year GWPs, including
updated GWP values and Global Temperature Change Potential (GTP)
values (Smith etal., 2012; Reisinger etal., 2012; Azar and Johansson,
2014). Quantitative studies show that the choice of metrics is critical
for the timing of CH
4
emission reductions among the Kyoto gases, but
that it rarely has a strong impact on overall global costs. The use of
dynamic GTP values (as alternative to GWPs) has been shown to
postpone emissions reductions of short-lived gases. Using different
438438
Assessing Transformation Pathways
6
Chapter 6
estimates for 100-year GWP from the various previous IPCC Assess-
ment Reports has no major impact on transition pathways.
6�3�2�6 The link between concentrations, radiative
forcing, and temperature
The assessment in this chapter focuses on scenarios that result in
alternative CO
2
eq concentrations by the end of the century. However,
temperature goals are also an important consideration in policy dis-
cussions. This raises the question of how the scenarios assessed in
this chapter relate to possible temperature outcomes. One complica-
tion for assessing this relationship is that scenarios can follow differ-
ent concentration pathways to the same end-of-century goal (see
Section 6.3.2.2), and this will lead to different temperature
responses. A second complication is that several uncertainties con-
found the relationship between emissions and temperature
responses, including uncertainties about the carbon cycle, climate
sensitivity, and the transient climate response (see WGI, Box 12.2).
This means that the temperature outcomes of different concentra-
tion pathways assessed here (see Section 6.3.2.1) are best expressed
in terms of a range of probable temperature outcomes (see Chapter
Figure 6�12 | Comparison of CMIP5 results (as presented in Working Group I) and MAGICC output for global temperature increase. Note that temperature increase is presented
relative to the 1986 – 2005 average in this figure (see also Figure 6.13). Panel a) shows concentration-driven runs for the RCP scenarios from MAGICC (lines) and one-standard
deviation ranges from CMIP5 models. Panel b) compares 2081 – 2100 period projections from MAGICC with CMIP5 for scenarios driven by prescribed RCP concentrations (four left-
hand bars of both model categories) and the RCP8.5 run with prescribed emissions (fifth bar; indicated by a star). Panel c) shows temperature increases for the concentration-driven
runs of a subset of CMIP5 models against cumulative CO
2
emissions back-calculated by these models from the prescribed CO
2
concentration pathways (full lines) and temperature
increase projected by the MAGICC model against cumulative CO
2
emissions (dotted lines) (based on WGI Figure SPM.10). Cumulative emissions are calculated from 2000 onwards.
Source: WG I AR5 (Section 12.5.4.2, Figure 12.46, TFE.8 Figure 1) and MAGICC calculations (RCP data (van Vuuren et al., 2011a), method as in Meinshausen et al., 2011c).
0
1
2
3
4
5
6
7
CMIP5
CMIP5
Individual Models
MAGICC
-1.64 * Standard Deviation
+1 Standard Deviation
+1.64 * Standard Deviation
-1 Standard Deviation
Median
MAGICC
0
1
2
3
4
5
6
7
MAGICC
RCP2.6
RCP4.5
RCP6.0
RCP8.5
0
1
2
3
4
RCP8.5*
RCP8.5
RCP6.0
RCP8.5
RCP2.6
RCP8.5*
RCP4.5
RCP6.0
RCP4.5
CMIP5
RCP 2.6
RCP 4.5
RCP 6.0
RCP 8.5
MAGICC
RCP 2.6
RCP 4.5
RCP 6.0
RCP 8.5
Temperature Increase [°C]
Temperature Increase [°C]
Temperature Increase [°C]
Cumulative CO
2
Emissions [GtCO
2
]
RCP2.6
0 2000
2000 2020 2040 2060 2080 2100
4000 6000 8000
5
th
Percentile
84
th
Percentile
95
th
Percentile
16
th
Percentile
c)
b)a)
439439
Assessing Transformation Pathways
6
Chapter 6
2 and Section 6.2.3 for a discussion of evaluating scenarios under
uncertainty). The definition of the temperature goals themselves
forms a third complication. Temperature goals might be defined in
terms of the long-term equilibrium associated with a given concen-
tration, in terms of the temperature in a specific year (e. g., 2100), or
based on never exceeding a particular level. Finally, the reference
year, often referred to as ‘pre-industrial’, is ambiguous given both
the lack of real measurements and the use of different reference
periods. Given all of these complications, a range of emissions path-
ways can be seen as consistent with a particular temperature goal
(see also Figure 6.12, 6.13, and 6.14).
Because of the uncertain character of temperature outcomes, probabilis-
tic temperature information has been created for the scenarios in the AR5
database that have reported information on at least CO
2
, CH
4
, N
2
O and
sulphur aerosol emissions. Several papers have introduced methods for
probabilistic statements on temperature increase for emission scenarios
(Meinshausen, 2006; Knutti etal., 2008; Schaeffer etal., 2008; Zickfeld
etal., 2009; Allen etal., 2009; Meinshausen etal., 2009; Ramanathan and
Xu, 2010; Rogelj etal., 2011). For this assessment, the method described
by Rogelj etal. (2012) and Schaeffer etal. (2014) is used, which employs
the MAGICC model based on the probability distribution of input param-
eters from Meinshausen (2009) (see also Meinshausen et al., 2011c).
Figure 6�13 | Changes in global temperature for the scenario categories above 1850 – 1900 reference level as calculated by MAGICC. (Observed temperatures in the 1985 – 2006
period were about 0.61 deg C above the reference level — see e. g. WG1 Table SPM.2). Panel a) shows temperature increase relative reference as calculated by MAGICC (10th
to 90th percentile for median MAGICC outcomes). Panel b) shows 2081 – 2100 temperature levels for the scenario categories and RCPs for the MAGICC outcomes. The bars for
the scenarios used in this assessment include both the 10th to 90th percentile range for median MAGICC outcomes (colored portion of the bars) and the 16th to 84th percentile
range of the full distribution of MAGICC outcomes from these scenarios, which also captures the Earth-System uncertainty. The bars for the RCPs are based on the 16th to 84th
of MAGICC outcomes based on the RCP emissions scenarios, capturing only the Earth-System uncertainty. Panel c) shows relationship between cumulative CO
2
emissions in the
2011 – 2100 period and median 2081 – 2100 temperature levels calculated by MAGICC. Panel d indicates the median temperature development of overshoot (>0.4 W / m
2
) and non-
overshoot scenarios for the first two scenario categories (25th to 75th percentile of scenario outcomes). Source: WG III AR5 Scenario Database (Annex II.10).
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
ppm CO
2
eq
430-480
480-530
530-580
580-720
720-1000
>1000
0
1
2
3
4
5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Temperature Increase [°C]
0
1
2
3
4
5
6
7
Temperature Increase [°C]
b)
2080-2100 Temperature Increase [°C]
a)
0
1
2
3
4
5
6
7
2080-2100 Temperature Increase [°C]
430-480 ppm CO
2
eq Overshoot
430-480 ppm CO
2
eq Non-Overshoot
530-580 ppm CO
2
eq Non-Overshoot
530-580 ppm CO
2
eq Overshoot
d)c)
Cumulative Emissions [GtCO
2
]
430-480
480-530
530-580
580-720
720-1000
>1000
RCP2.6
RCP4.5
RCP6
RCP8.5
2000 2020 2040 2060 2080 2100
2000 2020 2040 2060 2080 2100
0 2000 4000 6000 10,0008000
RCPsDatabase for AR5
With
Climate Uncertainty
Without
Median
84
th
16
th
Percentile
440440
Assessing Transformation Pathways
6
Chapter 6
MAGICC was run 600 times for each scenario. Probabilistic temperature
statements are based on the resulting distributions (see also the Meth-
ods and Metrics Annex; and the underlying papers cited). Because the
temperature distribution of these runs is based on a single probability
distribution in a single modelling framework, resulting probabilistic tem-
perature statements should be regarded as indicative.
An important consideration in the evaluation of this method is the con-
sistency between the distributions of key parameters used here and the
outcome of the WGI research regarding these same parameters. Carbon-
cycle parameters in the MAGICC model used in this chapter are based on
Earth-System Coupled Model Intercomparison Project (CMIP) 4 model
results from AR4, and a probability density function (PDF) for climate
sensitivity is assumed that corresponds to the assessment of IPCC AR4
(Meehl etal., 2007; Rogelj etal., 2012, Box 10.2). The MAGICC output
based on this approach has been shown to be consistent with the output
of the CMIP5 Earth-System models (see also WGI Sections 12.4.1.2 and
12.4.8). The MAGICC model captures the temperature outcomes of the
CMIP5 models reasonably well, with median estimates close to the mid-
dle of the CMIP5 uncertainty ranges (see panels a and b in Figure 6.12).
Figure 6�14 | The probability of staying below temperature levels for the different scenario categories as assessed by the MAGICC model (representing the statistics of 600 dif-
ferent climate realizations for each emission scenario). Panel a) probability in 2100 of being below 2 °C versus probability of staying below 2 °C throughout the 21st century.
Open dots indicate overshoot scenarios (>0.4 W / m
2
). Panel b) probability of staying below 1.5, 2.0, and 2.5 °C (10th to 90th percentile) during 21st century. Panel c) relationship
between peak concentration and the probability of exceeding 2 °C during the 21st century. Panel d) relationship between 2100 concentration and the probability of exceeding 2 °C
in 2100. Source: WG III AR5 Scenario Database (Annex II.10).
1.0 0.8 0.6 0.4 0.00.2
Probability of Staying Below 2°C (2100)
Scenario Category [ppm CO
2
eq]
0.0
0.2
0.4
0.6
0.8
1.0
400 500 600 700 800
400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Probability of Staying Below 2°C (Full Century)
Peak Concentration [ppm CO
2
eq]
Probability of Staying Below 2°C (2100)
2100 Concentration [ppm CO
2
eq]
0.0
0.2
0.4
0.6
0.8
1.0
720-
1000
>1000650-
720
580-
650
530-
580
430-
480
480-
530
430-480 ppm CO
2
eq
480-530 ppm CO
2
eq
530-580 ppm CO
2
eq
580-650 ppm CO
2
eq
650-720 ppm CO
2
eq
720-1000 ppm CO
2
eq
>1000 ppm CO
2
eq
Overshoot Scenarios
2.5 deg °C
2.0 deg °C
1.5 deg °C
Probability of Staying Below 2°C (Full Century)
d)c)
b)a)
441441
Assessing Transformation Pathways
6
Chapter 6
For lower-emission scenarios, the MAGICC uncertainty range is more
narrow, mainly due to the larger range methodologies representing
non-CO
2
forcings in the CMIP5 models, as well as the fact that MAGICC
does not reflect all of the structural uncertainty represented by the range
of CMIP5 models (see panels a and b in Figure 6.12, and WGI Figure
12.8 and Section 12.4.1.2). Uncertainty ranges are largest for emissions-
driven runs (only available for RCP 8.5 from CMIP5 models), since uncer-
tainties in carbon-cycle feedbacks play a larger role (see also WGI Sec-
tion 12.4.8.1). The relationship between the cumulative CO
2
emissions
and the transient temperature increase from MAGICC is well aligned
with the CMIP5 model results for the RCP pathways (Figure 6.12 panel
c, and WGI Section 12.5.4.2, Figure 12.46, TFE.8 Figure 1). WG I has esti-
mated that a cumulative CO
2
emissions budget of around 1000 GtCO
2
from 2011 onward is associated with a likely (>66 %) chance of main-
taining temperature change to less than 2 °C. For the database of sce-
narios assessed here, the majority of scenarios with a greater than 66 %
chance of limiting temperature change to less than 2 °C, based on the
MAGICC analysis, are those that reach between 430 and 480 ppm CO
2
eq,
and these are associated with cumulative emissions over the century of
630 – 1180 GtCO
2
(Table 6.3). The two budgets are not fully comparable,
however, since the WGI budget relates to the cumulative emissions at
the time of peak warming which are higher than the cumulative emis-
sions until 2100 in overshoot scenarios with net negative emissions by
the end of the century. In addition, the WGI AR5 estimate is based on a
single scenario for non-CO
2
substances, whereas the database assessed
here considers a much wider range of non-CO
2
emissions.
Based on the results of the MAGICC analysis, temperature outcomes
are similar across all scenarios in the next few decades, in part due
to physical inertia in the climate system (Figure 6.13, panel a). In the
second half of the century, however, temperatures diverge. Scenarios
leading to 2100 concentrations over 1000 ppm CO
2
eq lead to a tem-
perature increase of about 3 to 6 °C (66
th
percentile of the distribution
of temperature outcomes), while scenarios with 2100 concentrations
between 430 – 480 ppm CO
2
eq lead to a temperature increase of about
1.3 to 2.2 °C (66
th
percentile of the distribution of temperature out-
comes) (Figure 6.13, panels a and b). Cumulative CO
2
emissions for all
scenarios in the database correlate well to the temperature level — see
also WGI Section 12.5.4 (Figure 6.13, panel c). However, there is some
variation due to differences in emissions of other forcing agents, in par-
ticular CH
4
and sulphur, along with the timing of emissions reduction
and the associated extent of overshoot. In general, both the 2100 tem-
peratures and the relationship between the cumulative emissions and
2100 temperature change are roughly consistent with the correlation
for the RCPs in WGI (Figure 6.13, panel c). Scenarios that overshoot
the 2100 concentration goal by more than 0.4 W / m
2
result in higher
levels of temperature increase mid-century and prolonged periods of
relatively rapid rates of change in comparison to those without over-
shoot or with less overshoot (Figure 6.13, panel d). By 2100, however,
the different scenarios converge.
Defining temperature goals in terms of the chance of exceeding a par-
ticular temperature this century accounts for both the 2100 concentra-
tion and the pathway to get to this concentration (Figure 6.14). Over-
shoot scenarios of greater than 0.4 W / m
2
have a higher probability of
exceeding 2 °C prior to 2100 than in 2100 (Figure 6.14, panel a). In
general, the results suggest that the peak concentration during the 21st
century is a fundamental determinant of the probability of remaining
below a particular temperature goal (Figure 6.14, panel c). The CO
2
eq
concentration in 2100, on the other hand, is a proxy for the probability
of exceeding end-of-the-century temperature goals (panel d). Based on
the MAGICC results, only scenarios leading to 2100 concentrations of
430 – 480 ppm and a small number of scenarios leading to 2100 con-
centrations of 480 – 530 ppm have a probability of greater than 66 %
probability of maintaining temperature change below 2 °C throughout
the century. Scenarios that reach 2100 concentrations between 530
and 580 ppm CO
2
eq while exceeding this range (that is, exceeding
580 ppm CO
2
eq) during the course of the century have less than a 33 %
probability of limiting transient temperature change to below 2 °C over
the course of the century, based on the MAGICC results.
Other temperature levels in addition to 2 °C are relevant for mitigation
strategy. Based on the MAGICC results, scenarios leading to concentra-
tions between 430 and 480 ppm CO
2
eq have less than a 50 % prob-
ability of maintaining temperature change below 1.5 °C throughout the
21st century, and many have less than a 33 % probability of achieving
this goal. As noted in Section 6.3.2.1, there are scenarios in the litera-
ture that reach levels below 430 ppmCO
2
eq by 2100, but these were
not submitted to the database used for this assessment. Using the same
methods for assessing temperature implications of scenarios as used in
this assessment, the associated studies found that these scenarios have
a probability (also based on MAGICC) of more than 66 % of remaining
below 1.5 °C, after peaking earlier in the century (e. g., Luderer etal.,
2013, Rogelj etal., 2013a,b).
1
In contrast, the scenarios submitted to
this assessment that lead to CO
2
eq concentration below 580 ppm to
CO
2
eq by 2100 have more than a 50 % probability of limiting tem-
perature change to below 2.5 °C during the 21
st
century, based on the
MAGICC results, and many have more than a 66 % probability. (Section
6.9 discusses how the use of geoengineering techniques can change
the relationships between GHG emissions and radiative forcing.)
6�3�3 Treatment of impacts and adaptation in
transformation pathways
The importance of considering impacts and adaptation responses when
assessing the optimal level of mitigation in a cost-benefit framework
has been well studied in highly-aggregated models (see Box6.1. on
cost-benefit analysis). However the role impacts and adaptation in sce-
narios from large-scale integrated models has seen far less treatment.
Mitigation, impacts, and adaptation are interlinked in several important
1
In these scenarios, the cumulative CO
2
emissions range between 680 – 800 GtCO
2
from 2011 to 2050 and between 90 – 310 GtCO
2
from 2011 to 2100. Global
CO
2
eq emissions in 2050 are between 70 % and 95 % below 2010 emissions, and
they are between 110 % and 120 % below 2010 emissions in 2100.
442442
Assessing Transformation Pathways
6
Chapter 6
ways and should, ideally, be considered jointly in the context of achiev-
ing concentration goals such as those explored in this chapter. A few
studies from large-scale integrated models consider mitigation,
impacts, and adaptation simultaneously in their construction of scenar-
ios (see Reilly etal., 2007; Isaac and van Vuuren, 2009; Chum etal.,
2011; Nelson etal., 2014; Calvin etal., 2013; Zhou etal., 2013; Dowl-
ing, 2013). In the vast majority of cases, however, the scenarios dis-
cussed in this chapter do not consider these linkages, and this is consid-
ered a major gap in the transformation pathways literature. (For a
summary of integrated models that capture impacts and adaptation,
see, e. g., Füssel (2010) and Fisher-Vanden etal. (2013). For a compre-
hensive discussion of climate impacts, adaptation, and vulnerability,
see IPCC WGII AR5). Major efforts are now underway to incorporate
impacts and adaptation into large-scale integrated models, but these
efforts must overcome a range of challenges, including incorporating
the sectoral and regional character of impact and adaptation into inte-
grated models, which have higher spatial aggregation, and a lack of
data and empirical evidence on impacts and adaptation required for
model inputs.
Omitting climate impacts and adaptation responses from scenarios is
likely to lead to biased results for three main reasons. First, climate
impacts could influence the effectiveness of mitigation options. For
instance, electricity production could be affected by changes in cooling
water availability (Schaeffer etal., 2012) or air temperature, changes
in precipitation will alter hydroelectric power, and climate change
could impact biofuel crop productivities (Chum etal., 2011). Unfor-
tunately, the set of modelling studies that explore these issues is lim-
ited (Fisher-Vanden etal., 2011), so there is insufficient evidence today
to draw broad conclusions about how the omission of impacts and
adaptation responses would alter mitigation options and the resulting
scenarios reviewed in this chapter. Second, adaptation responses to
climate change could themselves alter emissions from human activi-
ties, either increasing or decreasing the emissions reductions required
to reach GHG-concentration goals. For example, a warmer climate is
likely to lead to higher demand for air conditioning (Mansur et al.,
2008), which will lead to higher emissions if this increased electric-
ity demand is met by electric power generated with fossil fuels. On
the other hand, a warmer climate will lead to reductions in heating
demand, which would lower emissions from fuels used in heating.
Also, impacts could potentially lead to lower economic growth and
thus lower emissions. Further, because electricity is relatively easier
to decarbonize than solid, liquid, or gaseous fuels, changing in heat-
ing and cooling demands could reduce the economic costs of mitiga-
tion (Isaac and van Vuuren, 2009; Zhou etal., 2013). Climate change
will also change the ability of the terrestrial biosphere to store car-
bon. Again, there is a limited number of studies that account for this
adaptive response to climate change (Bosello etal., 2010b; Eboli etal.,
2010; Anthoff etal., 2011) or optimal mitigation levels when adap-
tation responses are included (Patt et al., 2009). Finally, mitigation
strategies will need to compete with adaptation strategies for scarce
investment and R&D resources, assuming these occur contemporane-
ously. A number of studies account for competition for investment and
R&D resources. In a cost benefit framework, several modelling studies
Figure 6�15 | Cumulative global coal, oil, and gas use between 2010 and 2100 in baseline and mitigation scenarios compared to reserves and resources. Estimates of reserves and
resources (‘R+R’) are shown as shaded areas and historical cumulative use until 2010 is shown as dashed black line. Dots correspond to individual scenarios, of which the number
in each sample is indicated at the bottom of each panel. Note that the horizontal distribution of dots does not have a meaning, but avoids overlapping dots. Source: WG III AR5
Scenario Database (Annex II.10). Includes only scenarios based on idealized policy implementation. Reserve, resource, and historical cumulative use from Table 7.1 in Section 7.4.1.
0
20
40
60
80
Coal
Cumulative Coal Resource Use (2010-2100) [ZJ]
0
500
1000
1500
2000
Carbon Content [GtC]
N=221 N=237 N=239
R+R: 308−456 ZJ
0
20
40
60
80
Oil
Cumulative Oil Resource Use (2010-2100) [ZJ]
0
500
1000
1500
Carbon Content [GtC]
N=221 N=235 N=239
R+R: 24−34 ZJ
0
20
40
60
80
Gas
Cumulative Gas Resource Use (2010-2100) [ZJ]
0
500
1000
Carbon Content [GtC]
N=221 N=237 N=239
R+R: 72−205 ZJ
>720 ppm CO
2
eq
530−650 ppm CO
2
eq
430−530 ppm CO
2
eq
Min
75
th
Percentile
Max
Median
25
th
Percentile
443443
Assessing Transformation Pathways
6
Chapter 6
(Bosello etal., 2010a, 2010b; de Bruin etal., 2009) adaptation, and
mitigation are both decision variables and compete for investment
resources. Competition for investment resources is also captured in
studies measuring the economic impacts of climate impacts, but rather
than competing with mitigation investments, competition is between
investment in adaptation and consumption (Bosello etal., 2007) and
other capital investments (Darwin and Tol, 2001). Some simulation
studies that estimate the economic cost of climate damages add adap-
tation cost to the cost of climate impacts and do not capture crowd-
ing out of other expenditures, such as investment and R&D (Hope,
2006). No existing study, however, examines how this crowding out
will affect an economy’s ability to invest in mitigation options to reach
concentration goals.
6�3�4 Energy sector in transformation
pathways
The fundamental transformation required in the energy system to meet
long-term concentration goals is a phase-out in the use of freely emit-
ting fossil fuels, the timing of which depends on the concentration goal
(Fischedick et al., 2011). Baseline scenarios indicate that scarcity of
fossil fuels alone will not be sufficient to limit CO
2
eq concentrations to
levels such as 450, 550, or 650 ppm by 2100 (Verbruggen and Al Mar-
chohi, 2010; Riahi etal., 2012; Bauer etal., 2014b; Calvin etal., 2014b;
McCollum etal., 2014a, see also Section 7.4.1). Mitigation scenarios
indicate that meeting long-term goals will most significantly reduce
coal use, followed by unconventional oil and gas use, with conven-
tional oil and gas affected the least (Bauer etal., 2014a, 2014b; McCol-
lum etal., 2014a) (Figure 6.15). This will lead to strong re-allocation
effects on international energy markets (Section 6.3.6.6).
The reduction in freely emitting fossil fuels necessary for mitigation is
not necessarily equal to the reduction in fossil fuels more generally,
however, because fossil resources can be used in combination with
CCS to serve as a low-carbon energy source (McFarland etal., 2009;
Bauer etal., 2014b; McCollum etal., 2014a, see also Sections 7.5.5
and 7.11.2). This means that the total use of fossil fuels can exceed the
use of freely emitting fossil fuels.
To accommodate this reduction in freely emitting fossil fuels, trans-
formations of the energy system rely on a combination of three high-
level strategies: (1) decarbonization of energy supply, (2) an associated
switch to low-carbon energy carriers such as decarbonized electric-
ity, hydrogen, or biofuels in the end-use sectors, and (3) reductions in
energy demand. The first two of these can be illustrated in terms of
changes in the carbon intensity of energy. The last can be illustrated in
terms of energy intensity of GDP, energy per capita, or other indexed
measures of energy demand.
The integrated modelling literature suggests that the first of these two
(carbon intensity of energy) will make the largest break from past trends
in the long run on pathways toward concentration goals (Figure 6.16).
The fundamental reason for this is that the ultimate potential for end-
use demand reduction is limited; some energy will always be required
to provide energy services. Bringing energy system CO
2
emissions down
toward zero, as is ultimately required for meeting any concentration goal,
requires a switch from carbon-intensive (e. g., direct use of coal, oil, and
natural gas) to low-carbon energy carriers (most prominently electricity,
but also heat and hydrogen) in the end-use sectors in the long run.
At the same time, integrated modelling studies also sketch out a
dynamic in which energy intensity reductions equal or outweigh decar-
Figure 6�16 | Final energy intensity of GDP (left panel) and carbon intensity of primary energy (right panel) in mitigation and baseline scenarios, normalized to 1 in 2010 showing
the full scenario range. GDP is aggregated using base-year market exchange rates. Sources: WGIII AR5 Scenario Database (Annex II.10). Historic data: JRC / PBL (2013), IEA (2012a),
see Annex II.9; Heston etal. (2012); World Bank (2013); BP (2013).
-1.0
-0.5
0
0.5
1.0
1.5
2.0
1970 1990 2010 2030 2050 2070 2090
-1.0
-0.5
0
0.5
1.0
1.5
2.0
1970 1990 2010 2030 2050 2070 2090
430-530 ppm CO
2
eq
530-650 ppm CO
2
eq
Baselines
History
History
Final Energy Intensity of GDP (Relative to 2010) Carbon Intensity of Primary Energy (Relative to 2010)
444444
Assessing Transformation Pathways
6
Chapter 6
bonization of energy supply in the near term when the supply system is
still heavily reliant on largely carbon-intensive fossil fuels, and then the
trend is reversed over time (Figure 6.17, see Fisher etal. (2007, Figure
3.21)). At the most general level, this results directly from assumptions
about the flexibility to achieve end-use demand reductions relative to
decarbonization of supply in integrated models (Kriegler etal., 2014b),
about which there is a great deal of uncertainty (see Section 6.8). More
specifically, one reason for this dynamic is that fuel-switching takes
time to take root as a strategy because there is little incentive to
switch, say, to electricity early on when electricity may still be very
carbon-intensive. As electricity generation decreases in carbon inten-
sity through the use of low-carbon energy sources (see Section 7.11.3),
there is an increasing incentive to increase its use relative to sources
associated with higher emissions, such as natural gas. A second factor
is that there may be low-cost demand reduction options available in
the near term, although there is limited consensus on the costs of
reducing energy demand. Indeed, much of the energy reduction takes
place in baseline scenarios. Of importance, these trends can be very
Figure 6�17 | Development of carbon-intensity vs. final energy-intensity reduction relative to 2010 in selected baseline and mitigation scenarios reaching 530 – 580 ppm and
430 – 480 ppm CO
2
eq concentrations in 2100 (left panel) and relative to baseline in the same scenarios (right panel). Consecutive dots represent 10-year time steps starting in 2010
at the origin and going out to 2100. Source: WG III AR5 Scenario Database (Annex II.10). Sample includes only 2100 scenarios with idealized policy implementation for which a
baseline, a 530 – 580 ppm and a 430 – 480 ppm CO
2
eq scenario are available from the same set.
-50 0 50 100
0
20
40
60
80
100
Reduction Relative to 2010
Reduction in Carbon Intensity Relative to 2010 [%]
Reduction in Energy Intensity Relative to 2010 [%]
Baselines
530−580 ppm CO
2
eq
430−480 ppm CO
2
eq
0 20 40 60 80 100 120
0
20
40
60
80
100
Reduction Relative to Baseline
Reduction in Carbon Intensity Relative to Baseline [%]
Reduction in Energy Intensity Relative to Baseline [%]
Figure 6�18 | Global low-carbon primary energy supply (direct equivalent, see Annex II.4) vs. total final energy use by 2030 and 2050 for idealized implementation scenarios. Low-
carbon primary energy includes fossil energy with CCS, nuclear energy, bioenergy, and non-biomass renewable energy. Source: WG III AR5 Scenario Database (Annex II.10). Sample
includes baseline and idealized policy implementation scenarios. Historical data from IEA (2012a).
2010
0 200 400 600 800
0
100
200
300
400
500
600
2030
Final Energy Use [EJ/yr]
Low−Carbon Primary Energy Supply [EJ/yr]
1971
1980
1990
2000
0 200 400 600 800
0
100
200
300
400
500
600
2050
Final Energy Use [EJ/yr]
Low−Carbon Primary Energy Supply [EJ/yr]
1971
1980
1990
2000
2010
Baseline
530-580 ppm CO
2
eq
430-480 ppm CO
2
eq
445445
Assessing Transformation Pathways
6
Chapter 6
regional in character. For example, the value of fuel-switching will be
higher in countries that already have low-carbon electricity portfolios.
The decarbonization of the energy supply will require a significant
scaleup of low-carbon energy supplies, which may impose significant
challenges (see Section 7.11.2). The deployment levels of low-carbon
energy technologies are substantially higher than today in the vast
majority of scenarios, even under baseline conditions, and particularly
for the most stringent concentration categories. Scenarios based on an
idealized implementation approach in which mitigation begins imme-
diately across the world and with a full portfolio of supply options
indicate a scaleup of anywhere from a modest increase to upwards of
three times today’s low-carbon energy by 2030 to bring concentrations
to about 450 ppm CO
2
eq by 2100. A scaleup of anywhere from roughly
a tripling to over seven times today’s levels in 2050 is consistent with
this same goal Figure 6.18, Section 7.11.4). The degree of scaleup
depends critically on the degree of overshoot, which allows emissions
reductions to be pushed into the future.
The degree of low-carbon energy scaleup also depends crucially on the
degree that final energy use is altered along a transformation path-
way. All other things being equal, higher low-carbon energy technology
deployment tends to go along with higher final energy use and vice
versa (Figure 6.18, Figure 7.11). Final energy demand reductions will
occur both in response to higher energy prices brought about by mitiga-
tion as well as by approaches to mitigation focused explicitly on reduc-
ing energy demand. Hence, the relative importance of energy supply-
and-demand technologies varies across scenarios (Riahi etal., 2012).
A major advance in the literature since AR4 is the assessment of sce-
narios with limits on available technologies or variations in the cost
and performance of key technologies. These scenarios are intended as
a rough proxy for economic and various non-economic obstacles faced
by technologies. Many low-carbon supply technologies, such as nuclear
power, CO
2
storage, hydro, or wind power, face public acceptance
issues and other barriers that may limit or slow down their deployment
(see Section 7.9.4). In general, scenarios with limits on available tech-
nologies or variations in their cost and performance demonstrate the
simple fact that reductions in the availability and / or performance or an
increase in costs of one technology will necessarily result in increases
in the use of other options. The more telling result of these scenarios is
that limits on the technology portfolio available for mitigation can sub-
stantially increase the costs of meeting long-term goals. Indeed, many
models cannot produce scenarios leading to 450 ppm CO
2
eq when par-
ticularly important technologies are removed from the portfolio. This
topic is discussed in more detail in Section 6.3.6.3.
Delays in climate change mitigation both globally and at regional levels
simply alter the timing of the deployment of low-carbon energy sources
and demand reductions. As noted in Sections 6.3.2 and 6.4, less mitiga-
tion over the coming decades will require greater emissions reductions
in the decades that follow to meet a particular long-term climate goal.
The nature of technology transitions follows the emissions dynamic
directly. Delays in mitigation in the near term will lower the rate of
energy system transformation over the coming decades but will call for
a more rapid transformation in the decades that follow. Delays lead
to higher utilization of fossil fuels, and coal in particular, in the short
run, which can be prolonged after the adoption of stringent mitigation
action due to carbon lock-ins. To compensate for the prolonged use of
fossil fuels over the next decades, fossil fuel use — particularly oil and
gas — would need to be reduced much more strongly in the long run.
One study found that this leads to a reduction in overall fossil energy
use over the century compared to a scenario of immediate mitigation
(Bauer etal., 2014a). Another study (Riahi etal., 2014) found that if
2030 emissions are kept to below 50 GtCO
2
eq, then low-carbon energy
deployment is tripled between 2030 and 2050 in most scenarios reach-
ing concentrations of about 450 ppm CO
2
eq by 2100. In contrast, if
emissions in 2030 are greater than 55 GtCO
2
eq in 2030, then low-car-
bon energy deployment increases by five-fold in most scenarios meet-
ing this same long-term concentration goal (see Section 7.11.4, specifi-
cally Figure 7.15).
Beyond these high-level characteristics of the energy system trans-
formation lie a range of more detailed characteristics and tradeoffs.
Important issues include the options for producing low-carbon energy
and the changes in fuels used in end uses, and the increase in electric-
ity use in particular, both with and without mitigation. These issues are
covered in detail in Section 6.8 and Chapter 7 through 12.
6�3�5 Land and bioenergy in transformation
pathways
Scenarios suggest a substantial cost-effective, and possibly essential,
role for land in transformation pathways (Section 6.3.2.4 and Section
11.9), with baseline land-use emissions and sequestration an impor-
tant uncertainty (Section 6.3.1.4). Changes in land use and manage-
ment will result from a confluence of factors, only some of which are
due to mitigation. The key forces associated with mitigation are (1)
the demand for bioenergy, (2) the demand to store carbon in land by
reducing deforestation, encouraging afforestation, and altering soil
management practices, and (3) reductions in non-CO
2
GHG emissions
by changing management practices. Other forces include demand for
food and other products, such as forest products, land for growing
urban environments, and protecting lands for environmental, aesthetic,
and economic purposes. Currently, only a subset of models explicitly
model LUC in scenarios. The development of fully integrated land use
models is an important area of model development.
Scenarios from integrated models suggest the possibility of very dif-
ferent landscapes relative to today, even in the absence of mitigation.
Projected global baseline changes in land cover by 2050 typically
exhibit increases in non-energy cropland and decreases in ‘other’ land,
such as abandoned land, other arable land, and non-arable land (Fig-
ure 6.19). On the other hand, projected baseline pasture and forest
land exhibit both increases and decreases. The projected increases in
446446
Assessing Transformation Pathways
6
Chapter 6
non-energy cropland and decreases in forest area through 2050 are
typically projected to outpace historical changes from the previous 40
years (+165 and – 105 million hectares of crop and forest area
changes, respectively, from 1961 – 2005 (Food and Agriculture Organi-
zation of the United Nations (FAO), 2012). Energy cropland is typically
projected to increase as well, but there is less agreement across sce-
narios. Overall, baseline projections portray large differences across
models in the amount and composition of the land converted by agri-
cultural land expansion. These baseline differences are important
because they represent differences in the opportunity costs of land
use and management changes for mitigation. (See Chapter 11.9 for
regional baseline, and mitigation, land cover projections for a few
models and scenarios.)
Mitigation generally induces greater land cover conversion than in
baseline scenarios, but for a given level of mitigation, there is large
variation in the projections (Figure 6.19). Projections also suggest
additional land conversion with tighter concentration goals, but
declining additional conversion with increased mitigation stringency.
This is consistent with the declining relative role of land-related miti-
gation with the stringency of the mitigation goal (Rose etal., 2012).
However, additional land conversion with more stringent goals could
be substantial if there are only bioenergy incentives (see below).
A common, but not universal, characteristic of mitigation scenarios is
an expansion of energy cropland to support the production of mod-
ern bioenergy. There is also a clear tradeoff in the scenarios between
energy cropland cover and other cover types. Most scenarios project
reduced non-energy cropland expansion, relative to baseline expan-
sion, with some projections losing cropland relative to today. On the
other hand, there are projected pasture changes of every kind. Forest
changes depend on the incentives and constraints considered in each
scenario. Some of the variations in projected land cover change are
attributable to specific assumptions, such as fixed pasture acreage, pri-
oritized food provision, land availability constraints for energy crops,
and the inclusion or exclusion of afforestation options (e. g. Popp etal.,
2014). Others are more subtle outcomes of combinations of model-
ling assumption and structure, such as demands for food and energy,
land productivity and heterogeneity, yield potential, land-production
options, and land-conversion costs.
Which mitigation activities are available or incentivized has important
implications for land conversion (Figure 6.19). Bioenergy incentives
alone can produce energy cropland expansion, with increased forest
and other land conversion (Wise etal., 2009; Reilly etal., 2012). In gen-
eral, forest land contraction results when increased demand for energy
crops is not balanced by policies that incentivize or protect the storage
Figure 6�19 | Global land cover change by 2050 from 2005 for a sample of baseline and mitigation scenarios with different technology assumptions. ‘REM-MAg’ = REMIND-
MAgPIE. Sources: EMF27 Study (Kriegler etal., 2014a), Reilly etal. (2012), Melillo etal. (2009), Wise etal. (2009). Notes: default (see Section 6.3.1) fossil fuel, industry, and land
mitigation technology incentives assumed except as indicated by the following — ‘bioe’ = only land-based mitigation incentive is for modern bioenergy, ‘nobioe’ = land incentives
but not for modern bioenergy, ‘bioe+land’ = modern bioenergy and land carbon stocks incentives, ‘bioe+agint’ = modern bioenergy incentive and agricultural intensification
response allowed, ‘lowbio’ = global modern bioenergy constrained to 100 EJ / year, ‘noccs’ = CCS unavailable for fossil or bioenergy use. Other land cover includes abandoned land,
other arable land, and non-arable land.
-3000
-2000
-1000
0
1000
2000
3000
GCAM-EMF27
IMAGE-EMF27
REM-MAg-EMF27
Reilly et al.
Wise et al.
Reilly et al.-Bioe
Reilly et al.-Nobioe
Wise et al.-Bioe
Wise et al.-Bioe+Land
Melillo et al.-Bioe
Melillo et al.-Bioe+Agint
Reilly et al.-Bioe+Land
Wise et al.-Bioe
Wise et al-Bioe+Land
GCAM-EMF27
GCAM-EMF27-Lowbioe
GCAM-EMF27-Noccs
IMAGE-EMF27
IMAGE-EMF27-Lowbioe
IMAGE-EMF27-Noccs
REM-MAg-EMF27
REM-MAg-EMF27-Lowbioe
REM-MAg-EMF27-Noccs
Wise et al.-Bioe
Wise et al.-Bioe+Land
GCAM-EMF27
GCAM-EMF27-Lowbioe
GCAM-EMF27-Noccs
IMAGE-EMF27
REM-MAg-EMF27
REM-MAg-EMF27-Lowbioe
REM-MAg-EMF27-Noccs
Baseline 650-720 ppm CO
2
eq 580-650 ppm CO
2
eq 530-580 ppm CO
2
eq 430-480 ppm CO
2
eq
Acreage Change in 2050 from 2005 [Million ha]
Non-Energy Crops
Energy Crops
Forest
Pasture
Other
+6300
-6300 (-2660 Forest,-2130 Pasture, -1511 Other)
+3320
-3320
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Assessing Transformation Pathways
6
Chapter 6
of carbon in terrestrial systems. However, the degree of this forest con-
version will depend on a range of factors, including the potential for
agricultural intensification and underlying modelling approaches. For
example, Melillo etal. (2009) find twice as much forest land conversion
by 2050 when they ignore agricultural intensification responses. Forest
land expansion is projected when forests are protected, there are con-
straints on bioenergy deployment levels, or there are combined incen-
tives for bioenergy and terrestrial carbon stocks (e. g., Wise etal., 2009;
Reilly etal., 2012, and GCAM-EMF27 in Figure 6.19). Differences in for-
est land expansion result largely from differences in approaches to
incorporating land carbon in the mitigation regime. For example, in Fig-
ure 6.19, GCAM-EMF27 (all variants), Wise etal. (2009) (low bioe+land)
and Reilly et al. (2012)(low bioe and bioe+land) include an explicit
price incentive to store carbon in land, which serves to encourage affor-
estation and reduce deforestation of existing forests, and discourage
energy cropland expansion. In contrast, other scenarios consider only
avoided deforestation (REMIND-MAgPIE-EMF27), or land conversion
constraints (IMAGE-EMF27). Both protect existing forests, but neither
encourages afforestation. In other studies, Melillo et al. (2009) protect
existing natural forests based on profitability and Popp et al. (2011a)
(not shown) impose conservation policies that protect forest regardless
of cost. The explicit pricing of land carbon incentives can lead to large
land use carbon sinks in scenarios, and an afforestation incentive or
constraint on bioenergy use can result in less land conversion from bio-
energy, but not necessarily less land conversion as afforestation may
increase.
An important issue with respect to bioenergy, and therefore to land
transformation, is the availability and use of BECCS. As discussed in
Section 6.3.2, BECCS could be valuable for reaching lower-concentra-
tion levels, in part by facilitating concentration overshoot. The avail-
ability of CCS could therefore also have land-use implications. Con-
straints on the use of CCS would prohibit BECCS deployment. However,
CCS (for BECCS as well as fossil energy with CCS) may not increase
land conversion through 2050 relative to scenarios without BECCS.
Instead, the presence of BECCS could decrease near-term energy crop
expansion as some models project delayed mitigation with BECCS
(Rose etal., 2014a, 6.3.2.2). In addition to biomass feedstock require-
ments, BECCS land considerations include bioenergy CCS facility land,
as well as optimal siting relative to feedstock, geologic storage, and
infrastructure.
As noted above, land transformation is tightly linked to the role of
bioenergy in mitigation. To understand bioenergy’s role in transforma-
tion pathways, it is important to understand bioenergy’s role within
the energy system. The review by Chum etal. (2011) estimated techni-
cal potential for bioenergy of 300 and 500 EJ / year in 2020 and 2050,
respectively, and deployment of 100 to 300 EJ of biomass for energy
globally in 2050, while Rose etal. (2012) found bioenergy contribut-
ing up to 15 % of cumulative primary energy over the century under
climate policies. Rose etal. (2014a) analyze more recent results from
15 models (Figure 6.20). They find that modelled bioenergy structures
vary substantially across models, with differences in feedstock assump-
tions, sustainability constraints, and conversion technologies. Nonethe-
less, the scenarios project increasing deployment of, and dependence
on, bioenergy with tighter climate change goals, both in a given year
as well as earlier in time. Shares of total primary energy increase under
climate policies due to both increased deployment of bioenergy and
Figure 6�20 | Annual global modern biomass primary energy supply and bioenergy share of total primary energy supply (top panels) and BECCS share of modern bioenergy (bot-
tom panels) in baseline, 550 ppm and 450 ppm CO
2
eq scenarios in 2030, 2050, and 2100. Source: Rose etal. (2014a). Notes: All scenarios shown assume idealized implementation.
Results for 15 models shown (3 models project to only 2050). Also, some models do not include BECCS technologies and some no more than biopower options.
Baseline
530-580 ppm CO
2
eq
430-480 ppm CO
2
eq
0
10
20
30
40
50
Share of Total Primary Energy [%]
2030 2050
0
20
40
60
80
100
0 50 100 150 200 250 300 350
BECCS Share [%]
Total Modern Bioenergy [EJ/yr]
2030
0 50 100 150 200 250 300 350
Total Modern Bioenergy [EJ/yr]
2050
2100
0 50 100 150 200 250 300 350
Total Modern Bioenergy [EJ/yr]
2100
448448
Assessing Transformation Pathways
6
Chapter 6
shrinking energy systems. Bioenergy’s share of total regional electricity
and liquid fuels is projected to be up to 35 % and 75 %, respectively,
by 2050. However, there is no single vision about where biomass is
cost-effectively deployed within the energy system (electricity, liquid
fuels, hydrogen, and / or heat), due in large part to uncertainties about
relative technology options and costs over time. (See Chapter 7 for
more detail on bioenergy’s role in energy supply.) As noted above, the
availability of CCS, and therefore BECCS, has important implications for
bioenergy deployment. In scenarios that do include BECCS technolo-
gies, BECCS is deployed in greater quantities and earlier in time the
more stringent the goal, potentially representing 100 % of bioenergy in
2050 (Figure 6.20).
Models universally project that the majority of biomass supply for bio-
energy and bioenergy consumption will occur in developing and tran-
sitional economies. For instance, one study (Rose etal., 2014a) found
that 50 – 90 % of global bioenergy primary energy is projected to come
from non-OECD countries in 2050, with the share increasing beyond
2050. Developing and transitional regions are also projected to be the
home of the majority of agricultural and forestry mitigation.
Finally, a number of integrated models have explicitly modelled land
use with full emissions accounting, including indirect land cover
change and agricultural intensification. These models have suggested
that it could be cost-effective to tradeoff lower land carbon stocks
from land cover change and increase N
2
O emissions from agricultural
intensification for the long-run climate change management benefits
of bioenergy (Popp etal., 2014; Rose etal., 2014a).
Overall, the integrated modelling literature suggests opportunities
for large-scale global deployment of bioenergy and terrestrial carbon
gains. However, the transformations associated with mitigation will
be challenging due to the regional scale of deployments and imple-
mentation issues, including institution and program design, land use
and regional policy coordination, emissions leakage, biophysical and
economic uncertainties, and potential non-climate social implica-
tions. Among other things, bioenergy deployment is complicated by a
variety of social concerns, such as land conversion and food security
(See Section 6.6 and the Chapter 11 Bioenergy Annex). Coordination
between land-mitigation policies, regions, and activities over time will
affect forestry-, agricultural-, and bioenergy-mitigation costs and net
GHG mitigation effectiveness. When land options and bioenergy are
included in mitigation scenarios, it is typically under the assumption
of a highly idealized implementation, with immediate, global, and
comprehensive availability of land-related mitigation options. In these
cases, models are assuming a global terrestrial carbon-stock incentive
or global forest-protection policy, global incentives for bioenergy feed-
stocks, and global agriculture-mitigation policies. They also assume no
uncertainty, risk, or transactions costs. For a discussion of these issues,
see Lubowski and Rose (2013). The literature has begun exploring
more realistic policy contexts and found that there is likely less avail-
able mitigation potential in the near term than previously estimated,
and possibly unavoidable emissions leakage associated with getting
programs in place, as well as with voluntary mitigation supply mecha-
nisms (Section 11.9, Section 6.8). Additional exploration into the need
for and viability of large-scale land-based mitigation is an important
area for future research.
6�3�6 The aggregate economic implications of
transformation pathways
6�3�6�1 Overview of the aggregate economic
implications of mitigation
Mitigation will require a range of changes, including behavioural
changes and the use of alternative technologies. These changes will
affect economic output and the consumption of goods and services.
The primary source of information on these costs over multi-decade
or century-long time horizons are integrated models such as those
reviewed in this chapter.
Mitigation will affect economic conditions through several avenues,
only some of which are included in estimates from integrated models.
To a first-order, mitigation involves reductions in the consumption of
energy services, and perhaps agricultural products, and the use of more
expensive technologies. This first-order effect is the predominant fea-
ture and focus of the integrated modelling estimates discussed in this
chapter and will lead to aggregate economic losses. However, mitiga-
tion policies may interact with pre-existing distortions in labour, capi-
tal, energy, and land markets, and failures in markets for technology
adoption and innovation, among other things. These interactions might
increase or decrease economic impacts (Sections 3.6.3 and 6.3.6.5).
Estimates of the potential aggregate economic effects from mitigation
are generally expressed as deviations from a counter-factual baseline
scenario without mitigation policies; that is, the difference in economic
conditions relative to what would have happened without mitigation.
They can be expressed in terms of changes in these economic condi-
tions at a particular point in time (for example, reductions in total con-
sumption or GDP at a given point in time) or in terms of reductions in
the growth rates leading to these economic conditions (for example,
reductions in the rate of consumption or GDP growth). The estimates,
and those discussed in this section, generally do not include the ben-
efits from reducing climate change, nor do they consider the interac-
tions between mitigation, adaptation, and climate impacts (Section
6.3.3). In addition, the estimates do not take into account important
co-benefits and adverse side-effects from mitigation, such as impacts
on land use and health benefits from reduced air pollution (Sections
11.13.6 and 6.6).
A wide range of methodological issues attends the estimation of
aggregate economic costs in integrated models, one of which is
the metric itself. (For more discussion on these issues in estimating
aggregate economic costs, see Annex II.3.2 on mitigation costs met-
449449
Assessing Transformation Pathways
6
Chapter 6
rics and Chapter 3.) A change in welfare due to changes in house-
hold consumption is commonly measured in terms of equivalent and
compensating variation, but other, more indirect cost measures such
as GDP losses, consumption losses, and area under the marginal
abatement cost function are more widely used (Paltsev and Capros,
2013). For consistency, results in this section are presented preferen-
tially in terms of cost measures commonly reported by the models:
consumption losses and GDP losses for general-equilibrium models,
and area under the marginal abatement cost function or reduction
of consumer and producer surplus (in the following summarized with
the term abatement cost) for partial-equilibrium models. These cost
metrics differ in terms of whether or not general equilibrium effects
in the full economy have been taken into account and whether or
not the direct impact on households or the intermediate impact on
economic output is measured. They are therefore treated separately
in this chapter.
Emissions prices (carbon prices) are also assessed in this chapter. How-
ever, they are not a proxy for aggregate economic costs for two pri-
mary reasons. First, emissions prices measure marginal cost, that is, the
cost of an additional unit of emissions reduction. In contrast, total eco-
nomic costs represent the costs of all mitigation that has taken place.
Second, emissions prices can interact with other policies and measures,
such as regulatory policies or subsidies directed at low-carbon tech-
nologies, and will therefore indicate a lower marginal cost than is actu-
ally warranted if mitigation is achieved partly by these other measures.
Different methods can be used to sum costs over time. For this pur-
pose, in the absence of specific information from individual models
about the discount rate used in studies, the estimates of net pres-
ent value (NPV) costs in this chapter are aggregated ex-post using
a discount rate of 5 %. This is roughly representative of the aver-
age interest rate that underlies the discounting approach in most
models (Kriegler etal., 2014a). Other rates could have been used to
conduct this ex-post aggregation. Since mitigation costs tend to rise
over time, lower (higher) rates would lead to higher (lower) aggre-
gate costs than what are provided here. However, it is important to
note that constructing NPV metrics based on other rates is not the
same as actually evaluating scenarios under alternative discounting
assumptions and will not accurately reflect aggregate costs under
such assumptions.
Estimates of aggregate economic effects from integrated models vary
substantially. This arises because of differences in assumptions about
driving forces such as population and economic growth and the policy
environment in the baseline, as well as differences in the structures
and scopes of the models (Section 6.2). In addition, aggregate eco-
nomic costs are influenced by the future cost, performance, and avail-
ability of mitigation technologies (Section 6.3.6.3), the nature of inter-
national participation in mitigation (Section 6.3.6.4), and the policy
instruments used to reduce emissions and the interaction between
these instruments and pre-existing distortions and market failures
(Section6.3.6.5).
6�3�6�2 Global aggregate costs of mitigation in idealized
implementation scenarios
A valuable benchmark for exploring aggregate economic mitigation
costs is estimates based on the assumption of a stylized implementa-
tion approach in which a ubiquitous price on carbon and other GHGs
is applied across the globe in every sector of every country and rises
over time in a way that minimizes the discounted sum of costs over
time. These ‘idealized implementation’ scenarios are included in most
studies as a benchmark against which to compare results based on
less-idealized circumstances. One reason that these idealized scenarios
have been used as a benchmark is that the implementation approach
provides the lowest costs under idealized implementation conditions
of efficient global markets in which there are no pre-existing distor-
tions or interactions with other, non-climate market failures. For this
reason, they are often referred to as ‘cost-effective’ scenarios. However,
the presence of pre-existing market distortions, non-climate market
failures, or complementary policies means that the cost of the idealized
approach could be lower or higher than in an idealized implementation
environment, and that the idealized approach may not be the least-
cost strategy (see Section 6.3.6.5). Most of the idealized implementa-
tion scenarios assessed here consider these additional factors only to
a limited degree or not at all, and the extent to which a non-idealized
implementation environment is accounted for varies between them.
A robust result across studies is that aggregate global costs of mitiga-
tion tend to increase over time and with stringency of the concentration
goal (Figure 6.21). According to the idealized implementation scenarios
collected in the WG III AR5 Scenario Database (Annex II.10), the central
70 % (10 out of 14) of global consumption loss estimates for reaching
levels of 430 – 480 ppm CO
2
eq by 2100 range between 1 % to 4 % in
2030, 2 % to 6 % in 2050, and 3 % to 11 % in 2100 relative to consump-
tion in the baseline (Figure 6.21, panel c). These consumption losses cor-
respond to an annual average reduction of consumption growth by 0.06
to 0.20 percentage points from 2010 to 2030 (median of 0.09), 0.06 to
0.17 percentage points through 2050 (median of 0.09), and 0.04 to 0.14
percentage points over the century (median of 0.06). To put these losses
in context, studies assume annual average consumption growth rates
without mitigation between 1.9 % and 3.8 % per year until 2050 and
between 1.6 % and 3.0 % per year over the century. These growth rates
correspond to increases in total consumption by roughly a factor of 2 to
4.5 by 2050, and from roughly four-fold to over ten-fold over the century
(values are based on global projections in market exchange rates).
An important caveat to these results is that they do not account for a
potential model bias due to the fact that higher-cost models may have
not been able to produce low-concentration scenarios and have there-
fore not reported results for these scenarios (see discussion of model
failures in Section 6.2, and Tavoni and Tol, 2010). They also do not
capture uncertainty in model parameter assumptions (Webster etal.,
2012). Since scenario samples for different concentration levels do not
come from precisely the same models, it is informative to look at the
cost changes between different concentration levels as projected by
450450
Assessing Transformation Pathways
6
Chapter 6
⇒
9 28 60 60 34 9 28 60 54 32
0
10
20
30
40
50
60
70
80
90
2020 2030 2050 2100
10
0
10
1
10
2
10
3
10
4
CO
2
Price [USD
2010
/tCO
2
]
a) Carbon Prices 2020−2100
9 28 60 54 32
CO
2
Price [USD
2010
/tCO
2
]
b) Average Carbon Prices (2015-2100, 5% Discount Rate)
2020 2030 2050 2100
-2
0
2
4
6
8
10
12
7 16 46 40 14 7 16 46 32 14
Consumption Loss [% Baseline Consumption]
c) Consumption Losses 2020−2100
Consumption Losses GDP Losses Abatement Costs
8 20 49 35 16 1 7 12 11 12
0
1
2
3
4
5
6
[% Baseline Economy]
d) Mitigation Costs (NPV 2015-2100, 5% Discount Rate)
2020 2030 2050 2100
0
2
4
6
8
10
12
8 20 49 44 17 8 20 49 35 16
GDP Loss [% Baseline GDP]
e) GDP Losses 2020−2100
2020 2030 2050 2100
0
1
2
3
4
5
6
1 7 12 11 12 1 7 12 11 12
Abatement Costs [% Baseline GDP]
f) Abatement Costs 2020−2100
9 28 60 60 349 28 60 60 34
7 16 46 40 147 16 46 40 14
8 20 49 44 178 20 49 44 17
1 7 12 11 121 7 12 11 12
430−480 ppm CO
2
eq
480−530 ppm CO
2
eq
530−580 ppm CO
2
eq
580−650 ppm CO
2
eq
650−720 ppm CO
2
eq
311
22 311
1 1 1 11
13642 11
Min
75
th
Max
Median
25
th
Percentile
7 16 46 32 14
451451
Assessing Transformation Pathways
6
Chapter 6
individual models within a given study (Figure 6.22). This can partly
remove model bias, although the bias from a lack of models that could
not produce low-concentration scenarios remains. The large majority
of studies in the scenario database for AR5 report a factor 1.5 to 3
higher global consumption and GDP losses, and 2 to 4 times higher
abatement costs, for scenarios reaching 430 – 530 ppm CO
2
eq by 2100
compared to the 530 – 650 ppm CO
2
eq range.
Aggregate economic costs vary substantially, even in idealized scenar-
ios. The variation of cost estimates for individual CO
2
eq concentration
ranges can be attributed, among other things, to differences in assump-
tions about driving forces such as population and GDP and differences
in model structure and scope (see Section 6.2 for a discussion of model
differences). Diagnostic studies have indicated that the assumed avail-
ability and flexibility of low-carbon technologies to substitute fossil
energy is a key factor influencing the level of carbon prices for a given
level of emissions reductions (Kriegler etal., 2014a). The extent to which
carbon prices translate into mitigation costs through higher energy
prices is another factor that differs between models. Both the variation
of carbon prices and the variation of the economic impact of higher
prices are major determinants of the observed range of aggregate eco-
nomic costs for a given amount of emissions reductions. Assumptions
about the implementation environment can be another important driver
of costs. For example, the highest consumption and GDP losses in the
scenario sample are from a model with an emphasis on market imper-
fections, infrastructure lock-ins, and myopia (Waisman etal., 2012).
It is possible to control for several key sources of variation by relat-
ing mitigation costs to cumulative emissions reductions from baseline
emissions (Figure 6.23). As expected, carbon prices and mitigation
costs increase with the amount of mitigation. Since different models
have different capabilities for deep emissions reductions, the inter-
model spread in carbon price and cost estimates increases as well. In
other words, scenarios indicate greater consensus regarding the nature
of mitigation costs at higher-concentration levels than those at lower
levels. This increase in variation reflects the challenge associated with
modelling energy and other human systems that are dramatically dif-
ferent than those of today.
6�3�6�3 The implications of technology portfolios for
aggregate global economic costs
Because technology will underpin the transition to a low-carbon
economy, the availability, cost, and performance of technologies will
exert an influence on economic costs. Several multi-model studies and
a wide range of individual model studies have explored this space
(see Section 6.1.2.2). A precise understanding of the implications of
technology availability on costs is confounded by several factors. One
issue is that the sensitivities among technologies are not necessarily
comparable across models or scenarios. Some models do not repre-
sent certain technologies such as BECCS and therefore do not exhibit
a strong cost increase if these options are restricted. These models
may instead have difficulties in achieving tighter concentration goals
regardless of the restriction (Krey etal., 2014). In addition, assump-
tions about cost and performance can vary across models, even within
a single, multi-model study. Moreover, many limited technology sce-
narios are characterized by frequent model infeasibilities, as shown by
the fraction of models in the EMF27 study (Kriegler etal., 2014a) able
to meet a particular goal with different technology combinations at
the bottom of Figure 6.24. (See Section 6.2.4 regarding interpretation
of model infeasibility).
Despite these limitations, the literature broadly confirms that mitiga-
tion costs are heavily influenced by the availability, cost, and perfor-
mance of mitigation technologies. In addition, these studies indicate
that the influence of technology on costs generally increases with
increasing stringency of the concentration goal (Figure 6.24). The
effect on mitigation costs varies by technology, however, the ranges
reported by the different models tend to strongly overlap (Figure 6.24,
Krey etal., 2014), reflecting the general variation of mitigation costs
across models (Section 6.3.6.2, Fisher etal., 2007). In general, models
have been able to produce scenarios leading to about 550 ppm CO
2
eq
by 2100, even under limited technology assumptions. However, many
models could not produce scenarios leading to about 450 ppm CO
2
eq
by 2100 with limited technology portfolios, particularly when assump-
tions preclude or limit the use of BECCS (Azar etal., 2006; van Vliet
etal., 2009; Krey etal., 2014; Kriegler etal., 2014a).
As noted above, the lack of availability of CCS is most frequently
associated with the most significant cost increase (Edenhofer etal.,
2010; Tavoni etal., 2012; Krey etal., 2014; Kriegler etal., 2014a; Riahi
etal., 2014), particularly for concentration goals approaching 450 ppm
CO
2
eq, which are characterized by often substantial overshoot. One
fundamental reason for this is that the combination of biomass with
CCS can serve as a CDR technology in the form of BECCS (Azar etal.,
2006; Krey and Riahi, 2009; van Vliet et al., 2009; Edmonds et al.,
2013; Kriegler et al., 2013a; van Vuuren et al., 2013) (see Sections
6.3.2 and 6.9). In addition to the ability to produce negative emis-
sions when coupled with bioenergy, CCS is a versatile technology that
Figure 6�21 | Global mitigation costs of idealized implementation scenarios. Panels show the development of (a) carbon prices, (c) consumption losses, (e) GDP losses and
(f)abatement costs over time, and (b) the average carbon price (2015 – 2100), and (d) the NPV mitigation costs (2015 – 2100) discounted at a 5 % discount rate. Costs are expressed
as a fraction of economic output — or in the case of consumption losses — consumption in the baseline. The number of scenarios included in the boxplots is indicated at the bot-
tom of the panels, 2030 numbers also apply for 2020 and 2050. The number of scenarios outside the figure range is noted at the top. One model shows NPV consumption losses
of 13 % / 9.5 %, and GDP losses of 15 % / 11 % for 430 – 480 / 530 – 580 ppm CO
2
eq (see text). Source: WG III AR5 Scenario Database (Annex II.10).The scenario selection includes
all idealized implementation scenarios that reported costs or carbon prices to 2050 or 2100 (only the latter are included in aggregate cost and price plots) after removal of similar
scenarios (in terms of reaching similar goals with similar overshoots and assumptions about baseline emissions) from the same model.
452452
Assessing Transformation Pathways
6
Chapter 6
Figure 6�22 | Carbon price (left panel) and global mitigation cost changes (right panel) for idealized implementation scenarios relative to a reference concentration category
(530 – 650 ppm CO
2
eq in 2100). Results for NPV costs are shown by consumption losses, GDP losses, and abatement costs. Results are based on pairs of idealized implementation
scenarios, one in the 530 – 650 ppm CO
2
eq range and one in a neighbouring concentration range, from a single model and study. Cost changes were calculated on the basis of NPV
economic costs (discounted at 5 % per year) and carbon price changes on the basis of average discounted values for the period 2015 – 2100. See Figure 6.21 caption for further
explanation on the presentation of results. Source: WG III AR5 Scenario Database (Annex II.10).
1
N=9 N=66
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Relative to Reference Policy (530−650 ppm CO
2
eq)
Average Carbon Prices (2015−2100, 5% Discount Rate)
430−530 ppm CO
2
eq
650−720 ppm CO
2
eq
Consumption Losses
N=7 N=39
GDP Losses
N=7 N=43
Abatement Costs
N=1 N=17
−0.5
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Relative to Reference Policy (530−650 ppm CO
2
eq)
Mitigation Costs (NPV 2015−2100, 5% Discount Rate)
Min
Scenario
75
th
Percentile
Max
Median
25
th
Percentile
1
Mitigation Costs Lower Mitigation Costs Higher than in Reference Policy
Carbon Price Lower Carbon Price Higher than in Reference Policy
Figure 6�23 | Average carbon prices (left panel) and global mitigation costs (right panel) as a function of residual cumulative CO
2
emissions expressed as fraction of cumulative
baseline emissions over the period 2011 – 2100. Emissions reductions relative to baseline can be deduced by subtracting the fraction of residual cumulative emissions from unity.
Mitigation costs are reported in NPV consumption losses in percent baseline consumption for general equilibrium (GE) models and abatement costs in percent baseline GDP for
partial equilibrium (PE) models. A discount rate of 5 % per year was used for calculating average carbon prices and net present value mitigation costs. See description of Figure 6.21
for the selection of scenarios. Source: WG III AR5 Scenario Database (Annex II.10).
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
0
10
20
30
40
50
60
70
80
90
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
0
1
2
3
4
5
6
Cumulated CO
2
Emissions [Fraction of Baseline]
Consumption losses [% Baseline Consumption]
0
1
2
3
4
5
6
Abatement Costs [% Baseline GDP]
Mitigation Cost (NPV 2015-2100, 5% Discount Rate)
Cumulated CO
2
Emissions [Fraction of Baseline]
Average Carbon Price 2015−2100 [USD
2010
/tCO
2
]
430−480 ppm CO
2
eq
480−530 ppm CO
2
eq
530−580 ppm CO
2
eq
580−650 ppm CO
2
eq
650−720 ppm CO
2
eq
Partial Equilibrium Models
General Equilibrium Models
430−480 ppm CO
2
eq
480−530 ppm CO
2
eq
530−580 ppm CO
2
eq
580−650 ppm CO
2
eq
650−720 ppm CO
2
eq
PE Models
GE Models
Carbon Price
453453
Assessing Transformation Pathways
6
Chapter 6
can be combined with electricity, synthetic fuel, and hydrogen produc-
tion from several feedstocks and in energy-intensive industries such
as cement and steel. The CCS can also act as bridge technology that is
compatible with existing fossil-fuel dominated supply structures (see
Sections 7.5.5, 7.9, and 6.9 for a discussion of challenges and risks of
CCS and CDR). Bioenergy shares some of these characteristics with
CCS. It is also an essential ingredient for BECCS, and it can be applied
in various sectors of the energy system, including for the provision of
liquid low-carbon fuels for transportation (see Chapter 11, Bioenergy
Annex for a discussion of related challenges and risks). In contrast,
those options that are largely confined to the electricity sector (e. g.,
wind, solar, and nuclear energy) and heat generation tend to show a
lower value, both because they cannot be used to generate negative
emissions and because there are a number of low-carbon electricity
supply options available that can generally substitute each other (Krey
etal., 2014).
Scenarios also suggest that energy end-use technologies and mea-
sures have an important influence on mitigation costs. For example,
in the EMF27 and AMPERE multi-model studies, reductions in the
final energy demand of 20 – 30 % by 2050 and 35 – 45 % by 2100 led
to reductions in the cumulative discounted aggregate mitigation costs
on the order of 50 % (Krey etal., 2014; Kriegler etal., 2014a; Riahi
etal., 2014). An important caveat to these results is that the costs of
achieving these reductions were not considered nor were the policy
or technology drivers that led to them. Energy end-use measures are
important not just for reducing energy consumption, but also for facili-
tating the use of low-carbon fuels. For example, a number of studies
(Kyle and Kim, 2011; Riahi etal., 2012; Pietzcker etal., 2014; McCol-
lum etal., 2014b) show that allowing electricity or hydrogen in trans-
portation lowers mitigation costs by opening up additional supply
routes to the transportation sector (see Section 6.8 for more on this
topic). An increasing ability to electrify the end-use sectors and trans-
port in particular, in turn, tends to reduce the importance of CCS and
bioenergy technologies for achieving lower-concentration goals such
as 450 ppm CO
2
eq.
6�3�6�4 Economic implications of non-idealized
international mitigation policy implementation
Research has consistently demonstrated that delaying near-term global
mitigation as well as reducing the extent of international participation
in mitigation can significantly affect aggregate economic costs of miti-
Figure 6�24 | Relative increase of NPV mitigation costs (period 2015 – 2100, 5 % discount rate) from technology portfolio variations compared to a scenario with default technol-
ogy availability. Scenario names on the horizontal axis indicate the technology variation relative to the default assumptions: Low Energy Intensity = higher energy intensity improve-
ments leading to energy demand reductions of 20 – 30 % by 2050 and 35 – 45 % by 2100 relative to the default baseline; No CCS = unavailability of CCS; Nuclear Phase Out = No
addition of nuclear power plants beyond those under construction; existing plants operated until the end of their lifetime; Limited Solar / Wind = a maximum of 20 % global electric-
ity generation from solar and wind power in any year of these scenarios; Limited Bioenergy = maximum of 100 EJ / yr of modern bioenergy supply globally; Conventional Energy
Future = combining pessimistic assumptions for renewable energy (Limited Solar / Wind + Limited Bioenergy); Energy efficiency and Renewables = combining low energy intensity
with non-availability of CCS and nuclear phase-out; Limited Technology Future = all supply-side options constrained and energy intensity developing in line with historical records
in the baseline. Source: EMF27 study, adapted from (Kriegler etal., 2014a). Only those scenarios from the EMF27 study are included that reached the 430 – 480 and 530 – 580 ppm
CO
2
eq concentration ranges or were close to it (see footnotes in the figure).
* Number of models successfully vs. number of models attempting running the respective technology variation scenario
‡
Scenarios from two models reach concentration levels in 2100 that are slightly above the 430-480 ppm CO
2
eq category.
†
Scenarios from one model reach concentration levels in 2100 that are slightly below the 530-580 ppm CO
2
eq category
0/9*5/8*5/10*11/11*7/10*12/12*8/10*12/12*8/9*10/10*8/9*10/10*4/10*11/11*8/9*12/12*
Low Energy Intensity Limited Technology
Future
Energy Efficiency
and Renewables
Conventional
Energy Future
Limited BioenergyLimited Solar/WindNuclear Phase OutNo CCS
Mitigation Costs Lower Mitigation Costs Higher than for Default Technology Assumptions
0
1
2
3
4
5
Mitigation Costs Relative to Default Technology Assumptions
5.4
General Equilibrium Models
Partial Equilibrium Models
430-480
‡
ppm CO
2
eq
530
†
-580 ppm CO
2
eq
Min
75
th
Percentile
Max
Median
25
th
Percentile
454454
Assessing Transformation Pathways
6
Chapter 6
gation. One way in which aggregate mitigation costs are increased is
by delaying near-term global mitigation relative to what would be
warranted in the hypothetical idealized case that a long-term goal was
adopted and a least-cost approach to reach the global mitigation goal
was implemented immediately. This represents one manifestation of
not undertaking mitigation ‘when’ it is least expensive (Keppo and
Rao, 2007; Bosetti et al., 2009b; Krey and Riahi, 2009; Jakob etal.,
2012; Kriegler etal., 2013b; Luderer etal., 2013; Rogelj etal., 2013b;
Riahi etal., 2014). In scenarios in which near-term global mitigation is
limited, the increase in mitigation costs is significantly and positively
related to the gap in short-term mitigation with respect to the ideal-
ized scenarios (Figure 6.25). Costs are lower in the near-term, but
increase more rapidly in the transition period following the delayed
mitigation, and are also higher in the longer term. Future mitigation
costs are higher because delays in near-term mitgation not only require
deeper reductions in the long run to compensate for higher emissions
in the short term, but also produce a larger lock-in in carbon infrastruc-
ture, increasing the challenge of these accelerated emissions reduction
rates. The effects of delay on mitigation costs increase with the strin-
gency of the mitigation goal. Studies suggest that important transi-
tional economic metrics other than aggregate costs — for example,
reduced growth rates in economic output and consumption, escalating
energy prices, and increasing carbon rents — may be more affected by
delayed mitigation than aggregate costs (Kriegler etal., 2013b; Lud-
erer etal., 2014a).
Studies have consistently found that delays through 2030 have sub-
stantially more profound aggregate economic implications than delays
through 2020, both in terms of higher transitional impacts due to more
rapidly increasing mitigation costs at the time of adopting the long-
term strategy and higher long-term costs (Kriegler etal., 2013b; Rogelj
etal., 2013a; Luderer etal., 2014a). This is directly related to prolonged
delays in mitigation leading to both larger carbon lock-ins and higher
short term emissions that need to be compensated by deeper emis-
sions cuts in the long run (Sections 6.3.2 and 6.4). Moreover, delayed
mitigation further increases the dependence on the full availability
of mitigation options, especially on CDR technologies such as BECCS
(Luderer etal., 2013; Rogelj etal., 2013b; Riahi etal., 2014). (See Sec-
tion 6.3.6.3, Section 6.4).
Fragmented action or delayed participation by particular coun-
tries — that is, not undertaking mitigation ‘where’ it is least expen-
Figure 6�25 | Mitigation costs increase as a function of reduced near-term mitigation effort, expressed as relative change to immediate mitigation (idealized implementation) sce-
narios (referred to as the ‘mitigation gap’). Cost increase is shown both in the medium term (2030 – 2050, left panel) and in the long term (2050 – 2100, right panel), calculated on
undiscounted costs. The mitigation gap is calculated from cumulative CO
2
mitigation to 2030. Blue and yellow dots show scenarios reaching concentration goals of 430 – 530 ppm
and 530 – 650 ppm CO
2
eq, respectively. The shaded area indicates the range for the whole scenario set (two standard deviations). The bars in the lower panel indicate the mitigation
gap range where 75 % of scenarios with 2030 emissions, respectively, above and below 55 GtCO
2
are found. Not all model simulations of delayed additional mitigation until 2030
could reach the lower concentration goal of 430 – 530 ppm CO
2
eq (for 2030 emissions above 55 GtCO
2
eq, 29 of 48 attempted simulations could reach the goal; for 2030 emissions
below 55 GtCO
2
eq, 34 of 51 attempted simulations could reach the goal). Source: WG III AR5 Scenario Database (Annex II.10), differences between delayed mitigation to 2020 and
2030 and immediate mitigation categories.
Mitigation Costs Increase
(% Difference w.r.t. Immediate Mitigation)
Mitigation Gap till 2030 [%]Mitigation Gap till 2030 [%]
0
20
40
60
80
100
-20
0
+20
+40
+60
+80
+100
+120
+140
0
20
40
60
80
100
-20
0
+20
+40
+60
+80
+100
+120
+140
Medium Term (2030-2050) Long Term (2050-2100)
< 55 Gt CO
2
in 2030
> 55 Gt CO
2
in 2030
430-530 ppm CO
2
eq Obs.
530-650 ppm CO
2
eq Obs.
430-650 ppm CO
2
eq Range
Median
+1 Standard Deviation
-1 Standard Deviation
455455
Assessing Transformation Pathways
6
Chapter 6
sive — has also been broadly shown to increase global mitigation costs
(Edmonds et al., 2008; Calvin et al., 2009b; Clarke etal., 2009; Tol,
2009; Richels etal., 2009; Bosetti etal., 2009d; van Vliet etal., 2009;
Kriegler et al., 2014c). Fragmented action will influence aggregate
global economic costs not only because of misallocation of mitigation
across countries, but also through emissions leakage and trade-related
spillover effects (Arroyo-Curras etal., 2014; Babiker, 2005; Bauer etal.,
2014a; Blanford et al., 2014; Böhringer etal., 2012; Bosetti and De
Cian, 2013; Kriegler et al., 2014c). The range and strength of these
adverse effects and risks depends on the type of policy intervention
and the stringency of the mitigation effort. Border carbon adjustments
have been found to reduce economic impacts of exposed industries,
but not to yield significant global cost savings (Böhringer etal., 2012).
Some studies have indicated that the increased costs from fragmented
action could be counterbalanced by increased incentives to carry out
innovation, though only to a limited extent (Di Maria and Werf, 2007;
Golombek and Hoel, 2008; Gerlagh etal., 2009; De Cian and Tavoni,
2012; De Cian etal., 2014).
Multi model studies have indeed found that the smaller the propor-
tion of total global emissions included in a climate regime due to
fragmented action, the higher the costs and the more challenging
it becomes to meet any long-term goal. For example, only 2 (5) of
10 participating models could produce 450 ppm CO
2
eq overshoot
(550 ppm CO
2
eq not to exceed) scenarios under the regional frag-
mentation assumptions in the EMF22 scenarios (Clarke etal., 2009).
In these scenarios, the Annex I countries began mitigation immedi-
ately, followed by major emerging economies in 2030, and the rest
of the world in 2050 (see Table 6.1, (Clarke etal., 2009) (see Section
6.2 for a discussion of model infeasibility). Discounted global aggre-
gate mitigation costs over the century increased by 50 % to more
than double for those models that could produce these scenarios
(FIgure 6.26).
In general, when some countries act earlier than others, the increased
costs of fragmented action fall on early actors. However, aggregate
economic costs can also increase for late entrants, even taking into
account their lower near-term mitigation (Clarke etal., 2009; Jakob
etal., 2012). Late entrants benefit in early periods from lower mitiga-
tion; however, to meet long-term goals, they must then reduce emis-
sions more quickly once they begin mitigation, in just the same way
that global emissions must undergo a more rapid transition if they
are delayed in total. The increased costs of this rapid and deep miti-
gation can be larger than the reduced costs from delaying near-term
mitigation (Figure 6.26). The degree to which the late entrants’ miti-
gation costs increase with fragmented action depends on the extent
of carbon-intensive technologies and infrastructure put in place dur-
ing the period during which they delay reductions and the speed at
which emissions must be reduced after they begin emissions reduc-
tions. Indeed, in the face of a future mitigation commitment it is opti-
mal to anticipate emissions reductions, reducing the adjustment costs
of confronting mitigation policy with a more carbon-intensive capital
stock (Bosetti etal., 2009a; Richels etal., 2009). In addition, countries
may incur costs from international mitigation policy even if they do not
participate, for example, from a loss of fossil fuel revenues (Blanford
etal., 2014).
6�3�6�5 The interactions between policy tools and their
implementation, pre-existing taxes, market
failures, and other distortions
The aggregate economic costs reported in Section 6.3.6.2 have
assumed an idealized policy implementation and in many cases an
idealized implementation environment with perfectly functioning eco-
nomic markets devoid of market failures, institutional constraints, and
pre-existing tax distortions. Many models represent some of these dis-
tortions, but most models represent only a small portion of possible
distortions and market failures. The reality that assumptions of ideal-
ized implementation and idealized implementation environment will
not be met in practice means that real-world aggregate mitigation
costs could be very different from those reported here.
Under the assumption of a perfect implementation environment,
economic analysis has long demonstrated that the way to minimize
the aggregate economic costs of mitigation is to undertake mitiga-
tion where and when it is least expensive (Montgomery, 1972). This
implies that policies be flexible and comprehensive with a ubiquitous
price on GHG emissions, as might be achieved by a cap-and-trade
policy or carbon tax (Goulder and Parry, 2008). The literature pre-
sented thus far in this section has assumed such an approach. Even
Figure 6�26 | Impact of fragmented action on the relative mitigation costs of three
representative regions: Annex I without Russia; Brasil, Russia, India, and China (BRIC);
and Rest of the World (ROW) from the EMF22 Study. In this study, Annex I (without
Russia) joins immediately, BRIC in 2030, and ROW in 2050 (see Table 6.1). The vertical
axis shows the increase in mitigation costs between full participation and fragmented
action scenarios. Thus, values above 0 indicate that fragmented action increases costs.
Mitigation costs are calculated relative to baseline over 2015 – 2100 both in NPV at 5 %
discount rate (left bars) and as maximum losses over the century (right bars). Source:
EMF22 data base.
-100
0
+100
+200
+300
+400
+500
Mitigation Cost Increase Relative to Full Participation [%]
Outlier
Mean
10
th
Percentile
75
th
Percentile
90
th
Percentile
Net Present Value
Maximum Losses
Median
25
th
Percentile
BRIC Rest of WorldAnnex I
Without Russia
456456
Assessing Transformation Pathways
6
Chapter 6
scenarios with fragmented or limited near-term emissions reductions
have typically assumed efficient, full-economy carbon prices for all
countries undertaking mitigation. However, real-world approaches
may very well deviate from this approach. For example, some policies
may only address particular sectors, such as power generation; other
policies may regulate the behaviour of particular sectors through
command and control measures, for example, through renewable
portfolio standards for power generation or fuel economy standards
for transport.
In an idealized implementation environment, the literature shows that
approaches that exclude sectors or regulate reductions by sector will
lead to higher aggregate mitigation costs, particularly for goals requir-
ing large emissions reductions where coverage and flexibility are most
important (Paltsev etal., 2008). A wide range of recent studies have
corroborated this general result, including the large scale multi-model
comparison studies such as EMF22 (Böhringer et al., 2009), EMF24
(Fawcett etal., 2014), and EMF28 (Knopf etal., 2013) along with a
wide range of individual papers. As an example, a survey of results
(OECD, 2009) indicates that exempting energy-intensive industries
increases mitigation costs for achieving concentrations of 550 ppm by
50 % in 2050, and that excluding non-CO
2
GHG emissions increases
the mitigation costs by 75 % in 2050. The EMF22 study (Böhringer
etal., 2009) find that differential prices for the European Union (EU)
Emission Trading Scheme (ETS) and non-ETS emissions in the EU and
the inclusion of a renewable portfolio standard could double the miti-
gation costs for the EU goals for 2020. Wise etal. (2009) found that the
failure to include changes in land use emissions in mitigation policy
could double global carbon prices in a 450 ppm CO
2
scenario. At the
same time, it is important to recognize that mitigation may not be the
only objective of these sectoral approaches and regulatory policies.
They may also be designed to address other policy priorities such as
energy security and local environmental concerns.
Climate policies will interact with pre-existing policy structures as
well as with other market failures beyond the market failure posed
by climate change — that is, a non-idealized implementation environ-
ment — and these interactions can either increase or decrease policy
costs. A number of authors have argued that costs could be much
lower or even negative compared to those produced by studies assum-
ing idealized policy and implementation environments (Bosquet, 2000;
Bye etal., 2002; Waisman etal., 2012). The results of these studies
rest on one or several assumptions — that mitigation policy be used
not only to address the climate externality, but also to achieve other
policy priorities such as sustainable development; the use of mitigation
policy instruments for the correction of the implementation environ-
ment including removal of market failures and pre-existing distortions;
and / or on optimistic views of climate-related innovation and technol-
ogy development, adoption, and penetration.
Because technology is so critical to the economic costs of mitigation,
the economic costs and efficacy of climate policies more generally will
necessarily be influenced by market failures in markets for technology
adoption and those for development and R&D (Jaffe, 2012). There are
numerous market failures, such as research and adoption spillovers,
limited foresight, limited information, and imperfect capital markets,
which can cause underinvestment in mitigation technologies, dis-
cussed in more detail in Section 15.6 (Thollander etal., 2010; Allcott,
2011, 2013; Kalkuhl etal., 2012, among many others). Studies indi-
cate aggregate mitigation costs could be lower if these market fail-
ures could be removed through complementary policies (Jaffe etal.,
2005; Thollander etal., 2010). Additionally, literature that focuses in
particular on failures in markets for investments in technology and
R&D has found large reductions in aggregate mitigation costs as a
result of correcting these failures, for example, through the recycling
of revenue from climate policies or otherwise using public funds (Bos-
quet, 2000; Edenhofer etal., 2010; Waisman etal., 2012). The litera-
ture has also shown the value of related complementary policies to
enhance labor flexibility (Guivarch etal., 2011) or impact the mobility
of demand, such as transportation infrastructures or urban and fiscal
policies lowering real estate prices and urban sprawl (Waisman etal.,
2012).
Interactions with pre-existing policies and associated distortions will
also influence economic costs. The EU ETS offers an example where
an efficient policy tool (cap-and-trade system) that is applied on par-
tial sectors (partial coverage) and interacts with pre-existing distor-
tions (high energy taxes) and other energy policies (renewable energy
requirements) is affected by over-allocation of permits and slower than
expected economic growth (Ellerman and Buchner, 2008; Ellerman,
2010; Batlle etal., 2012). Paltsev et al (2007) show that pre-existing
distortions (e. g., energy taxes) can greatly increase the cost of a policy
that targets emission reduction. In contrast, literature has also looked
into the use of carbon revenues to reduce pre-existing taxes (generally
known as the ‘double dividends’ literature). This literature indicates
that total mitigation costs can be reduced through such recycling of
revenues (Goulder, 1995; Bovenberg and Goulder, 1996). Nonetheless,
a number of authors have also cautioned against the straight gener-
alization of such results indicating that the interplay between carbon
policies and pre-existing taxes can differ markedly across countries
showing empirical cases where a ‘double dividend’ does not exist
as discussed in Section 3.6.3.3 (Fullerton and Metcalf, 1997; Babiker
etal., 2003; Metcalf etal., 2004).
6�3�6�6 Regional mitigation costs and effort-sharing
regimes
The costs of climate change mitigation will not be identical across
countries (Clarke etal., 2009; Hof etal., 2009; Edenhofer etal., 2010;
Lüken etal., 2011; Luderer etal., 2012b; Tavoni etal., 2013; Aboumah-
boub etal., 2014; Blanford etal., 2014). The regional variation in costs
will be influenced by the nature of international participation in miti-
gation, regional mitigation potentials, and transfer payments across
regions. In the idealized setting of a universal carbon price leading to
reductions where they would be least expensive, and in the absence
457457
Assessing Transformation Pathways
6
Chapter 6
of transfer payments, the total aggregate economic costs of mitiga-
tion would vary substantially across countries and regions. In results
collected from modelling studies under these circumstances, relative
aggregate costs in the OECD-1990, measured as a percentage change
from, or relative to, baseline conditions, are typically lower than the
global average, those in Latin America are typically around the global
average, and those in other regions are higher than the global average
(Figure 6.27) (Clarke etal., 2009; Tavoni etal., 2013).
The variation in these relative regional costs can be attributed to sev-
eral factors (Stern etal., 2012; Tavoni etal., 2013). First, costs are driven
by relative abatement with respect to emissions in a baseline, or no-
policy, scenario, which are expected to be higher in developing coun-
tries (see Section 6.3.2 for more discussion). Second, developing coun-
tries are generally characterized by higher energy and carbon
intensities due to the structure of economies in economic transition.
This induces a higher economic feedback for the same level of mitiga-
tion (Luderer et al., 2012b). Third, domestic abatement is only one
determinant of policy costs, since international markets would interact
with climate policies (Leimbach etal., 2010). For some regions, notably
the fossil energy exporting countries, higher costs would originate from
unfavourable terms of trade effects of the mitigation policy (OECD,
2008; Luderer etal., 2012a; Massetti and Tavoni, 2011; Aboumahboub
etal., 2014; Blanford etal., 2014), while some regions could experience
increased bio-energy exports (Persson etal., 2006; Wise etal., 2009;
Leimbach etal., 2010). A final consideration is that the total costs (as
opposed to costs measured as a percentage change from baseline con-
ditions) and associated mitigation investments are also heavily influ-
enced by baseline emissions, which are projected to be larger in the
developing regions than the developed regions (see Section6.3.1).
A crucial consideration in the analysis of the aggregate economic
costs of mitigation is that the mitigation costs borne in a region can
be separated from who pays those costs. Under the assumption of
efficient markets, effort-sharing schemes have the potential to yield
a more equitable cost distribution between countries (Ekholm etal.,
2010b; Tavoni etal., 2013). Effort-sharing approaches will not mean-
ingfully change the globally efficient level of regional abatement, but
can substantially influence the degree to which mitigation costs or
investments might be borne within a given country or financed by
other countries (e. g. Edenhofer etal., 2010). A useful benchmark for
consideration of effort-sharing principles is the analysis of a frame-
work based on the creation of endowments of emission allowances
and the ability to freely exchange them in an international carbon
market. Within this framework, many studies have analyzed differ-
ent effort-sharing allocations according to equity principles and other
indicators (see Section 3.3, Section 4.6.2) (den Elzen and Höhne, 2008,
2010; Höhne etal., 2014).
Comparing emission allocation schemes from these proposals is com-
plex because studies explore different regional definitions, timescales,
starting points for calculations, and measurements to assess emission
allowances such as CO
2
only or as CO
2
eq (see Höhne etal., 2014). The
range of results for a selected year and concentration goal is relatively
large due to the fact that the range includes fundamentally different
effort-sharing approaches and other variations among the assump-
tions of the studies.
Nonetheless, it is possible to provide a general comparison and charac-
terization of these studies. To allow comparison of substantially different
proposals, Höhne etal. (2014) developed a categorization into seven cat-
Figure 6�27 | Regional mitigation costs relative to global average for scenarios reaching 430 – 530 ppm CO
2
eq in 2100 (left panel) and 530 – 650 ppm CO
2
eq in 2100 (right panel).
Values above (below) 1 indicate that the region has relative mitigation costs higher (lower) than global average. Relative costs are computed as the cumulative costs of mitigation
over the period 2020 – 2100, discounted at a 5 % discount rate, divided by cumulative discounted economic output over that period. Scenarios assume no carbon trading across
regions. The numbers below the regions names indicate the number of scenarios in each box plot. Source: WGIII AR5 Scenario Database (Annex II.10), idealized implementation
and default (see Section 6.3.1) technology scenarios.
93# of Scenarios 101 79 6787 84929988105
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Regional Mitigation Costs Relative to Global Average
530-650 ppm CO
2
eq430-530 ppm CO
2
eq
OECD-1990 ASIA LAM MAF EIT
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Regional Mitigation Costs Relative to Global Average
OECD-1990 ASIA LAM MAF EIT
Regional Costs Higher than Global Average
Regional Costs Higher than Global Average
Mean
Outlier
Global Costs
10
th
Percentile
75
th
Percentile
90
th
Percentile
Median
25
th
Percentile
458458
Assessing Transformation Pathways
6
Chapter 6
egories based on three equity principles (see Chapter 4): responsibility,
capability, and equality (Table 6.5). The first three categories represent
these equity principles alone. The following three categories represent
combinations of these principles. ‘Equal cumulative per capita emissions’
combines equality (per capita) with responsibility (cumulative account-
ing for historical emissions); ‘responsibility, capability, and need’ includes
approaches that put high emphasis on historical responsibility and at the
same time on capability plus the need for sustainable development;
‘staged approaches’ includes those that already constitute a compro-
mise over several principles. Finally, the last category, ‘equal marginal
abatement costs’ (implemented in the models as uniform carbon tax
with no compensatory transfers), represents the initial allocation to that
which would emerge from a global price on carbon. This is used as a
reference against which to compare the implications of other regimes.
Table 6�5 | Categories of effort-sharing proposals. Source: Höhne etal. (2014)
Categories
Responsibility
Capability
Equality
Description References
Responsibility X
The concept to use historical emissions to derive emission goals
was first directly proposed by Brazil in the run-up of the Kyoto
negotiations (UNFCCC, 1997), without allocations. Allowances
based only on this principle were quantified by only a few studies.
Berk and den Elzen (2001)*, Den Elzen etal.
(2005); Den Elzen and Lucas (2005)
Capability X
Frequently used for allocation relating reduction goals or reduction
costs to GDP or human development index (HDI). This includes
also approaches that are focused exclusively on basic needs.
Den Elzen and Lucas (2005); Knopf etal. (2011); Jacoby
etal. (2009); Miketa and Schrattenholzer (2006);
Kriegler etal. (2013b) and Tavoni etal. (2013) **
Equality X
A multitude of studies provide allocations based on immediate or
converging per capita emissions (e. g. Agarwal and Narain, 1991;
Meyer, 2000). Later studies refine the approach using also per capita
distributions within countries (e. g. Chakravarty etal., 2009).
Berk and den Elzen (2001)*, Kriegler etal. (2013b) and Tavoni
etal. (2013)**, Böhringer and Welsch (2006); Bows and Anderson
(2008); Chakravarty etal. (2009); Criqui etal.(2003); Den Elzen
and Lucas (2005); Den Elzen and Meinshausen (2006); Den
Elzen etal.(2005, 2008); Edenhofer etal. (2010); Hof etal.
(2010b); Höhne and Moltmann (2008, 2009); Knopf etal.(2009,
2011); Kuntsi-Reunanen and Luukkanen (2006); Nabel etal.
(2011); Miketa and Schrattenholzer (2006); Peterson and Klepper
(2007); Onigkeit etal. (2009); Van Vuuren etal. (2009a, 2010)
Responsibility,
capability, and need
X X
Recent studies used responsibility and capability explicitly
as a basis, e. g., Greenhouse Development Rights
(Baer etal., 2008); or ‘Responsibility, Capability, and
Sustainable Development’(Winkler etal., 2011)
Baer etal. (2008); Baer (2013); Höhne and Moltmann
(2008, 2009); Winkler etal. (2011)
Equal cumulative per
capita emissions
X X
Several studies allocate equal cumulative per capita emission rights
based on a global carbon budget (Pan, 2005, 2008). Studies diverge on
how they assign the resulting budget for a country to individual years.
Bode (2004); Nabel etal. (2011); Jayaraman
etal. (2011); Schellnhuber etal. (2009);
Staged approaches X X X
A suite of studies propose or analyze approaches, where
countries take differentiated commitments in various stages.
Also approaches based on allocation for sectors such as the
Triptych approach (Phylipsen etal., 1998) or sectoral approaches
are included here. Categorization to a stage and the respective
commitments are determined by indicators using all four equity
principles. Finally, studies using equal percentage reduction goals,
also called grandfathering, are also placed in this category.
Bosetti and Frankel (2012); Criqui etal. (2003); Den Elzen
and Lucas (2005); Den Elzen and Meinshausen (2006); Den
Elzen etal. (2007, 2008, 2012); Hof etal.(2010a); Höhne and
Moltmann (2008, 2009); Höhne etal.(2005, 2006); Knopf etal.
(2011); Vaillancourt and Waaub (2004); Peterson and Klepper
(2007); Böhringer and Welsch (2006); Knopf etal.(2011)
Berk and den Elzen (2001)
Equal Marginal Abatement
Costs (for reference)
Modelling studies often use the allocations that would emerge from a
global carbon price as a reference case for comparing other allocations.
Peterson and Klepper (2007), Van Vuuren etal. (2009a),
Kriegler etal. (2013b) and Tavoni etal. (2013) **
*
Not included in the quantitative results, because either too old or pending clarifications of the data.
**
This is a model comparison study of seven integrated models as part of the LIMITS research project: PBL, IIASA, FEEM, ECN*, PIK, PNNL, NIES*. Each of these models repre-
sents one data point. Some of these model studies are more extensively described in a particular model study (Kober etal., 2014).
459459
Assessing Transformation Pathways
6
Chapter 6
The range of allowances can be substantial even within specific cate-
gories of effort sharing, depending on the way the principle is imple-
mented (Figure 6.28). For some effort-sharing categories, the ranges
are smaller because only a few studies were found. Despite the ranges
within a category, distributional impacts differ significantly with under-
lying criteria for effort sharing.
The concentration goal is significant for the resulting emissions allow-
ances (Figure 6.29). Indeed, for many regions, the concentration goal is
of equal or larger importance for emission allowances than the effort-
sharing approach. For concentration levels between 430 and 480 in
2100, the allowances in 2030 under all effort-sharing approaches in
OECD-1990 are approximately half of 2010 emissions with a large
range, roughly two-thirds in the EITs, roughly at the 2010 emissions
level or slightly below in ASIA, slightly above the 2010 level in the
Middle East and Africa, and well below the 2010 level in Latin Amer-
ica. For these same concentration levels, allowances in OECD-1990
and EITs are a fraction of today’s emissions in 2050, and allowances
for Asia and Latin America are approximately half of 2010 emission
levels in 2050. For higher concentration levels, most studies show a
significant decline in allowances below current levels for OECD-1990
and EITs by 2050. Most studies show a decline in allowances below
current levels for the Latin America region, mostly increasing above
current levels for the Africa and Middle East region, and an inconsis-
tent picture for ASIA.
The creation of endowments of emissions allowances would gener-
ate payment transfers across regions in a global carbon market. These
transfer payments would depend on the regional abatement opportu-
nities, the distribution of allowances, and the concentration goal. To
the extent that regional mitigation levels represent the cost-effective
mitigation strategy across regions, the size of these allocations relative
to domestic emissions provide an indication of the degree to which
allowances would be transferred to or from any region. If allocations
are higher than the ‘equal marginal abatement cost’ allocation in a
particular country, then the country could possibly improve its financial
position by reducing emissions and selling the remaining allowances.
If allocations are lower than the ‘equal marginal abatement cost’ allo-
cation, the country could possibly purchase allowances and therefore
provide transfers.
Box 6�2 | Least-developed countries in integrated models
There are significant data and information deficits pertaining to
least-developed countries(LDCs) and limits to the modelling of the
specific features and characteristics of LDCs. For this reason, the
integrated modelling literature provides relatively little informa-
tion on the specific implications of transformation pathways for
LDCs. Based on the limited available literature, LDCs contribute
little to future GHG emissions until 2050 even though they are
projected to grow faster than global emissions. Post-2050 emis-
sions trends for LDCs depend on highly uncertain projections of
their long-term economic growth prospects. One study in the
available integrated modelling literature suggests that LDC’s
contribution to global emissions increases by about 50 % between
2000 and 2100 (Calvin etal., 2009b).The mitigation challenges
for LDCs are particularly significant given their ambitions for
economic growth, poverty alleviation, and sustainable develop-
ment on the one hand, and their limited means for mitigation
in terms of technology and finance on the other hand. Tradeoffs
can include, among other things, a prolonged use of traditional
bioenergy and a reduction in final energy use. Potential synergies
include accelerated electrification (Calvin et al., 2014a).
The literature on the transformation pathways has also indicated
the need for large deployment of low-carbon technologies.
These projections pose critical challenges and uncertainties for
LDCs when taking into account issues related to deployment,
institutions and program design, and non-climate socioeconomic
implications. In particular, many scenarios rely on technologies
with potentially large land footprints, such as bioenergy and
afforestation or reforestation, to achieve mitigation goals. The
scenarios surveyed in the chapter universally project the major-
ity of bioenergy primary energy will occur in developing econo-
mies (50 – 90 % in non-OECD in 2050, see Section 6.3.5). These
abatement patterns imply significant challenges for developing
countries in general, and LDCs in particular, where large land-use
abatement potentials lie.
The literature related to effort-sharing and distributional impli-
cations of mitigation in LDCs is relatively scarce. The literature
suggests that there are tradeoffs between food security and
mitigation (e. g. Reilly etal., 2012) with negative impacts for poor,
developing countries due to the high share of their incomes spent
on food. Mitigation might increase the rural-urban gap and dete-
riorate the living standards of large sections of the population in
developing countries (e. g. Liang and Wei, 2012). In contrast, policy
and measures aligned to development and climate objectives
can deliver substantial co-benefits and help avoid climate risks in
developing countries (Shukla etal., 2009). Modelling studies that
use the ‘low carbon society’ framework arrive at a similar conclu-
sion about co-benefits in developing countries and LDCs (Kainuma
etal., 2012; Shrestha and Shakya, 2012). Spillover effects from
trade-related mitigation policies may pose certain risks for LDCs
such as induced factor mobility, unemployment, and international
transport-related impacts on food and tourism sectors (Nurse,
2009; ICTSD, 2010; Pentelow and Scott, 2011). Downscaling of
integrated modelling to the level of LDCs is a key area for future
research.
460460
Assessing Transformation Pathways
6
Chapter 6
Figure 6�28 | Emission allowances in 2030 relative to 2010 emissions by effort-sharing category for mitigation scenarios reaching 430 – 480 ppm CO
2
eq in 2100. GHG emissions
(all gases and sectors) in GtCO
2
eq in 1990 and 2010 were 13.4 and 14.2 for OECD-1990, 8.4 and 5.6 for EIT, 10.7 and 19.9 for ASIA, 3.0 and 6.2 for MAF, 3.3 and 3.8 for LAM.
Emissions allowances are shown compared to 2010 levels, but this does not imply a preference for a specific base-year. For the OECD-1990 in the category ‘responsibility, capabil-
ity, need’ the emission allowances in 2030 is – 106 % to – 128 % (20th to 80th percentile) below 2010 level (therefore not shown here). The studies with the ‘Equal cumulative
per capita emissions’ approaches do not have the regional representation MAF. For comparison in orange: ‘Equal marginal abatement cost’ (allocation based on the imposition of
a global carbon price) and baseline scenarios. Source: Adapted from Höhne etal.(2014). Studies were placed in this CO
2
eq concentration range based on the level that the studies
themselves indicate. The pathways of the studies were compared with the characteristics of the range, but concentration levels were not recalculated.
-100
-80
-60
-40
-20
0
+20
+40
+60
+80
+100
Capability
Equality
Responsibility, Capability, Need
Equal Cumulative per Capita Emissions
Staged Approaches
Equal Marginal Abatement Costs
Baseline Scenarios
Capability
Equality
Responsibility, Capability, Need
Equal Cumulative per Capita Emissions
Staged Approaches
Equal Marginal Abatement Costs
Baseline Scenarios
Capability
Equality
Responsibility, Capability, Need
Equal Cumulative per Capita Emissions
Staged Approaches
Equal Marginal Abatement Costs
Baseline Scenarios
Capability
Equality
Responsibility, Capability, Need
Equal Cumulative per Capita Emissions
Staged Approaches
Equal Marginal Abatement Costs
Baseline Scenarios
Capability
Equality
Responsibility, Capability, Need
Equal Cumulative per Capita Emissions
Staged Approaches
Equal Marginal Abatement Costs
Baseline Scenarios
Emission Allowances in 2030 Relative to 2010 Emissions [%]
# of Scenarios
10 11 2 4 9 11 23 10 11 2 3 9 2311 10 11 2 4 9 11 23 10 11 2 0 9 11 23 10 11 2 3 9 11 23
OECD-1990 EIT ASIA MAF LAM
Min
80
th
Percentile
Max
20
th
Percentile
Figure 6�29 | Emission allowances in 2050 relative to 2010 emissions for different 2100 CO
2
eq concentration ranges by all effort-sharing categories except ‘equal marginal abate-
ment costs’. For comparison in orange: baseline scenarios. Source: Adapted from Höhne etal. (2014). Studies were placed in the CO
2
eq concentration ranges based on the level that
the studies themselves indicate. The pathways of the studies were compared with the characteristics of the ranges, but concentration levels were not recalculated.
-100
11 36 6 43 26 21 11 35 6 43 26 21 11 36 6 43 26 21 7 32 6 40 26 21 8 35 6 40 2126
-80
-60
-40
-20
0
+20
+40
+60
+80
+100
<430 ppm CO
2
eq
430-480 ppm CO
2
eq
480-530 ppm CO
2
eq
530-580 ppm CO
2
eq
580-720 ppm CO
2
eq
Baseline Scenarios
<430 ppm CO
2
eq
430-480 ppm CO
2
eq
480-530 ppm CO
2
eq
530-580 ppm CO
2
eq
580-720 ppm CO
2
eq
Baseline Scenarios
<430 ppm CO
2
eq
430-480 ppm CO
2
eq
480-530 ppm CO
2
eq
530-580 ppm CO
2
eq
580-720 ppm CO
2
eq
Baseline Scenarios
<430 ppm CO
2
eq
430-480 ppm CO
2
eq
480-530 ppm CO
2
eq
530-580 ppm CO
2
eq
580-720 ppm CO
2
eq
Baseline Scenarios
<430 ppm CO
2
eq
430-480 ppm CO
2
eq
480-530 ppm CO
2
eq
530-580 ppm CO
2
eq
580-720 ppm CO
2
eq
Baseline Scenarios
Emission Allowances in 2050 Relative to 2010 Emissions [%]
OECD-1990 EIT ASIA MAF LAM
# of Scenarios
Min
80
th
Percentile
Max
20
th
Percentile
461461
Assessing Transformation Pathways
6
Chapter 6
Multi-model studies indicate that the size of the carbon market
transfers would be significant in relation to the total global aggre-
gate economic costs of mitigation, of the order of hundreds of bil-
lions of United States dollars per year before mid-century (Clarke
et al., 2009; Luderer et al., 2012b; Tavoni et al., 2013). Transfers
through emissions allowances are also particularly high if the carbon
price is high, because the transfers are based on the quantity of the
allowances traded and the price of those allowances. Higher prices
are associated with more ambitious mitigation. For some regions,
financial flows could be on the same order of magnitude as the
investment requirements for emissions reductions (McCollum etal.,
2013b). Transfers are particularly high for some regions for the cat-
egories ‘equal per capita cumulative emissions’ and ‘responsibility,
capability, and need’ in general and for ‘staged approaches’ in some
of studies.
The transfers associated with different effort-sharing schemes have
a direct impact on the regional distribution of mitigation policy costs
(Luderer etal., 2012b). These costs are sensitive both to local abate-
ment costs and to size and direction of transfers, both of which are
related to the effort-sharing scheme as well as the carbon price and
the associated climate goal (Russ and Criqui, 2007; den Elzen etal.,
2008; Edenhofer et al., 2010; Ekholm et al., 2010b; Luderer et al.,
2012b). Given the large uncertainty about future transfers and car-
bon prices, the regional distribution of costs under different sharing
schemes varies widely (Luderer etal., 2012b; Tavoni etal., 2013). For
example, emerging economies like China could incur relatively high
expenditures (den Elzen etal., 2012; Johansson etal., 2014), but this
would change when cumulative past emissions are also accounted for
(Jiahua, 2008; Ding etal., 2009; He etal., 2009). Moreover, the uneven
regional distribution of relative mitigation costs observed in Figure
6.27 in the case without transfers is not significantly alleviated when
emissions rights are equalized per capita by 2050 and the concentra-
tion goal is stringent, as shown in Figure 6.30.
Optimal transfers can also be devised as a way to provide economic
incentives to regions to participate in international climate agree-
ments. When accounting for the strategic behaviour of the various
regions and countries, the literature suggests that climate coalitions,
which are self-enforcing and stable, can indeed be effective only in the
presence of significant compensatory payments across regions (Finus
etal., 2003; Nagashima etal., 2009; Bréchet etal., 2011). Transfers
would also occur in the case that different regional social costs of
carbon were equalized to maximize efficiency (Landis and Bernauer,
2012).
The impacts of mitigation policies on global fossil fuel trade depend
on the type of fuel, time horizon, and stringency of mitigation efforts.
Recent model intercomparison studies focusing on low-concentration
goals (430 – 530 CO
2
eq in 2100) have found an unambiguous decrease
in coal trade over the first half of the century (Cherp etal., 2014; Jewell
etal., 2013). In contrast, studies indicate that natural gas trade could
potentially increase over the coming decades as gas serves as a transi-
tion fuel and substitutes for coal (Cherp etal., 2014). Studies present
a less clear picture regarding the future of oil trade for concentration
goals in this range. In general, however, studies find oil trade to be less
sensitive to mitigation policy than coal and gas trade through 2030,
and perhaps even to 2050 (Bauer etal., 2014a, 2014b; Cherp etal.,
2014; Jewell etal., 2013; McCollum etal., 2014a).
These changes in trade patterns will have important implications
for the future trade revenues of fossil-exporting countries. There is
high agreement among integrated models that revenues from coal
trade are likely to fall for major exporters (Lüken etal., 2011; Bauer
etal., 2014a, 2014b). For oil and gas, on the other hand, the effect of
stringent climate policies on export revenues is less clear, with results
varying across models. Notwithstanding these differences, the gen-
eral conclusion of recent intercomparison exercises is that there is
likely to be a decrease in oil and gas revenues for exporting coun-
tries over the first half of the century (IEA, 2009; Haurie and Vielle,
2010; Bauer etal., 2014a, 2014b; Tavoni etal., 2013; McCollum etal.,
2014a). There are several studies that diverge from the bulk of the lit-
erature and argue that conventional oil exporters could in the short-
term benefit from climate policies under certain conditions related to
the cost of oil alternatives (biofuels and unconventional oil), the price
elasticity of oil and the cost of backstop technologies (Persson etal.,
2007; Johansson et al., 2009; Nemet and Brandt, 2012). Because
exporters of these resources can benefit from the cheaper extraction
costs and less carbon-intensive nature of conventional oil (relative to
unconventional oil deposits and coal- or gas-derived liquids), mitiga-
Figure 6�30 | Regional mitigation costs relative to global average for a 450 ppm CO
2
eq
concentration goal for a per capita effort-sharing scheme from the LIMITS multi-model
study. Values above (below) 1 indicate that the region has relative mitigation costs
higher (lower) than global average ones. Values below 0 are possible for regions who
are large net sellers of carbon allowances. Mitigation costs are computed relative to
the baseline, over 2020 – 2100 in NPV at a 5 % discount rate. Emission allocations are
based on linear convergence from 2020 levels to equal per capita by 2050, with per
capita equalization thereafter. Regions are allowed to trade emission rights after 2020
without any constraint. Source: WG III AR5 Scenario Database (Annex II.10), LIMITS per
capita scenarios.
Regional Costs
Lower than Global Average
Regional Costs
Higher than Global Average
−2
−1
0
1
2
3
4
5
OECD-1990 ASIA LAM MAF
EIT
GCAM
IMAGE
MESSAGE
REMIND
TIAM−ECN
Global Costs
WITCH
Regional Mitigation Costs Relative to Global Average
462462
Assessing Transformation Pathways
6
Chapter 6
tion efforts could potentially have a positive impact on export rev-
enues for conventional oil. These dynamics depend critically on future
commodity prices. No global studies have, as yet, systematically
explored the impact of stringent climate policies on unconventional
gas trade and export revenues, particularly those where methane
leakage from extraction activities could be an issue. The deployment
of fossil fuels is generally higher in scenarios with CCS. The availabil-
ity of CCS would thus reduce the adverse effect of mitigation on the
value of fossil fuel assets.
6.4 Integrating long- and
short-term perspectives
6�4�1 Near-term actions in a long-term
perspective
Stabilizing atmospheric concentrations of GHGs and radiative forc-
ing is a long-term endeavour. Whether a particular long-term mitiga-
tion goal will be met, and what the costs and other implications will
be of meeting it, will depend on decisions to be made and uncertain-
ties to be resolved over many decades in the future. For this rea-
son, transformation pathways to long-term climate goals are best
understood as a process of sequential decision making and learn-
ing. The most relevant decisions are those that must be made in the
near term with the understanding that new information and oppor-
tunities for strategic adjustments will arrive often in the future, but
largely beyond the reach of those making decisions today. An impor-
tant question for decision makers today is therefore how near-term
decisions will influence choices available to future decision makers.
Some decisions may maintain a range of future options, while oth-
ers may constrain the future set of options for meeting long-term
climate goals.
6�4�2 Near-term emissions and long-term
transformation pathways
A key outcome of current decision making will be the level of near-
term global emissions. Scenarios can provide important insights into
the implications of the near-term (i. e., 2020 – 2030) emissions level
for long-term climate outcomes. As discussed in Section 6.1.2, a num-
ber of multi-model studies have been designed specifically for this
purpose, exploring delays in global mitigation, in which near-term
emissions are held fixed to particular levels, and fragmented action,
in which only a subset of regions initially respond to a long-term
goal (see Table 6.1). These scenarios are typically designed as coun-
terpoint to idealized implementation scenarios in which mitigation
begins immediately, timing of reductions is unconstrained, and full
participation is assumed from the outset. This distinction is essential
for characterizing the relationship between the path emissions fol-
low through 2030 and the possible climate outcomes through the
end of the century. Among idealized implementation scenarios with
2100 concentrations in the range of 430 – 530 ppm CO
2
eq, emissions
in 2020 fall almost exclusively below the range of global GHG emis-
sions implied by the Cancún Pledges (see Section 13.13.1.3 for more
details), as in Rogelj etal. (2013a) (Figure 6.31, top panel). However,
several scenarios with delayed mitigation imposed either through
global delays or delayed participation have 2020 emissions in the
possible range of the Cancún Agreements and in some cases 2030
emissions even higher than this range while still remaining consistent
with the long-term goal (the cost implications of delay are discussed
in Section 6.3.6.4).
A second distinction that can play a critical role is the extent to which
CDR options are available and deployed. In scenarios designed with a
forcing goal applied only at the end of the century, particularly concen-
trations in the range of 430 – 530 ppm CO
2
eq, idealized implementation
scenarios often choose to temporarily overshoot the 2100 concentra-
tion (Section 6.3.2). As noted in Section 6.3.2, CDR options, typically
represented in integrated models by BECCS but also afforestation in
some cases, facilitate more rapid declines in emissions, amplifying
this overshoot pattern (Krey etal., 2014). A large number of scenarios
reaching CO
2
eq concentrations below 530 ppm CO
2
eq by 2100 deploy
CDR technologies at large enough scales that net global emissions
become negative in the second half of the century. The availability of
CDR options, as well as the representation of intertemporal flexibility,
varies significantly across models and studies. The spread in reliance
on CDR options across scenarios reveals a strong impact on the timing
of emissions pathways. In scenarios reaching the the 2100 concentra-
tion range of 430 – 530 ppm CO
2
eq in which global net CO
2
emissions
remain positive through the century, near-term emissions are gener-
ally lower than if the scenario deploys CDR technologies to a large
enough scale to lead to net negative total global CO
2
emissions later
in the century (Figure 6.31, top panel). More generally, the scenarios
indicate that a reliance on large-scale CDR, whether or not emissions
become net negative, leads to higher near-term emissions (van Vuuren
and Riahi, 2011).
The interaction between delayed mitigation and CDR options is also
important. Very few scenarios are available to demonstrate emissions
pathways consistent with 2100 concentrations of 430 – 530 ppmCO
2
eq
in which mitigation effort is delayed in some form and global carbon
emissions do not become net negative. Whether these circumstances
are not represented because they have been under-examined or
because they have been examined and the scenarios failed is a crucial
distinction, yet one that it is currently not possible to fully report (see
discussion of model infeasibility in Section 6.3.2). However, there are
instances where the combination of delay and limited options for CDR
has been explored and has resulted in model infeasibilities (Luderer
etal., 2013; Rogelj etal., 2013b; Riahi etal., 2014), which supports the
notion that this combination presents important challenges. For exam-
ple, in the AMPERE study, seven out of nine models could not produce
463463
Assessing Transformation Pathways
6
Chapter 6
Figure 6�31 | Near-term global GHG emissions from mitigation scenarios reaching 430 – 530 ppmCO
2
eq (top panel) and 530 – 650 ppmCO
2
eq (bottom panel) in 2100. Includes
only scenarios for which temperature exceedance probabilities were calculated (see Section 6.3.2). Individual model results are indicated with a data point when 2 °C exceedance
probability, based on the MAGICC results, is below 50 % for top panel or when 2.5 °C exceedance probability is below 50 % for bottom panel. For these below-50 % scenarios the
interquartile range is shown by a black rectangular frame. Colours refer to scenario classification in terms of whether net CO
2
emissions become negative before 2100 (Negative vs.
No Negative) and the timing of international participation in climate mitigation (Immediate vs. Delay 2020 / 2030). Number of reported individual results is shown in legend. The
range of global GHG emissions in 2020 implied by the Cancún Pledges is based on an analysis of alternative interpretations of national pledges (see Section 13.13.1.3 for details).
Source: WG III AR5 Scenario Database (Annex II.10). Historic data: JRC / PBL (2013), IEA (2012a), see Annex II.9. Note: Only four reported scenarios were produced based on delayed
mitigation without net negative emissions while still lying below 530 ppm CO
2
eq by 2100. They do not appear in the top panel because the model had insufficient coverage of
non-gas species to enable a temperature calculation (see Section 6.3.2). Delay in these scenarios extended only to 2020, and their emissions fell in the same range as the ‘No Nega-
tive / Immediate’ category. Note: Delayed scenarios include both delayed global action and fragmented action scenarios.
<50
GtCO
2
eq
Full Range for All Scenarios with
Calculated 2°C Exceedance
Probability
0
10
20
30
40
50
60
70
80
2000 2010 2020 2030
0
10
20
30
40
50
60
70
80
2000 2010 2020 2030
Annual GHG Emissions [GtCO
2
eq/yr] Annual GHG Emissions [GtCO
2
eq/yr]
Full Range for All Scenarios with
Calculated 2.5°C Exceedance
Probability
No Negative/Immediate (68)
No Negative/Delay 2020 (0)
No Negative/Delay 2030 (4)
Negative/Immediate (28)
Negative/Delay 2020 (0)
Negative/Delay 2030 (25)
Interquartile Range for Scenarios
with 2.5°C Exceedance Probability
<50%
No Negative/Immediate (17)
No Negative/Delay 2020 (0)
No Negative/Delay 2030 (0)
Negative/Immediate (116)
Negative/Delay 2020 (21)
Negative/Delay 2030 (27)
Ranges for 530-650 ppm CO
2
eq
Range for Cancún Pledges
Base Year Variation
in Model Scenarios
<50
GtCO
2
eq
>55
GtCO
2
eq
50-55
GtCO
2
eq
>55
GtCO
2
eq
50-55
GtCO
2
eq
Interquartile Range for Scenarios
with 2°C Exceedance Probability
<50%
530-650 ppm CO
2
eq in 2100
430-530 ppm CO
2
eq in 2100
History
Ranges for 430-530 ppm CO
2
eq
Range for Cancún Pledges
Base Year Variation
in Model Scenarios
History
464464
Assessing Transformation Pathways
6
Chapter 6
a scenario with global delay through 2030 and a restriction on CCS
technology that reached 450 CO
2
eq by 2100 (one of the remaining
two had net negative global emissions through other channels and the
other did not run past 2050). Several individual modelling team stud-
ies have also explored this space, and have found situations in which
they could not reach solutions for more ambitious goals and delayed
mitigation or constrained technology, including O’Neill etal. (2010),
Edmonds etal. (2008), and Edmonds etal. (2013). Studies have found
that delayed reductions through 2020 do not have as substantial an
effect on the cost and challenge more broadly of meeting 2100 con-
centration levels such as 450 ppm CO
2
eq as delayed reductions
through 2030 (Kriegler etal., 2013b; Luderer etal., 2013; Rogelj etal.,
2013b)
The implications of delayed mitigation, CDR options, and overshoot
for possible temperature outcomes are also significant. Numerous
studies have attempted to place the possible outcome of the Cancún
Agreements in the context of longer-term climate goals (Höhne etal.,
2012; UNEP, 2012). Due to the factors discussed above, but also varia-
tion in assumptions about baseline growth, mitigation costs, trad-
eoffs between sectors such as energy and land use, and the evolution
of non-gas forcing agents, models have found that a wide range of
near-term emissions could be consistent with a given long-term out-
come. Among scenarios with 2100 concentrations between 430 and
530 ppmCO
2
eq, focusing on those scenarios in the AR5 database for
which temperature implications were calculated (see Section 6.3.2),
near-term global emissions range from 22 to 56 GtCO
2
eq in 2020
and from 18 to 66 GtCO
2
eq in 2030 (Figure 6.31, top panel). How-
ever, based on the MAGICC results, not all pathways in this range
are consistent with at least a 50 % chance of remaining below 2 °C,
in particular those that rely on net negative global emissions. Path-
ways reaching the same 2100 concentration with higher emissions
in 2030 tend to have more overshoot; when forcing stays higher for
longer, the likelihood of reaching a temperature threshold increases.
Based on the MAGICC results, very few scenarios in the 430 – 530 ppm
CO
2
eq range have a 50 % chance of remaining below 1.5 °C, and
none with delay or limited deployment of CDR technologies; most
have a probability between 0 and 25 %. A few studies have explored
scenarios that lead to concentrations below 430 ppm CO
2
eq in 2100
(e. g., Luderer etal., 2013, Rogelj et al., 2013a, b), some of which
have been found to have more than a 66 % chance of returning to
1.5 °C by the end of the century after peaking at higher levels; these
scenarios are characterized by immediate emissions reductions fol-
lowed by very low mid-century emissions and extensive deployment
of CDR technologies. Based on the MAGICC results, nearly all sce-
narios reaching 2100 concentrations in the range of 530 – 650 ppm
CO
2
eq, have a greater than 50 % chance of exceeding 2 °C by 2100,
but many have a probability of less than 50 % of exceeding 2.5 °C
(Figure 6.31, bottom panel). Because of the higher long-term forcing
range, some growth in emissions can occur, and the preferred least-
cost range is similar to the delayed range and largely consistent with
the global GHG emissions reductions through 2020 implied by the
Cancún Pledges (see Section 13.13.1.3).
Whether due to delayed mitigation or widespread use of CDR options
or some combination of the two, higher levels of emissions in the near-
term imply an emissions pathway shifted in time, resulting in steeper
reductions later to remain consistent with a given long-term forcing
goal. As discussed in Section6.3.2, emissions in 2030 have been used
as a rough indicator for understanding the relationship between near-
term and long-term mitigation. Higher emissions in 2030 require more
rapid decreases in emissions from 2030 through 2050, both to make
up for the larger cumulative emissions up through 2030 and because
emissions must be reduced from a higher 2030 level (Figure 6.32).
Emissions decline rates for any scenario that meets 2100 concentra-
tion goals such as 450 or 550 ppm CO
2
eq must at some point push
beyond historical experience, because emissions have in general fol-
lowed growth, with past instances of decline associated only with
large-scale disruptions such as the collapse of the Soviet Union or spe-
cial cases of policy intervention such as France and Sweden (see Chap-
ter 5). Less mitigation over the coming decades will only exacerbate
the required departure from the past to meet long-term goals — path-
ways with emissions above 55GtCO
2
eq in 2030 indicate decline rates
between 2030 and 2050 of around 6 % for scenarios in the range of
430 – 530 ppmCO
2
eq in 2100 (Figure 6.32).
6�4�3 The importance of near-term
technological investments and
development of institutional capacity
While it is clear that some mitigation effort in the near term is crucial
to preserve the option of achieving low-concentration goals, whether
these goals are met in the long run depends to a greater extent on
the potential for deep GHG-emissions reductions several decades from
now. Thus efforts to begin the transformation to lower concentra-
tions must also be directed toward developing the technologies and
institutions that will enable deep future emissions cuts rather than
exclusively on meeting particular near-term goals. The way in which
countries begin low-carbon technology deployment and the imple-
mentation of climate change mitigation policies may well turn out to
be quite different from the approach that proves best in the long run.
The benefit of beginning to create and improve technologies as well
as to develop appropriate institutional capacity today is that these
present-day activities create opportunities to make early and mid-
course corrections.
The likelihood of a unified global policy for a deep GHG-emissions
reduction is low for the near future. Rather, the expectation is that a
‘mosaic’ of national and regional policies will emerge over the years
to come. Individual countries will bring different views and values to
bear on their decisions, which will likely lead to a wide variety of policy
approaches, some more economically efficient than others. Flexible
market-based policies with maximal sectoral and geographic coverage
are generally understood to deliver emissions reductions at the lowest
economic cost (see Section6.3.6.5 for a discussion of issues that influ-
ence the efficiency of implementation approaches). Although the added
Figure 6�32 | The implications of different 2030 GHG emissions levels for the pace of CO
2
emissions reductions to 2050 in mitigation scenarios reaching 430 – 530 ppmCO
2
eq
by 2100. Left-hand panel shows the development of GHG emissions to 2030. Right-hand panel denotes the corresponding annual CO
2
emissions reduction rates for the period
2030 – 2050. The scenarios are grouped according to different emissions levels by 2030 (colored in dark, medium and light green). The range of global GHG emissions in 2020
implied by the Cancún Pledges is based on an analysis of alternative interpretations of national pledges (see Section 13.13.1.3 for details). The right-hand panel compares the
median and interquartile range across scenarios from recent intermodelling comparisons with explicit 2030 interim goals with the range of scenarios in the WG III AR5 Scenario
Database (Annex II.10). Extreme scenarios with very high net negative emissions (>20 GtCO
2
/ yr) in 2100 are reported separetly as diamonds. Annual rates of historical emissions
change between 1900-2010 (sustained over a period of 20 years) and average annual emissions change between 2000-2010 are shown in grey. Sources: Intermodelling compari-
sons with explicit interim goals (AMPERE: Riahi etal., 2013; LIMITS: Kriegler etal., 2013b; ROSE: Luderer etal., 2013) and the WGIII AR5 Scenario Database (Annex II.10). Historic
data: JRC/PBL (2013), IEA (2012a), see Annex II.9. Note: Only scenarios with default technology assumptions are shown. Scenarios with non-optimal timing of mitigation due to
exogenous carbon price trajectories are excluded.
2005 2010 2015 2020 2025 2030
20
25
30
35
40
45
50
55
60
65
Annual GHG Emissions [GtCO
2
eq/yr]
GHG Emissions Pathways to 2030 of Mitigation
Scenarios Reaching 430-530 ppm CO
2
eq in 2100
Implications for the Pace of Annual Average
CO
2
Emissions Reductions from 2030 to 2050
Depending on Different 2030 GHG Emissions Levels
Annual Rate of Change in CO
2
Emissions (2030-2050) [%]
AR5 Scenario Range
Interquartile Range
and Median of Model
Comparisons with
2030 Targets
Scenarios with High
Net Negative Emissions
>20 GtCO
2
/yr in 2100
History
1900-2010
2000-2010
n=76
Cancún
Pledges
<50 GtCO
2
eq
Annual
GHG Emissions
in 2030
50-55 GtCO
2
eq
>55 GtCO
2
eq
−12
−9
−6
−3
0
3
6
n = 76
465465
Assessing Transformation Pathways
6
Chapter 6
a scenario with global delay through 2030 and a restriction on CCS
technology that reached 450 CO
2
eq by 2100 (one of the remaining
two had net negative global emissions through other channels and the
other did not run past 2050). Several individual modelling team stud-
ies have also explored this space, and have found situations in which
they could not reach solutions for more ambitious goals and delayed
mitigation or constrained technology, including O’Neill etal. (2010),
Edmonds etal. (2008), and Edmonds etal. (2013). Studies have found
that delayed reductions through 2020 do not have as substantial an
effect on the cost and challenge more broadly of meeting 2100 con-
centration levels such as 450 ppm CO
2
eq as delayed reductions
through 2030 (Kriegler etal., 2013b; Luderer etal., 2013; Rogelj etal.,
2013b)
The implications of delayed mitigation, CDR options, and overshoot
for possible temperature outcomes are also significant. Numerous
studies have attempted to place the possible outcome of the Cancún
Agreements in the context of longer-term climate goals (Höhne etal.,
2012; UNEP, 2012). Due to the factors discussed above, but also varia-
tion in assumptions about baseline growth, mitigation costs, trad-
eoffs between sectors such as energy and land use, and the evolution
of non-gas forcing agents, models have found that a wide range of
near-term emissions could be consistent with a given long-term out-
come. Among scenarios with 2100 concentrations between 430 and
530 ppmCO
2
eq, focusing on those scenarios in the AR5 database for
which temperature implications were calculated (see Section 6.3.2),
near-term global emissions range from 22 to 56 GtCO
2
eq in 2020
and from 18 to 66 GtCO
2
eq in 2030 (Figure 6.31, top panel). How-
ever, based on the MAGICC results, not all pathways in this range
are consistent with at least a 50 % chance of remaining below 2 °C,
in particular those that rely on net negative global emissions. Path-
ways reaching the same 2100 concentration with higher emissions
in 2030 tend to have more overshoot; when forcing stays higher for
longer, the likelihood of reaching a temperature threshold increases.
Based on the MAGICC results, very few scenarios in the 430 – 530 ppm
CO
2
eq range have a 50 % chance of remaining below 1.5 °C, and
none with delay or limited deployment of CDR technologies; most
have a probability between 0 and 25 %. A few studies have explored
scenarios that lead to concentrations below 430 ppm CO
2
eq in 2100
(e. g., Luderer etal., 2013, Rogelj et al., 2013a, b), some of which
have been found to have more than a 66 % chance of returning to
1.5 °C by the end of the century after peaking at higher levels; these
scenarios are characterized by immediate emissions reductions fol-
lowed by very low mid-century emissions and extensive deployment
of CDR technologies. Based on the MAGICC results, nearly all sce-
narios reaching 2100 concentrations in the range of 530 – 650 ppm
CO
2
eq, have a greater than 50 % chance of exceeding 2 °C by 2100,
but many have a probability of less than 50 % of exceeding 2.5 °C
(Figure 6.31, bottom panel). Because of the higher long-term forcing
range, some growth in emissions can occur, and the preferred least-
cost range is similar to the delayed range and largely consistent with
the global GHG emissions reductions through 2020 implied by the
Cancún Pledges (see Section 13.13.1.3).
Figure 6�32 | The implications of different 2030 GHG emissions levels for the pace of CO
2
emissions reductions to 2050 in mitigation scenarios reaching 430 – 530 ppmCO
2
eq
by 2100. Left-hand panel shows the development of GHG emissions to 2030. Right-hand panel denotes the corresponding annual CO
2
emissions reduction rates for the period
2030 – 2050. The scenarios are grouped according to different emissions levels by 2030 (colored in dark, medium and light green). The range of global GHG emissions in 2020
implied by the Cancún Pledges is based on an analysis of alternative interpretations of national pledges (see Section 13.13.1.3 for details). The right-hand panel compares the
median and interquartile range across scenarios from recent intermodelling comparisons with explicit 2030 interim goals with the range of scenarios in the WG III AR5 Scenario
Database (Annex II.10). Extreme scenarios with very high net negative emissions (>20 GtCO
2
/ yr) in 2100 are reported separetly as diamonds. Annual rates of historical emissions
change between 1900-2010 (sustained over a period of 20 years) and average annual emissions change between 2000-2010 are shown in grey. Sources: Intermodelling compari-
sons with explicit interim goals (AMPERE: Riahi etal., 2013; LIMITS: Kriegler etal., 2013b; ROSE: Luderer etal., 2013) and the WGIII AR5 Scenario Database (Annex II.10). Historic
data: JRC/PBL (2013), IEA (2012a), see Annex II.9. Note: Only scenarios with default technology assumptions are shown. Scenarios with non-optimal timing of mitigation due to
exogenous carbon price trajectories are excluded.
2005 2010 2015 2020 2025 2030
20
25
30
35
40
45
50
55
60
65
Annual GHG Emissions [GtCO
2
eq/yr]
GHG Emissions Pathways to 2030 of Mitigation
Scenarios Reaching 430-530 ppm CO
2
eq in 2100
Implications for the Pace of Annual Average
CO
2
Emissions Reductions from 2030 to 2050
Depending on Different 2030 GHG Emissions Levels
Annual Rate of Change in CO
2
Emissions (2030-2050) [%]
AR5 Scenario Range
Interquartile Range
and Median of Model
Comparisons with
2030 Targets
Scenarios with High
Net Negative Emissions
>20 GtCO
2
/yr in 2100
History
1900-2010
2000-2010
n=76
Cancún
Pledges
<50 GtCO
2
eq
Annual
GHG Emissions
in 2030
50-55 GtCO
2
eq
>55 GtCO
2
eq
−12
−9
−6
−3
0
3
6
n = 76
cost of inefficient policies in the near term may be smaller than in the
long-term when mitigation requirements will be much larger, their
implementation now may lead to ‘institutional lock-in’ if policy reform
proves difficult. Thus a near-term focus on developing institutions to
facilitate flexible mitigation strategies, as well as political structures to
manage the large capital flows associated with carbon pricing (see e. g.
Kober et al., 2014), could provide substantial benefits over the coming
decades when mitigation efforts reach their full proportions.
R&D investments to bring down the costs of low-emitting technology
options, combined with early deployment of mitigation technologies to
improve long-term performance through learning-by-doing, are among
the most important steps that can be taken in the near term (see e. g.
Sagar and van der Zwaan, 2006). R&D investments are important for
bringing down the costs of known low-carbon energy alternatives to
the current use of predominantly fossil fuels, to develop techniques that
today only exist on the drawing board, or for generating new concepts
that have not yet been invented. Early deployment of climate change
mitigation technologies can lead to both incremental and fundamental
improvements in their long-term performance through the accumula-
tion of experience or learning by doing. Mitigation policy is essential
for spurring R&D and learning by doing, because it creates commit-
ments to future GHG-emissions reductions that create incentives today
for investments in these drivers of technological innovation, and avoids
further lock-in of long-lived carbon-intensive capital stock.
Even if policies requiring GHG-emissions reductions are not imple-
mented immediately, market participants may act in anticipation of
future mitigation. Commitments to emissions reductions in the future
will create incentives for investments in climate change mitigation
technologies today, which can serve both to reduce current emissions
and avoid further lock-in of long-lived carbon-intensive capital stock
and infrastructure (see, for example, Bosetti etal., 2009c; Richels etal.,
2009).
466466
Assessing Transformation Pathways
6
Chapter 6
6.5 Integrating technological
and societal change
Technological change occurs as innovations create new possibilities
for processes and products, and market demand shifts over time in
response to changes in preferences, purchasing power, and other soci-
etal factors. Societal changes can be viewed as both a requirement for
and a result of global climate change mitigation. Because the use of
improved and new technologies is an inherent element of society’s
transformation required for climate change mitigation, technological
and societal changes necessarily interact. Their analysis therefore needs
to be integrated.
6�5�1 Technological change
The development and deployment of technology is central to long-
term mitigation, since established fossil fuel-based energy supply will
need to be replaced by new low-carbon energy techniques. The impor-
tance of technological change raises key questions about whether cur-
rent technology is sufficient for deep GHG-emissions reductions, the
best ways to improve the technologies needed for deep emissions
reductions, and the degree to which current efforts in this regard are
adequate to the upcoming challenge. Essential questions also surround
the appropriate timing of investments in technological change relative
to other efforts to reduce GHG emissions.
A primary question regarding technological change is whether cur-
rent technology is sufficient for the deep emissions reductions ulti-
mately needed to stabilize GHG concentrations. Arguments have
been made on both sides of this debate (see Hoffert etal. (2002),
and Pacala and Socolow (2004), for complementary perspectives on
this question). The integrated modelling literature provides limited
information regarding the sufficiency of current technology, because
virtually all baseline and mitigation scenarios assume that technol-
ogy will improve significantly over time, especially for technologies
with a large potential for advancement (see Riahi etal., 2013, and
van der Zwaan etal., 2013, for two recent cross-model comparison
examples). There is generally more agreement about the rate of incre-
mental cost and performance improvements for mature technologies
than for emerging technologies upon which transformation pathways
may depend (see McCollum etal., 2013b, for a cross-model study on
the investment dimension of this matter). Nonetheless, the literature
makes clear that improvements in technology and the availability of
advanced technologies can dramatically alter the costs of climate
change mitigation (see also Section 6.3.6.3). The current scientific
literature also emphasizes that the development and deployment
of CDR technologies (see Section 6.9), are a further requirement for
particular transformation pathways, for example those leading to
450 ppm CO
2
eq by 2100 yet assuming substantial near-term delays
in mitigation.
Various steps can be observed in the life of a technology, from inven-
tion through innovation, demonstration, commercialization, diffusion,
and maturation (see e. g. Grübler et al., 1999). Both investments in
R&D and the accumulation of experience through learning by doing
play important roles in the mechanisms behind technological change.
These forces are complemented by economies of scale. All these driv-
ers of technological change are complementary yet and interlinked
(Clarke and Weyant, 2002; Goulder and Mathai, 2000; Sagar and van
der Zwaan, 2006; Stoneman, 2013).
Although technological change has received extensive attention and
analysis in the context of transformation pathways (for recent exam-
ples, see IPCC, 2011; GEA, 2012), a clear systematic understanding of
the subject matter is still not available. For this reason, most of the
scenarios developed since the 1970s for energy and climate change
analysis make exogenous assumptions about the rate of technological
change. Only since the late 1990s has the effect of induced innova-
tion been considered in a subset of integrated models used for the
development of these scenarios (such as in Messner, 1997; Goulder
and Schneider, 1999; van der Zwaan etal., 2002; Carraro etal., 2003).
This restricted treatment is due to limitations in the ability to repre-
sent the complexity of technological change, and also results from the
incomplete empirical evidence on the magnitude of the effects of tech-
nological change (Popp, 2006b). More recently, empirical data on tech-
nological change have been incorporated in some integrated models
(see e. g., Fisher-Vanden, 2008), which advances the endogenous rep-
resentation of technological progress. Unsettled issues remain, how-
ever, including the proper accounting for opportunity costs of climate-
related knowledge generation, the treatment of knowledge spillovers
and appropriability, and the empirical basis for parameterizing techno-
logical relationships (Gillingham etal., 2008).
The relation between mitigation and innovation, and the presence of
market failures associated with both, raises the question of the proper
combination of innovation and mitigation policy for reducing GHG
emissions over the long term. The modelling literature broadly indicates
that relying solely on innovation policies would not be sufficient to sta-
bilize GHG concentrations (see e. g. Bosetti etal., 2011; Kalkuhl etal.,
2013), as evidenced by the fact that although most reference scenarios
assume substantial technological change, none of them lead to emis-
sions reductions on the level of those needed to bring CO
2
eq concen-
trations to levels such as 650 ppm CO
2
eq or below by 2100 (see Section
6.3.2). Climate policies such as carbon pricing could induce significant
technological change, provided the policy commitment is credible, long
term, and sufficiently strong (Popp, 2006a; Bosetti etal., 2011), while
at the same time contributing to emission reductions. The positive
effect of climate policies on technological change, however, does not
necessarily obviate the need for specific policies aimed at incentivizing
R&D investments. Market failures associated with innovation provide
the strongest rationale for subsidizing R&D (see Section 15.6).
The joint use of R&D subsidies and climate policies has been shown
to possibly generate further advantages, with some studies indi-
467467
Assessing Transformation Pathways
6
Chapter 6
cating benefits of the order of 10 – 30 % overall climate control cost
reductions (D. Popp, 2006; V. Bosetti et al., 2011). Climate-specific
R&D instruments can step up early innovation and ultimately reduce
mitigation costs (Gerlagh etal., 2009), although R&D subsidies could
raise the shadow value of CO
2
in the short term because of rebound
effects from stimulating innovation (Otto and Reilly, 2008) (See Sec-
tion 6.3.6.5 for further discussion of combining policy instruments to
reduce aggregate mitigation costs). In the absence of explicit efforts
to address innovation market failures, carbon taxes might be increased
or differentiated across regions to indirectly address the under provi-
sion of R&D (Golombek and Hoel, 2008; Hart, 2008; Greaker and Pade,
2009; Heal and Tarui, 2010; De Cian and Tavoni, 2012).
Although there is no definitive conclusion on the subject matter, sev-
eral studies suggest that the benefits of increased technological
change for climate change mitigation may be sufficiently high to jus-
tify upfront investments and policy support in innovation and diffu-
sion of energy efficiency and low-carbon mitigation technologies (see
e. g. Dowlatabadi, 1998; Newell etal., 1999; Nordhaus, 2002; Buon-
anno etal., 2003; Gerlagh and van der Zwaan, 2003). For example, it
has been suggested that the current rates of investments are rela-
tively low and that an average increase several times from current
clean energy R&D expenditures may be closer towards optimality to
stabilize GHG concentrations (Popp, 2006a; Nemet and Kammen,
2007; Bosetti et al., 2009a; IEA, 2010a; Marangoni and M. Tavoni,
2014) (Table 6.6). Bridging a possible ‘R&D gap’ is particularly impor-
tant and challenging, given that public energy R&D investments in
OECD countries have generally been decreasing as a share of total
research budgets over the past 30 years (from 11 % down to 4 %,
according to recent International Energy Agency (IEA) R&D statistics).
On the other hand, in the private sector the rate of innovation (if
measured by clean energy patents) seems to have accelerated over
the past 10 years.
An unequivocal call for energy innovation policy can be questioned,
however, when all inventive activities are accounted for. It might also
not be straightforward to determine the overall effect of mitigation
policy on technological innovation, since low-carbon energy R&D may
crowd out other inventive activity and result in lower overall welfare
(Goulder and Schneider, 1999). The degree of substitutability between
different inputs of production has been shown to drive the outcome of
scenarios from integrated models (Otto etal., 2008; Acemoglu etal.,
2009; Carraro etal., 2010). Innovation is found to play an important
role in attempts to hedge against future uncertainties such as related
to climate change impacts, technological performance and policy
implementation (Loschel, 2002; Bohringer and Löschel, 2006; Baker
and Shittu, 2008; Bosetti and Tavoni, 2009).
6�5�2 Integrating societal change
Individual behaviour, social preferences, historical legacies, and insti-
tutional structures can influence the use of technologies and mitiga-
tion more generally. Technological transitions necessarily encompass
more than simply improving and deploying technology. Because they
co-evolve with technologies, social determinants of individual and col-
lective behaviours can be either causes or consequences of transfor-
mation pathways. Moreover, governance and policies can influence
these factors and thereby affect transformation pathways. This more
complex framing of transformation pathways implies the need for a
broader perspective on mitigation that explicitly considers the obsta-
cles to deployment and mitigation more generally.
Research on these societal change elements is analytically diverse
and often country-specific, which complicates comparative modelling
exercises of the type reviewed in this chapter. The difficulty in repre-
senting these processes in models has meant that societal change
research has often been divorced from the literature on transformation
pathways. However, significant bodies of literature show how societal
changes can affect the costs and acceptability of mitigation, and the
interactions of climate policies and other dimensions of public policies
beyond the energy sector.
Non-optimal or real world institutional conditions can influence how
technological pathways evolve even under an economy-wide price on
carbon. Because of the heterogeneity of the carbon impact of differ-
ent sectors, the impact of a carbon price differs widely across sec-
tors (Smale etal., 2006; Houser etal., 2009; Fischer and Fox, 2011;
Monjon and Quirion, 2011) Demailly etal., 2008). Even in less energy-
intensive sectors, pre-existing characteristics in the national econ-
Table 6�6 | Preliminary findings on energy efficiency and clean energy R&D investments, as suggested in the literature to date, and as needed to attain concentration goals. For
reference, current public R&D expenditures are approximately 10 Billion USD / yr.
Study
Foreseen total clean
energy R&D investments
Notes
Nemet and Kammen (2007) based on Davis and Owens (2003) 17 – 27 Billion USD / yr For the period 2005 – 2015
IEA (2010a) 50 – 100 Billion USD / yr To achieve the ‘Blue Map’ scenario in 2050. Roughly half of the
investments are reserved for advanced vehicle R&D.
Bosetti etal. (2009a) 70 – 90 Billion USD / yr Average to 2050 for a range of climate concentration goals.
A large share is reserved for low-carbon fuel R&D.
468468
Assessing Transformation Pathways
6
Chapter 6
omy — such as inflexible labour markets — can complicate the deploy-
ment of technologies (Guivarch etal., 2011). A further obstacle is the
uneven impacts of a carbon price on household purchasing power,
particularly for lower-income brackets (Combet etal., 2010; Grainger
and Kolstad, 2010).
Policy uncertainty can have implications for low-carbon technol-
ogy investment. High levels of uncertainty force risk-averse firms
not to adopt technologies by merit order in terms of net present
value (Kahneman and Tversky, 1979; Pindyck, 1982; Majd and Pin-
dyck, 1987). Hallegatte etal. (2008) show the importance of the dif-
ference in investment rules in a managerial economy (Roe, 1994)
and a shareholder economy (Jensen, 1986). Hadjilambrinos (2000)
and Finon and Romano (2009) show how differences in regulatory
regimes may explain differences in technological choices in the elec-
tricity industries. Bosetti etal. (2011) show that investment uncer-
tainty increases the costs and reduces the pace of transformation
pathways. Perceived policy risks can not only dampen investment but
can also encourage perverse outcomes such as non-additionality in
the CDM (Hultman etal., 2012b). This raises the potential for linking
mitigation policies, energy sector regulatory reforms, and financial
policies to increase the risk-averse returns of mitigation investments
(Hourcade and Shukla, 2013).
Changes in institutional structures will be required to facilitate the
technological change envisaged in the scenarios reviewed in this
chapter. Historically, political and institutional pre-conditions, chang-
ing decision routines, and organizational skills help explain why coun-
tries with similar dependence on oil imports adopted very different
energy responses to oil shocks (Hourcade and Kostopoulou, 1994;
Hultman etal., 2012a). Similar issues arise in a low-carbon transition.
New policies and institutional structures might be developed to man-
age infrastructures such as those associated with large quantities of
intermittent resources on the electric grid, CO
2
transport and storage,
dispersed generation or storage of electricity, or nuclear waste and
materials.
Although modelling exercises have been able to assess the possible
changes in the energy supply portfolio and the pressures to deploy
energy efficiency technologies, such changes are difficult in practice
to separate from the evolution of preference and lifestyles. The litera-
ture on energy-efficiency investments highlights the frequent incon-
gruity between perceived economic benefits for energy efficiency
and actual consumer behaviour that seems often to ignore profitable
investments. Such behaviour has been shown to stem from perceived
unreliability, unfounded expectations for maintenance, information
failures, property rights, split incentives, and differentiation across
income.
Finally, social factors influence the changes in the way energy systems
couple with other large-scale systems of production such as the built
environment, transportation, and agriculture. The way that energy is
used and consumed in urban areas (such as in transportation, heat-
ing, and air-conditioning) is often driven by the structure and form of
the urban infrastructure (Leck, 2006). Recent modelling exercises dem-
onstrated the tradeoff between commuting costs and housing costs
and their impact on the urban sprawl and the mobility needs (Gusdorf
and Hallegatte, 2007; Gusdorf etal., 2008). In many cases, the price of
real estate is as powerful a driver of mobility demand as the price of
transportation fuel, and therefore affects the price of carbon needed
for meeting a given climate objective (Waisman etal., 2012; Lampin
etal., 2013). The transport contribution to carbon can be affected by,
for example, just-in-time processes and geographical splits of the pro-
ductive chains (Crassous and Hourcade, 2006).
6.6 Sustainable development
and transformation path-
ways, taking into account
differences across regions
Averting the adverse social and environmental effects of climate
change is fundamental to sustainable development (WCED, 1987, and
Chapter 4). Yet, climate change is but one of many challenges fac-
ing society in the 21st century. Others include, for instance, providing
access to clean, reliable, and affordable energy services to the world’s
poorest; maintaining stable and plentiful employment opportuni-
ties; limiting air pollution, health damages, and water impacts from
energy and agriculture; alleviating energy security concerns; minimiz-
ing energy-driven land use requirements and biodiversity loss; and
maintaining the security of food supplies. A complex web of interac-
tions and feedback effects links these various policy objectives, all of
which are important for sustainable development (see Section 4.8 and
Table4.1).
Implementation of mitigation policies and measures therefore may
be adequately described within a multi-objective framework and may
be aligned with other objectives to maximize synergies and minimize
tradeoffs. Because the relative importance of individual objectives dif-
fers among diverse stakeholders and may change over time, transpar-
ency on the multiple effects that accrue to different actors at different
points of time is important for decision making (see Sections 2.4, 3.6.3,
3.7.1, and 4.8).
Although the scientific literature makes very clear that a variety of
policies and measures exist for mitigating climate change, the impacts
of each of these options along other, non-climate dimensions have
received less attention. To the extent these mitigation side-effects are
positive, they can be deemed ‘co-benefits’; if adverse, they imply ‘risks’
with respect to the other non-climate objectives (see Annex I for defini-
tions). Despite their importance for mitigation strategies, side-effects
are often not monetized or even quantified in analyses of climate
change (see e. g. Levine etal., 2007).
469469
Assessing Transformation Pathways
6
Chapter 6
Table 6�7 | Potential co-benefits (green arrows ↑) and adverse side-effects (orange arrows ↑) of the main sectoral mitigation measures; arrows pointing up / down denote a positive / negative effect on the respective objective or concern; a
question mark (?) denotes an uncertain net effect. Co-benefits and adverse side-effects depend on local circumstances as well as on the implementation practice, pace, and scale (see Tables 7.3, 8.4, 9.7, 10.5, 11.9, 11.12). Column two
provides the contribution of different sectoral mitigation strategies to stringent mitigation scenarios reaching atmospheric CO
2
eq concentrations of 430 – 530 ppm in 2100. The interquartile ranges of the scenario results for the year 2050
show that there is flexibility in the choice of mitigation strategies within and across sectors consistent with low-concentration goals (see Sections 6.4 and 6.8). Scenario results for energy supply and end-use sectors are based on the AR5
Scenario Database (see Annex II.10). For an assessment of macroeconomic, cross-sectoral effects associated with mitigation policies (e. g., on energy prices, consumption, growth, and trade), see Sections 3.9, 6.3.6, 13.2.2.3, and 14.4.2. The
uncertainty qualifiers in brackets denote the level of evidence and agreement on the respective effects. Abbreviations for evidence: l = limited, m = medium, r = robust; for agreement: l = low, m = medium, h = high.
Sectoral mitigation
measures
Integrated model results for
stringent mitigation scenarios
Effect on additional objectives / concerns
Economic Social Environmental Other
Energy Supply
Deploymen t
1
Rate of
change
[% / yr]
For possible upstream effects of biomass supply for bioenergy, see AFOLU.
2010 2050
Nuclear replacing
coal power
10
EJ / yr
(4 – 22)
17 – 47
EJ / yr
(– 2 – 2)
1 – 4
↑
↑
↑
Energy security (reduced exposure
to fuel price volatility) (m / m)
Local employment impact (but
uncertain net effect) (l / m)
Legacy cost of waste and
abandoned reactors (m / h)
↓
↑
↑
Health impact via
Air pollution and coal mining
accidents (m / h)
Nuclear accidents and waste
treatment, uranium mining and
milling (m / l)
Safety and waste concerns (r / h)
↓
↑
Ecosystem impact via
Air pollution (m / h) and coal mining (l / h)
Nuclear accidents (m / m)
Proliferation risk (m / m)
Renewable energy
(wind, photovoltaic
(PV), concentrated
solar power (CSP),
hydro, geothermal,
bioenergy)
replacing coal
62
EJ / yr
(66 – 125)
194 – 282
EJ / y
(0.2 – 2)
3 – 4
↑
↑
↑
↑
Energy security (resource sufficiency,
diversity in the near / medium term) (r / m)
Local employment impact (but
uncertain net effect) (m / m)
Irrigation, flood control, navigation, water
availability (for multipurpose use of
reservoirs and regulated rivers) (m / h)
Extra measures to match demand
(for PV, wind and some CSP) (r / h)
↓
↓
↑
?
↑
Health impact via
Air pollution (except bioenergy) (r / h)
Coal mining accidents (m / h)
Contribution to (off-grid)
energy access (m / l)
Project-specific public acceptance
concerns
(e. g., visibility of wind) (l / m)
T
hreat of displacement (for
large hydro) (m / h)
↓
↓
↑
↑
↓
↑
Ecosystem impact via
Air pollution (except bioenergy) (m / h)
Coal mining (l / h)
Habitat impact (for some hydro) (m / m)
Landscape and wildlife impact (for wind) m / m)
Water use (for wind and PV) (m / m)
Water use (for bioenergy, CSP, geothermal,
and reservoir hydro) (m / h)
Higher use of critical metals
for PV and direct drive
wind turbines (r / m)
Fossil CCS
replacing coal
0 Gt
CO
2
/ yr
stored
(0)
4 – 12
CO
2
/ yr
stored
(0)
NA
↑
↑
Preservation vs. lock-in of human and
physical capital in the fossil industry (m / m)
↑
↑
↑
Health impact via
Risk of CO
2
leakage (m / m)
Upstream supply-chain activities (m / h)
Safety concerns (CO
2
storage
and transport) (m / h)
↑
↑
Ecosystem impact via upstream
supply-chain activities (m / m)
Water use (m / h)
Long-term monitoring
of CO
2
storage (m / h)
BECCS replacing coal
0 Gt
CO
2
/ yr
(0)
0 – 6
CO
2
/ yr
NA See fossil CCS where applicable. For possible upstream effect of biomass supply, see agriculture, forestry, and other land use (AFOLU).
Methane leakage
prevention, capture
or treatment
NA NA NA
↑
Energy security (potential to use
gas in some cases) (l / h)
↓
↑
Health impact via reduced
air pollution (m / m)
Occupational safety at coal mines (m / m)
↓
Ecosystem impact via reduced air pollution (l / m)
1)
Deployment levels for baseline scenarios (in parentheses) and stringent mitigation scenarios leading to 430 – 530 ppm CO
2
eq in 2100 (in italics). Ranges correspond to the 25th to 75th percentile interquartile across the scenario
ensemble of the AR5 Scenario Database (for mitigation scenarios, only assuming idealized policy implementation). Data for 2010 is historic data from IEA (2012c, 2012d).
470470
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Chapter 6
Sectoral mitigation
measures
Integrated model
results for stringent
mitigation scenarios
Effect on additional objectives / concerns
Economic Social Environmental Other
Transport Scenario results For possible upstream effects of low-carbon electricity, see Energy Supply. For possible upstream effects of biomass supply, see AFOLU.
Reduction of fuel
carbon intensity:
electricity,
hydrogen(H
2
),
compressed natural
gas (CNG), biofuels
Interquartile ranges
for the whole
sector in 2050 with
430 – 530 ppm CO
2
eq
concentrations in
2100 (see Figures
6.37 & 6.38):
1) Final energy low-
carbon fuel shares
27 – 41 %
2) Final energy
reduction relative
to baseline
20 – 45 %
↑
↑
Energy security (diversification,
reduced oil dependence and exposure
to oil price volatility) (m / m)
Technological spillovers (e. g., battery
technologies for consumer electronics) (l / l)
?
↓
↑
↓
↓
Health impact via urban air pollution by
CNG, biofuels: net effect unclear (m / l)
Electricity, H
2
: reducing most pollutants (r / h)
Diesel: potentially increasing pollution (l / m)
Health impact via reduced noise
(electrification and fuel cell LDVs) (l / m)
Road safety (silent electric LDVs at low speed) (l / l)
↓
↑
?
Ecosystem impact of electricity and hydrogen via
Urban air pollution (m / m)
Material use (unsustainable resource mining) (l / l)
Ecosystem impact of biofuels: see AFOLU
Reduction of
energy intensity
↑
Energy security (reduced oil dependence
and exposure to oil price volatility) (m / m)
↓
↑
Health impact via reduced urban air pollution (r / h)
Road safety (via increased crash-worthiness) (m / m)
↓
Ecosystem and biodiversity impact via
reduced urban air pollution (m / h)
Compact urban
form and improved
transport
infrastructure
Modal shift
↑
↑
?
Energy security (reduced oil dependence
and exposure to oil price volatility) (m / m)
Productivity (reduced urban congestion
and travel times, affordable and
accessible transport) (m / h)
Employment opportunities in the public
transport sector vs car manufacturing jobs (l / m)
↓
↑
↓
↑
↑
Health impact for non-motorized modes via
Increased physical activity (r / h)
Potentially higher exposure to air pollution (r / h)
Noise (modal shift and travel reduction) (r / h)
Equitable mobility access to employment
opportunities, particularly in developing
countries (DCs) (r / h)
Road safety (via modal shift and / or infrastructure
for pedestrians and cyclists) (r / h)
↓
↓
Ecosystem impact via reduced
Urban air pollution (r / h)
Land-use competition (m / m)
Journey distance
reduction and
avoidance
↑
↑
Energy security (reduced oil dependence
and exposure to oil price volatility) (r / h)
Productivity (reduced urban congestion,
travel times, walking) (r / h)
↓
Health impact (for non-motorized
transport modes) (r / h)
↓
↑
↓
Ecosystem impact via
Urban air pollution (r / h)
New / shorter shipping routes (r / h)
Land-use competition from transport infrastructure (r / h)
Buildings Scenario results
For possible upstream effects of fuel switching and RES, see Energy Supply.
Fuel switching,
incorporation of
renewable energy,
green roofs, and
other measures
reducing GHG
emissions intensity
Interquartile ranges
for the whole
sector in 2050 with
430 – 530 ppm CO
2
eq
concentrations in
2100 (see Figures
6.37 & 6.38):
1) Final energy low-
carbon fuel shares
51 – 60 %
2) Final energy
reduction relative
to baseline
14 – 35 %
↑
↑
↑
↑
Energy security (m / h)
Employment impact (m / m)
Lower need for energy subsidies (l / l)
Asset values of buildings (l / m)
↓
↑
↓
↑
Fuel poverty (residential) via
Energy demand (m / h)
Energy cost (l / m)
Energy access (for higher energy cost) (l / m)
Productive time for women / children
(for replaced traditional cookstoves) (m / h)
↓
↓
↓
↓
↑
Health impact in residential buildings via
Outdoor air pollution (r / h)
Indoor air pollution (in DCs) (r / h)
Fuel poverty (r / h)
Ecosystem impact (less outdoor air pollution) (r / h)
Urban biodiversity (for green roofs) (m / m)
Reduced Urban Heat
Island (UHI) effect (l / m)
Retrofits of existing
buildings (e� g�, cool
roof, passive solar, etc�)
Exemplary new
buildings
Efficient equipment
↑
↑
↑
↑
↑
↑
Energy security (m / h)
Employment impact (m / m)
Productivity (for commercial buildings) (m / h)
Lower need for energy subsidies (l / l)
Asset values of buildings (l / m)
Disaster resilience (l / m)
↓
↓
↑
↑
Fuel poverty (for retrofits and
efficient equipment) (m / h)
Energy access (higher cost for housing due
to the investments needed) (l / m)
Thermal comfort (for retrofits and
exemplary new buildings) (m / h)
Productive time for women and children (for
replaced traditional cookstoves) (m / h)
↓
↓
↓
↓
↓
↓
↓
Health impact via
Outdoor air pollution (r / h)
Indoor air pollution (for efficient cookstoves) (r / h)
Improved indoor environmental conditions (m / h)
Fuel poverty (r / h)
Insufficient ventilation (m / m)
Ecosystem impact (less outdoor air pollution) (r / h)
Water consumption and sewage production (l / l)
Reduced UHI effect
(retrofits and new
exemplary buildings) (l / m)
Behavioural
changes reducing
energy demand
↑
↑
Energy security (m / h)
Lower need for energy subsidies (l / l)
↓
↓
Health impact via less outdoor air pollution (r / h) and
improved indoor environmental conditions (m / h)
Ecosystem impact (less outdoor air pollution) (r / h)
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Chapter 6
Sectoral mitigation
measures
Integrated model
results for stringent
mitigation scenarios
Effect on additional objectives / concerns
Economic Social Environmental Other
Industry Scenario results For possible upstream effects of low-carbon energy supply (incl CCS), see energy supply and of biomass supply, see AFOLU.
CO
2
and non-CO
2
GHG emissions
intensity reduction
Interquartile ranges
for the whole
sector in 2050 with
430 – 530 ppm CO
2
eq
concentrations in
2100 (see Figures
6.37 & 6.38):
1) Final energy low-
carbon fuel shares:
44 – 57 %
2) Final energy
reduction relative
to baseline:
22 – 38 %
↑
Competitiveness and productivity (m / h)
↓
Health impact via reduced local air pollution
and better work conditions (for perfluorinated
compounds (PFCs) from aluminium) (m / m)
↓
↑
Ecosystem impact via reduced local air pollution
and reduced water pollution (m / m)
Water conservation (l / m)
Technical energy
efficiency
improvements via
new processes and
technologies
↑
↑
↑
↑
Energy security (via lower
energy intensity) (m / m)
Employment impact (l / l)
Competitiveness and productivity (m / h)
Technological spillovers in DCs (due
to supply chain linkages) (l / l)
↓
↑
↑
↑
Health impact via reduced local pollution (l / m)
New business opportunities (m / m)
Water availability and quality (l / l)
Safety, working conditions and job satisfaction (m / m)
↓
↓
Ecosystem impact via
Fossil fuel extraction (l / l)
Local pollution and waste (m / m)
Material efficiency
of goods, recycling
↓
↑
↑
↑
National sales tax revenue in medium term (l / l)
Employment impact in waste
recycling market (l / l)
Competitiveness in manufacturing (l / l)
New infrastructure for industrial clusters (l / l)
↓
↑
↓
Health impacts and safety concerns (l / m)
New business opportunities (m / m)
Local conflicts (reduced resource extraction) (l / m)
↓
↓
Ecosystem impact via reduced local air and water
pollution and waste material disposal (m / m)
Use of raw / virgin materials and natural resources
implying reduced unsustainable resource mining (l / l)
Product demand
reductions
↓
National sales tax revenue (medium term) (l / l)
↑
Wellbeing via diverse lifestyle choices (l / l)
↓
Post-consumption waste (l / l)
AFOLU Scenario results Note: co-benefits and adverse side-effects depend on the development context and the scale of the intervention (size).
Supply side: Forestry,
land-based agriculture,
livestock, integrated
systems and bioenergy
(marked by †)
Demand side: Reduced
losses in the food
supply chain, changes
in human diets, changes
in demand for wood
and forestry products
Ranges for cumulative
land-related emissions
reductions relative to
baseline for CH
4
, CO
2
,
and N
2
O in idealized
implementation
scenarios with
450 CO
2
eq ppm
concentrations
in 2100 (see
Table 11.10):
CH
4
: 2 – 18 %
CO
2
:
– 104 – 423 %
N
2
O: 8 – 17 %
†
↑
↓
↑†
↑†
↑†
↑†
↑
↑
Employment impact via
Entrepreneurship development (m / h)
Use of less labor-intensive (m / m)
Technologies in agriculture
Diversification of income sources
and access to markets (r / h)
Additional income to (sustainable)
landscape management (m / h)
Income concentration (m / m)
Energy security (resource sufficiency) (m / h)
Innovative financing mechanisms for
sustainable resource management (m / h)
Technology innovation and transfer (m / m)
↑†
↓†
↑
↑†
↓†
†
↑
↑
Food-crops production through integrated systems
and sustainable agriculture intensification (r / m)
Food production (locally) due to large-scale
monocultures of non-food crops (r / l)
Cultural habitats and recreational areas
via (sustainable) forest management
and conservation (m / m)
Human health and animal welfare e. g., through less
pesticides, reduced burning practices and practices
like agroforestry and silvo-pastoral systems (m / h)
Human health when using burning practices
(in agriculture or bioenergy) (m / m)
Gender, intra- and inter-generational equity via
Participation and fair benefit sharing (r / h)
Concentration of benefits (m / m)
↑
↑†
↑†
↑
↓
↑
↑
Provision of ecosystem services via
Ecosystem conservation and
sustainable management as well
as sustainable agriculture (r / h)
Large-scale monocultures (r / h)
Land use competition (r / m)
Soil quality (r / h)
Erosion (r / h)
Ecosystem resilience (m / h)
Albedo and evaporation (r / h)
Institutional aspects:
↑↓†
↑↓
↑
Tenure and use
rights at the local
level (for indigenous
people and local
communities)
especially when
implementing
activities in natural
forests (r / h)
Access to
participative
mechanisms for
land management
decisions (r / h)
Enforcement of
existing policies for
sustainable resource
management (r / h)
Human Settlements and Infrastructure For co-benefits and adverse side-effects of compact urban form and improved transport infrastructure, see also Transport.
Compact development and infrastructure
↑
↑
Innovation, productivity and efficient
resource use and delivery (r / h)
Higher rents and property values (m / m)
↑
Health from increased physical activity: see Transport
↑
Preservation of open space (m / m)
Increased accessibility
↑
Commute savings (r / h)
↑
↑
Health from increased physical activity: see Transport
Social interaction and mental health (m / m)
↑
Air quality and reduced ecosystem
and health impacts (m / h)
Mixed land use
↑
↑↑
Commute savings (r / h)
Higher rents and property values (m / m)
↑
↑
Health from increased physical activity (r / h)
Social interaction and mental health
(l / m)
↑
Air quality and reduced ecosystem
and health impacts (m / h)
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Chapter 6
6�6�1 Co-benefits and adverse side-effects
of mitigation measures: Synthesis of
sectoral information and linkages to
transformation pathways
One source of information on side-effects emerges from literature
exploring the nature of individual technological or sectoral mitigation
measures. These studies are covered in Chapters 7 – 12. Based on those
assessments, Table 6.7 provides an aggregated but qualitative over-
view of the potential co-benefits and adverse side-effects that could be
realized if certain types of mitigation measures are enacted in different
sectors: energy supply-side transformations; technological and behav-
ioural changes in the transport, buildings, and industry end-use sec-
tors; and modified agriculture, forestry, and land use practices. These
co-benefits and adverse side-effects can be classified by the nature of
their sustainable development implications: economic, social, or envi-
ronmental (see Sections 4.2 and 4.8 for a discussion of the three pillars
of sustainable development). Other types of impacts are also possible
and are highlighted in the table where relevant.
Whether or not any of these side-effects actually materialize, and to
what extent, will be highly case- and site-specific, as they will depend
importantly on local circumstances and the scale, scope, and pace
of implementation, among other factors. Measures undertaken in
an urbanized area of the industrialized world, for instance, may not
yield the same impacts as when enacted in a rural part of a devel-
oping country (Barker et al., 2007). Such detailed considerations are
not reflected in Table 6.7, which is meant to give an aggregated sense
of the potential co-benefits and adverse side-effects throughout the
world when mitigation policies are in place. Details are discussed in
each of the respective sectoral chapters (see Chapters 7 – 12). Note
that in addition to the qualitative information on potential side-effects
summarized below, Table 6.7 also provides quantitative information
for each sector regarding the mid-century contribution of the respec-
tive (group of) mitigation measures to reach stringent mitigation goals
(see Sections 6.8, 7.11, and 11.9 for the underlying data).
The compilation of sectoral findings in Table 6.7 suggests that the
potential for co-benefits clearly outweighs that of adverse side-effects
in the case of energy end-use mitigation measures (transport, buildings,
and industry), whereas the evidence suggests this may not be the case
for all supply-side and AFOLU measures. Although no single category
of mitigation measures is completely devoid of risk, Table 6.7 high-
lights that certain co-benefits are valid across all sectors. For instance,
by contributing to a phaseout of conventional fossil fuels, nearly all
mitigation measures have major health and environmental benefits for
society, owing to significant reductions in both outdoor and indoor air
pollution, and lead to improved energy security at the national level for
most countries. In addition to the many sector-specific co-benefits and
adverse side-effects, sectoral employment and productivity gains, tech-
nological spillovers, and more equitable energy / mobility access offer
examples of co-benefits that are possible across all demand sectors.
While energy demand reductions additionally mitigate risks associated
with energy supply technologies (see also Rogelj et al., 2013b), the
upstream effects of fuel switching are more complex and depend to a
large extent on local circumstances (see Section 7.11).
Moreover, while nearly all mitigation measures for reducing (fuel)
carbon and energy intensity have higher up-front investment require-
ments than conventional technologies, their often lower operating
costs, and sometimes even lifecycle costs, can contribute to reduced
energy service prices for consumers, depending on local and national
institutional settings (see Section 7.9.1). If, on the other hand, energy
prices rise as a consequence, so do the political challenges of imple-
mentation, such as those associated with the provision of universal
energy access and associated economic, social, environmental, and
health risks for the poorest members of society (Markandya et al.,
2009; Sathaye etal., 2011; Rao, 2013). Well-designed policies are thus
important to avoid perverse incentives of climate policies, including
increasing traditional biomass use for heating and cooking (see Bollen
etal., 2009a, b, and Section 9.7.1).
In addition to furthering the achievement of various global goals
for sustainability, namely those of the major environmental conven-
tions (e. g., the United Nations’ Convention to Combat Desertification
(UNCCD, 2004), Convention on Biological Diversity (CBD, 1992), ‘Sus-
tainable Energy for All’ initiative, and the Millennium Development
Goals (MDG)), mitigation can potentially yield positive side-effects in
the impacts, adaptation, and vulnerability (IAV) dimensions (see Sec-
tion 6.6.2.5 and 11.7, Haines et al., 2009; Rogelj et al., 2013c). For
instance, decentralized renewable energy systems can help to build
adaptive capacity in rural communities (Venema and Rehman, 2007),
and sustainable agricultural practices (e. g., conservation tillage and
water management) can improve drought resistance and soil conser-
vation and fertility (Uprety etal., 2012).
6�6�2 Transformation pathways studies with
links to other policy objectives
As indicated above, the overall nature and extent of the co-benefits
and risks arising from global transformation pathways depends impor-
tantly on which mitigation options are implemented and how. The full
systems-level welfare impacts for multi-objective decision making are
therefore best viewed from an integrated perspective that permits
the full accounting of the impacts of each of the objectives on social
welfare (see Section 3.5.3) (Bell etal., 2008; Sathaye etal., 2011; Rao
etal., 2013). Taking such a perspective poses a significant challenge,
since the costs of mitigation need to be weighed against the multiple
benefits and adverse side-effects for the other objectives. To compli-
cate matters further, these other objectives are traditionally measured
in different units (e. g., health benefits of reduced air pollution in terms
of deaths avoided). In addition, combining the different objectives into
a single overall welfare formulation implies subjective choices about
the ranking or relative importance of policy priorities. Such a ranking
is highly dependent on the policy context (see Sections 2.4 and 3.6.3).
473473
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Chapter 6
Since AR4, a number of scenario studies have been conducted to
shed light on the global implications of transformation pathways for
other objectives. Earlier scenario literature primarily focused on the
health and ecosystem benefits of mitigation via reduced air pollu-
tion; some evidence of co-benefits for employment and energy secu-
rity was also presented in AR4. More recent studies have broadened
their focus to include energy security, energy access, biodiversity
conservation, water, and land-use requirements (see Section 11.13.7
for a review of scenario studies focusing on water and land use and
implications for food security). Many of these newer analyses use
globally consistent methods, meaning they employ long-term, multi-
region frameworks that couple models of both bio-geophysical and
human processes, thereby permitting the consideration of targeted
policies for the additional objectives in their own right. While the
majority of these studies focus on two-way interactions (e. g., the
effect of mitigation on air pollution in a given country or across
groups of countries — or vice versa), a few recent analyses have
looked at three or more objectives simultaneously (Section 6.6.2.7).
Important to note in this context is that many of the non-technical
measures listed in Table 6.7 (e. g., behavioral changes) are not fully
taken into account by models, though the state-of-the-art continues
to improve.
6�6�2�1 Air pollution and health
Greenhouse gas and air pollutant emissions typically derive from the
same sources, such as power plants, factories, and cars. Hence, miti-
gation strategies that reduce the use of fossil fuels typically result
in major cuts in emissions of black carbon (BC), sulfur dioxide (SO
2
),
nitrogen oxides (NO
x
), and mercury (Hg), among other harmful species.
Together with tropospheric ozone and its precursors (mainly deriving
from AFOLU and fossil fuel production / transport processes), these pol-
lutants separately or jointly cause a variety of detrimental health and
ecosystem effects at various scales (see Section7.9.2). The magnitude
of these effects varies across pollutants and atmospheric concentra-
tions (as well as the concentrations of pollutants created via further
chemical reactions) and is due to different degrees of population
exposure, whether indoor or outdoor or in urban or rural settings (see
Barker etal., 2007; Bollen etal., 2009b; Markandya etal., 2009; Smith
etal., 2009; Sathaye etal., 2011; GEA, 2012). The term ‘fine particulate
matter (PM
2.5
)’ is frequently used to refer to a variety of air pollutants
that are extremely small in diameter and therefore cause some of the
most serious health effects.
The literature assessed in AR4 focused on air pollution reductions
in individual countries and regions, pointing to large methodologi-
cal differences in, for example, the type of pollutants analyzed, sec-
toral focus, and the treatment of existing air pollution policy regimes.
As confirmed by recent literature (Friel etal., 2009; Wilkinson etal.,
2009; Woodcock etal., 2009; Markandya etal., 2009; Haines etal.,
2009; Smith etal., 2009; Nemet etal., 2010), AR4 showed that the
monetized air quality co-benefits from mitigation are of a similar
order of magnitude as the mitigation costs themselves (see Sec-
tions3.6.3 and 5.7.1). For instance, taking into account new findings
on the relationship between chronic mortality and exposure to PM
and ozone as well as the effect of slowing climate change on air
quality, West etal. (2013) calculate global average monetized co-
benefits of avoided mortality of 55 – 420 USD
2010
/ tCO
2
. They find that
the values for East Asia far exceed the marginal mitigation costs in
2030. (See Section 5.7 for a broader review of this issue, as well as a
discussion of the importance of baseline conditions for these results.)
Furthermore, it has been noted that reductions in certain air pollut-
ants can potentially increase radiative forcing (see Sections 1.2.5,
5.2, and WGI Chapter 7). This is an important adverse side-effect,
and one that is not discussed here due to the lack of scenario stud-
ies addressing the associated tradeoff between health and climate
benefits.
The available evidence indicates that transformation pathways lead-
ing to 430 – 530 ppm CO
2
eq in 2100 will have major co-benefits in
terms of reduced air pollution (Figure 6.33, top right panel). Recent
integrated modelling studies agree strongly with earlier findings by
van Vuuren etal. (2006) and Bollen etal. (2009a) in this regard. For
example, Rose etal. (2014b) find that national air pollution policies
may no longer be binding constraints on pollutant emissions depend-
ing on the stringency of climate policies. In China, for instance, miti-
gation efforts consistent with a global goal of 3.7 W / m
2
(2.8 W / m
2
)
in 2100 result in SO
2
emissions 15 to 55 % (25 – 75 %) below refer-
ence levels by 2030 and 40 to 80 % (55 – 80 %) by 2050. Chaturvedi
and Shukla (2014) find similar results for India. Globally, Rafaj etal.
(2013b) calculate that stringent mitigation efforts would simulta-
neously lead to near-term (by 2030) reductions of SO
2
, NO
x
, and
PM
2.5
on the order of 40 %, 30 %, and 5 %, respectively, relative to
a baseline scenario. Riahi etal. (2012) find that by further exploit-
ing the full range of opportunities for energy efficiency and ensuring
access to modern forms of energy for the world’s poorest (hence less
indoor / household air pollution), the near-term air pollution co-bene-
fits of mitigation could be even greater: 50 % for SO
2
, 35 % for NO
x
,
and 30 % for PM
2.5
by 2030. Additionally, Amann etal. (2011) and Rao
etal. (2013) find significant reductions in air quality control costs due
to mitigation policies (see Section 6.6.2.7). Riahi etal. (2012) further
estimate that stringent mitigation efforts can help to reduce globally
aggregated disability-adjusted life years (DALYs) by more than 10 mil-
lion by 2030, a decrease of one-third compared to a baseline scenario.
The vast majority of these co-benefits would accrue in urban house-
holds of the developing world. Similarly, West etal. (2013) find that
global mitigation (RCP4.5) can avoid 0.5 ± 0.2, 1.3 ± 0.5, and 2.2
± 0.8 million premature deaths in 2030, 2050, and 2100, relative to
a baseline scenario that foresees decreasing PM and ozone (O
3
) con-
centrations. Regarding mercury, Rafaj etal. (2013a) show that under
a global mitigation regime, atmospheric releases from anthropogenic
sources can be reduced by 45 % in 2050, relative to a a baseline sce-
nario without climate measures.
474474
Assessing Transformation Pathways
6
Chapter 6
Min
75
th
Percentile
Max
Median
25
th
Percentile
Stringent
Climate Policy
Stringent
Climate Policy
Baseline Baseline
Change from 2005 [%]
IPCC AR5 Scenario Ensemble
Impact of Climate Policy on Air Pollutant Emissions (Global, 2005-2050)
Air Quality Levels of GEA Scenarios in Bottom PanelEnergy Security Levels of GEA Scenarios in Bottom Panel
Increased
Pollution
Decreased
Pollution
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Only Energy Security Only Air Quality Only Mitigation All Three Objectives
Policy Choices
Total Global Policy Costs 2010-2050 [% of Global GDP]
w)
Costs of Achieving
Energy Security
Levels Shown in
Top Left Panel
x)
Costs of Achieving
Air Pollution Levels
Shown in Top Right
Panel
w + x + y > z
y)
Costs of Achieving
Stringent
Mitigation Targets
(430-530 ppm
CO
2
eq in 2100)
z)
Costs of Integrated
Approaches that
Achieve all Three
Objectives
Simultaneosly;
Highest Cost-
Effectiveness
Global Energy Assessment Scenario Ensemble (n=624)
Policy Costs of Achieving Different Objectives
Co-Benefits of Climate Change Mitigation for Energy Security and Air Quality
0
50
100
150
200
250
300
350
400
450
5
6
7
8
9
10
11
12
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
[EJ/yr]
Baseline Stringent
Climate
Policy
Baseline Stringent
Climate
Policy
Baseline Stringent
Climate
Policy
Shannon-Wiener-Diversity Index
[ZJ]
LIMITS Model Inter-Comparison
Impact of Climate Policy on Energy Security
Energy Trade
(Global, 2050)
Cumulative Oil Extraction
(Global, 2010-2050)
Electricity Diversity
(Global, 2050)
Improved
Energy
Security
Improved
Energy
Security
Improved
Energy
Security
-100
-50
0
50
Black Carbon Sulfur Dioxide
⇒
475475
Assessing Transformation Pathways
6
Chapter 6
Several studies published since AR4 have analyzed the potential cli-
mate impacts of methane mitigation and local air pollutant emissions
control (West etal., 2006, 2007; Shine etal., 2007; Reilly etal., 2007;
Ramanathan and Carmichael, 2008; Jerrett et al., 2009; Anenberg
etal., 2012). For instance, Shindell etal. (2012) identify 14 different
methane and BC mitigation measures that, in addition to slowing the
growth in global temperatures in the medium term (~0.5 °C lower
by 2050, central estimate), lead to important near-term (2030) co-
benefits for health (avoiding 0.7 to 4.7 million premature deaths from
outdoor air pollution globally) and food security (increasing annual
crop yields globally by 30 to 135 million metric tons due to ozone
reductions; see Section 11.13.7 for a further discussion of the rela-
tionship between mitigation and food security). Smith and Mizrahi
(2013) also acknowledge the important co-benefits of reducing cer-
tain short-lived climate forcers (SLCF) but at the same time conclude
that (1) the near-to-medium term climate impacts of these measures
are likely to be relatively modest (0.16 °C lower by 2050, central esti-
mate; 0.04 – 0.35 °C considering the various uncertainties), and (2) the
additional climate benefit of targeted SLCF measures after 2050 is
comparatively low.
6�6�2�2 Energy security
A number of analyses have studied the relationship between mitiga-
tion and energy security. The assessment here focuses on energy secu-
rity concerns that relate to (1) the sufficiency of resources to meet
national energy demand at competitive and stable prices, and (2) the
resilience of energy supply (see Section 7.9.1 for a broader discus-
sion). A number of indicators have been developed to quantitatively
express these concerns (Kruyt etal., 2009; Jewell, 2011; Jewell etal.,
2014). The most common indicators of sufficiency of energy supply are
energy imports (see SRREN (IPCC, 2011) Figure 9.6) and the adequacy
of the domestic resource base (Gupta, 2008; Kruyt etal., 2009; Le Coq
and Paltseva, 2009; IEA, 2011; Jewell, 2011; Jewell etal., 2013). Resil-
ience of energy systems is commonly measured by the diversity of
energy sources and carriers (Stirling, 1994, 2010; Grubb etal., 2006;
Bazilian and Roques, 2009; Skea, 2010) and the energy intensity of
GDP (Gupta, 2008; Kruyt etal., 2009; Jewell, 2011; Cherp etal., 2012).
Recent studies show that mitigation policies would likely increase
national energy sufficiency and resilience (Figure 6.33, top left panel).
Mitigation policies lead to major reductions in the import dependency of
many countries, thus making national and regional energy systems less
vulnerable to price volatility and supply disruptions (Criqui and Mima,
2012; Shukla and Dhar, 2011; Jewell etal., 2013). One multi-model study
finds that in stringent mitigation scenarios, global energy trade would
be 10 – 70 % lower by 2050 and 40 – 74 % by 2100 than in the baseline
scenario (Jewell etal., 2013). Most of the decrease in regional import
dependence would appear after 2030 since mitigation decreases the
use of domestic coal in the short term, which counteracts the increase
in domestic renewables (Akimoto etal., 2012; Jewell etal., 2013). At
the same time mitigation leads to much lower extraction rates for fos-
sil resources (Kruyt etal., 2009; Jewell et al., 2013; McCollum etal.,
2014a). The IEA, for example, finds that rapid deployment of energy
efficiency technologies could reduce oil consumption by as much as 13
million barrels a day (IEA, 2012). Mitigation actions could thus alleviate
future energy price volatility, given that perceptions of resource scarcity
are a key driver of rapid price swings. This would mean that domestic
fossil resources could act as a ‘buffer of indigenous resources’ (Turton
and Barreto, 2006). Improved energy security of importers, however,
could adversely impact the ‘demand security’ of exporters (Luft, 2013);
indeed, most of the modeling literature indicates that climate mitigation
would decrease oil export revenues of oil exporters (IEA, 2009; Haurie
and Vielle, 2010; Bauer etal., 2014a, 2014b; Tavoni etal., 2013; McCol-
lum etal., 2013a). However, three recent studies argue that if the cost of
alternatives to conventional oil is high enough, conventional oil export-
ers could benefit from climate policies, particularly in the near term
(Persson etal. 2007; Johansson etal. 2009; Nemet and Brandt, 2012).
Although there is broad agreement in the literature about the overall
negative effect on oil export revenues, the distribution of this effect will
differ between exporters of conventional vs unconventional oil export-
ers. (See Section 6.3.6.6 regarding the impacts that these trade shifts
would have on major energy exporters.)
Studies also indicate that mitigation would likely increase the resil-
ience of energy systems (Figure 6.33, top left panel). The diversity
of energy sources used in the transport and electricity sectors would
rise relative to today and to a baseline scenario in which fossils
remain dominant (Grubb etal., 2006; Riahi etal., 2012; Cherp etal.,
2014; Jewell etal., 2013). Additionally, energy trade would be much
less affected by fluctuations in GDP growth and by uncertainties
in fossil resource endowments and energy demand growth (Cherp
etal., 2014; Jewell etal., 2013). These developments (mitigation and
energy-efficiency improvements) would make energy systems more
resilient to various types of shocks and stresses and would help
insulate economies from price volatility and supply disruptions (see
Chapters8 – 10).
Figure 6�33 | Co-benefits of mitigation for energy security and air quality in scenarios with stringent climate policies (reaching 430 – 530 ppm CO
2
eq concentrations in 2100).
Upper panels show co-benefits for different energy security indicators and air pollutant emissions. Lower panel shows related global policy costs of achieving the energy security,
air quality, and mitigation objectives, either alone (w, x, y) or simultaneously (z). Integrated approaches that achieve these objectives simultaneously show the highest cost-
effectiveness due to synergies (w+x+y>z). Policy costs are given as the increase in total energy system costs relative to a no-policy baseline; hence, they only capture the mitigation
component and do not include the monetized co-benefits of, for example, reduced health impacts or climate damages. In this sense, costs are indicative and do not represent full
uncertainty ranges. Sources: LIMITS model intercomparison (Jewell etal., 2013; Tavoni etal., 2013), WGIII AR5 Scenario Database (Annex II.10, includes only scenarios based on
idealized policy implementation and full technology availability), Global Energy Assessment (GEA) scenarios (Riahi etal., 2012; McCollum etal., 2013a).
476476
Assessing Transformation Pathways
6
Chapter 6
6�6�2�3 Energy access
According to the literature, providing universal energy access (see Sec-
tion 7.9.1 for a broader discussion) would likely result in negligible
impacts on GHG emissions globally (PBL, 2012; Riahi etal., 2012). Rog-
elj et al (2013c) find that the United Nation’s (UN) energy access goals
for 2030 are fully consistent with stringent mitigation measures while
other scenario analyses indicate that deployment of renewable energy
in LDCs can help to promote access to clean, reliable, and afford-
able energy services (Kaundinya etal., 2009; Reddy etal., 2009). In
addition, a number of recent integrated modelling studies ensure, by
design, that developing country household final energy consumption
levels are compatible with minimal poverty thresholds (Ekholm etal.,
2010a; van Ruijven etal., 2011; Daioglou etal., 2012; Narula etal.,
2012; Krey etal., 2012). An important message from these studies is
that the provision of energy access in developing countries should not
be confused with broader economic growth. The latter could have a
pronounced GHG affect, particularly in today’s emerging economies
(see Section 6.3.1.3).
The primary risk from mitigation is that an increase in energy prices
for the world’s poor could potentially impair the transition to univer-
sal energy access by making energy less affordable (see Sections 6.6.1
and 7.9.1). A related concern is that increased energy prices could also
delay structural changes and the build-up of physical infrastructure
(Goldemberg etal., 1985; Steckel etal., 2013; Jakob and Steckel, 2014).
Isolating these effects has proven to be difficult in the integrated mod-
elling context because these models typically aggregate consumption
losses from climate policies (see Section 6.3.6).
6�6�2�4 Employment
The potential consequences of climate policies on employment are
addressed in the scientific literature in different ways. One strand
of literature analyzes the employment impacts associated with the
deployment of specific low-carbon technologies, such as renewables
or building retrofits (see Sections 7.9.1 and 9.7.2.1). This literature
often finds a significant potential for gross job creation, either directly
or indirectly; however, a number of issues are left unresolved regard-
ing the methodologies used in computing those impacts on one hand
and the gap between this potential and net employment impacts in
a particular sector on the other hand (see Wei etal., 2010). The net
effect is typically addressed in general equilibrium literature. Although
many integrated models used to develop long-term scenarios are gen-
eral equilibrium models, they usually assume full employment and are
therefore not well-suited to addressing gross versus net employment-
related questions.
According to the literature, employment benefits from mitigation
depend on the direction and strength of income / output and substitu-
tion impacts of mitigation. These impacts are governed by two inter-
related sets of factors related to mitigation technologies and general
equilibrium effects. One set involves the characteristics of mitigation
technologies, including (1) their costs per job created, which deter-
mines the crowding out of jobs in other sectors when capital is con-
strained (Frondel etal., 2010); (2) the portion of the low-carbon tech-
nologies that is imported, which determines domestic job creation and
the net positive impact on the trade balance; and (3) the availability of
skills in the labor force, as well as its capacity to adapt (Babiker and
Eckaus, 2007; Fankhauser etal., 2008; Guivarch etal., 2011), which
determines the pace of job creation and the real cost of low-carbon
technology deployment in terms of increased wages due to skilled
labor scarcities.
A second set of factors encompasses all the general equilibrium effects,
some of which are triggered by the above parameters and others by
the net income effects of higher carbon prices (see Section 3.6.3).
Recycling the revenues from carbon pricing and subsequently lower-
ing labor taxes changes the relative prices of labor and energy (and
to a lesser extent the costs of production inputs), which in turn leads
to a redirection of technology choices and innovation towards more
labor-intensive techniques. In addition, by contributing to higher
energy costs, climate policies change the relative prices of energy- and
non-energy intensive goods and services, thereby causing households
to consume more of the latter. These mechanisms operate differently
in developed, emerging, and developing economies, particularly with
respect to the various forms of informal labor. Some of the mechanisms
operate over the medium (more labor-intensive techniques) and long
term (structural change) (Fankhauser et al., 2008). Others, however,
operate over the short term and might therefore be influenced by near-
term mitigation policies.
6�6�2�5 Biodiversity conservation
The concept of biodiversity can be interpreted in different ways. Mea-
suring it therefore presents a challenge. One indicator that has been
used in the integrated modelling literature for assessing the biodiversity
implications of global transformation pathways is that of mean species
abundance (MSA), which uses the species composition and abundance
of the original ecosystem as a reference situation. According to PBL
(2012), globally averaged MSA declined continuously from approxi-
mately 76 % in 1970 to 68 % in 2010 (relative to the undisturbed states
of ecosystems). This was mostly due to habitat loss resulting from con-
version of natural systems to agriculture uses and urban areas.
The primary biodiversity-related side-effects from mitigation involve
the potentially large role of reforestation and afforestation efforts
and of bioenergy production. These elements of mitigation strategy
could either impose risks or lead to co-benefits, depending on where
and how they are implemented (see Table 6.7). The integrated model-
ling literature does not at this time provide an explicit enough treat-
ment of these issues to effectively capture the range of transforma-
tion pathways. One study (PBL, 2012) suggests that it is possible to
stabilize average global biodiversity at the 2020 / 2030 level (MSA =
477477
Assessing Transformation Pathways
6
Chapter 6
65 %) by 2050 even if land-use mitigation measures are deployed.
Such an achievement represents more than a halving of all biodiver-
sity loss projected to occur by mid-century in the baseline scenario
and is interpreted to be in accordance with the Aichi Biodiversity Tar-
gets (CBD, 2010). Of critical importance in this regard are favourable
institutional and policy mechanisms for reforestation / afforestation
and bioenergy that complement mitigation actions (as described in
Section11.13).
6�6�2�6 Water use
The last decades have seen the world’s freshwater resources come
under increasing pressure. Almost three billion people live in water-
scarce regions (Molden, 2007), some two billion in areas of severe
water stress in which demand accounts for more than 40 % of total
availability (PBL, 2012). Water withdrawals for energy and industrial
processes (currently 20 % globally) and municipal applications (10 %)
are projected to grow considerably over the next decades, jointly sur-
passing irrigation (70 %) as the primary water user by 2050 (Alcamo
and Henrichs, 2002; Shiklomanov and Rodda, 2003; Molden, 2007;
Fischer et al., 2007; Shen et al., 2008; Bruinsma, 2011). This growth
is projected to be greatest in areas already under high stress, such as
South Asia.
Renewable energy technologies such as solar PV and wind power will
reduce freshwater withdrawals for thermal cooling relative to fossil
alternatives. On the other hand, CCS and some forms of renewable
energy, especially bioenergy, could demand a significant amount of
water (see Table 6.7 and Section 7.9.2). For bioenergy in particular, the
overall effect will depend importantly on which feedstocks are grown,
where, and if they require irrigation (see Section 11.13.7). Similarly,
reforestation and afforestation efforts, as well as attempts to avoid
deforestation, will impact both water use and water quality. The net
effects could be either positive (Townsend et al., 2012) or negative
(Jackson etal., 2005), depending on the local situation (see Section
11.7).
When accounting for the system dynamics and relative econom-
ics between alternative mitigation options (both in space and time),
recent integrated modelling scenarios generally indicate that stringent
mitigation actions, combined with heightened water-use efficiency
measures, could lead to significant reductions in global water demand
over the next several decades. PBL (2012), for instance, calculates a
25 % reduction in total demand by 2050, translating to an 8 % decline
in the number of people living in severely water-stressed regions
worldwide. Other studies by Hanasaki etal. (2013) and Hejazi etal.
(2013) find the co-benefits from mitigation to be of roughly the same
magnitude: reductions of 1.0 – 3.9 % and 1.2 – 5.5 %, respectively, in
2050. Hejazi etal. (2013) note, however, that water scarcity could be
exacerbated if mitigation leads to more intensive production of bio-
energy crops. In contrast, Akimoto et al. (2012) find that stringent
mitigation increases water-stressed populations globally (+3 % in
2050) as a result of decreases in annual water availability in places
like South Asia.
6�6�2�7 Integrated studies of multiple objectives
Integrated scenario research is just beginning to assess multiple sustain-
able development objectives in parallel. This emerging literature gener-
ally finds that mitigation goals can be achieved more cost-effectively if
the objectives are integrated and pursued simultaneously rather than in
isolation. Recent examples of such studies include Bollen etal. (2010)
and the Global Energy Assessment (GEA) (McCollum etal., 2011, 2013a;
Riahi etal., 2012). These two analyses are unique from other integrated
studies (see e. g., Shukla et al., 2008; Skea and Nishioka, 2008; Stra-
chan etal., 2008; IEA, 2011; Shukla and Dhar, 2011; PBL, 2012; Akimoto
etal., 2012; Howells etal., 2013) because they attempt to quantify key
interactions in economic terms on a global scale, employing varying
methodologies to assess the interactions between climate change, air
pollution, and energy security policies. Bollen et al. (2010) employ a
cost-benefit social welfare optimization approach while the GEA study
employs a cost-effectiveness approach (see Section 3.7.2.1). Despite
these differences, the two studies provide similar insights. Both suggest
that near-term synergies can be realized through decarbonization and
energy efficiency and that mitigation policy may be seen as a strategic
entry point for reaping energy security and air quality co-benefits. The
GEA study in particular finds major cost savings from mitigation policy
in terms of reduced expenditures for imported fossil fuels and end-of-
pipe air pollution control equipment (see bottom panel of Figure 6.33).
The magnitude of these co-benefits depends importantly on the future
stringency of energy security and air pollution policies in the absence of
mitigation policy. If these are more aggressive than currently planned,
then the co-benefits would be smaller.
Another class of sustainable development scenarios are the Low-Carbon
Society (LCS) assessments (Kainuma et al., 2012), which collectively
indicate that explicit inclusion of mitigation co-benefits in the cost cal-
culation results in a lower-carbon price in the LCS scenarios than in a
scenario that only considers mitigation costs (Shukla etal., 2008). A key
message from these studies is that co-benefits are neither automatic nor
assured, but result from conscious and carefully coordinated policies and
implementation strategies, such as lifestyle changes, green manufactur-
ing processes, and investments into energy efficient devices, recycling
measures, and other targeted actions (Shukla and Chaturvedi, 2012).
Finally, studies suggest that co-benefits could influence the incen-
tives for global climate agreements discussed in Section 13.3 (Pittel
and Rübbelke, 2008; Bollen etal., 2009b; Wagner, 2012). At the pres-
ent time, however, international policy regimes for mitigation and its
important co-benefits remain separate (Holloway etal., 2003; Swart
etal., 2004; Nemet etal., 2010; Rao etal., 2013). Dubash etal. (2013)
propose a methodology for operationalizing co-benefits in mitigation
policy formulation, thus helping to bring the varied policy objectives
closer together (see Section 15.2).
478478
Assessing Transformation Pathways
6
Chapter 6
6.7 Risks of transformation
pathways
Mitigation will be undertaken within the context of a broad set of pol-
icy objectives, existing societal structures, institutional frameworks, and
physical infrastructures. The relationship between these broader char-
acteristics of human societies and the particular implications of mitiga-
tion activities will be both complex and uncertain. Mitigation will also
take place under uncertainty about the underlying physical processes
that govern the climate. All of these indicate that there is a range of
different risks associated with different transformation pathways.
The various risks associated with transformation pathways can be
grouped into several categories, and many of these are discussed else-
where in this chapter. One set of risks is associated with the linkage
of mitigation with other policy objectives, such as clean air, energy
security, or energy access. These linkages may be positive (co-benefits)
or negative (risks). These relationships are discussed in Section 6.6.
Another set of risks is associated with the possibility that particular
mitigation measures might be taken off the table because of perceived
negative side-effects and that stabilization will prove more challenging
that what might have been expected (Strachan and Usher, 2012). These
issues are discussed in Section 6.3 as well as elsewhere in the chapter,
including Section 6.9 for CDR options. Another risk is that the economic
costs may be higher or lower than anticipated, because the implications
of mitigation cannot be understood with any degree of certainty today,
for a wide range of reasons. This issue is discussed in Section 6.3.6. It is
important to emphasize that both the economic costs and the economic
benefits of mitigation are uncertain. One of the most fundamental risks
associated with mitigation is that any transformation pathway may
not maintain temperatures below a particular threshold, such as 2 °C
or 1.5 °C above preindustrial levels due to limits in our understanding
of the relationship between emissions and concentrations and, more
importantly, the relationship between GHG concentrations and atmo-
spheric temperatures. This topic is discussed in Section 6.3.2.
A broad risk that underpins all the mitigation scenarios in this chapter
is that every long-term pathway depends crucially not just on actions by
today’s decision makers, but also by future decision-makers and future
generations. Indeed, mitigation must be framed within a sequential-
decision making not just because it is good practice, but more funda-
mentally because decision makers today cannot make decisions for
those in the future. A consistent risk is that future decision makers may
not undertake the mitigation that is required to meet particular long-
term goals. In this context, actions today can be seen as creating or
limiting options to manage risk rather than leading to particular goals.
This topic is discussed in Sections 6.3 and 6.4 through the exploration
of the consequences of different levels of near-term mitigation. This
issue is particularly important in the context of scenarios that lead to
concentration goals such as 450 ppm CO
2
eq by 2100. The vast majority
of these scenarios temporarily overshoot the long-term goal and then
descend to it by the end of the century through increasing emissions
reductions. When near-term mitigation is not sufficiently strong, future
mitigation must rely heavily on CDR technologies such as BECCS, put-
ting greater pressure on future decision makers and highlighting any
uncertainties and risks surrounding these technologies. While these sce-
narios are possible in a physical sense, they come with a very large risk
that future decision makers will not take on the ambitious action that
would ultimately be required. Indeed, studies have shown that delayed
and fragmented mitigation can lead to a relaxation of long-term goals
if countries that delay their participation in a global mitigation strategy
are not willing or unable to pick up the higher costs of compensating
higher short-term emissions (Blanford etal., 2014; Kriegler etal., 2014c).
6.8 Integrating sector
analyses and
transformation scenarios
6�8�1 The sectoral composition of GHG
emissions along transformation
pathways
Options for reducing GHG emissions exist across a wide spectrum of
human activities. The majority of these options fall into three broad
areas: energy supply, energy end-use, and AFOLU. The primary focus
of energy supply options is to provide energy from low- or zero-car-
bon energy sources; that is, to decarbonize energy supply. Options
in energy end-use sectors focus either on reducing the use of energy
and / or on using energy carriers produced from low-carbon sources,
including electricity generated from low-carbon sources. Direct
options in AFOLU involve storing carbon in terrestrial systems (for
example, through afforestation). This sector is also the source of bio-
energy. Options to reduce non-CO
2
emissions exist across all these
sectors, but most notably in agriculture, energy supply, and industry.
These sectors and the associated options are heavily interlinked. For
example, energy demand reductions may be evident not only as direct
emissions reductions in the end-use sectors but also as emissions
reductions from the production of energy carriers such as electricity
(‘indirect emissions’, see Annex A.II.5). Replacing fossil fuels in energy
supply or end-use sectors by bioenergy reduces emissions in these sec-
tors, but may increase land-use emissions in turn (see Chapter 11, Bio-
energy Appendix). In addition, at the most general level, sectoral miti-
gation actions are linked by the fact that reducing emissions through a
mitigation activity in one sector reduces the required reductions from
mitigation activities in other sectors to meet a long-term CO
2
-equiva-
lent concentration goal.
The precise set of mitigation actions taken in any sector will depend on
a wide range of factors, including their relative economics, policy struc-
479479
Assessing Transformation Pathways
6
Chapter 6
tures, and linkages to other objectives (see Section 6.6) and interac-
tions among measures across sectors. Both integrated models, such as
those assessed in this chapter, and sectorally focused research, such as
that assessed in Chapters 7 – 11, offer insights into the options for miti-
gation across sectors. The remainder of this section first assesses the
potential for mitigation within the sectors based on integrated studies
and then in each of the emitting sectors based on the combined assess-
ments from sectoral and integrated studies. Important questions are
how consistent the results from integrated modelling studies are with
sectorally-focused literature and how they complement each other.
6�8�2 Mitigation from a cross-sectoral
perspective: Insights from integrated
models
Integrated models are a key source of research on the tradeoffs and
synergies in mitigation across sectors. In scenarios from these models,
energy sector emissions are the dominant source of GHG emissions in
baseline scenarios, and these emissions continue to grow over time
relative to net AFOLU CO
2
emissions and non-CO
2
GHG emissions (Sec-
tion 6.3.1 and Figure 6.34). Within the energy sector, direct emissions
from energy supply, and electricity generation in particular, are larger
than the emissions from any single end-use sector (Figure 6.34). Direct
emissions, however, do not provide a full representation of the impor-
tance of different activities causing the emissions, because the con-
sumption of energy carriers such as electricity by the end-use sectors,
leads to indirect emissions from the production of those energy car-
riers (consumption-based approach). An alternative perspective is to
allocate these indirect energy supply emissions to the end-use sectors
that use these supplies (see, for example, in Figure 6.34). At present,
indirect emissions from electricity use are larger than direct emissions
in buildings and constitute an important share of industrial emissions
while they are small in transport compared to direct CO
2
emissions.
In mitigation scenarios from integrated models, decarbonization of the
electricity sector takes place at a pace more rapid than reduction of
direct emissions in the energy end-use sectors (see Sections7.11.3 and
Figure 6.35). For example, in 450 ppm CO
2
eq scenarios, the electricity
sector is largely decarbonized by 2050, whereas deep reductions in direct
emissions in the end-use sectors largely arise beyond mid-century. More
so than any other energy supply technology, the availability of BECCS
and its role as a primary CDR technology (Sections 6.3.2 and 6.9) has a
substantial effect on this dynamic, allowing for energy supply sectors to
serve as a net negative emissions source by mid-century and allowing for
more gradual emissions reductions in other sectors. In contrast, sectoral
studies show available pathways to deep reductions in emissions (both
direct and indirect) already by mid-century (see, e. g., Chapter 9).
Figure 6�34 | Direct (left panel) and direct and indirect emissions (right panel) of CO
2
and non-CO
2
GHGs across sectors in baseline scenarios. Note that in the case of indirect emis-
sions, only electricity emissions are allocated from energy supply to end-use sectors. In the left panel electricity sector emissions are shown (“Electricity*”) in addition to energy sup-
ply sector emissions which they are part of, to illustrate their large role on the energy supply side. The numbers at the bottom refer to the number of scenarios included in the ranges
that differ across sectors and time due to different sectoral resolution and time horizon of models. Source: WG III AR5 Scenario Database (Annex II.10). Includes only baseline scenar-
ios. Note that scenarios from the AMPERE study were excluded due to large overlap with the EMF27 study. Historical data: JRC / PBL (2013), IEA (2012), see Annex II.9 and Annex II.5.
n=
0
20
40
60
80
Direct Emissions
Direct Emissions [GtCO
2
eq/yr]
93 93 78 80 80 65 80 80 65 103 103 88 131 131 118 121 121 107
2030
2050
2100
103 103 88
0
20
40
60
80
Direct and Indirect Emissions
Direct and Indirect Emissions [GtCO
2
eq/yr]
77 77 68 68 68 59 68 68 59
2030
2050
2100
CO
2
Electricity
CO
2
Transport
CO
2
Buildings
CO
2
Industry
CO
2
Energy Supply
CO
2
Net AFOLU
Non−CO
2
(All Sectors)
Actual 2010 Level
Min
75
th
Percentile
Max
Median
25
th
Percentile
CO
2
Energy Supply
excl. Electricity Generation
CO
2
Transport
CO
2
Buildings
CO
2
Industry
Actual 2010 Level
147 147 127
Transport Buildings Industry Energy
Supply
Energy
Supply
Net AFOLU Non−CO
2
Transport Buildings Industry
Electricity*
480480
Assessing Transformation Pathways
6
Chapter 6
Within the end-use sectors, deep emissions reductions in transport
are generally the last to emerge in integrated modelling studies
because of the assumption that options to switch to low-carbon
energy carriers in transport are more limited than in buildings and
industry, and also because of the expected high growth for mobility
and freight transport (Section 8.9.1). In the majority of baseline sce-
narios from integrated models, net AFOLU CO
2
emissions largely dis-
appear by mid-century, with some models projecting a net sink after
2050 (Section 6.3.1.4). There is a wide uncertainty in the role of affor-
estation and reforestation in mitigation, however. In some mitigation
scenarios the AFOLU sectors can become a significant carbon sink
(Section 6.3.2.4).
6�8�3 Decarbonizing energy supply
Virtually all integrated modelling studies indicate that decarbonization
of electricity is critical for mitigation, but there is no general consensus
regarding the precise low-carbon technologies that might support this
decarbonization (Fischedick etal., 2011; Clarke etal., 2012) (Section
7.11.3). These studies have presented a wide range of combinations
of renewable energy sources (Krey and Clarke, 2011; Luderer etal.,
2014b), nuclear power (Bauer et al., 2012; Rogner and Riahi, 2013),
and CCS-based technologies (McFarland et al., 2009; Bauer et al.,
2014a; McCollum etal., 2014a; van der Zwaan etal., 2014) as both
viable and cost-effective (see Section 7.11). The breadth of different,
potentially cost-effective strategies raises the possibility not only that
future costs and performances of competing electricity technologies
are uncertain today, but also that regional circumstances, including
both energy resources and links to other regional objectives (e. g.,
national security, local air pollution, energy security, see Section 6.6),
might be as important decision making factors as economic costs (Krey
etal., 2014). The one exception to this flexibility in energy supply sur-
rounds the use of BECCS. CDR technologies such as BECCS are fun-
damental to many scenarios that achieve low-CO
2
eq concentrations,
particularly those based on substantial overshoot as might occur if
near-term mitigation is delayed (Sections 6.3.2 and 6.4). In contrast to
the electricity sector, decarbonization of the non-electric energy-supply
sector (e. g., liquid fuels supply) is progressing typically at much lower
pace (Section 7.11.3, Figures 7.14 and 7.15) and could therefore con-
stitute a bottleneck in the transformation process.
6�8�4 Energy demand reductions and fuel
switching in end-use sectors
The two major groups of options in energy end-use sectors are those
that focus on reducing the use of energy and / or those that focus on
using energy carriers produced from low-carbon sources. Three impor-
tant issues are therefore the potential for fuel switching, the potential
Figure 6�35 | Direct emissions of CO
2
and non-CO
2
GHGs across sectors in mitigation scenarios that reach around 450 (430 – 480) ppm CO
2
eq concentrations in 2100 with using
CCS (left panel) and without using CCS (right panel). The numbers at the bottom of the graphs refer to the number of scenarios included in the ranges that differ across sectors and
time due to different sectoral resolution and time horizon of models. White dots in the right panel refer to emissions of individual scenarios to give a sense of the spread within the
ranges shown due to the small number of scenarios. Source: WG III AR5 Scenario Database (Annex II.10). Includes only scenarios based on idealized policy implementation that
provide emissions at the sectoral level. Note that scenarios from the AMPERE study were excluded due to large overlap with the EMF27 study. Historical data: JRC / PBL (2013), IEA
(2012), see Annex II.9.
29 29 29 22 22 22 22 22 22 36 36 36 32 32 32 36 36 36
2030
2050
2100
Transport Buildings Industry Electricity Net
AFOLU
Non−CO
2
Transport Buildings Industry Electricity Net
AFOLU
Non−CO
2
5 5 5 3 3 3 3 3 3 5 5 5 6 6 6 6 6 6
20
-20
10
-10
0
Direct GHG Emissions [GtCO
2
eq/yr]
Direct GHG Emissions [GtCO
2
eq/yr]
n=
450 ppm CO
2
eq with CCS 450 ppm CO
2
eq without CCS
2030
2050
2100
20
-20
10
-10
0
CO
2
Transport
CO
2
Buildings
CO
2
Industry
CO
2
Electricity
CO
2
Net AFOLU
Non−CO
2
(All Sectors)
Actual 2010 Level
Individual
Scenarios
Min
75
th
Percentile
Max
Median
25
th
Percentile
481481
Assessing Transformation Pathways
6
Chapter 6
for reductions of energy use per unit of output / service, and the rela-
tionship and timing between the two. In general, as discussed in Sec-
tion6.3.4, integrated studies indicate that energy intensity (per unit of
GDP) reductions outweigh decarbonization of energy supply in the
near term when the energy-supply system is still heavily reliant on
largely carbon-intensive fossil fuels (Figure 6.17). Over time, the miti-
gation dynamic switches to one focused on carbon-intensity reduc-
tions (see AR4, Fisher etal., 2007, Section 3.3.5.2). From the perspec-
tive of end-use sectors, decarbonization of energy involves both the
decarbonization of existing sources, for example, by producing electric-
ity from low-carbon sources or using liquid fuels made from bioenergy,
and an increase in the use of lower-carbon fuels, for example, through
an increase in the use of electricity (Edmonds etal., 2006; Kyle etal.,
2009; Sugiyama, 2012; Williams etal., 2012; Krey etal., 2014; Yama-
moto etal., 2014). It should be noted that there is generally an autono-
mous increase in electrification in baseline scenarios that do not
assume any climate policies, which reflects a trend toward more con-
venient grid-based fuels due to higher affluence (Nakicenovic etal.,
1998; Schäfer, 2005), as well as electricity typically showing a slower
cost increase over time compared to other energy carriers (Edmonds
etal., 2006; Krey etal., 2014).
The comparison between integrated and sectoral studies is difficult
with regard to the timing and tradeoffs between fuel switching and
energy reduction, because few sectoral studies have attempted to
look concurrently at both fuel switching and energy-reduction strat-
egies. Instead, the majority of sectoral studies have focused most
heavily on energy reduction, asking how much energy use for a par-
ticular activity can be reduced with state-of-the-art technology. One
reason for this focus on energy reduction is that sectoral research
is more commonly focused on near-term actions based on available
mitigation technologies and, in the near-term, major fuel sources
such as liquid fuels and electricity may have high-carbon intensities.
This means that energy reductions will have substantial near-term
mitigation effects. In the longer term, however, these fuel sources
will be largely decarbonized along low-concentration transformation
pathways, meaning that energy reductions will not so clearly lead to
reductions in indirect emissions (note that this does not mean they
do not continue to be important, because they decrease the need for
utilizing energy sources and the associated co-benefits and risks, see
Section 6.6).
This evolution can be clearly seen through a comparison of direct and
indirect emissions in end-use sectors in integrated modelling scenarios
(Figure 6.36). In 2010, the largest part of emissions from the buildings
sector are the indirect emissions from electricity. This trend continues in
baseline scenarios (Figure 6.36). However, in deep emission-reduction
scenarios, indirect emissions from electricity are largely eliminated by
2050, and in many scenarios, the electricity sector even becomes a sink
for CO
2
through the use of BECCS (Figure 6.35, left panel). There are
only minimal indirect emissions from electricity in the transport sector
today and by 2050 in mitigation scenarios. Those scenarios that decar-
bonize the transportation sector through electrification do so by taking
advantage of a largely decarbonized electricity sector. The industrial
sector lies between the buildings and transport sectors. Of importance,
the observed trends can be very regional in character. For example,
the value of electrification will be higher in countries or regions that
already have low-carbon electricity portfolios.
The primary distinction between sectoral studies and integrated model-
ling studies with regard to end-use options for fuel switching and end-
use reductions is that integrated models typically represent end-use
options at a more aggregated scale than sectoral studies. In addition,
however, there is an important difference in the way that the two types
of studies attempt to ascertain opportunities (see Section 8.9). Long-
term mitigation scenarios from integrated models achieve reductions
from baseline emissions based almost exclusively on the imposition of a
carbon price and generally assume functioning markets and may not
Figure 6�36 | Direct CO
2
emissions vs. indirect CO
2
emissions from electricity in the transport, buildings, and industry sectors in 2050 for baseline and mitigation scenarios reach-
ing 430 – 480 ppm and 530 – 580 ppm CO
2
eq in 2100. Source: WG III AR5 Scenario Database (Annex II.10). Includes only scenarios based on idealized policy implementation that
provide emissions at the sectoral level. Historical data from JRC / PBL (2013), IEA (2012a), see Annex II.9.
0 5 10 15 20 0 5 10 15 20 0 5 10 15 20
Transport Buildings Industry
Direct CO
2
Emissions [GtCO
2
/yr] Direct CO
2
Emissions [GtCO
2
/yr] Direct CO
2
Emissions [GtCO
2
/yr]
Indirect CO
2
Emissions [GtCO
2
/yr]
430−480 ppm CO
2
eq
530−580 ppm CO
2
eq
Baselines
0
10
15
20
5
0
10
15
20
5
0
10
15
20
5
2010
2010
2010
482482
Assessing Transformation Pathways
6
Chapter 6
fully represent existing barriers, in particular in end-use sectors. In con-
trast, sectoral studies explore options for energy-demand reduction
based on engineering and / or local details and do so based on cost-
effectiveness calculations regarding a typically much richer portfolio of
tailored options. They also recognize that there are many boundaries to
consumer rationality and thus not all options that are cost-effective hap-
pen automatically in a baseline, but are mobilized by mitigation policies.
It is also challenging to compare the potential for energy reductions
across sectoral and integrated studies, because of difficulties to discern
the degree of mitigation that has occurred in the baseline itself in these
studies. Therefore any comparisons must be considered approximate at
best. It is important to note that the emphasis on economic instruments
like carbon pricing in integrated studies leads to a negative correlation
between energy-demand reduction and the option of switching to low-
carbon energy carriers at modest cost. Therefore, integrated studies that
foresee a significant potential for switching to electricity, for example, in
an end-use sector at modest costs, usually show a lower need for reduc-
ing energy demand in this sector and the other way around. It should
also be noted that there is not always a clear cut distinction between
sectoral and integrated studies. Some sectoral studies, in particular
those that provide estimates for both energy savings and fuel switching,
are in fact integrated studies with considerable sectoral detail such as
the IEA World Energy Outlook (IEA, 2010b, 2012b) or the Energy Tech-
nology Perspectives report (IEA, 2008, 2010c) (see Annex II.10).
In general, in the transport sector, the opportunities for energy-use
reductions and fuel switching are broadly consistent between inte-
grated and sectoral studies (Figures 6.37 and 6.38, Section 8.9).
However, the underlying mechanisms utilized in these studies may
be different. Comprehensive transport sector studies tend to include
technical efficiency measures, switching to low-carbon fuels, behav-
ioural changes that affect both the modal split and the amount of
transport services demanded, and a broader set of infrastructural
characteristics such as compact cities. In integrated studies, these
factors are not always addressed explicitly, and the focus is usually
on technical efficiency measures, fuel switching and service demand
reduction. Regarding fuel choice, the majority of integrated stud-
ies indicate a continued reliance on liquid and gaseous fuels, sup-
ported by an increase in the use of bioenergy up to 2050. Many inte-
grated studies also include substantial shares of electricity through,
for example, the use of electric vehicles for light-duty transporta-
tion, usually during the second-half of the century. Hydrogen has
also been identified by numerous studies as a potential long-term
solution should storage, production, and distribution challenges be
overcome (Section 8.9.1). While electricity and hydrogen achieve
substantial shares in some scenarios, many integrated modelling sce-
narios show no dominant transport fuel source in 2100. This prevails
in scenarios leading to 430 – 530 ppm CO
2
eq concentration levels in
2100 with the median values for the share of electricity and hydro-
Figure 6�37 | Sectoral final energy demand reduction relative to baseline in the energy end-use sectors, transport, buildings, and industry by 2030 and 2050 in mitigation scenarios
reaching 430 – 530 ppm and 530 – 650 ppm CO
2
eq in 2100 (see Section 6.3.2) compared to sectoral studies assessed in Chapters 8 – 10. Filled circles correspond to sectoral studies
with full sectoral coverage while empty circles correspond to studies with only partial sectoral coverage (e. g., heating and cooling only for buildings). Source: WG III AR5 Scenario
Database (Annex II.10). Includes only scenarios based on idealized policy implementation. Sectoral studies as provided by Chapters 8, 9, and 10, see Annex II.10.
N= 161 225 161 225 126 189 126 189 126 189 126 189
Min
75
th
Percentile
Max
Median
25
th
Percentile
2030 2050
Transport
Final Energy Demand Reduction Relative to Baseline [%]
2030 2050
Buildings
Final Energy Demand Reduction Relative to Baseline [%]
2030 2050
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
Industry
Final Energy Demand Reduction Relative to Baseline [%]
530−650 ppm CO
2
eq
430−530 ppm CO
2
eq
Sectoral Studies (Partial)
Sectoral Studies (Full)
483483
Assessing Transformation Pathways
6
Chapter 6
gen in 2100 being 22 % and 25 % of final energy, respectively (Sec-
tion 8.9.1, Figure 8.9.4).
Detailed building sector studies indicate energy savings potential
by 2050 on the upper end of what integrated studies show (Section
9.8.2, Figure 9.19), and both sectoral and integrated studies show
modest opportunities for fuel switching due to the already high
level of electricity consumption in the buildings sector, particularly
in developed countries (Figures 6.37 and 6.38). Building sector stud-
ies have focused largely on identifying options for saving energy
whereas fuel switching as a means for reducing emissions is not con-
sidered in detail by most studies. In general, both sectoral and inte-
grated studies indicate that electricity will supply a dominant share
of building energy demand over the long term, especially if heating
demand decreases due to a combination of efficiency gains, better
architecture and climate change. Best case new buildings can reach
90 % lower space heating and cooling energy use compared to the
existing stock (Section 9.3.3), while for existing buildings, deep ret-
rofits can achieve heating and cooling energy savings in the range of
50 – 90 % (Section 9.3.4).
Detailed industry sector studies tend to be more conservative regard-
ing savings in industrial final energy compared to baseline, but on the
other hand foresee a greater potential for switching to low-carbon
fuels, including electricity, heat, hydrogen and bioenergy than inte-
grated studies (Figures 6.37 and 6.38). Sectoral studies, which are
often based on micro unit-level analyses, indicate that the broad appli-
cation of best available technologies for energy reduction could lead to
about 25 % of energy savings in the sector with immediate deployment
and similar contributions could be achieved with new innovations
and deployment across a large number of production processes (Sec-
tion 10.4). Integrated models in general (with exceptions, see Section
10.10.1) treat the industry sector in a more aggregated fashion and
mostly do not provide detailed sub-sectoral material flows, options for
reducing material demand, and price-induced inter-input substitution
possibilities explicitly (Section10.10.1). Similar to the transportation
sector, there is no single perceived near- or long-term configuration
for industrial energy (see Sections 10.4 and 10.7). Multiple pathways
may be pursued or chosen depending on process selection and tech-
nology development. For the industry sector to achieve near-zero
emission with carbonaceous energy, carriers will need CCS facilities
though market penetration of this technology is still highly uncertain
and only limited examples are in place so far. Some integrated studies
indicate a move toward electricity whereas others indicate a continued
reliance on liquid or solid fuels, largely supported through bioenergy
(Section 10.10.1, Figure 10.14). Due to the heterogeneous character of
the industry sector a coherent comparison between sectoral and inte-
grated studies remains difficult.
Figure 6�38 | Development of final energy low-carbon fuel shares in the energy end-use sectors transport, buildings, and industry by 2030 and 2050 in baseline and mitigation
scenarios reaching 430 – 530 ppm and 530 – 650 ppm CO
2
eq in 2100 (see Section 6.3.2) compared to sectoral studies assessed in Chapters 8 – 10. Low-carbon fuels include electric-
ity, hydrogen, and liquid biofuels in transport, electricity in buildings and electricity, heat, hydrogen, and bioenergy in industry. Filled symbols correspond to sectoral studies with
additional climate policies whereas empty symbols correspond to studies with baseline assumptions. Source: WG III AR5 Scenario Database (Annex II.10). Includes only scenarios
based on idealized policy implementation. Sectoral studies as provided by Chapters 8, 9, and 10, see AnnexII.10. Historical data from IEA (2012c; d).
N=
154 130 182 154 130 182 124 103 110 124 103 110 107 86 95 107 86 95
Baselines
530−650 ppm CO
2
eq
430−530 ppm CO
2
eq
Sectoral Studies (Base)
Sectoral Studies (Policy)
Actual 2010 Level
2030 2050
0
20
40
60
100 100 100
80
Transport
Low-Carbon Fuel Share in Final Energy [%]
2030 2050
0
20
40
60
80
Buildings
Low-Carbon Fuel Share in Final Energy [%]
2030 2050
0
20
40
60
80
Industry
Low-Carbon Fuel Share in Final Energy [%]
Min
75
th
Percentile
Max
Median
25
th
Percentile
484484
Assessing Transformation Pathways
6
Chapter 6
6�8�5 Options for bioenergy production,
reducing land-use change emissions, and
creating land-use GHG sinks
As noted in Section 6.3.5, land use has three primary roles in miti-
gation: bioenergy production, storage of carbon in terrestrial systems,
mitigation of non-CO
2
GHGs. It also influences mitigation through
biogeophysical factors such as albedo. Integrated modelling studies
are the primary means by which the tradeoffs and synergies between
these different roles, in particular the first two, might unfold over the
rest of the century. The integrated modelling studies sketch out a wide
range of ways in which these forces might affect the land surface, from
widespread afforestation under comprehensive climate policies to
widespread deforestation if carbon storage on land is not included in
the mitigation policy (Sections 6.3.5 and 11.9).
Sectoral studies complement integrated modelling studies by explor-
ing the ability of policy and social structures to support broad
changes in land-use practices over time (Section 11.6). In general,
sectoral studies point to the challenges associated with making
large-scale changes to the land surface in the name of mitigation,
such as challenges associated with institutions, livelihoods, social
and economic concerns, and technology and infrastructure. These
challenges raise questions about transformation pathways (Section
11.6). For example, although increasing the land area covered by nat-
ural forests could enhance biodiversity and a range of other ecosys-
tem services, afforestation occurring through large-scale plantations
could negatively impact biodiversity, water, and other ecosystem
services (Sections 11.7 and 11.13.6). Similarly, the use of large land
areas for afforestation or dedicated feedstocks for bioenergy could
increase food prices and compromise food security if land normally
used for food production is converted to bioenergy or forests (Sec-
tion 11.4). The degree of these effects is uncertain and depends on
a variety of sector-specific details regarding intensification of land
use, changes in dietary habits, global market interactions, and bio-
physical characteristics and dynamics. The implications of transfor-
mation pathways that rely heavily on reductions of non-CO
2
GHGs
from agriculture depend on whether mitigation is achieved through
reduced absolute emissions, or through reduced emissions per unit of
agricultural product (Section 11.6), and the role of large-scale inten-
sive agriculture, which has often not been implemented sustainably
(e. g., large areas of monoculture food or energy crops or intensive
livestock production, potentially damaging ecosystem services). Fur-
thermore, sector studies are beginning to elucidate implementation
issues, such as the implications of staggered and / or partial regional
adoption of land mitigation policies, as well as institutional design.
For example, realizing large-scale bioenergy without compromising
the terrestrial carbon stock might require strong institutional condi-
tions, such as an implemented and enforced global price on land car-
bon. Finally, sector studies will continue to provide revised and new
characterizations of mitigation technologies that can be evaluated in
a portfolio context (Section 11.9).
6.9 Carbon and radiation
management and
other geo- engineering
options including
environmental risks
Some scientists have argued that it might be useful to consider, in
addition to mitigation and adaptation measures, various intentional
interventions into the climate system as part of a broader climate
policy strategy (Keith, 2000; Crutzen, 2006). Such technologies have
often been grouped under the blanket term ‘geoengineering’ or, alter-
natively, ‘climate engineering’ (Keith, 2000; Vaughan and Lenton,
2011). Calls for research into these technologies have increased in
recent years (Caldeira and Keith, 2010; Science and Technology Com-
mittee, 2010), and several assessments have been conducted (Royal
Society, 2009; Edenhofer etal., 2011; Ginzky etal., 2011; Rickels etal.,
2011). Two categories of geoengineering are generally distinguished.
Removal of GHGs, in particular carbon dioxide termed ‘carbon diox-
ide removal’ or CDR, would reduce atmospheric GHG concentrations.
The boundary between some mitigation and some CDR methods is
not always clear (Boucher etal., 2011, 2013). ‘Solar radiation manage-
ment’ or SRM technologies aim to increase the reflection of sunlight to
cool the planet and do not fall within the usual definitions of mitiga-
tion and adaptation. Within each of these categories, there is a wide
range of techniques that are addressed in more detail in Sections 6.5
and 7.7 of the WG I report.
Many geoengineering technologies are presently only hypothetical.
Whether or not they could actually contribute to the avoidance of future
climate change impacts is not clear (Blackstock etal., 2009; Royal Soci-
ety, 2009). Beyond open questions regarding environmental effects and
technological feasibility, questions have been raised about the socio-
political dimensions of geoengineering and its potential implications
for climate politics (Barrett, 2008; Royal Society, 2009; Rickels etal.,
2011). In the general discussion, geoengineering has been framed in a
number of ways (Nerlich and Jaspal, 2012; Macnaghten and Szerszyn-
ski, 2013; Luokkanen etal., 2013; Scholte etal., 2013), for instance, as
a last resort in case of a climate emergency (Blackstock etal., 2009;
McCusker etal., 2012), or as a way to buy time for implementing con-
ventional mitigation (Wigley, 2006; Institution of Mechanical Engineers,
2009; MacCracken, 2009). Most assessments agree that geoengineer-
ing technologies should not be treated as a replacement for conven-
tional mitigation and adaptation due to the high costs involved for
some techniques, particularly most CDR methods, and the potential
risks, or pervasive uncertainties involved with nearly all techniques
(Royal Society, 2009; Rickels etal., 2011). The potential role of geoengi-
neering as a viable component of climate policy is yet to be determined,
and it has been argued that geoengineering could become a distraction
from urgent mitigation and adaptation measures (Lin; Preston, 2013).
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Chapter 6
6�9�1 Carbon dioxide removal
6�9�1�1 Proposed carbon dioxide removal methods and
characteristics
Proposed CDR methods involve removing CO
2
from the atmosphere
and storing the carbon in land, ocean, or geological reservoirs. These
methods vary greatly in their estimated costs, risks to humans and
the environment, potential scalability, and notably in the depth of
research about their potential and risks. Some techniques that fall
within the definition of CDR are also regarded as mitigation mea-
sures such as afforestation and BECCS (see Glossary). The term
‘negative emissions technologies’ can be used as an alternative to
CDR (McGlashan etal., 2012; McLaren, 2012; Tavoni and Socolow,
2013).
The WGI report (Section 6.5.1) provides an extensive but not exhaus-
tive list of CDR techniques (WGI Table 6.14). Here only techniques that
feature more prominently in the literature are covered. This includes
(1) increased land carbon sequestration by reforestation and affores-
tation, soil carbon management, or biochar (see WGIII Chapter 11);
(2) increased ocean carbon sequestration by ocean fertilization; (3)
increased weathering through the application of ground silicates to
soils or the ocean; and (4) chemical or biological capture with geologi-
cal storage by BECCS or direct air capture (DAC). CDR techniques can
be categorized in alternative ways. For example, they can be catego-
rized (1) as industrial technologies versus ecosystem manipulation; (2)
by the pathway for carbon dioxide capture (e. g. McLaren, 2012; Cal-
deira etal., 2013); (3) by the fate of the stored carbon (Stephens and
Keith, 2008); and (4) by the scale of implementation (Boucher etal.,
2013). Removal of other GHGs, e. g., CH
4
and N
2
O, have also been pro-
posed (Boucher and Folberth, 2010; de Richter and Caillol, 2011; Sto-
laroff etal., 2012).
All CDR techniques have a similar slow impact on rates of warming as
mitigation measures (van Vuuren and Stehfest, 2013) (see WGI Section
6.5.1). An atmospheric ‘rebound effect’ (see WGI Glossary) dictates
that CDR requires roughly twice as much CO
2
removed from the atmo-
sphere for any desired net reduction in atmospheric CO
2
concentration,
as some CO
2
will be returned from the natural carbon sinks (Lenton
and Vaughan, 2009; Matthews, 2010). Permanence of the storage res-
ervoir is a key consideration for CDR efficacy. Permanent (larger than
tens of thousands of years) could be geological reservoirs while non-
permanent reservoirs include oceans and land (the latter could, among
others, be affected by the magnitude of future climate change) (see
WGI Section 6.5.1). Storage capacity estimates suggest geological res-
ervoirs could store several thousand GtC; the oceans a few thousand
GtC in the long term, and the land may have the potential to store
the equivalent to historical land-use loss of 180 ± 80GtC (also see
Table 6.15 of WG I)(IPCC, 2005; House etal., 2006; Orr, 2009; Mat-
thews, 2010).
Ocean fertilization field experiments show no consensus on the effi-
cacy of iron fertilization (Boyd et al., 2007; Smetacek et al., 2012).
Modelling studies estimate between 15 ppm and less than 100 ppm
drawdown of CO
2
from the atmosphere over 100 years (Zeebe and
Archer, 2005; Cao and Caldeira, 2010) while simulations of mechanical
upwelling suggest 0.9 Gt / yr (Oschlies etal., 2010). The latter technique
has not been field tested. There are a number of possible risks including
downstream decrease in productivity, expanded regions of low-oxygen
concentration, and increased N
2
O emissions (See WGI Section 6.5.3.2)
(low confidence). Given the uncertainties surrounding effectiveness
and impacts, this CDR technique is at a research phase with no active
commercial ventures. Furthermore, current international governance
states that marine geoengineering including ocean fertilization is to
be regulated under amendments to the London Convention / London
Protocol on the Prevention of Marine Pollution by Dumping of Wastes
and Other Matter, only allowing legitimate scientific research (Güssow
etal., 2010; International Maritime Organization, 2013).
Enhanced weathering on land using silicate minerals mined, crushed,
transported, and spread on soils has been estimated to have a poten-
tial capacity, in an idealized study, of 1 GtC / yr (Köhler etal., 2010).
Ocean-based weathering CDR methods include use of carbonate or
silicate minerals processed or added directly to the ocean (see WGI
Section 6.5.2.3). All of these measures involve a notable energy
demand through mining, crushing, and transporting bulk materials.
Preliminary hypothetical cost estimates are in the order of 23 – 66
USD / tCO
2
(Rau and Caldeira, 1999; Rau et al., 2007) for land and
51 – 64 USD / tCO
2
for ocean methods (McLaren, 2012). The confidence
level on the carbon cycle impacts of enhanced weathering is low
(WGI Section 6.5.3.3).
The use of CCS technologies (IPCC, 2005) with biomass energy also
creates a carbon sink (Azar etal., 2006; Gough and Upham, 2011).
BECCS is included in the RCP2.6 (van Vuuren etal., 2007, 2011b) and
a wide range of scenarios reaching similar and higher concentration
goals. From a technical perspective, BECCS is very similar to a com-
bination of other techniques that are part of the mitigation portfolio:
the production of bio-energy and CCS for fossil fuels. Estimates of the
global technical potential for BECCS vary greatly ranging from 3 to
more than 10 GtCO
2
/ yr (Koornneef etal., 2012; McLaren, 2012; van
Vuuren etal., 2013), while initial cost estimates also vary greatly from
around 60to 250 USD / tCO
2
(McGlashan etal., 2012; McLaren, 2012).
Important limiting factors for BECCS include land availability, a sus-
tainable supply of biomass and storage capacity (Gough and Upham,
2011; McLaren, 2012). There is also a potential issue of competition for
biomass under bioenergy-dependent mitigation pathways.
Direct air capture uses a sorbent to capture CO
2
from the atmosphere
and the long-term storage of the captured CO
2
in geological reservoirs
(GAO, 2011; McGlashan etal., 2012; McLaren, 2012). There are a number
of proposed capture methods including adsorption of CO
2
using amines
in a solid form and the use of wet scrubbing systems based on calcium
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6
Chapter 6
or sodium cycling. Current research efforts focus on capture methodolo-
gies (Keith etal., 2006; Baciocchi etal., 2006; Lackner, 2009; Eisenberger
etal., 2009; Socolow etal., 2011) with storage technologies assumed
to be the same as CCS (IPCC, 2005). A U. S. Government Accountabil-
ity Office (GAO) (2011) technology assessment concluded that all DAC
methods were currently immature. A review of initial hypothetical cost
estimates, summarizes 40 – 300 USD / tCO
2
for supported amines and
165 – 600 USD / tCO
2
for sodium or calcium scrubbers (McLaren, 2012)
reflecting an ongoing debate across very limited literature. Carbon diox-
ide captured through CCS, BECCS, and DAC are all intended to use the
same storage reservoirs (in particular deep geologic reservoirs), poten-
tially limiting their combined use under a transition pathway.
6�9�1�2 Role of carbon dioxide removal in the context of
transformation pathways
Two of the CDR techniques listed above, BECCS and afforestation, are
already evaluated in the current integrated models. For concentration
goals on the order of 430 – 530 ppm CO
2
eq by 2100, BECCS forms an
essential component of the response strategy for climate change in
the majority of scenarios in the literature, particularly in the context of
concentration overshoot. As discussed in Section 6.2.2, BECCS offers
additional mitigation potential, but also an option to delay some of the
drastic mitigation action that would need to happen to reach lower
GHG-concentration goals by the second half of the century. In sce-
narios aiming at such low-concentration levels, BECCS is usually com-
petitive with conventional mitigation technologies, but only after these
have been deployed at very large scale (see Azar etal., 2010; Tavoni
and Socolow, 2013). At same time, BECCS applications do not feature
in less ambitious mitigation pathways (van Vuuren etal., 2011a). Key
implications of the use of BECCS in transition pathways is that emis-
sion reduction decisions are directly related to expected availability and
deployment of BECCS in the second half of the century and that scenar-
ios might temporarily overshoot temperature or concentration goals.
The vast majority of scenarios in the literature show CO
2
emissions of
LUC become negative in the second half of the century — even in the
absence of mitigation policy (see Section 6.3.2). This is a consequence
of demographic trends and assumptions on land-use policy. Addition-
ally afforestation as part of mitigation policy is included in a smaller
set of models. In these models, afforestation measures increase for
lower-concentration categories, potentially leading to net uptake of
carbon of around 10GtCO
2
/ yr.
There are broader discussions in the literature regarding the techno-
logical challenges and potential risks of large-scale BECCS deploy-
ment. The potential role of BECCS will be influenced by the sustain-
able supply of large-scale biomass feedstock and feasibility of capture,
transport, and long-term underground storage of CO
2
as well as the
perceptions of these issues. The use of BECCS faces large challenges in
financing, and currently no such plants have been built and tested at
scale. Integrated modeling studies have therefore explored the sensi-
tivities regarding the availability of BECCS in the technology portfolio
by limiting bioenergy supply or CCS storage (Section 6.3.6.3).
Only a few papers have assessed the role of DAC in mitigation scenar-
ios (e. g. Keith etal., 2006; Keller etal., 2008a; Pielke Jr, 2009; Nemet
and Brandt, 2012; Chen and Tavoni, 2013). These studies generally
show that the contribution of DAC hinges critically on the stringency
of the concentration goal, the costs relative to other mitigation tech-
nologies, time discounting and assumptions about scalability. In these
models, the influence of DAC on the mitigation pathways is similar to
that of BECCS (assuming similar costs). That is, it leads to a delay in
short-term emission reduction in favour of further reductions in the
second half of the century. Other techniques are even less mature and
currently not evaluated in integrated models.
There are some constraints to the use of CDR techniques as empha-
sized in the scenario analysis. First of all, the potential for BECCS,
afforestation, and DAC are constrained on the basis of available land
and / or safe geologic storage potential for CO
2
. Both the potential for
sustainable bio-energy use (including competition with other demands,
e. g., food, fibre, and fuel production) and the potential to store >100
GtC of CO
2
per decade for many decades are very uncertain (see previ-
ous section) and raise important societal concerns. Finally, the large-
scale availability of CDR, by shifting the mitigation burden in time,
could also exacerbate inter-generational impacts.
6�9�2 Solar radiation management
6�9�2�1 Proposed solar radiation management methods
and characteristics
SRM geoengineering technologies aim to lower the Earth’s tempera-
ture by reducing the amount of sunlight that is absorbed by the Earth’s
surface, and thus countering some of the GHG induced global warm-
ing. Most techniques work by increasing the planetary albedo, thus
reflecting a greater fraction of the incoming sunlight back to space. A
number of SRM methods have been proposed:
• Mirrors (or sunshades) placed in a stable orbit between the Earth
and Sun would directly reduce the insolation the Earth receives
(Early, 1989; Angel, 2006). Studies suggest that such a technology
is unlikely to be feasible within the next century (Angel, 2006).
• Stratospheric aerosol injection would attempt to imitate the global
cooling that large volcanic eruptions produce (Budyko and Miller,
1974; Crutzen, 2006; Rasch etal., 2008). This might be achieved by
lofting sulphate aerosols (or other aerosol species) or their precur-
sors to the stratosphere to create a high-altitude reflective layer
that would need to be continually replenished. Section 7.7.2.1 of
WGI assessed that there is medium confidence that up to 4 W/m
2
of forcing could be achieved with this approach.
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Chapter 6
• Cloud brightening might be achieved by increasing the albedo of
certain marine clouds through the injection of cloud condensation
nuclei, most likely sea salt, producing an effect like that seen when
ship-tracks of brighter clouds form behind polluting ships (Latham,
1990; Latham etal., 2008, 2012). Section 7.7.2.2 of WGI assessed
that too little was known about marine cloud brightening to pro-
vide a definitive statement on its potential efficacy, but noted that
it might be sufficient to counter the radiative forcing that would
result from a doubling of CO
2
levels.
• Various methods have been proposed that could increase the
albedo of the planetary surface, for example in urban, crop, and
desert regions (President’s Science Advisory Committee. Environ-
mental Pollution Panel, 1965; Gaskill, 2004; Hamwey, 2007; Ridg-
well etal., 2009). These methods would likely only be possible on a
much smaller scale than those listed above. Section 7.7.2.3 of WG I
discusses these approaches.
This list is non-exhaustive and new proposals for SRM methods may be
put forward in the future. Another method that is discussed alongside
SRM methods aims to increase outgoing thermal radiation through the
modification of cirrus clouds (Mitchell and Finnegan, 2009) (see WG I
Section 7.7.2.4).
As SRM geoengineering techniques only target the solar radiation
budget of the Earth, the effects of CO
2
and other GHGs on the Earth
System would remain, for example, greater absorption and re-emis-
sion of thermal radiation by the atmosphere (WG I Section 7.7), an
enhanced CO
2
physiological effect on plants (WG I Section 6.5.4), and
increased ocean acidification (Matthews etal., 2009). Although SRM
geoengineering could potentially reduce the global mean surface air
temperature, no SRM technique could fully return the climate to a
pre-industrial or low-CO
2
-like state. One reason for this is that global
mean temperature and global mean hydrological cycle intensity can-
not be simultaneously returned to a pre-industrial state (Govindasamy
and Caldeira, 2000; Robock etal., 2008; Schmidt etal., 2012; Kravitz
etal., 2013; MacMartin etal., 2013; Tilmes etal., 2013). Section 7.7.3
of WGI details the current state of knowledge on the potential climate
consequences of SRM geoengineering. In brief, simulation studies sug-
gest that some SRM geoengineering techniques applied to a high-CO
2
climate could create climate conditions more like those of a low-CO
2
climate (Moreno-Cruz et al., 2011; MacMartin et al., 2013), but the
annual mean, seasonality, and interannual variability of climate would
be modified compared to the pre-industrial climate (Govindasamy and
Caldeira, 2000; Lunt etal., 2008; Robock etal., 2008; Ban-Weiss and
Caldeira, 2010; Moreno-Cruz etal., 2011; Schmidt etal., 2012; Kravitz
etal., 2013; MacMartin etal., 2013). SRM geoengineering that could
reduce global mean temperatures would reduce thermosteric sea-level
rise and would likely also reduce glacier and ice-sheet contributions to
sea-level rise (Irvine etal., 2009, 2012; Moore etal., 2010).
Model simulations suggest that SRM would result in substantially
altered global hydrological conditions, with uncertain consequences
for specific regional responses such as precipitation and evaporation in
monsoon regions (Bala etal., 2008; Schmidt etal., 2012; Kravitz etal.,
2013; Tilmes etal., 2013). In addition to the imperfect cancellation of
GHG-induced changes in the climate by SRM, CO
2
directly affects the
opening of plant stomata, and thus the rate of transpiration of plants
and in turn the recycling of water over continents, soil moisture, and
surface hydrology (Bala etal., 2007; Betts etal., 2007; Boucher etal.,
2009; Spracklen etal., 2012).
Due to these broadly altered conditions that would result from an
implementation of geoengineering, and based on experience from
studies of the detection and attribution of climate change, it may take
many decades of observations to be certain whether SRM is respon-
sible for a particular regional trend in climate (Stone etal., 2009; Mac-
Mynowski etal., 2011). These detection and attribution problems also
imply that field testing to identify some of the climate consequences of
SRM geoengineering would require deployment at a sizeable fraction
of full deployment for a period of many years or even decades (Robock
etal., 2010; MacMynowski etal., 2011).
It is important to note that in addition to affecting the planet’s climate,
many SRM methods could have serious non-climatic side-effects. Any
stratospheric aerosol injection would affect stratospheric chemistry
and has the potential to affect stratospheric ozone levels. Tilmes etal.
(2009) found that sulphate aerosol geoengineering could delay the
recovery of the ozone hole by decades (WG I Section7.7.2.1). Strato-
spheric aerosol geoengineering would scatter light, modifying the
optical properties of the atmosphere. This would increase the diffuse-
to-direct light ratio, which would make the sky appear hazier (Kravitz
etal., 2012), reduce the efficacy of concentrated solar power facili-
ties (Murphy, 2009), and potentially increase the productivity of some
plant species, and preferentially those below the canopy layer, with
unknown long-term ecosystem consequences (Mercado etal., 2009).
The installations and infrastructure of SRM geoengineering techniques
may also have some negative effects that may be particularly acute for
techniques that are spatially extensive, such as desert albedo geoengi-
neering. SRM would have very little effect on ocean acidification and
the other direct effects of elevated CO
2
concentrations that are likely to
pose significant risks (see WG I Section 6.5.4).
6�9�2�2 The relation of solar radiation management to
climate policy and transformation pathways
A key determinant of the potential role, if any, of SRM in climate policy
is that some methods might act relatively quickly. For example, strato-
spheric aerosol injection could be deployable within months to years,
if and when the technology is available, and the climate response to
the resulting changes in radiative forcing could occur on a timescale
of a decade or less (e. g. Keith, 2000; Matthews and Caldeira, 2007;
Royal Society, 2009; Swart and Marinova, 2010; Goes etal., 2011).
Mitigating GHG emissions would affect global mean temperatures
only on a multi-decadal to centennial time-scale because of the inertia
488488
Assessing Transformation Pathways
6
Chapter 6
in the carbon cycle (van Vuuren and Stehfest, 2013). Hence, it has been
argued that SRM technologies could potentially complement mitiga-
tion activities, for example, by countering global GHG radiative forcing
while mitigation activities are being implemented, or by providing a
back-up strategy for a hypothetical future situation where short-term
reductions in radiative forcing may be desirable (Royal Society, 2009;
Rickels etal., 2011). However, the relatively fast and strong climate
response expected from some SRM techniques would also impose
risks. The termination of SRM geoengineering forcing either by policy
choice or through some form of failure would result in a rapid rise
of global mean temperature and associated changes in climate, the
magnitude of which would depend on the degree of SRM forcing that
was being exerted and the rate at which the SRM forcing was with-
drawn (Wigley, 2006; Matthews and Caldeira, 2007; Goes etal., 2011;
Irvine etal., 2012; Jones etal., 2013). It has been suggested that this
risk could be minimized if SRM geoengineering was used moderately
and combined with strong CDR geoengineering and mitigation efforts
(Ross and Matthews, 2009; Smith and Rasch, 2012). The potential of
SRM to significantly impact the climate on short time-scales, at poten-
tially low cost, and the uncertainties and risks involved in this raise
important socio-political questions in addition to natural scientific and
technological considerations in the section above.
The economic analysis of the potential role of SRM as a climate change
policy is an area of active research and has, thus far, produced mixed
and preliminary results (see Klepper and Rickels, 2012). Estimates of
the direct costs of deploying various proposed SRM methods differ sig-
nificantly. A few studies have indicated that direct costs for some SRM
methods might be considerably lower than the costs of conventional
mitigation, but all estimates are subject to large uncertainties because
of questions regarding efficacy and technical feasibility (Coppock,
1992; Barrett, 2008; Blackstock etal., 2009; Robock etal., 2009; Pierce
etal., 2010; Klepper and Rickels, 2012; McClellan etal., 2012).
However, SRM techniques would carry uncertain risks, do not directly
address some impacts of anthropogenic GHG emissions, and raise a
range of ethical questions (see WGIII Section 3.3.8) (Royal Society, 2009;
Goes et al., 2011; Moreno-Cruz and Keith, 2012; Tuana et al., 2012).
While costs for the implementation of a particular SRM method might
potentially be low, a comprehensive assessment would need to consider
all intended and unintended effects on ecosystems and societies and
the corresponding uncertainties (Rickels etal., 2011; Goes etal., 2011;
Klepper and Rickels, 2012). Because most proposed SRM methods would
require constant replenishment and an increase in their implementation
intensity if emissions of GHGs continue, the result of any assessment
of climate policy costs is strongly dependent on assumptions about the
applicable discount rate, the dynamics of deployment, the implementa-
tion of mitigation, and the likelihood of risks and side-effects of SRM
(see Bickel and Agrawal, 2011; Goes etal., 2011). While it has been sug-
gested that SRM technologies may ‘buy time’ for emission reductions
(Rickels etal., 2011), they cannot substitute for emission reductions in
the long term because they do not address concentrations of GHGs and
would only partially and imperfectly compensate for their impacts.
The acceptability of SRM as a climate policy in national and interna-
tional socio-political domains is uncertain. While international com-
mitment is required for effective mitigation, a concern about SRM is
that direct costs might be low enough to allow countries to unilater-
ally alter the global climate (Bodansky, 1996; Schelling, 1996; Barrett,
2008). Barrett (2008) and Urpelainen (2012) therefore argue that SRM
technologies introduce structurally obverse problems to the ‘free-rider’
issue in climate change mitigation. Some studies suggest that deploy-
ment of SRM hinges on interstate cooperation, due to the complexity
of the climate system and the unpredictability of outcomes if states do
not coordinate their actions (Horton, 2011). In this case, the political
feasibility of an SRM intervention would depend on the ability of state-
level actors to come to some form of agreement.
The potential for interstate cooperation and conflict will likely depend
on the institutional context in which SRM is being discussed, as well
as on the relative importance given to climate change issues at the
national and international levels. Whether a broad international agree-
ment is possible is a highly contested subject (see Section 13.4.4)
(SRMGI, 2012). Several researchers suggest that a UN-based institu-
tional arrangement for decision making on SRM would be most effec-
tive (Barrett, 2008; Virgoe, 2009; Zürn and Schäfer, 2013). So far there
are no legally binding international norms that explicitly address SRM,
although certain general rules and principles of international law are
applicable (see WGII, Chapter 13, p.37). States parties to the UN Con-
vention on Biological Diversity have adopted a non-binding decision on
geoengineering that establishes criteria that could provide guidance for
further development of international regulation and governance (CBD
Decision IX / 16 C (ocean fertilization) and Decision X / 33(8)(w); see also
LC / LP Resolutions LC-LP.1(2008) and LC-LP.2(2010), preamble).
Commentators have identified the governance of SRM technologies
as a significant political and ethical challenge, especially in ensuring
legitimate decision making, monitoring, and control (Victor, 2008;
Virgoe, 2009; Bodansky, 2012). Even if SRM would largely reduce the
global temperature rise due to anthropogenic climate change, as cur-
rent modelling studies indicate, it would also imply a spatial and tem-
poral redistribution of risks. SRM thus introduces important questions
of intra- and intergenerational justice, both distributive and procedural
(see Wigley, 2006; Matthews and Caldeira, 2007; Goes et al., 2011;
Irvine etal., 2012; Tuana et al., 2012; Bellamy etal., 2012; Preston,
2013). Furthermore, since the technologies would not remove the need
for emission reductions, in order to to effectively ameliorate climate
change over a longer-term SRM regulation would need to be based
on a viable relation between mitigation and SRM activities, and con-
sider the respective and combined risks of increased GHG concentra-
tions and SRM interventions. The concern that the prospect of a viable
SRM technology may reduce efforts to mitigate and adapt has featured
prominently in discussions to date (Royal Society, 2009; Gardiner,
2011; Preston, 2013).
Whether SRM field research or even deployment would be socially and
politically acceptable is also dependent on the wider discursive con-
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Assessing Transformation Pathways
6
Chapter 6
text in which the topic is being discussed. Bellamy etal. (2013) show
that the success of mitigation policies is likely to have an influence on
stakeholder acceptability of SRM. While current evidence is limited to
few studies in a very narrow range of cultural contexts, in a first review
of early studies on perceptions of geoengineering, Corner etal. (2012)
find that participants of different studies tend to prefer CDR over SRM
and mitigation over geoengineering. Considerations that influence
opinions are, amongst others, the perceived ‘naturalness’ of a technol-
ogy, its reversibility, and the capacity for responsible and transparent
governance (Corner etal., 2012). Furthermore, the way that the topic is
framed in the media and by experts plays an important role in influenc-
ing opinions on SRM research or deployment (Luokkanen etal., 2013;
Scholte etal., 2013). The direction that future discussions may take is
impossible to predict, since deepened and highly differentiated informa-
tion is rapidly becoming available (Corner etal., 2012; Macnaghten and
Szerszynski, 2013).
6�9�3 Summary
Despite the assumption of some form of negative CO
2
emissions
in many scenarios, including those leading to 2100 concentrations
approaching 450 ppm CO
2
eq, whether proposed CDR or SRM geoen-
gineering techniques can actually play a useful role in transformation
pathways is uncertain as the efficacy and risks of many techniques are
poorly understood at present. CDR techniques aim to reduce CO
2
(or
potentially other GHG) concentrations. A broad definition of CDR would
cover afforestation and BECCS, which are sometimes classified as miti-
gation techniques, but also proposals that are very distinct from mitiga-
tion in terms of technical maturity, scientific understanding, and risks
such as ocean iron fertilization. The former are often included in current
integrated models and scenarios and are, in terms of their impact on
the climate, directly comparable with techniques that are considered
to be conventional mitigation, notably fossil CCS and bio-energy use.
Both BECCS and afforestation may play a key role in reaching low-GHG
concentrations, but at a large scale have substantial land-use demands
that may conflict with other mitigation strategies and societal needs
such as food production. Whether other CDR techniques would be able
to supplement mitigation at any significant scale in the future depends
upon efficacy, cost, and risks of these techniques, which at present are
highly uncertain. The properties of potential carbon storage reservoirs
are also critically important, as limits to reservoir capacity and longevity
will constrain the quantity and permanence of CO
2
storage. Further-
more, some CDR techniques such as ocean iron fertilization may pose
transboundary risks. The impacts of CDR would be relatively slow: cli-
mate effects would unfold over the course of decades.
In contrast to CDR, SRM would aim to cool the climate by shielding
sunlight. These techniques would not reduce elevated GHG concentra-
tions, and thus not affect other consequences of high-GHG concentra-
tions, such as ocean acidification. Some SRM proposals could potentially
cause a large cooling within years, much quicker than mitigation or CDR,
and a few studies suggest that costs might be considerably lower than
CDR for some SRM techniques. It has thus been suggested that SRM
could be used to quickly reduce global temperatures or to limit tempera-
ture rise while mitigation activities are being implemented. However,
to avoid warming, SRM would need to be maintained as long as GHG
concentrations remain elevated. Modelling studies show that SRM may
be able to reduce global average temperatures but would not perfectly
reverse all climatic changes that occur due to elevated GHG concentra-
tions, especially at local to regional scales. For example, SRM is expected
to weaken the global hydrological cycle with consequences for regional
precipitation patterns and surface hydrology, and is expected to change
the seasonality and variability of climate. Because the potential climate
impacts of any SRM intervention are uncertain and evidence is very lim-
ited, it is too early to conclude how effective SRM would be in reducing
climate risks. SRM approaches may also carry significant non-climatic
side-effects. For example, sulphate aerosol injection would modify
stratospheric chemistry, potentially reducing ozone levels, and would
change the appearance of the sky. The risks of SRM interventions and
large-scale experiments, alongside any potential benefits, raise a num-
ber of ethical and political questions that would require public engage-
ment and international cooperation to address adequately.
6.10 Gaps in knowledge
and data
The questions that motivate this chapter all address the broad char-
acteristics of possible long-term transformation pathways toward sta-
bilization of GHG concentrations. The discussion has not focused on
today’s global or country-specific technology strategies, policy strate-
gies, or other elements of a near-term strategy. It is therefore within
this long-term strategic context that gaps in knowledge and data
should be viewed.
Throughout this chapter, a number of areas of further development
have been highlighted. Several areas would be most valuable to fur-
ther the development of information and insights regarding long-term
transformation pathways. These include the following: development of
a broader set of socioeconomic and technological storylines to support
the development of future scenarios; scenarios pursuing a wider set
of climate goals including those related to temperature change; more
mitigation scenarios that include impacts from, and adaptations to, a
changing climate, including energy and land-use systems critical for
mitigation; expanded treatment of the benefits and risks of CDR and
SRM options; expanded treatment of co-benefits and risks of mitiga-
tion pathways; improvements in the treatment and understanding of
mitigation options and responses in end-use sectors in transforma-
tion pathways; and more sophisticated treatments of land use and
land use-based mitigation options in mitigation scenarios. In addition,
a major weakness of the current integrated modelling suite is that
regional definitions are often not comparable across models. An impor-
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Assessing Transformation Pathways
6
Chapter 6
tant area of advancement would be to develop some clearly defined
regional definitions that can be met by most or all models.
6.11 Frequently Asked
Questions
FAQ 6�1 Is it possible to bring climate change
under control given where we are
and what options are available to us?
What are the implications of delay-
ing mitigation or limits on technology
options?
Many commonly discussed concentration goals, including the goal
of reaching 450 ppm CO
2
eq by the end of the 21st century, are both
physically and technologically possible. However, meeting long-term
climate goals will require large-scale transformations in human societ-
ies, from the way that we produce and consume energy to how we
use the land surface, that are inconsistent with both long-term and
short-term trends. For example, to achieve a 450 ppm CO
2
eq concen-
tration by 2100, supplies of low-carbon energy — energy from nuclear
power, solar power, wind power, hydroelectric power, bioenergy, and
fossil resources with carbon dioxide capture and storage — might
need to increase five-fold or more over the next 40 years. The pos-
sibility of meeting any concentration goal therefore depends not just
on the available technologies and current emissions and concentra-
tions, but also on the capacity of human societies to bear the asso-
ciated economic implications, accept the associated rapid and large-
scale deployment of technologies, develop the necessary institutions
to manage the transformation, and reconcile the transformation with
other policy priorities such as sustainable development. Improvements
in the costs and performance of mitigation technologies will ease the
burden of this transformation. If the world’s countries cannot take on
sufficiently ambitious mitigation over the next 20 years, or obstacles
impede the deployment of important mitigation technologies at large
scale, goals such as 450 ppm CO
2
eq by 2100 may no longer be pos-
sible.
FAQ 6�2 What are the most important
technologies for mitigation? Is there a
silver bullet technology?
Limiting CO
2
eq concentrations will require a portfolio of options,
because no single option is sufficient to reduce CO
2
eq concentrations
and eventually eliminate net CO
2
emissions. A portfolio approach can
be tailored to local circumstances to take into account other priorities
such as those associated with sustainable development. Technology
options include a range of energy supply technologies such as nuclear
power, solar energy, wind power, and hydroelectric power, as well as
bioenergy and fossil resources with carbon dioxide capture and storage.
In addition, a range of end-use technologies will be needed to reduce
energy consumption, and therefore the need for low-carbon energy,
and to allow the use of low-carbon fuels in transportation, buildings,
and industry. Halting deforestation and encouraging an increase in for-
ested land will help to halt or reverse LUC CO
2
emissions. Furthermore,
there are opportunities to reduce non-CO
2
emissions from land use and
industrial sources. Many of these options must be deployed to some
degree to stabilize CO
2
eq concentrations. At the same time, although a
portfolio approach is necessary, if emissions reductions are too modest
over the coming two decades, it may no longer be possible to reach a
goal of 450 ppm CO
2
eq by the end of the century without large-scale
deployment of carbon dioxide removal technologies. Thus, while no
individual technology is sufficient, carbon dioxide removal technologies
could become necessary in such a scenario.
FAQ 6�3 How much would it cost to bring climate
change under control?
Aggregate economic mitigation cost metrics are an important criterion
for evaluating transformation pathways and can indicate the level of
difficulty associated with particular pathways. However, the broader
socio-economic implications of mitigation go beyond measures of
aggregate economic costs, as transformation pathways involve a range
of tradeoffs that link to other policy priorities. Global mitigation cost
estimates vary widely due to methodological differences along with
differences in assumptions about future emissions drivers, technolo-
gies, and policy conditions. Most scenario studies collected for this
assessment that are based on the idealized assumptions that all coun-
tries of the world begin mitigation immediately, there is a single global
carbon price applied to well-functioning markets, and key technologies
are available, find that meeting a 430 – 480 ppm CO
2
eq goal by cen-
tury’s end would entail a reduction in the amount global consumers
spend of 1 – 4 % in 2030, 2 – 6 % in 2050, and 3 – 11 % in 2100 rela-
tive to what would happen without mitigation. To put these losses in
context, studies assume that consumption spending might grow from
four- to over ten-fold over the century without mitigation. Less ambi-
tious goals are associated with lower costs this century. Substantially
higher and lower estimates have been obtained by studies that con-
sider interactions with pre-existing distortions, non-climate market
failures, and complementary policies. Studies explicitly exploring the
implications of less-idealized policy approaches and limited technol-
ogy performance or availability have consistently produced higher cost
estimates. Delaying mitigation would reduce near-term costs; however
subsequent costs would rise more rapidly to higher levels.
491491
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Chapter 6
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