1439
26
North America
Coordinating Lead Authors:
Patricia Romero-Lankao (Mexico), Joel B. Smith (USA)
Lead Authors:
Debra J. Davidson (Canada), Noah S. Diffenbaugh (USA), Patrick L. Kinney (USA), Paul Kirshen
(USA), Paul Kovacs (Canada), Lourdes Villers Ruiz (Mexico)
Contributing Authors:
William Anderegg (USA), Jessie Carr (USA), Anthony Cheng (USA), Thea Dickinson (Canada),
Ellen Douglas (USA), Hallie Eakin (USA), Daniel M. Gnatz (USA), Mary Hayden (USA),
Maria Eugenia Ibarraran Viniegra (Mexico), Blanca E. Jiménez Cisneros (Mexico), Rob de Loë
(Canada), Michael D. Meyer (USA), Catherine Ngo (USA), Amrutasri Nori-Sarma (India),
Greg Oulahen (Canada), Diana Pape (USA), Ana Peña del Valle (Mexico), Roger Pulwarty
(USA), Ashlinn Quinn (USA), Fabiola S. Sosa-Rodriguez (Mexico), Daniel Runfola (USA),
Landy Sánchez Peña (Mexico), Bradley H. Udall (USA), Fiona Warren (Canada),
Kate Weinberger (USA), Tom Wilbanks (USA)
Review Editors:
Ana Rosa Moreno (Mexico), Linda Mortsch (Canada)
Volunteer Chapter Scientist:
William Anderegg (USA)
This chapter should be cited as:
Romero-Lankao
, P., J.B. Smith, D.J. Davidson, N.S. Diffenbaugh, P.L. Kinney, P. Kirshen, P. Kovacs, and L. Villers Ruiz,
2014: North America. In: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects.
Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate
Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D. Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. Ebi,
Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)].
Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1439-1498.
26
1440
Executive Summary ......................................................................................................................................................... 1443
26.1. Introduction .......................................................................................................................................................... 1446
26.2. Key Trends Influencing Risk, Vulnerability, and Capacities for Adaptation .......................................................... 1448
26.2.1. Demographic and Socioeconomic Trends ........................................................................................................................................ 1448
26.2.1.1. Current Trends ................................................................................................................................................................. 1448
Box 26-1. Adapting in a Transboundary Context: The Mexico-USA Border Region ........................................................ 1448
26.2.1.2. Future Trends ................................................................................................................................................................... 1450
26.2.2. Physical Climate Trends .................................................................................................................................................................. 1452
26.2.2.1. Current Trends ................................................................................................................................................................. 1452
26.2.2.2. Climate Change Projections ............................................................................................................................................ 1454
26.3. Water Resources and Management ...................................................................................................................... 1456
26.3.1. Observed Impacts of Climate Change on Water Resources ............................................................................................................. 1456
26.3.1.1. Droughts and Floods ....................................................................................................................................................... 1456
26.3.1.2. Mean Annual Streamflow ................................................................................................................................................ 1456
26.3.1.3. Snowmelt ........................................................................................................................................................................ 1456
26.3.2. Projected Climate Change Impacts and Risks ................................................................................................................................. 1456
26.3.2.1. Water Supply ................................................................................................................................................................... 1456
26.3.2.2. Water Quality .................................................................................................................................................................. 1457
26.3.2.3. Flooding .......................................................................................................................................................................... 1457
26.3.2.4. Instream Uses .................................................................................................................................................................. 1458
26.3.3. Adaptation ..................................................................................................................................................................................... 1458
26.4. Ecosystems and Biodiversity ................................................................................................................................ 1458
26.4.1. Overview ........................................................................................................................................................................................ 1458
26.4.2. Tree Mortality and Forest Infestation .............................................................................................................................................. 1459
26.4.2.1. Observed Impacts ............................................................................................................................................................ 1459
26.4.2.2. Projected Impacts and Risks ............................................................................................................................................ 1459
26.4.3. Coastal Ecosystems ........................................................................................................................................................................ 1459
26.4.3.1. Observed Climate Impacts and Vulnerabilities ................................................................................................................. 1459
26.4.3.2. Projected Impacts and Risks ............................................................................................................................................ 1459
Box 26-2. Wildfires ......................................................................................................................................................... 1460
26.4.4. Ecosystems Adaptation, and Mitigation .......................................................................................................................................... 1460
26.5. Agriculture and Food Security .............................................................................................................................. 1462
26.5.1. Observed Climate Change Impacts ................................................................................................................................................. 1462
26.5.2. Projected Climate Change Risks ..................................................................................................................................................... 1462
Table of Contents
1441
North America Chapter 26
26
26.5.3. A Closer Look at Mexico ................................................................................................................................................................. 1463
26.5.4. Adaptation ..................................................................................................................................................................................... 1463
26.6. Human Health ....................................................................................................................................................... 1464
26.6.1. Observed Impacts, Vulnerabilities, and Trends ................................................................................................................................. 1464
26.6.1.1. Storm-Related Impacts .................................................................................................................................................... 1464
26.6.1.2. Temperature Extremes ..................................................................................................................................................... 1464
26.6.1.3. Air Quality ....................................................................................................................................................................... 1464
26.6.1.4. Pollen .............................................................................................................................................................................. 1465
26.6.1.5. Water-borne Diseases ...................................................................................................................................................... 1465
26.6.1.6. Vector-borne Diseases ..................................................................................................................................................... 1465
26.6.2. Projected Climate Change Impacts ................................................................................................................................................. 1465
26.6.3. Adaptation Responses .................................................................................................................................................................... 1466
26.7. Key Economic Sectors and Services ...................................................................................................................... 1466
26.7.1. Energy ............................................................................................................................................................................................ 1466
26.7.1.1. Observed Impacts ............................................................................................................................................................ 1466
26.7.1.2. Projected Impacts ............................................................................................................................................................ 1466
26.7.1.3. Adaptation ...................................................................................................................................................................... 1466
26.7.2. Transportation ................................................................................................................................................................................ 1467
26.7.2.1. Observed Impacts ............................................................................................................................................................ 1467
26.7.2.2. Projected Impacts ............................................................................................................................................................ 1467
26.7.2.3. Adaptation ...................................................................................................................................................................... 1467
26.7.3. Mining ............................................................................................................................................................................................ 1467
26.7.3.1. Observed Impacts ............................................................................................................................................................ 1467
26.7.3.2. Projected Impacts ............................................................................................................................................................ 1467
26.7.3.3. Adaptation ...................................................................................................................................................................... 1468
26.7.4. Manufacturing ................................................................................................................................................................................ 1468
26.7.4.1. Observed Impacts ............................................................................................................................................................ 1468
26.7.4.2. Projected Impacts ............................................................................................................................................................ 1468
26.7.4.3. Adaptation ...................................................................................................................................................................... 1468
26.7.5. Construction and Housing .............................................................................................................................................................. 1468
26.7.5.1. Observed Impacts ............................................................................................................................................................ 1468
26.7.5.2. Projected Impacts ............................................................................................................................................................ 1468
26.7.5.3. Adaptation ...................................................................................................................................................................... 1468
26.7.6. Insurance ........................................................................................................................................................................................ 1469
26.7.6.1. Observed Impacts ............................................................................................................................................................ 1469
26.7.6.2. Projected Impacts ............................................................................................................................................................ 1469
26.7.6.3. Adaptation ...................................................................................................................................................................... 1469
1442
Chapter 26 North America
26
26.8. Urban and Rural Settlements ................................................................................................................................ 1469
26.8.1. Observed Weather and Climate Impacts ......................................................................................................................................... 1469
26.8.2. Observed Factors and Processes Associated with Vulnerability ....................................................................................................... 1470
26.8.2.1. Urban Settlements ........................................................................................................................................................... 1470
26.8.2.2. Rural Settlements ............................................................................................................................................................ 1471
26.8.3. Projected Climate Risks on Urban and Rural Settlements ............................................................................................................... 1472
26.8.4. Adaptation ..................................................................................................................................................................................... 1472
26.8.4.1. Evidence of Adaptation ................................................................................................................................................... 1472
26.8.4.2. Opportunities and Constraints ......................................................................................................................................... 1473
Box 26-3. Climate Responses in Three North American Cities ....................................................................................... 1474
26.9. Federal and Subnational Level Adaptation ........................................................................................................... 1475
26.9.1. Federal Level Adaptation ................................................................................................................................................................ 1475
26.9.2. Subnational Level Adaptation ......................................................................................................................................................... 1475
26.9.3. Barriers to Adaptation .................................................................................................................................................................... 1476
26.9.4. Maladaptation, Trade-offs, and Co-benefits .................................................................................................................................... 1476
26.10.Key Risks, Uncertainties, Knowledge Gaps, and Research Needs ......................................................................... 1476
26.10.1. Key Multi-sectoral Risks ................................................................................................................................................................ 1476
26.10.2. Uncertainties, Knowledge Gaps, and Research Needs ................................................................................................................... 1477
References ....................................................................................................................................................................... 1478
Frequently Asked Questions
26.1: What impact are climate stressors having on North America? ....................................................................................................... 1478
26.2: Can adaptation reduce the adverse impacts of climate stressors in North America? ...................................................................... 1478
1443
North America Chapter 26
26
Executive Summary
Overview
North America’s climate has changed and some societally relevant changes have been attributed to anthropogenic causes (very
high confidence). {Figure 26-1} Recent climate changes and individual extreme events demonstrate both impacts of climate-
related stresses and vulnerabilities of exposed systems (very high confidence). {Figure 26-2}
Observed climate trends in North America
include an increased occurrence of severe hot weather events over much of the USA, decreases in frost days, and increases in heavy precipitation
over much of North America (high confidence). {26.2.2.1} The attribution of observed changes to anthropogenic causes has been established
for some climate and physical systems (e.g., earlier peak flow of snowmelt runoff and declines in the amount of water stored in spring snowpack
in snow-dominated streams and areas of western USA and Canada (very high confidence). {Figure 26-1} Evidence of anthropogenic climatic
influence on ecosystems, agriculture, water resources, infrastructure, and urban and rural settlements is less clearly established, though, in
many areas, these sectors exhibit substantial sensitivity to climate variability (high confidence). {26.3.1-2, 26.4.2.1-2, 26.4.3.1, 26.5.1, 26.7.1.1,
26.7.2, 26.8.1; Figure 26-2; Box 26-3}
Many climate stresses that carry risk—particularly related to severe heat, heavy precipitation, and declining snowpack—will
increase in frequency and/or severity in North America in the next decades (very high confidence). Global warming of approximately
2°C (above the preindustrial baseline) is very likely to lead to more frequent extreme heat events and daily precipitation extremes over most
areas of North America, more frequent low-snow years, and shifts toward earlier snowmelt runoff over much of the western USA and Canada.
{26.2.2.2} Together with climate hazards such as higher sea levels and associated storm surges, more intense droughts, and increased precipitation
variability, these changes are projected to lead to increased stresses to water, agriculture, economic activities, and urban and rural settlements
(high confidence). {26.3.2, 26.5.2, 26.7.1.2, 26.8.3} Global warming of approximately 4°C is very likely to cause larger changes in extreme heat
events, daily-scale precipitation extremes and snow accumulation and runoff, as well as emergence of a locally novel temperature regime
throughout North America. {26.2.2.2} This higher level of global temperature change is likely to cause decreases in annual precipitation over
much of the southern half of the continent and increases in annual precipitation over much of the northern half of the continent. {26.2.2.2} The
higher level of warming would present additional and substantial risks and adaptation challenges across a range of sectors (high confidence).
{26.3.3, 26.5.2, 26.6.2, 26.7.2.2, 26.8.3}
We highlight below key findings on impacts, vulnerabilities, projections, and adaptation responses relevant to specific North American sectors:
ecosystems, water, agriculture, human health, urban and rural settlements, infrastructure, and the economy. We then highlight challenges and
opportunities for adaptation, and future risks and adaptive capacity for three key climate-related risks.
Sector-Specific Climate Risks and Adaptation Opportunities
North American ecosystems are under increasing stress from rising temperatures, carbon dioxide (CO
2
) concentrations, and sea
levels, and are particularly vulnerable to climate extremes (very high confidence).
Climate stresses occur alongside other anthropogenic
influences on ecosystems, including land use changes, non-native species, and pollution, and in many cases will exacerbate these pressures
(very high confidence). {26.4.1, 26.4.3}. Evidence since the Fourth Assessment Report (AR4) highlights increased ecosystem vulnerability to
multiple and interacting climate stresses in forest ecosystems, through wildfire activity, regional drought, high temperatures, and infestations
(medium confidence); {26.4.2.1; Box 26-2} and in coastal zones due to increasing temperatures, ocean acidification, coral reef bleaching,
increased sediment load in runoff, sea level rise (SLR), storms, and storm surges (high confidence). {26.4.3.1} In the near term, conservation and
adaptation practices can buffer against climate stresses to some degree in these ecosystems, both through increasing system resilience, such as
forest management to reduce vulnerability to infestation, and in reducing co-occurring non-climate stresses, such as careful oversight of fishing
pressure (medium confidence). {26.4.4}
Water resources are already stressed in many parts of North America due to non-climate change anthropogenic forces, and are
expected to become further stressed due to climate change (high confidence). {26.3}
Decreases in snowpacks are already influencing
seasonal streamflows (high confidence). {26.3.1} Though indicative of future conditions, recent floods, droughts, and changes in mean flow
1444
Chapter 26 North America
26
conditions cannot yet be attributed to climate change (medium to high confidence). {26.3.1-2} The 21st century is projected to witness decreases
in water quality and increases in urban drainage flooding throughout most of North America under climate change as well as a decrease in
instream uses such as hydropower in some regions (high confidence). {26.3.2.2-4} In addition, there will be decreases in water supplies for
urban areas and irrigation in North America except in general for southern tropical Mexico, northwest coastal USA, and west coastal Canada
(high to medium confidence). {26.3.2.1} Many adaptation options currently available can address water supply deficits; adaptation responses
to flooding and water quality concerns are more limited (medium confidence). {26.3.3}
Effects of temperature and climate variability on yields of major crops have been observed (high confidence). {25.5.1} Projected
increases in temperature, reductions in precipitation in some regions, and increased frequency of extreme events would result in
net productivity declines in major North American crops by the end of the 21st century without adaptation, although the rate of
decline varies by model and scenario, and some regions, particularly in the north, may benefit (very high confidence). {26.5.2}
Given that North America is a significant source of global food supplies, projected productivity declines here may affect global food security
(medium confidence). At 2°C, adaptation has high potential to offset projected declines in yields for many crops, and many strategies offer
mitigation co-benefits; but effectiveness of adaptation would be reduced at 4°C (high confidence). {26.5.3} Adaptation capacity varies widely
among producers, and institutional support—currently lacking in some regions—greatly enhances adaptive potential (medium confidence).
{26.5.4}
Human health impacts from extreme climate events have been observed, although climate change-related trends and attribution
have not been confirmed to date. Extreme heat events currently result in increases in mortality and morbidity in North America (very high
confidence), with impacts that vary by age, location, and socioeconomic factors (high confidence). {26.6.1.2} Extreme coastal storm events can
cause excess mortality and morbidity, particularly along the East Coast of the USA, and the Gulf Coast of both Mexico and the USA (high
confidence). {26.6.1.1} A range of water-, food-, and vector-borne infectious diseases, air pollutants, and airborne pollens are influenced by
climate variability and change (medium confidence). {26.6.1.3-6} Further climate warming in North America will impose stresses on the health
sector through more severe extreme events such as heat waves and coastal storms, as well as more gradual changes in climate and CO
2
levels.
{26.6.2} Human health impacts in North America from future climate extremes can be reduced by adaptation measures such as targeted and
sustainable air conditioning, more effective warning and response systems, enhanced pollution controls, urban planning strategies, and resilient
health infrastructure (high confidence). {26.6.3}
Observed impacts on livelihoods, economic activities, infrastructure, and access to services in North American urban and rural
settlements have been attributed to SLR, changes in temperature and precipitation, and occurrences of such extreme events as
heat waves, droughts, and storms (high confidence). {26.8.2.1}
Differences in the severity of climate impacts on human settlements are
strongly influenced by context-specific social and environmental factors and processes that contribute to risk, vulnerability, and adaptive
capacity such as hazard magnitude, populations access to assets, built environment features, and governance (high confidence). {26.8.2.1-2}.
Some of these processes (e.g., the legacy of previous and current stresses) are common to urban and rural settlements, while others are more
pertinent to some types of settlements than others. For example, human and capital risks are highly concentrated in some highly exposed
urban locations, while in rural areas, geographic isolation and institutional deficits are key sources of vulnerability. Among the most vulnerable
are indigenous peoples due to their complex relationship with their ancestral lands and higher reliance on subsistence economies, and those
urban centers where high concentrations of populations and economic activities in risk-prone areas combine with several socioeconomic and
environmental sources of vulnerability (high confidence). {26.8.2.1-2} Although larger urban centers would have higher adaptation capacities,
future climate risks from heat waves, droughts, storms, and SLR in cities would be enhanced by high population density, inadequate infrastructures,
lack of institutional capacity, and degraded natural environments (medium evidence, high agreement). {26.8.3}
Much of North American infrastructure is currently vulnerable to extreme weather events and, unless investments are made to
strengthen them, would be more vulnerable to climate change (medium confidence).
Water resources and transportation infrastructure
are in many cases deteriorating, thus more vulnerable to extremes than strengthened ones (high confidence). Extreme events have caused
significant damage to infrastructure in many parts of North America; risks to infrastructure are particularly acute in Mexico but are a big
concern in all three countries (high confidence). {26.7}
1445
North America Chapter 26
26
Most sectors of the North American economy have been affected by and have responded to extreme weather, including hurricanes,
flooding, and intense rainfall (high confidence). {Figure 26-2}
Despite a growing experience with reactive adaptation, there are few
examples of proactive adaptation anticipating future climate change impacts, and these are largely found in sectors with longer term decision
making, including energy and public infrastructure. Knowledge about lessons learned and best adaptive practices by industry sector are not
well documented in the published literature. {26.7} There is an emerging concern that dislocation in one sector of the economy may have an
adverse impact on other sectors as a result of supply chain interdependency (medium confidence). {26.7} Slow-onset perils—such as SLR,
drought, and permafrost thaw—are an emerging concern for some sectors, with large regional variation in awareness and adaptive capacity
(medium confidence).
Adaptation Responses
Adaptation—including through technological innovation, institutional strengthening, economic diversification, and infrastructure
design—can help to reduce risks in the current climate, and to manage future risks in the face of climate change (medium
confidence). {26.8.4, 26.9.2}
There is increasing attention to adaptation among planners at all levels of government but particularly at the
municipal level, with many jurisdictions engaging in assessment and planning processes. These efforts have revealed the significant challenges
and sources of resistance facing planners at both the planning and implementation stages, particularly the adequacy of informational, institutional,
financial, and human resources, and lack of political will (medium confidence). {26.8.4.2, 26.9.3} Specific strategies introduced into policy to
date tend to be incremental rather than transformational. Fiscal constraints are higher for Mexican jurisdictions and sectors than for Canada or
the USA. The literature on sectoral-level adaptation is stronger in the areas of technological and engineering adaptation strategies than in social,
behavioral, and institutional strategies. Adaptation actions have the potential to result in synergies or trade-offs with mitigation and other
development actions and goals (high confidence). {26.8.4.2, 26.9.3}
1446
Chapter 26 North America
26
26.1. Introduction
This chapter assesses literature on observed and projected impacts,
vulnerabilities, and risks as well as on adaptation practices and options
in three North American countries: Canada, Mexico, and the USA. The
North American Arctic region is assessed in Chapter 28: Polar Regions.
North America ranges from the tropics to frozen tundra, and contains a
diversity of topography, ecosystems, economies, governance structures,
and cultures. As a result, risk and vulnerability to climate variability and
change differ considerably across the continent depending on geography,
scale, hazard, socio-ecological systems, ecosystems, demographic sectors,
cultural values, and institutional settings. This chapter seeks to take
account of this diversity and complexity as it affects and is projected to
affect vulnerabilities, impacts, risks, and adaptation across North America.
No single chapter would be adequate to cover the range and scope of the
literature about climate change vulnerabilities, impacts, and adaptations
in the three focus countries of this assessment. (Interested readers are
encouraged to review these reports: Lemmen et al., 2008; INECC and
SEMARNAT, 2012a; NCADAC, 2013.) We therefore attempt to take a more
integrative and innovative approach. In addition to describing current and
future climatic and socioeconomic trends of relevance to understanding
risk and vulnerability in North America (Section 26.2), we contrast climate
impacts, vulnerabilities, and adaptations across and within the three
countries in the following key sectors: water resources and management
(Section 26.3); ecosystems and biodiversity (Section 26.4); agriculture
and food security (Section 26.5); human health (Section 26.6); and key
economic sectors and services (Section 26.7). We use a comparative and
place-based approach to explore the factors and processes associated
with differences and commonalities in vulnerability, risk, and adaptation
between urban and rural settlements (Section 26.8); and to illustrate
and contrast the nuanced challenges and opportunities adaption entails
at the city, subnational, and national levels (Sections 26.8.4, 26.9; Box
26-3). We highlight two case studies that cut across sectors, systems, or
national boundaries. The first, on wildfires (Box 26-2), explores some of
the connections between climatic and physical and socioeconomic
process (e.g., decadal climatic oscillation, droughts, wildfires land use,
and forest management) and across systems and sectors (e.g., fires direct
and indirect impacts on local economies, livelihoods, built environments,
and human health). The second takes a look at one of the worlds
longest borders between a high-income (USA) and middle-income
country (Mexico) and briefly reflects on the challenges and opportunities
of responding to climate change in a transboundary context (Box 26-1).
We close with a section (26.10) summarizing key multi-sectoral risks
and uncertainties and discussing some of the knowledge gaps that will
need to be filled by future research.
Findings from the Fourth Assessment Report
This section summarizes key findings on North America, as identified in
Chapter 13 of the Fourth Assessment Report (AR4) focused on Mexico
(Magrin et al., 2007) and Chapter 14 on Canada and the USA (Field et
al., 2007). It focuses on observed and projected impacts, vulnerabilities,
and risks, as well as on adaptation practices and options, and highlights
areas of agreement and difference between the AR4’s two chapters and
our consolidated North American chapter.
Observed Impacts and Processes Associated with Vulnerability
Both WGII AR4 Section 14.2 and our chapter (Figure 26-2) find that, over
the past decades, economic damage from severe weather has increased
dramatically. Our chapter confirms that although Canada and the USA
have considerably more adaptive capacity than Mexico, their vulnerability
depends on the effectiveness and timing of adaptation and the distribution
of capacity, which vary geographically and between sectors (WGII AR4
Sections 14.2.6, 14.4-5; Sections 26.2.2, 26.8.2).
WGII AR4 Chapters 13 and 14 did not assess impacts, vulnerabilities,
and risks in urban and rural settlements, but rather assessed literature
on future risks in the following sectors:
Ecosystems: Both AR4 and our chapter find that ecosystems are under
increased stress from increased temperatures, climate variability,
and other climate stresses (e.g., sea level rise (SLR) and storm-surge
flooding), and that these stresses interact with developmental and
environmental stresses (e.g., as salt intrusion, pollution, population
growth, and the rising value of infrastructure in coastal areas) (WGII
AR4 Sections 13.4.4, 14.2.3, 14.4.3). Differential capacities for range
shifts and constraints from development, habitat fragmentation,
invasive species, and broken ecological connections would alter
ecosystem structure, function, and services in terrestrial ecosystems
(WGII AR4 Sections 14.2, 14.4). Both reports show that dry soils
and warm temperatures are associated with increased wildfire
activity and insect outbreaks in Canada and the USA (WGII AR4
Sections 14.2, 14.4; Section 26.4.2.1).
Water resources: AR4 projects millions in Mexico to be at risk from
the lack of adequate water supplies due to climate change (WGII AR4
Section 13.4.3); our chapter, however, finds that water resources
are already stressed by non-climatic factors, such as population
pressure that will be compounded by climate change (Section 26.3.1).
Both reports find that in the USA and Canada rising temperatures
would diminish snowpack and increase evaporation (Section 26.2.2.1),
thus affecting seasonal availability of water (WGII AR4 Section
14.2.1; Section 26.3.1). The reports also agree that these effects
will be amplified by water demand from economic development,
agriculture, and population growth, thus imposing further constraints
to over-allocated water resources and increasing competition
among agricultural, municipal, industrial, and ecological uses (WGII
AR4 Sections 14.4.1, 14.4.6; Section 26.3.3). Both agree water quality
will be further stressed (WGII AR4 Sections 13.4.3, 14.4.1; Section
26.3.2.2). There is more information available now on water
adaptation than in AR4 (WGII AR4 Sections 13.5.1.3, 14.5.1;
Section 26.3.3), and it is possible to attribute changes in extreme
precipitation, snowmelt, and snowpack to climate change (WGII
AR4 Sections 13.2.4, 14.2.1; Section 26.3.1).
Agriculture: The AR4 noted that while increases in grain yields in
the USA and Canada are projected by most scenarios (WGII AR4
Section 14.4.4), in Mexico the picture is mixed for wheat and maize,
with different projected impacts depending on scenario used (WGII
AR4 Section 13.4.2). Research since the AR4 has offered more
cautious projections of yield change in North America due to shifts
in temperature and precipitation, particularly by 2100; and significant
harvest losses due to recent extreme weather events have been
observed (Section 26.5.1). Furthermore, our chapter reports on recent
research that underscores the context-specific nature of adaptation
1447
North America Chapter 26
26
c
apacity and of institutional support and shows that these factors,
which greatly enhance adaptive potential, are currently lacking in
some regions (Section 26.5.3).
Health: AR4 focused primarily on a set of future health risks. These
include changes in the geographical distribution and transmission
of diseases such as dengue (WGII AR4 Section 13.4.5) and increases
in respiratory illness, including exposure to pollen and ozone (WGII
A
R4 Section 14.4) and in mortality from hot temperatures and
extreme weather in Canada and the USA. AR4 also projects that
climate change impacts on infrastructure and human health in cities
of Canada and the USA would be compounded by aging infrastructure,
maladapted urban form and building stock, urban heat islands, air
pollution, population growth, and an aging population (WGII AR4
Sections 14.4-5). Without increased investments in measures such
Understanding causes of trends
(a) Degree of understanding of causes of changes
in climatic extreme events in the USA
(b) Degree of understanding of the climate
influence in key impacts in North America
More knowledgeLess knowledge
Adequacy of data to detect trends
Less knowledge More knowledge
Understanding of climate Influence
More knowledgeLess knowledge
Adequacy of data to detect trends
Less knowledge More knowledge
1. Earlier peak flow of snowmelt runoff in snow-dominated streams and rivers in western North
America (Section 26.3.1)
2. Declines in the amount of water stored in spring snowpack in snow-dominated areas of western
North America (Section 26.3.1)
3. Northward and upward shifts in species’ distributions in multiple taxa of terrestrial species, although
not all taxa and regions (Section 26.4.1),
4. Increases in coastal flooding (Section 26.8.1)
5. Increases in wildfire activity, including fire season length and area burned by wildfires in the western
USA and boreal Canada (Box 26-2)
6. Storm-related disaster losses in the USA (most of the increase in insurance claims paid has been
attributed to increasing exposure of people and assets in areas of risk; Sections 26.7.6.1, 26.8.1)
7. Increases in bark beetle infestation levels in pine tree species in western North America (Section
26.4.2.1)
8. Yield increases due in part to increasing temperatures in Canada and higher precipitation in the USA;
yield variances attributed to climate variability in Ontario and Quebec; yield losses attributed to
climate-related extremes across North America (Section 26.5.1)
9. Increases in tree mortality rates in old-growth forests in the western USA and western Canada from
1960 to 2007 (Section 26.4.2.1)
10. Changes in flooding in some urban areas due to extreme rainfall (Sections 26.3.1, 26.8.2.1)
Trend detected and attributed
11. Changes in storm-related mortality in the USA (Section 26.6.1.2)
12.
Changes in heat-related mortality in the USA (Section 26.6.1.2)
13. Increase in water supply shortages due to drought (Sections 26.3, 26.8.1)
14. Changes in cold-related mortality (Section 26.6.1.2)
Trend not detected
Trend detected but not attributed
Extreme precipitation
Heat waves
C
old waves
Hail
Tornadoes
H
urricanes
Extreme snow
Ocean waves
Floods
Droughts
T
hunderstorm winds
Ice
Winds
Extreme extratropical
cyclones
1
7
2
6
458
3
9
14
13
11 12 10
Figure 26-1 | (a) Detection and attribution of climate change impacts. Comparisons of the adequacy of currently available data to detect trends and the degree of understanding
of causes of those changes in climatic extreme events in the USA (Peterson et al., 2013), and (b) degree of understanding of the climate influence in key impacts in North
America. Note that “climate influence” means that the impact has been documented to be sensitive to climate, not that it has been attributed to climate change. Red circles
indicate that formal detection and attribution to climate change has been performed for the given impact; yellow circles indicate that a trend has been detected from background
variability in the given impact, but formal attribution to climate change has not occurred and the trend could be due to other drivers; and white circles indicate that a trend has
not currently been detected.
1448
Chapter 26 North America
26
a
s early warning and surveillance systems, air conditioning, and
access to health care, hot temperatures and extreme weather in
Canada and the USA are predicted to result in increased adverse
health impacts (WGII AR4 Sections 14.4-5). Our chapter provides
a more detailed assessment of these future risks (Section 26.6),
besides assessing a richer literature on observed health impacts
(Section 26.6.1).
Adaptation: AR4 found that Mexico has early warning and risk
management systems, yet it faces planning and management barriers.
In Canada and the USA, a decentralized response framework has
resulted in adaptation that tends to be reactive, unevenly distributed,
and focused on coping with rather than preventing problems (WGII
AR4 Section 14.5). Both chapters see “mainstreaming” climate issues
into decision making as key to successful adaptation (WGII AR4
Sections 13.5, 14.5). The current chapter provides a summary of
the growing empirical literature on emerging opportunities and
constraints associated with recent institutional adaptation planning
activities since the AR4 (Sections 26.3.3, 26.4.4, 26.5.4, 26.6.3,
26.8.4, 26.9).
In summary, scholarship on climate change impacts, adaptation, and
vulnerability has grown considerably since the AR4 in North America,
particularly in Canada and the USA. It is possible now not only to detect
and attribute to anthropogenic climate change some impacts such as
changes in extreme precipitation, snowmelt, and snowpack, but also to
examine trends showing increased insect outbreaks, wildfire events, and
c
oastal flooding. These latter trends have been shown to be sensitive
to climate, but, like the local climate patterns that cause them, have
not yet been positively attributed to anthropogenic climate change (see
Figure 26-1).
26.2. Key Trends Influencing Risk, Vulnerability,
and Capacities for Adaptation
26.2.1. Demographic and Socioeconomic Trends
26.2.1.1. Current Trends
Canada, Mexico, and USA share commonalities but also differ in key
dimensions shaping risk, vulnerability, and adaptation such as population
dynamics, economic development, and institutional capacity. During the
last years, the three countries, particularly the USA, have suffered economic
losses from extreme weather events (Figure 26-2). Hurricanes, droughts,
floods, and other climate-related hazards produce risk as they interact
with increases in exposed populations, infrastructure, and other assets
and with the dynamics of such factors shaping vulnerability as wealth,
population size and structure, and poverty (Figures 26-2 and SPM.1).
Population growth has been slower in Canada and USA than in Mexico
(UN DESA Population Division, 2011). Yet population growth in Mexico
also decreased from 3.4% between 1970 and 1980 to 1.5% yearly during
2000–2010. Populations in the three countries are aging at different
Box 26-1 | Adapting in a Transboundary Context: The Mexico-USA Border Region
Extending over 3111 km (1933 miles; U.S. Census Bureau, 2011), the border between the USA and Mexico, which can be defined in
different ways (Varady and Ward, 2009), illustrates the challenges and opportunities of responding to climate change in a transboundary
context. Changing regional climate conditions and socioeconomic processes combined shape differentiated vulnerabilities of exposed
populations, infrastructure, and economic activities.
Since at least 1999, the region has experienced high temperatures and aridity anomalies leading to drought conditions (Woodhouse
et al., 2010; Wilder et al., 2013) affecting large areas on both sides of the border, and considered the most extreme in over a century
of recorded precipitation patterns for the area (Cayan et al., 2010; Seager and Vecchi, 2010; Nielsen-Gammon, 2011). Streamflow in
already oversubscribed rivers such as the Colorado and Rio Grande (Nakaegawa et al., 2013) has decreased. Climatological conditions
for the area have been unprecedented, with sustained high temperatures that may have exceeded any experienced for 1200 years.
Although these changes cannot conclusively be attributed to anthropogenic climate change, they are consistent with climate change
projections (Woodhouse et al., 2010).
The population of the Mexico-USA border is rapidly growing and urbanizing, doubling from just under 7 million in 1983 to more than
15 million in 2012 (Peach and Williams, 2000). Since 1994, rapid growth in the area has been fueled by rapid economic development
subsequent to passage of the North American Free Trade Agreement (NAFTA). Between 1990 and 2001 the number of assembly
factories or maquiladoras in Mexico grew from 1700 to nearly 3800, with 2700 in the border area. By 2004, it was estimated that
more than 1 million Mexicans were employed in more than 3000 maquiladoras located along the border (Border Indicators Task
Force, 2011; EPA and SEMARNAT, 2012).
Continued next page
1449
North America Chapter 26
26
rates (Figure 26-2). In 2010, 14.1% of the population in Canada was
60 years and older, compared to 12.7% in the USA and 6.1% in Mexico
(UN DESA Population Division, 2011). Urban populations have grown
faster than rural populations, resulting in a North America that is highly
urbanized (Canada 84.8%, Mexico 82.8%, and USA 85.8%). Urban
populations are also expanding into peri-urban spaces, producing rapid
changes in population and land use dynamics that can exacerbate risks
from such hazards as floods and wildfires (Eakin et al., 2010; Romero-
Lankao et al., 2012a). Mexico has a markedly higher poverty rate (34.8%)
than Canada (9.1%) and the USA (12.5%) (Figure 26-2), with weather
events and climate affecting poor people’s livelihood assets, including
crop yields, homes, food security, and sense of place (Chapter 13;
Section 26.8.2). Between 1970 and 2012, a 10% increase in single-
person households—who can be vulnerable because of isolation and low
income and housing quality (Roorda et al., 2010)—has been detected
in the USA (Vespa et al., 2013).
While concentrations of growing populations, water, sanitation,
transportation and energy infrastructure, and industrial and service
sectors in urban areas can be a source of risk, geographic isolation and
high dispersion of rural populations also introduce risk because of long
distances to essential services (Section 26.8.2). Rural populations are
more vulnerable to climate events due to smaller labor markets, lower
income levels, and reduced access to public services. Rural poverty could
also be aggravated by changes in agricultural productivity, particularly
in Mexico, where 65% of the rural population is poor, agricultural income
is seasonal, and most households lack insurance (Scott, 2007). Food price
increases, which may also result from climate events, would contribute
to food insecurity (Lobell et al., 2011; World Bank, 2011).
Migration is a key trend affecting North America, recently with movements
between urban centers and from rural Mexico into Mexico’s cities, and
in the USA. Rates of migration from rural Mexico are positively associated
with natural disaster occurrence and increased poverty trends (Saldaña-
Zorilla and Sandberg, 2009), and with decreasing precipitation (Nawrotski
et al., 2013). Studies of migration induced by past climate variability
and change indicate a preference for short-range domestic movement,
a complex relationship to assets with indications that the poorest are
Box 26-1 (continued)
Notwithstanding this growth, challenges to adaptive capacity include high rates of poverty in a landscape of uneven economic
development (Wilder et al., 2013). Large sections of the urban population, particularly in Mexico, live in informal housing lacking the
health and safety standards needed to respond to hazards, and with no insurance (Collins et al., 2011). Any effort to increase regional
capacity to respond to climate needs to take existing gaps into account. In addition, there is a prevalence of incipient or actual conflict
(Mumme, 1999), given by currently or historically contested allocation of land and water resources (e.g., an over-allocated Colorado
River ending in Mexico above the Sea de Cortes (Getches, 2003)). Climate change, therefore, would bring additional significant
consequences for the region’s water resources, ecosystems, and rural and urban settlements.
The impacts of regional climatic and non-climatic stresses compound existing urban vulnerabilities that are different across countries.
For instance, besides degrading highly diverse ecosystems (Wilder et al., 2013), residential growth in flood-prone areas in Ciudad
Juárez has not been complemented with the provision of determinants of adaptive capacity to residents, such as housing, health
care, and drainage infrastructure. As a result, although differences in mean hazard scores are not significant between Ciudad Juárez
(Mexico) and El Paso (USA), social vulnerability and average risk are three times and two times higher in Ciudad Juárez than in El
Paso respectively (Collins, 2008).
Projected warming and drying would impose additional burdens on already stressed water resources and ecosystems and compound
existing vulnerabilities for populations, infrastructure, and economic activities (Wilder et al., 2013). The recent drought in the region
illustrated the multiple dimensions of climate-related events, including notable negative impacts on the agricultural sector, water
supplies, food security, and risk of wildfire (discussed in Box 26-2) (Wehner et al., 2011; Hoerling et al., 2012; Schwalm et al., 2012).
Adaptation opportunities and constraints are shared across international borders, creating the need for cooperation among local,
national, and international actors. Although there are examples of efforts to manage transborder environmental issues, such as the
USA-Mexico International Boundary and Water Commission agreement (United States and Mexico International Boundary and Water
Commission, 2012), constraints to effective cooperation and collaboration include different governance structures (centralized in
Mexico, decentralized in the USA), institutional fragmentation, asymmetries in the use and dissemination of information, and language
(Wilder et al., 2010, 2013; Megdal and Scott, 2011).
1450
Chapter 26 North America
26
l
ess able to migrate, and the role of preexisting immigrant networks in
facilitating international migration (Oppenheimer, 2013).
North America has become more economically integrated following the
1994 North American Free Trade Agreement. Prior to a 2007–2008
reduction in trade, the three countries registered dynamic growth in
industry, employment, and global trade of agricultural and manufactured
goods (Robertson et al., 2009). Notwithstanding North America’s
economic dynamism, increased socioeconomic disparities (Autor et al.,
2
008) have affected such determinants of vulnerability as differentiated
human development and institutional capacity within and across
countries.
26.2.1.2. Future Trends
The North American population is projected to continue growing, reaching
between 531.8 (SRES B2) and 660.1 (A2) millionby 2050 (IIASA, 2007).
0
400 800 1200 1600200
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Continued next page
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(a) Significant weather events taking place during 1993–2012
Figure 26-2 | Extreme events illustrating vulnerabilities for Mexico, the USA, and Canada. This figure offers a graphic illustration of location of extreme events and relevant
vulnerability trends. The observed extreme events have not been attributed to anthropogenic climate change, yet they are climate-sensitive sources of impact illustrating vulnerability
of exposed systems, particularly if projected future increases in the frequency and/or intensity of such events should materialize. The figure contains three elements. (a) A map with
significant weather and climate events taking place during 1993–2012 (data derived from NatCatSERVICE, 2013). The categories “Severe storm” and “Winter storm” are
aggregations of multiple types of storms; e.g., hailstorms are shown as Winter storms and tornadoes as Severe storms. Boxed numbers refer to the years in which the extreme events
occurred. Hurricanes are placed offshore of the point of initial landfall, and placement of all other boxes (which may span multiple subnational jurisdictions) is weighted towards areas
with the highest expected impacts (defined by estimated affected populations when finer subnational detail was not available). The map includes only events with overall losses
US$1 billion in the USA, or ≥US$500 million in Mexico and Canada, adjusted to 2012 values; hence, it does not include events of small and medium impact. Additionally, losses do
not capture the impacts of disasters on populations’ livelihoods and well-being. (b) A map (facing page) with population density per ~0.0083
˚ gridbox at 1-km resolution highlighting
exposure and represented using 2011 Landsat data (Bright et al., 2012). Note that a ~0.0083
˚ grid box is approximately1 km
2
, but this approximation varies by latitude. (c)
Four panels (facing page) with trends in socio-demographic indicators used in the literature to measure vulnerability to hazards (Romero-Lankao et al., 2012b): poverty rates,
percentage of elderly, GDP per capita and total population (U.S. Census Bureau, 2011; Statistics Canada, 2012, CEPAL, 2013).
1451
North America Chapter 26
26
Figure 26-2 (continued)
Percent of people
0
2
4
6
8
10
12
14
1980 1985 1990 1995 2000 2005 2010
% of population older than 65
0
5
10
15
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40
45
50
1980 1985 1990 1995 2000 2005 2010
2012 US dollars (thousands)
0
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1980 1985 1990 1995 2000 2005 2010
People (millions)
Canada
United States
Mexico
0
0 –100
100–10,000
10,000–50,000
50,000 +
Population
(per ~0.0083˚ gridbox)
(b) Population density at 1 km resolution
(c) Trends in socioeconomic indicators
% of population below poverty line
Elderly population
Per capita gross domestic product
Total population
1452
Chapter 26 North America
26
T
he percentage of elderly people (older than 64 years) is also projected
to continue to increase, by 23.4 to 26.9% in Canada, 12.4 to 18.4% in
Mexico, and 17.3 to 20.9% in the USA by 2050 (B2 and A2, respectively)
(IIASA, 2007). The elderly are highly vulnerable to extreme weather
events (heat waves in particular, Figure 26-2) (Martiello and Giacchi,
2010; Diffenbaugh and Scherer, 2011; Romero-Lankao, 2012; White-
Newsome et al., 2012). Numbers of single-person households and female-
headed households—both of which are vulnerable because of low
income and housing quality—are anticipated to increase (Roorda et al.,
2010). Institutional capacity to address the demands posed by increasing
numbers of vulnerable populations may also be limited, with resulting
stress on health and the economy.
Three other shifts are projected to influence impacts, vulnerabilities, and
adaptation to climate change in North America: urbanization, migration,
and socioeconomic disparity. With small differences between countries,
both the concentration of growing populations in some urban areas and
the dispersion of rural populations are projected to continue to define
North America by 2050. Assuming no change in climate, between 2005
and 2030 the population of Mexico City Metro Area will increase by
17.5%, while between 2007 and 2030 available water will diminish by
11.2% (Romero-Lankao, 2010). Conversely, education, a key determinant
of adaptive capacity (Chapter 13), is expected to expand to low-income
households, minorities, and women, which could increase the coping
capacity of households and have a positive impact on economic growth
(Goujon et al., 2004). However, the continuation of current patterns of
economic disparity and poverty would hinder future adaptive capacity.
Inequality in Mexico is larger (Figure 26-2), having a Gini coefficient
(according to which the higher the number the higher economic disparity)
of 0.56, in contrast to 0.317 for Canada and 0.389 for the USA (OECD,
2010). Mexico is one of five countries in the world that is projected to
experience the highest increases in poverty due to climate-induced
extreme events (52% increase in rural households, 95.4% in urban wage-
labor households; Coupled Model Intercomparison Project Phase 3
(CMIP3), A2) (Ahmed et al., 2009).
Some studies project increased North American migration in response
to climate change. Feng, Krueger, and Oppenheimer (2010) estimated
the emigration of an additional 1.4 to 6.7 million Mexicans by 2080
based on projected maize yield declines, range depending on model
(B1, United Kingdom Meteorological Office (UKMO), and Geophysical
Fluid Dynamics Laboratory (GFDL)). Oppenheimer speculates that the
indirect impacts of migration “could be as substantial as the direct effects
of climate change in the receiving area, because the arrival migrants
can increase pressure on climate sensitive urban regions (Oppenheimer,
2013, p. 442).
26.2.2. Physical Climate Trends
Some processes important for climate change in North America are
assessed eslewhere in the Fifth Assessment Report, including WGI AR5
Chapter 2 (Observations: Atmosphere and Surface), WGI AR5 Chapter 4
(Observations: Cryosphere), WGI AR5 Chapter 12 (Long-term Climate
Change: Projections, Commitments, and Irreversibility), WGI AR5 Chapter
14 (Climate Phenomena and Their Relevance for Future Regional Climate
Change), WGI AR5 Annex I (Atlas of Global and Regional Climate
P
rojections), and Chapter 21 of this volume (Regional Context). In
addition, comparisons of emissions, concentrations, and radiative
forcing in the Representative Concentration Pathways (RCPs) and
Special Report on Emission Scenarios (SRES) scenarios can be found in
WGI AR5 Annex II (Climate System Scenario Tables).
2
6.2.2.1. Current Trends
It is very likely that mean annual temperature has increased over the
past century over most of North America (WGI AR5 Figure SPM.1b;
Figure 26-3). Observations also show increases in the occurrence of
severe hot events over the USA over the late 20th century (Kunkel et
al., 2008), a result in agreement with observed late-20th-century
increases in extremely hot seasons over a region encompassing
northern Mexico, the USA, and parts of eastern Canada (Diffenbaugh and
Scherer, 2011). These increases in hot extremes have been accompanied
by observed decreases in frost days over much of North America
(Alexander et al., 2006; Brown et al., 2010; see also WGI AR5 Section
2.6.1), decreases in cold spells over the USA (Kunkel et al., 2008; see
also WGI AR5 Section 2.6.1), and increasing ratio of record high to low
daily temperatures over the USA (Meehl et al., 2009). However, warming
has been less pronounced and less robust over areas of the central and
southeastern USA (e.g., Alexander et al., 2006; Peterson et al., 2008;
see also WGI AR5 Section 2.6.1; WGI AR5 Figure SPM.1b; Figure 26-3).
It is possible that this pattern of muted temperature change has been
influenced by changes in the hydrologic cycle (e.g., Pan et al., 2004;
Portmann et al., 2009), as well as by decadal-scale variability in the
ocean (e.g., Meehl et al., 2012; Kumar et al., 2013b).
It is very likely that annual precipitation has increased over the past
century over areas of the eastern USA and Pacific Northwest (WGI AR5
Figure 2.29; Figure 26-3). Observations also show increases in heavy
precipitation over Mexico, the USA, and Canada between the mid-20th
and the early 21st century (DeGaetano, 2009; Peterson and Baringer,
2009; Pryor et al., 2009; see also WGI AR5 Section 2.6.2). Observational
analyses of changes in drought are more equivocal over North America,
with mixed sign of trend in dryness over Mexico, the USA, and Canada
(Dai, 2011; Sheffield et al., 2012; see also WGI AR5 Section 2.6.2; WGI
AR5 Figure 2.42). There is also evidence for earlier occurrence of peak
flow in snow-dominated rivers globally (Rosenzweig, 2007; WGI AR5
Section 2.6.2). Observed snowpack and snow-dominated runoff have
been extensively studied in the western USA and western Canada, with
observations showing primarily decreasing trends in the amount of
water stored in spring snowpack from 1960 to 2002 (with the most
prominent exception being the central and southern Sierra Nevada;
Mote, 2006) and primarily earlier trends in the timing of peak runoff over
the 1948–2000 period (Stewart et al., 2006; WGI AR5 Section 4.5; WGI
AR5 Figure 4.21). Observations also show decreasing mass and length
of glaciers in North America (WGI AR5 Section 4.3; WGI AR5 Figures
4.9, 4.10, 4.11). Further, in assessing changes in the hydrology of the
western USA, it has been concluded that “up to 60% of the climate-related
trends of river flow, winter air temperature, and snowpack between
1950 and 1999 are human-induced” (Barnett et al., 2008, p. 1080).
Observational limitations prohibit conclusions about trends in severe
thunderstorms (WGI AR5 Section 2.6.2) and tropical cyclones (WGI AR5
1453
North America Chapter 26
26
Annual Precipitation
Change
Diagonal Lines
Trend not
statistically
significant
White
Insufficient
data
Solid Color
Strong
agreement
Very strong
agreement
Little or
no change
Gray
Divergent
changes
Solid Color
Significant
trend
Diagonal Lines
White Dots
Annual Temperature Change
late 21st century
mid 21st century
Difference from 19862005 mean (%)
Difference from 19862005 mean
(˚C)
Trend over 19012012
(˚C over period)
(mm/year per decade)
Trend in annual precipitation over 19512010
Figure 26-3 | Observed and projected changes in annual average temperature and precipitation. (Top panel, left) Map of observed annual average temperature change from
1901–2012, derived from a linear trend. [WGI AR5 Figures SPM.1 and 2.21] (Bottom panel, left) Map of observed annual precipitation change from 1951–2010, derived from a
linear trend. [WGI AR5 Figures SPM.2 and 2.29] For observed temperature and precipitation, trends have been calculated where sufficient data permit a robust estimate (i.e., only
for grid boxes with greater than 70% complete records and more than 20% data availability in the first and last 10% of the time period). Other areas are white. Solid colors
indicate areas where trends are significant at the 10% level. Diagonal lines indicate areas where trends are not significant. (Top and bottom panel, right) CMIP5 multi-model
mean projections of annual average temperature changes and average percent changes in annual mean precipitation for 2046–2065 and 2081–2100 under RCP2.6 and 8.5,
relative to 1986–2005. Solid colors indicate areas with very strong agreement, where the multi-model mean change is greater than twice the baseline variability (natural internal
variability in 20-yr means) and ≥90% of models agree on sign of change. Colors with white dots indicate areas with strong agreement, where ≥66% of models show change
greater than the baseline variability and ≥66% of models agree on sign of change. Gray indicates areas with divergent changes, where ≥66% of models show change greater
than the baseline variability, but <66% agree on sign of change. Colors with diagonal lines indicate areas with little or no change, where <66% of models show change greater
than the baseline variability, although there may be significant change at shorter timescales such as seasons, months, or days. Analysis uses model data and methods building
from WGI AR5 Figure SPM.8. See also Annex I of WGI AR5. [Boxes 21-2 and CC-RC]
20 0 20 40
5 0
5
2510
2.5
2.5 50
10
50
25
100
RCP8.5RCP2.6
0 2 4 6
late 21st century
mid 21st century
RCP8.5RCP2.6
1454
Chapter 26 North America
26
S
ection 2.6.3) over North America. The most robust trends in extratropical
cyclones over North America are determined to be toward more frequent
and intense storms over the northern Canadian Arctic and toward less
frequent and weaker storms over the southeastern and southwestern
coasts of Canada over the 1953–2002 period (Wang et al., 2006; see
also WGI AR5 Section 2.7.4).
WGI concludes that “Global mean sea level (GMSL) has risen by 0.19
(0.17 to 0.21) m over the period 1901–2010” and that “it is very likely
that the mean rate was 1.7 (1.5 to 1.9) mm yr
–1
between 1901 and 2010
and increased to 3.2 (2.8 to 3.6) mm yr
–1
between 1993 and 2010
(WGI AR5 Chapter 3 ES). In addition, observed changes in extreme sea
level have been caused primarily by increases in mean sea level (WGI
AR5 Section 3.7.5). Regional variations in the observed rate of SLR can
result from processes related to atmosphere and ocean variability (such
as lower rates along the west coast of the USA) or vertical land motion
(such as high rates along the US Gulf Coast), but the persistence of the
observed regional patterns is unknown (WGI AR5 Section 3.7.3).
26.2.2.2. Climate Change Projections
WGI AR5 Chapters 11 and 12 assess near- and long-term future climate
change, respectively. WGI AR5 Chapter 14 assesses processes that are
important for regional climate change, with WGI AR5 Section 14.8.3
focused on North America. Many of the WGI AR5 conclusions are drawn
from Annex I of the WGI contribution to the AR5.
The CMIP5 ensemble projects very likely increases in mean annual
temperature over North America, with very likely increases in temperature
over all land areas in the mid- and late-21st-century periods in RCP2.6
and RCP8.5 (Figure 26-3). Ensemble-mean changes in mean annual
temperature exceed 2°C over most land areas of all three countries in
the mid-21st-century period in RCP8.5 and the late-21st-century period
in RCP8.5, and exceed 4°C over most land areas of all three countries
in the late-21st-century period in RCP8.5. However, ensemble-mean
changes in mean annual temperature remain within 2°C above the late-
20th-century baseline over most North American land areas in both the
mid- and late-21st-century periods in RCP2.6. The largest changes in
mean annual temperature occur over the high latitudes of the USA and
Canada, as well as much of eastern Canada, including greater than 6°C
in the late-21st-century period in RCP8.5. The smallest changes in mean
annual temperature occur over areas of southern Mexico, the Pacific
Coast of the USA, and the southeastern USA.
The CMIP5 ensemble projects warming in all seasons over North America
beginning as early as the 2016–2035 period in RCP2.6, with the greatest
warming occurring in winter over the high latitudes (WGI AR5 Annex I;
Figure 26-3) (Diffenbaugh and Giorgi, 2012). The CMIP5 and CMIP3
ensembles suggest that the response of warm-season temperatures to
elevated radiative forcing is larger as a fraction of the baseline variability
than the response of cold-season temperatures (Diffenbaugh and
Scherer, 2011; Kumar et al., 2013b), and the CMIP3 ensemble suggests
that the response of temperature in low-latitude areas of North America
is larger as a fraction of the baseline variability than the response of
temperature in high-latitude areas (Diffenbaugh and Scherer, 2011). In
addition, CMIP3 and a high-resolution climate model ensemble suggest
t
hat the signal-to-noise ratio of 21st century warming is far greater over
the western USA, northern Mexico, and the northeastern USA than over
the central and southeastern USA (Diffenbaugh et al., 2011), a result
that is similar to the observed pattern of temperature trend significance
in the USA (Figure 26-3).
Most land areas north of 45°N exhibit likely or very likely increases in
mean annual precipitation in the late-21st-century period in RCP8.5
(Figure 26-3). The high-latitude areas of North America exhibit very likely
changes in mean annual precipitation throughout the illustrative RCP
periods, with very likely increases occurring in the mid-21st-century
period in RCP2.6 and becoming generally more widespread at higher
levels of forcing. In contrast, much of Mexico exhibits likely decreases
in mean annual precipitation beginning in the mid-21st-century period
in RCP8.5, with the area of likely decreases expanding to cover most of
Mexico and parts of the south-central and southwestern USA in the
late-21st-century period in RCP8.5. Likely changes in mean annual
precipitation are much less common at lower levels of forcing. For
example, likely changes in mean annual precipitation in the mid- and
late-21st-century periods in RCP2.6 are primarily confined to increases
over areas of Canada and Alaska, with no areas of Mexico and very few
areas of the contiguous USA exhibiting differences that exceed the
baseline variability in more than 66% of the models.
CMIP5 projects increases in winter precipitation over Canada and Alaska,
consistent with projections of a poleward shift in the dominant cold-
season storm tracks (Yin, 2005; see also WGI AR5 Section 14.8.3),
extratropical cyclones (Trapp et al., 2009), and areas of moisture
convergence (WGI AR5 Section 14.8.3), as well as with projections of a
shift toward positive North Atlantic Oscillation (NAO) trends (Hori et al.,
2007; see also WGI AR5 Section 14.8.3). CMIP5 also projects decreases
in winter precipitation over the southwestern USA and much of Mexico
associated with the poleward shift in the dominant stormtracks and the
expansion of subtropical arid regions (Seager and Vecchi, 2010; see WGI
AR5 Section 14.8.3). However, there are uncertainties in hydroclimatic
change in western North America associated with the response of the
tropical Pacific sea surface temperatures (SSTs) to elevated radiative
forcing (particularly given the influence of tropical SSTs on the Pacific
North American (PNA) pattern and north Pacific storm tracks; Cayan et
al., 1999; Findell and Delworth, 2010; Seager and Vecchi, 2010; see
also WGI AR5 Section 14.8.3), and not all CMIP5 models simulate the
observed recent hydrologic trends in the region (Kumar et al., 2013a).
For seasonal-scale extremes, CMIP5 projects substantial increases in the
occurrence of extremely hot seasons over North America in early, middle,
and late-21st-century periods in RCP8.5 (Diffenbaugh and Giorgi, 2012;
Figure 26-4). For example, during the 2046–2065 period in RCP8.5,
more than 50% of summers exceed the respective late-20th-century
maximum seasonal temperature value over most of the continent.
CMIP3 projects similar increases in extremely hot seasons, including
greater than 50% of summers exceeding a mid-20th-century baseline
throughout much of North America by the mid-21st-century in the A2
scenario (Duffy and Tebaldi, 2012), and greater than 70% of summers
exceeding the highest summer temperature observed on record over
much of the western USA, southeastern USA, and southern Mexico by
the mid-21st-century in the A2 scenario (Battisti and Naylor, 2009).
CMIP5 also projects substantial decreases in snow accumulation over
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North America Chapter 26
26
0 10 20 30 40 50
% of years
20 40 60 80 1000
% of years
% of years
% change in 20-year return value
–2 2 5 10 20–5 30
0
20 40 60
80
100
(c) Summer Extreme Dry
RCP8.5 2080–2099
(a) Summer Extreme Hot
RCP8.5 2046–2065
(d) March Extreme Low Snow
RCP8.5 2070–2099
(b) Extreme Precipitation
RCP4.5 2046–2065
Figure 26-4 | Projected changes in extremes in North America. (a) The percentage of years in the 2046–2065 period of Representative Concentration Pathway 8.5 in which the
summer temperature is greater than the respective maximum summer temperature of the 1986–2005 baseline period (Diffenbaugh and Giorgi, 2012). (b) The percentage
difference in the 20-year return value of annual precipitation extremes between the 2046–2065 period of RCP4.5 and the 1986–2005 baseline period (Kharin et al., 2013). The
hatching indicates areas where the differences are not significant at the 5% level. (c) The percentage of years in the 2080–2099 period of RCP8.5 in which the summer
precipitation is less than the respective minimum summer precipitation of the 1986–2005 baseline period (Diffenbaugh and Giorgi, 2012). (d) The percentage of years in the
2070–2099 period of RCP8.5 in which the March snow water equivalent is less than the respective minimum March snow water equivalent of the 1976–2005 period
(Diffenbaugh et al., 2012). The black (white) stippling indicates areas where the multi-model mean exceeds 1.0 (2.0) standard deviations of the multi-model spread. (a-d) The
RCPs and time periods are those used in the peer-reviewed studies in which the panels appear. The 2046–2065 period of RCP8.5 and the 2046–2065 period of RCP4.5 exhibit
global warming in the range of 2°C to 3°C above the preindustrial baseline (WGI AR5 Figure 12.40). The 2080–2099 and 2070–2099 periods of RCP8.5 exhibit global warming
in the range of 4°C to 5°C above the preindustrial baseline (WGI AR5 Figure 12.40).
1456
Chapter 26 North America
26
t
he USA and Canada (Diffenbaugh et al., 2012; Figure 26-4), suggesting
that the increases in cold-season precipitation over these regions reflect
a shift towards increasing fraction of precipitation falling as rain rather
than snow (Diffenbaugh et al., 2012). Over much of the western USA
and western Canada, greater than 80% of years exhibit March snow
amount that is less than the late-20th-century median value beginning
in the mid-21st-century period in RCP8.5, with the ensemble-mean
change exceeding 2 standard deviations of the ensemble spread.
Likewise, greater than 60% of years exhibit March snow amount that is
less than the late-20th-century minimum value in the late-21st-century
period in RCP8.5, with the ensemble-mean change exceeding 2 standard
deviations of the ensemble spread (Diffenbaugh and Giorgi, 2012; Figure
26-4). CMIP5 also projects increases in the occurrence of extremely dry
summer seasons over much of Mexico, the USA, and southern Canada
(Figure 26-4). The largest increases occur over southern Mexico, where
greater than 30% of summers in the late-21st-century period in RCP8.5
exhibit seasonal precipitation that is less than the late-20th-century
minimum summer precipitation.
For daily-scale extremes, almost all areas of North America exhibit very
likely increases of at least C in the warmest daily maximum temperature
by the late-21st-century period in RCP8.5. Likewise, most areas of Canada
exhibit very likely increases of at least 10°C in the coldest daily minimum
temperature by the late-21st-century period in RCP8.5, while most areas
of the USA exhibit very likely increases of at least 5°C and most areas
of Mexico exhibit very likely increases of at least 3°C (Sillmann et al.,
2013; see also WGI AR5 Figure 12.13). In addition, almost all areas of
North America exhibit very likely increases of 5 to 20% in the 20-year
return value of extreme precipitation by the mid-21st-century period in
RCP4.5 (Figure 26-4), while most areas of the USA and Canada exhibit
very likely increases of at least 5% in the maximum 5-day precipitation
by the late-21st-century period in RCP8.5 (Sillmann et al., 2013; see also
WGI AR5 Figure 12.13). Further, almost all areas of Mexico exhibit very
likely increases in the annual maximum number of consecutive dry days
by the late-21st-century period in RCP8.5 (Sillmann et al., 2013; see also
WGI AR5 Figure 12.13).
26.3. Water Resources and Management
Water withdrawals are exceeding stressful levels in many regions of
North America such as the southwestern USA, northern and central
Mexico (particularly Mexico City), southern Ontario, and the southern
Canadian Prairies (CONAGUA, 2010; Romero-Lankao, 2010; Sosa-
Rodriguez, 2010; Averyt et al., 2011; Environment Canada, 2013a).
Water quality is also a concern with 10 to 30% of the surface monitoring
sites in Mexico having polluted water (CONAGUA, 2010), and about 44%
of assessed stream miles and 64% of assessed lake areas in the USA not
clean enough to support their uses (EPA, 2004). Stations in Canada’s
16 most populated drainage basins reported at least fair quality, with
many reporting good or excellent quality (Environment Canada, 2013b).
In basins outside of the populated areas there are some cases of
declining water quality where impacts are related to resource extraction,
agriculture, and forestry (Hebben, 2009).
Water management infrastructure in most areas of North America is in
need of repair, replacement, or expansion (Section 26.7). Climate change,
l
and use changes and population growth, and demand increases will
add to these stresses (Karl et al., 2009).
26.3.1. Observed Impacts of Climate Change
on Water Resources
2
6.3.1.1. Droughts and Floods
As reported in WGI AR5 Chapter 10 and in Section 26.2.2.1, it is not
possible to attribute changes in drought frequency in North America to
anthropogenic climate change (Prieto-González et al., 2011; Axelson et al.,
2012; Orlowsky and Senevirantne, 2013; Figure 26-1). Few discernible
trends in flooding have been observed in the USA (Chapter 3). Changes in
the magnitude or frequency of flood events have not been attributed to
climate change. Floods are generated by multiple mechanisms (e.g., land
use, seasonal changes, and urbanization); trend detection is confounded
by flow regulation, teleconnections, and long-term persistence (Section
26.2.2.1; Collins, 2009; Kumar et al., 2009; Smith et al., 2010; Villarini
and Smith, 2010; Villarini et al., 2011; Hirsch and Ryberg, 2012; INECC
and SEMARNAT, 2012a; Prokoph et al., 2012; Peterson et al., 2013).
26.3.1.2. Mean Annual Streamflow
Whereas annual precipitation and runoff increases have been found in
the midwestern and northwestern USA, decreases have been observed
in southern states (Georgakakos et al., 2013). Chapter 3 notes the
correlation between changes in streamflow and observed regional
changes in temperature and precipitation. Kumar et al. (2009) suggest
that human activities have influenced observed trends in streamflow,
making attribution of changes to climate difficult in many watersheds.
Nonetheless, earlier peak flow of snowmelt runoff in snow-dominated
streams and rivers in western North America has been formally detected
and attributed to anthropogenic climate change (Barnett et al., 2008;
Das et al., 2011; Figure 26-1).
26.3.1.3. Snowmelt
Warm winters produced earlier runoff and discharge but less snow
water equivalent and shortened snowmelt seasons in many snow-
dominated areas of North America (Barnett et al., 2005; Rood et al.,
2008; Reba et al., 2011; see also Section 26.2.2; Chapter 3).
26.3.2. Projected Climate Change Impacts and Risks
26.3.2.1. Water Supply
Most of this assessment focuses on surface water as there are few
groundwater studies (Tremblay et al., 2011; Georgakakos et al., 2013).
Impacts and risks vary by region and model used.
In arid and semiarid western USA and Canada and in most of Mexico,
except the southern tropical area, water supplies are projected to be
further stressed by climate change, resulting in less water availability
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North America Chapter 26
26
a
nd increased drought conditions (Seager et al., 2007; Cayan et al.,
2010; MacDonald, 2010; Martínez Austria and Patiño mez, 2010;
Montero Martínez et al., 2010; CONAGUA, 2011; Prieto-González et al.,
2011; Bonsal et al., 2012; Diffenbaugh and Field, 2013; Orlowsky and
Seneviratne, 2013; Sosa-Rodriguez, 2013). Compounding factors include
saltwater intrusion, and increased groundwater and surface water
pollution (Leal Asencio et al., 2008).
In the southwest and southeast USA, ecosystems and irrigation are
projected to be particularly stressed by decreases in water availability
due to the combination of climate change, growing water demand, and
water transfers to urban and industrial users (Seager et al., 2009;
Georgakakos et al., 2013). In the Colorado River basin, crop irrigation
requirements for pasture grass are projected to increase by 20% by
2040 and by 31% by 2070 (Dwyer et al., 2012). In the Rio Grande basin,
New Mexico, runoff is projected to decrease by 8 to 30% by 2080 due
to climate change. Water transfers may entail significant transaction
costs associated with adjudication and potential litigation, and might
have economic, environmental, social, and cultural impacts that vary
by water user (Hurd and Coonrod, 2012). In Mexico, water shortages
combined with increased water demands are projected to increase
surface and groundwater over-exploitation (CONAGUA, 2011).
Other parts of North American are projected to have different climate
risks. The vulnerability of water resources over the tropical southern
region of Mexico is projected to be low for 2050: precipitation decreases
from 10 to 5% in the summer and no precipitation changes in the
winter. After 2050, greater winter precipitation is projected, increasing
the possibility of damaging hydropower and water storage dams by
floods, while precipitation is projected to decrease by 40 to 35% in the
summer (Martínez Austria and Patiño Gómez, 2010).
Throughout the 21st century, cities in northwest Washington are
projected to have drawdown of average seasonal reservoir