361
5
Coastal Systems
and Low-Lying Areas
Coordinating Lead Authors:
Poh Poh Wong (Singapore), Iñigo J. Losada (Spain)
Lead Authors:
Jean-Pierre Gattuso (France), Jochen Hinkel (Germany), Abdellatif Khattabi (Morocco),
Kathleen L. McInnes (Australia), Yoshiki Saito (Japan), Asbury Sallenger (USA)
Contributing Authors:
So-Min Cheong (Republic of Korea), Kirstin Dow (USA), Carlos M. Duarte (Australia/Spain),
Kristie L. Ebi (USA), Lucy Faulkner (UK), Masahiko Isobe (Japan), Jack Middelburg
(Netherlands), Susanne Moser (USA), Mark Pelling (UK), Edmund Penning-Rowsell (UK),
Sybil Seitzinger (USA), Marcel Stive (Netherlands), Richard S.J. Tol (Netherlands),
Athanasios Vafeidis (Greece/Germany)
Review Editors:
Robert J. Nicholls (UK), Filipe Santos (Portugal)
Volunteer Chapter Scientist:
Sara Amez (Spain)
This chapter should be cited as:
Wong
, P.P., I.J. Losada, J.-P. Gattuso, J. Hinkel, A. Khattabi, K.L. McInnes, Y. Saito, and A. Sallenger, 2014: Coastal
systems and low-lying areas. In: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A:
Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach,
M.D. Mastrandrea, 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. 361-409.
5
362
Executive Summary ........................................................................................................................................................... 364
5.1. Introduction ............................................................................................................................................................ 366
5.2. Coastal Systems ...................................................................................................................................................... 366
5.3. Drivers .................................................................................................................................................................... 367
5.3.1. Introduction ...................................................................................................................................................................................... 367
5.3.2. Relative Sea Level Rise ..................................................................................................................................................................... 367
5.3.2.1.Global Mean Sea Level ......................................................................................................................................................... 368
5.3.2.2.Regional Sea Level ............................................................................................................................................................... 369
5.3.2.3.Local Sea Level ..................................................................................................................................................................... 369
5.3.3. Climate-Related Drivers .................................................................................................................................................................... 370
5.3.3.1.Severe Storms ...................................................................................................................................................................... 370
5.3.3.2.Extreme Sea Levels ............................................................................................................................................................... 370
5.3.3.3.Winds and Waves ................................................................................................................................................................. 371
5.3.3.4.Sea Surface Temperature ...................................................................................................................................................... 371
5.3.3.5.Ocean Acidification .............................................................................................................................................................. 372
5.3.3.6.Freshwater Input .................................................................................................................................................................. 372
5.3.4. Human-Related Drivers ..................................................................................................................................................................... 372
5.3.4.1.Socioeconomic Development ............................................................................................................................................... 372
5.3.4.2.Nutrients .............................................................................................................................................................................. 373
5.3.4.3.Hypoxia ................................................................................................................................................................................ 373
5.3.4.4.Sediment Delivery ................................................................................................................................................................ 373
5.4. Impacts, Vulnerabilities, and Risks .......................................................................................................................... 374
5.4.1. Introduction ...................................................................................................................................................................................... 374
5.4.2. Natural Systems ................................................................................................................................................................................ 375
5.4.2.1.Beaches, Barriers, and Sand Dunes ....................................................................................................................................... 375
5.4.2.2.Rocky Coasts ........................................................................................................................................................................ 376
5.4.2.3.Wetlands and Seagrass Beds ................................................................................................................................................ 377
5.4.2.4.Coral Reefs ........................................................................................................................................................................... 378
5.4.2.5.Coastal Aquifers ................................................................................................................................................................... 379
5.4.2.6.Estuaries and Lagoons ......................................................................................................................................................... 379
5.4.2.7.Deltas ................................................................................................................................................................................... 380
Table of Contents
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Coastal Systems and Low-Lying Areas Chapter 5
5
5.4.3. Human Systems ................................................................................................................................................................................ 381
5.4.3.1.Human Settlements .............................................................................................................................................................. 381
5.4.3.2.Industry, Infrastructure, Transport, and Network Industries .................................................................................................. 383
5.4.3.3.Fisheries, Aquaculture, and Agriculture ................................................................................................................................. 384
5.4.3.4.Coastal Tourism and Recreation ........................................................................................................................................... 384
5.4.3.5.Health .................................................................................................................................................................................. 385
5.4.4. Summary: Detection and Attribution ................................................................................................................................................. 386
5.5. Adaptation and Managing Risks ............................................................................................................................. 386
5.5.1. Introduction ...................................................................................................................................................................................... 386
5.5.2. Adaptation Measures ....................................................................................................................................................................... 387
5.5.3. Adaptation Decision Making and Governance ................................................................................................................................. 388
5.5.3.1.Decision Analysis .................................................................................................................................................................. 388
5.5.3.2.Institution and Governance Analysis .................................................................................................................................... 388
Box 5-1. London’s Thames Estuary 2100 Plan: Adaptive Management for the Long Term .............................................. 389
5.5.4. Implementation and Practice ............................................................................................................................................................ 390
5.5.4.1.Frameworks .......................................................................................................................................................................... 390
5.5.4.2.Principles, Guidance, and Experiences .................................................................................................................................. 390
5.5.5. Global Adaptation Costs and Benefits .............................................................................................................................................. 392
5.5.6. Adaptation Opportunities, Constraints, and Limits ........................................................................................................................... 393
5.5.7. Synergies and Trade-Offs between Mitigation and Adaptation ......................................................................................................... 394
5.5.8. Long-Term Commitment to Sea Level Rise and Adaptation .............................................................................................................. 394
5.6. Information Gaps, Data Gaps, and Research Needs ............................................................................................... 395
References ......................................................................................................................................................................... 396
Frequently Asked Questions
5.1: How does climate change affect coastal marine ecosystems? .......................................................................................................... 374
5.2: How is climate change influencing coastal erosion? ........................................................................................................................ 376
5.3: How can coastal communities plan for and adapt to the impacts of climate change, in particular sea level rise? ........................... 387
364
Chapter 5 Coastal Systems and Low-Lying Areas
5
Executive Summary
Coastal systems are particularly sensitive to three key drivers related to climate change: sea level, ocean temperature, and ocean
acidity (very high confidence). {5.3.2, 5.3.3.4, 5.3.3.5} Despite the lack of attribution of observed coastal changes, there is a long-term
commitment to experience the impacts of sea level rise because of a delay in its response to temperature (high confidence). {5.5.8} In contrast,
coral bleaching and species ranges can be attributed to ocean temperature change and ocean acidity. {5.4.2.2, 5.4.2.4} For many other coastal
changes, the impacts of climate change are difficult to tease apart from human-related drivers (e.g., land use change, coastal development,
pollution) (robust evidence, high agreement).
Coastal systems and low-lying areas will increasingly experience adverse impacts such as submergence, coastal flooding, and
coastal erosion due to relative sea level rise (RSLR; very high confidence).
In the absence of adaptation, beaches, sand dunes, and cliffs
currently eroding will continue to do so under increasing sea level (high confidence). {5.4.2.1, 5.4.2.2} Large spatial variations in the projected
sea level rise together with local factors means RSLR at the local scale can vary considerably from projected global mean sea level rise (GMSLR)
(very high confidence). {5.3.2} Changes in storms and associated storm surges may further contribute to changes in sea level extremes but the
small number of regional storm surge studies, and uncertainty in changes in tropical and mid-latitude cyclones at the regional scale, means
that there is low confidence in projections of storm surge change {5.3.3.2} Both RSLR and impacts are also influenced by a variety of local
processes unrelated to climate (e.g., subsidence, glacial isostatic adjustment, sediment transport, coastal development) (very high confidence).
Acidification and warming of coastal waters will continue with significant negative consequences for coastal ecosystems (high
confidence). The increase in acidity will be higher in areas where eutrophication or coastal upwellings are an issue. It will have negative impacts
for many calcifying organisms (high confidence). {5.4.2.2} Warming and acidification will lead to coral bleaching, mortality, and decreased
constructional ability (high confidence), making coral reefs the most vulnerable marine ecosystem with little scope for adaptation. {5.4.2.4,
Box CC-OA} Temperate seagrass and kelp ecosystems will decline with the increased frequency of heat waves and sea temperature extremes as
well as through the impact of invasive subtropical species (high confidence). {5.4.2.3}
The population and assets exposed to coastal risks as well as human pressures on coastal ecosystems will increase significantly
in the coming decades due to population growth, economic development, and urbanization (high confidence).
The exposure of
people and assets to coastal risks has been rapidly growing and this trend is expected to continue. {5.3.4.1, 5.4.3.1} Humans have been the
primary drivers of changes in coastal aquifers, lagoons, estuaries, deltas, and wetlands (very high confidence) and are expected to further
exacerbate human pressures on coastal ecosystems resulting from excess nutrient input, changes in runoff, and reduced sediment delivery
(high confidence). {5.3.4.2, 5.3.4.3, 5.3.4.4}
For the 21st century, the benefits of protecting against increased coastal flooding and land loss due to submergence and erosion
at the global scale are larger than the social and economic costs of inaction (limited evidence, high agreement). Without adaptation,
hundreds of millions of people will be affected by coastal flooding and will be displaced due to land loss by year 2100; the majority of those
affected are from East, Southeast, and South Asia (high confidence). {5.3.4.1, 5.4.3.1} At the same time, protecting against flooding and erosion
is considered economically rational for most developed coastlines in many countries under all socioeconomic and sea level rise scenarios
analyzed, including for the 21st century GMSLR of above 1 m (limited evidence, high agreement). {5.5.5}
The relative costs of adaptation vary strongly between and within regions and countries for the 21st century (high confidence).
Some low-lying developing countries (e.g., Bangladesh, Vietnam) and small islands are expected to face very high impacts and associated
annual damage and adaptation costs of several percentage points of gross domestic product (GDP). {5.5.5} Developing countries and small
islands within the tropics dependent on coastal tourism will be impacted directly not only by future sea level rise and associated extremes but
also by coral bleaching and ocean acidification and associated reductions in tourist arrivals (high confidence). {5.4.3.4}
365
5
Coastal Systems and Low-Lying Areas Chapter 5
The analysis and implementation of coastal adaptation toward climate-resilient and sustainable coasts has progressed more
significantly in developed countries than in developing countries (high confidence).
Given ample adaptation options, more proactive
responses can be made and based on technological, policy related, financial, and institutional support. Observed successful adaptation includes
major projects (e.g., Thames Estuary, Venice Lagoon, Delta Works) and specific practices in both developed countries (e.g., Netherlands,
Australia) and developing countries (e.g., Bangladesh). {5.5.4.2} More countries and communities carry out coastal adaptation measures
including those based on integrated coastal zone management, local communities, ecosystems, and disaster reduction, and these measures are
mainstreamed into relevant strategies and management plans (high confidence). {5.5.4, 5.5.5}
366
Chapter 5 Coastal Systems and Low-Lying Areas
5
5.1. Introduction
This chapter presents an updated picture of the impacts, vulnerability,
and adaptation of coastal systems and low-lying areas to climate change,
w
ith sea level rise perceived as the most important risk for human
systems. Unlike the coastal chapter in the previous assessment (Fourth
Assessment Report, AR4), materials pertinent to the oceans are not
covered here but in two new ocean chapters (Chapters 6 and 30). As in
AR4, polar coasts are in another chapter (Chapter 28); small islands are
also considered separately (Chapter 29) so an in-depth discussion is not
provided herein.
The topics covered in this chapter follow the outline for sectoral chapters
approved by the IPCC. An Executive Summary summarizes the key
messages with a line of sight to the supporting sections in the chapter.
This chapter consists of six sections, with this first section dealing with
progress in knowledge from AR4 to AR5 (Fifth Assessment Report),
scope of chapter, and new developments. Section 5.2 defines the coastal
systems and climate and non-climate drivers. The coastal systems include
both natural systems and human systems, and this division is generally
followed throughout the chapter. The climate and non-climate drivers
are assessed in Section 5.3, followed by the impacts, vulnerabilities, and
risks in Section 5.4. Section 5.5 deals with adaptation and managing
risks. Information gaps, data gaps, and research needs are assessed in
Section 5.6. There is one box on a specific example and reference to
three cross-chapter boxes.
In AR4, the coastal chapter assessed the impact of climate change and
a global sea level rise up to 0.59 m in the 2090s. The coastal systems
were considered to be affected mainly by higher sea levels, increasing
temperatures, changes in precipitation, larger storm surges, and increased
ocean acidity. Human activities had continued to increase their pressure
on the coasts with rapid urbanization in coastal areas and growth of
megacities with consequences on coastal resources. Regionally, South,
Southeast, and East Asia; Africa; and small islands were identified as
most vulnerable. The AR4 chapter offered a range of adaptation
measures, many under the Integrated Coastal Zone Management (ICZM)
framework that could be carried out in both developed and developing
countries, but recognized that the latter would face more challenges.
Various issues on increasing the adaptive capacity or increasing the
resilience of coastal communities were discussed. The unavoidability of
sea level rise in the long term, even with stringent mitigation, was
noted, with adaptation becoming an urgent issue.
A number of key issues related to the coasts have arisen since AR4.
There is now better understanding of the natural systems, their ecosystem
functions, their services and benefits to humanity, and how they can be
affected by climate change. Their linkages landward to the watersheds
and seaward to the seas and oceans need to be considered for a more
integrated assessment of climate change impacts. The global mean sea
level rise (GMSLR) is projected to be 0.28 to 0.98 m by 2100 (Table 5-2),
although with regional variations and local factors the local sea level
rise can be higher than that projected for the GMSLR. This has serious
implications for coastal cities, deltas, and low-lying states. While higher
rates of coastal erosion are generally expected under rising sea levels, the
complex inter-relationships between the geomorphological and ecological
a
ttributes of the coastal system (Gilman et al., 2006; Haslett, 2009) and
the relevant climate and oceanic processes need to be better established
at regional and local scales. Such complex inter-relationships can be
influenced by different methods and responses of coastal management.
Also of concern is ocean acidification. Together with warming, it causes
coral reefs to lose their structural integrity, negatively implicating reef
communities and shore protection (Sheppard et al., 2005; Manzello et al.,
2008; see Boxes CC-OA, CC-CR). Acidification has potential impacts of
reduced calcification in shellfish and impacts on commercial aquaculture
(Barton et al., 2012). Since AR4, a significant number of new findings
regarding the impacts of climate change on human settlements and key
coastal systems such as rocky coasts, beaches, estuaries, deltas, salt
marshes, mangroves, coral reefs, and submerged vegetation have
become available and are reviewed in this chapter. However, uncertainties
regarding projections of potential impacts on coastal systems remain
generally high.
This chapter also provides advances in both vulnerability assessments
and the identification of potential adaptation actions, costs, benefits,
and trade-offs. A large number of new studies estimate the costs of
inaction versus potential adaptation. Coastal adaptation has become
more widely used, with a wider range of approaches and frameworks
such as integrated coastal management, ecosystem-based adaptation,
community-based adaptation, and disaster risk reduction and management.
Climate change will interact differently with the variety of human
activities and other drivers of change along coastlines of developed and
developing countries. For example, on the coastlines of developed
countries, changes in weather and climate extremes and sea level rise may
impact the demand for housing, recreational facilities, and construction
of renewable energy infrastructure on the coast (Hadley, 2009), including
critical infrastructures such as transportation, ports, and naval bases.
Along the coasts of developing countries, weather and climate extremes
affect a wide range of economic activities supporting coastal communities
and pose an additional risk to many of the fastest growing low-lying
urban areas, such as in Bangladesh and China (McGranahan et al., 2007;
Smith, 2011).
5.2. Coastal Systems
Coastal systems and low-lying areas, also referred to as coasts in this
assessment, include all areas near mean sea level. Generally, there is
no single definition for the coast and the coastal zone/area, where the
latter emphasizes the area or extent of the coastal ecosystems. In
relation to exposure to potential sea level rise, the low-elevation coastal
zone (LECZ) has been used in recent years with reference to specific
area and population up to 10 m elevation (Vafeidis et al., 2011).
Coastal systems are conceptualized to consist of both natural and
human systems (Figure 5-1). The natural systems include distinct coastal
features and ecosystems such as rocky coasts, beaches, barriers and
sand dunes, estuaries and lagoons, deltas, river mouths, wetlands, and
coral reefs. These elements help define the seaward and landward
boundaries of the coast. In spite of providing a wide variety of regulating,
provisioning, supporting, and cultural services (MEA, 2005), they have
367
Coastal Systems and Low-Lying Areas Chapter 5
5
been altered and heavily influenced by human activities, with climate
change constituting only one among many pressures these systems are
facing. The human systems include the built environment (e.g., settlements,
water, drainage, as well as transportation infrastructure and networks),
human activities (e.g., tourism, aquaculture, fisheries), as well as formal
and informal institutions that organize human activities (e.g., policies,
laws, customs, norms, and culture). The human and natural systems form
a tightly coupled socio-ecological system (Berkes and Folke, 1998;
Hopkins et al., 2012).
5.3. Drivers
5.3.1. Introduction
In AR4, changes in climate drivers (i.e., any climate-induced factor that
directly or indirectly causes a change), including sea level rise, were
projected for different Special Report on Emissions Scenarios (SRES)
emissions scenarios (IPCC, 2000). Consequently, to date, most of the
impacts and vulnerability assessments of climate change in coastal areas
are based on SRES A2, A1B, B2, and A1F1 scenarios. Since AR4 a new
scenario process has been initiated to replace the SRES scenarios with
Representative Concentration Pathways (RCPs) and Shared Socioeconomic
Pathways (SSPs) (Moss et al., 2010). The RCPs are scenarios specifying
concentrations, rather than emissions, thereby avoiding differences in
concentrations of long-lived greenhouse gas (GHG) and aerosol
concentrations for the same emissions scenarios that can arise from the
use of different models (van Vuuren et al., 2011). For a comparison
between RCP and SRES scenarios, see WGI AR5 Box 1.2. In addition,
Extended Concentration Pathways (ECPs) have been introduced for the
2100–2300 period (Meinhausen et al., 2011), providing the opportunity
to assess the long-term commitment to sea level rise, which is virtually
certain to continue beyond 2500 unless global temperature declines
(WGI AR5 Chapter 1; Section 13.5.2).
The SSPs provide representative qualitative story lines (narratives) of world
development together with quantitative pathways of key socioeconomic
variables such as gross domestic product (GDP) and population. A
preliminary list of five SSPs has been proposed (Arnell et al., 2011;
O’Neill et al., 2012), and work to further refine them is ongoing (Kriegler
et al. 2012; Van Vuuren et al., 2012). SSPs do not include assumptions
on mitigation policy and are thus independent from RCPs in the sense
that the same SSP may lead to different concentration levels and
consequently rises in sea level depending on the level of mitigation
reached (Arnell et al., 2011; O’Neill et al., 2012). Table 5-1 summarizes
the main climate-related drivers for the coastal systems.
5.3.2. Relative Sea Level Rise
Assessments of coastal impacts, vulnerability, and adaptation need to
consider relative sea level rise (RSLR), which includes climate-induced
GMSLR (Section 5.3.2.1) and regional variations (Section 5.3.2.2) as
well as local non-climate-related sea level changes (Section 5.3.2.3).
RSLR poses a significant threat to coastal systems and low-lying areas
around the globe, leading to inundation and erosion of coastlines and
contamination of freshwater reserves and food crops (Nicholls, 2010).
Sea level rise due to thermal expansion as the oceans warm, together
with meltwater from glaciers, icecaps, and ice sheets of Greenland and
Antarctica, are the major factors that contribute to RSLR globally.
However, regional variations in the rate of rise occur because of ocean
circulation patterns and interannual and decadal variability (e.g., Zhang
R
isk on coastal systems
Adaptation
C
limate
Drivers
Exposure and vulnerability
N
atural systemsHuman systemsClimate-related
H
uman-related
Natural
variability
Anthropogenic
climate change
• Settlements
• Infrastructure
• Food production
• Tourism
Health
Relative sea level rise
• Storms
• Extreme sea level
Temperature
• CO
2
concentration
• Freshwater input
• Ocean acidification
Socioeconomic
development
• Nutrients
Hypoxia
• Sediment delivery
Rocky coasts
• Beaches
• Wetlands and
s
eagrasses
• Coral reefs
• Aquifers
• Estuaries and lagoons
Deltas
Figure 5-1 | Climate, just as anthropogenic or natural variability, affects both climate and human related drivers. Risk on coastal systems is the outcome of integrating drivers'
associated hazards, exposure, and vulnerability. Adaptation options can be implemented either to modify the hazards or exposure and vulnerability, or both.
368
Chapter 5 Coastal Systems and Low-Lying Areas
5
and Church, 2012; Ganachaud et al., 2013) and glacial isostatic rebound
and tectonic movement. Subsidence of coastal land from sediment
compaction due to building loads, harbor dredging, changes in sediment
supply that cause erosion/accretion, and subsurface resource extraction
(e.g., groundwater, gas and petroleum; Syvitski et al., 2009) may also
contribute to RSLR locally and therefore requires consideration in
coastal impact studies. Sea level impacts are most pronounced during
episodes of extreme sea levels and these are discussed in Section 5.3.3.
5.3.2.1. Global Mean Sea Level
It is very likely that global mean sea level rose at a mean rate of 1.7
[1.5 to 1.9] mm yr
–1
between 1900 and 2010 and at a rate 3.2 [2.8 to
3.6] mm yr
–1
from 1993 to 2010 (WGI AR5 Section 13.2.2). Ocean thermal
expansion and melting of glaciers have been the largest contributors,
accounting for more than 80% of the GMSLR over the latter period
(WGI AR5 Section 13.3.1). Future rates of GMSLR during the 21st century
are projected to exceed the observed rate for the period 1971–2010 of
2.0 [1.7 to 2.3] mm yr
–1
for all RCP scenarios (WGI AR5 Table 13.1). Table
5-2 summarizes the likely ranges of 21st century GMSLR as established
by the Working Group I contribution to this Assessment Report.
From a coastal risk management perspective (Nicholls et al., 2013)
assessments of impacts, vulnerabilities, and adaptation have been using
GMSLR scenarios above the ranges put forward by WGI reports of AR4
(Meehl et al., 2007; Table 10.7) and AR5 (WGI AR5 Table 13.5). The ranges
estimated by WGI of AR4 and AR5 include only those components of
GMSLR that can be quantified using process-based models (i.e., models
derived from the laws of physics; WGI AR5 Glossary). The ranges given
in AR4 thus explicitly excluded contributions to GMSLR resulting from
changes in ice flows from the ice sheets of Greenland and Antarctica
because at that time process-based models were not able to assess this
with sufficient confidence (Meehl et al., 2007; WGI AR5 Section 4.4.5).
Since then, understanding has increased and the likely range of GMSLR
given in AR5 now includes ice sheet flow contributions. Likely, however,
Climate-related driver Physical /chemical effects Trends Projections Progress since AR4
S
ea level Submergence, fl ood damage,
e
rosion; saltwater intrusion; rising
water tables / impeded drainage;
w
etland loss (and change).
G
lobal mean sea level very likely
i
ncrease (Section 5.3.2.2; WGI AR5
Sections 3.7.2, 3.7.3).
G
lobal mean sea level very likely
i
ncrease (see Table 5.1; WGI AR5
Section 13.5.1).
R
egional variability (Section 5.3.2.2;
WGI AR5 Chapter 13).
I
mproved confi dence in
c
ontributions to observed sea level.
More information on regional and
l
ocal sea level rise.
S
torms: tropical cyclones (TCs),
extratropical cyclones (ETCs)
S
torm surges and storm waves,
coastal fl ooding, erosion; saltwater
i
ntrusion; rising water tables /
impeded drainage; wetland loss
(
and change). Coastal infrastructure
damage and fl ood defense failure.
T
Cs (Box 5-1, WGI AR5 Section
2.6.3): low confi dence in trends
i
n frequency and intensity due to
limitations in observations and
r
egional variability.
ETCs (Section 5.3.3.1; WGI AR5
S
ection 2.6.4): likely poleward
movement of circulation features
but low confi dence in intensity
c
hanges.
T
Cs (Box 5-1): likely decrease to no
change in frequency; likely increase
i
n the most intense TCs.
ETCs (Section 5.3.3.1): high
c
onfi dence that reduction of
ETCs will be small globally. Low
c
onfi dence in changes in intensity.
L
owering of confi dence of observed
trends in TCs and ETCs since AR4.
M
ore basin-specifi c information on
storm track changes.
Winds Wind waves, storm surges, coastal
c
urrents, land coastal infrastructure
damage.
Low confi dence in trends in mean
a
nd extreme wind speeds (Section
5.3.3.2, SREX, WGI AR5 Section
3
.4.5).
Low confi dence in projected mean
w
ind speeds. Likely increase in
TC extreme wind speeds (Section
5
.3.3.2, SREX).
Winds not specifi cally addressed
i
n AR4.
Waves Coastal erosion, overtopping and
coastal fl ooding.
Likely positive trends in Hs in high
latitudes (Section 5.3.3.2; WGI AR5
S
ection 3.4.5).
Low confi dence for projections
overall but medium confi dence for
S
outhern Ocean increases in Hs
(Section 5.3.3.2).
Large increase in number of wave
projection studies since AR4.
E
xtreme sea levels Coastal fl ooding erosion, saltwater
intrusion.
H
igh confi dence of increase due to
global mean sea level rise (Section
5
.3.3.3; WGI AR5 Chapter 13).
H
igh confi dence of increase due
to global mean sea level rise, low
c
onfi dence of changes due to storm
changes (Section 5.3.3.3; WGI AR5
Section 13.5).
L
ocal subsidence is an important
contribution to regional sea level
r
ise in many locations.
Sea surface temperature (SST) Changes to stratifi cation and
circulation; reduced incidence
of sea ice at higher latitudes;
increased coral bleaching and
mortality, poleward species
migration; increased algal blooms.
High confi dence that coastal SST
increase is higher than global SST
increase (Section 5.3.3.4).
High confi dence that coastal
SSTs will increase with projected
temperature increase (Section
5.3.3.4).
Emerging information on coastal
changes in SSTs.
Freshwater input Altered fl ood risk in coastal
lowlands; altered water quality /
salinity; altered fl uvial sediment
supply; altered circulation and
nutrient supply.
Medium confi dence (limited
evidence) in a net declining trend in
annual volume of freshwater input
(Section 5.3.3.6).
Medium confi dence for general
increase in high latitudes and
wet tropics and decrease in other
tropical regions (Section 5.3.3.6).
Emerging information on
freshwater input.
Ocean acidity Increased CO
2
fertilization;
decreased seawater pH and
carbonate ion concentration (or
“ocean acidifi cation”).
High confi dence of overall increase,
with high local and regional
variability (Section 5.3.3.5).
High confi dence of increase at
unprecedented rates but with
local and regional variability (Box
CC-OA).
Coastal ocean acidifi cation not
specifi cally addressed in AR4.
Considerable progress made in
chemical projections and biological
impacts.
Table 5-1 | Main climate-related drivers for coastal systems, their trends due to climate change, and their main physical and ecosystem effects.
SREX = IPCC 2012 Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation.
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Coastal Systems and Low-Lying Areas Chapter 5
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m
eans that there is still a 0 to 33% probability of GMSLR beyond this
range, and coastal risk management needs to consider this. WGI does
not assign probabilities to GMSLR beyond the likely range, because this
cannot be done with the available process-based models. WGI, however,
assigns medium confidence that 21st century GMSLR does not exceed
the likely range by several tenths of a meter (WGI AR5 Section 13.5.1).
When using other approaches such as semi-empirical models, evidence
from past climates and physical constraints on ice-sheet dynamics
GMSLR upper bounds of up to 2.4 m by 2100 have been estimated, but
there is low agreement on these higher estimates and no consensus on
a 21st century upper bound (WGI AR5 Section 13.5.3). Coastal risk
management is thus left to choose an upper bound of GMSLR to
consider based on which level of risk is judged to be acceptable in the
specific case. The Dutch Delta Programme, for example, considered a
21st century GMSLR of 1.3 m as the upper bound.
It is virtually certain that sea level rise will continue beyond the 21st
century, although projections beyond 2100 are based on fewer and
simpler models that include lower resolution coupled climate models
for thermal expansion and ice sheet models coupled to climate models
to project ice sheet contributions. The basis for the projections are the
Extended Concentration Pathways (ECPs), and projections are provided
for low, medium, and high scenarios that relate to atmospheric GHG
concentrations <500, 500 to 700, and >700 ppm respectively (WGI AR5
Section 13.5.2). Projections of GMSLR up to 2500 are also summarized
in Table 5-2.
5.3.2.2. Regional Sea Level
Sea level rise will not be uniform in space and time. Natural modes of
climate variability influence sea levels in different regions of the globe
and this will affect the rate of rise on interannual and interdecadal time
periods. For example, in the equatorial Pacific, sea levels can vary from
the global mean by up to 40 cm due to El Niño-Southern Oscillation
(ENSO; e.g., Walsh et al., 2012) and this can strongly influence trends
on decadal scales. Regional variations in the rate of sea level rise on
the coast can arise from climate and ocean dynamic processes such as
changes in winds and air pressure, air-sea heat and freshwater fluxes,
and ocean currents and their steric properties (Timmermann et al., 2010;
WGI AR5 FAQ 13.1). Although the vast majority of coastlines are
experiencing sea level rise, coastlines near current and former glaciers
and ice sheets are experiencing relative sea level fall (Milne et al., 2009;
W
GI AR5 FAQ 13.1). This is because the gravitational attraction of the
ice sheet decreases as it melts and exerts less pull on the oceans and
also because the land tends to rise as the ice melts, the shape of the
sea floor changes under the reduced load of the ice sheets, and the
change in mass distribution alters the Earth’s rotation (WGI AR5 FAQ
13.1; Gomez et al., 2010). In terms of absolute sea level change,
approximately 70% of the global coastlines are projected to experience
sea level change that is within 20% of the global mean sea level change
(WGI AR5 Section 13.6.5).
5.3.2.3. Local Sea Level
Besides the effect of long-term vertical land movement on regional sea
level, RSLR can occur locally due to subsidence or uplifts of coastal
plains as well as due to other natural causes. Natural subsidence can
occur because of sediment compaction and loading, as in the Mississippi
River, and other deltas (Törnqvist et al., 2008; Dokka, 2011; Marriner et
al., 2012). Tectonic movements, both sustained and abrupt, have brought
about relative sea level changes. The Great East Japan Earthquake in
2011 caused subsidence of up to 1.2 m of the Pacific coast of northeast
Japan (Geospatial Information Authority of Japan, 2011). The Sumatra-
Andaman earthquake in 2004 and subsequent earthquakes in 2005
produced vertical deformation ranging from uplift of 3 m to subsidence
of 1 m (Briggs et al., 2006). These movements are especially important
in coastal zones located near active plate margins.
Anthropogenic causes of RSLR include sediment consolidation from
building loads, reduced sediment delivery to the coast, and extraction
of subsurface resources such as gas, petroleum, and groundwater.
Subsidence rates may also be sensitive to the rates of oil and gas
removal (e.g., Kolker et al., 2011). Syvitski et al. (2009) estimate that
the majority of the world’s largest deltas are currently subsiding at rates
that are considerably larger than the current rates of sea level rise
because of coastal sediment starvation due to substantial dam building
over the 20th century or sediment compaction through natural or
anthropogenic activities. Many large cities on deltas and coastal plains
have subsided during the last 100 years: ~4.4 m in eastern Tokyo, ~3 m
in the Po delta, ~2.6 m in Shanghai, and ~1.6 m in Bangkok (Syvitski et
al., 2009; Teatini et al., 2011). Loads from massive buildings and other
large structures can also increase sediment compaction and subsidence
(Mazzotti et al., 2009). RSLR can exceed GMSLR by an order of magnitude,
reaching more than 10 cm yr
–1
, and it is estimated that the delta surface
Emission
scenario
Representative
Concentration
Pathway (RCP)
2100 CO
2
concentration
(ppm)
Mean sea level rise (m)
Emission
scenario
Mean sea level rise (m)
2046–2065 2100 2200 2300 2500
Low 2.6 421 0.24 [0.17–0.32] 0.44 [0.28–0.61] Low 0.35–0.72 0.41–0.85 0.50–1.02
Medium low 4.5 538 0.26 [0.19–0.33] 0.53 [0.36–0.71] Medium 0.26–1.09 0.27–1.51 0.18–2.32
Medium high 6.0 670 0.25 [0.18–0.32] 0.55 [0.38–0.73]
High 0.58–2.03 0.92–3.59 1.51–6.63
High 8.5 936 0.29 [0.22–0.38] 0.74 [0.52–0.98]
Table 5-2 | Projections of global mean sea level rise in meters relative to 1986–2005 are based on ocean thermal expansion calculated from climate models, the contributions
from glaciers, Greenland and Antarctica from surface mass balance calculations using climate model temperature projections, the range of the contribution from Greenland and
Antarctica due to dynamical processes, and the terrestrial contribution to sea levels, estimated from available studies. For sea levels up to and including 2100, the central values
and the 5–95% range are given whereas for projections from 2200 onwards, the range represents the model spread due to the small number of model projections available and
the high scenario includes projections based on RCP6.0 and RCP8.5. Source: WGI AR5 Summary for Policymakers and Sections 12.4.1, 13.5.1, and 13.5.4.
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Chapter 5 Coastal Systems and Low-Lying Areas
5
a
rea vulnerable to flooding could increase by 50% for 33 deltas around
the world under the sea level rise as projected for 2100 by the IPCC
AR4 (Syvitski et al., 2009).
Clearly large regional variations in the projected sea level rise, together
with local factors such as subsidence, indicates that RSLR can be much
larger than projected GMSLR and therefore is an important consideration
in impact assessments (very high confidence).
5.3.3. Climate-Related Drivers
Increasing GHGs in the atmosphere produce changes in the climate
system on a range of time scales that impact the coastal physical
environment. On shorter time scales, physical coastal impacts such as
inundation, erosion, and coastal flooding arise from severe storm-
induced surges, wave overtopping, and rainfall runoff. On longer time
scales, wind and wave climate change can cause changes in sediment
transport at the coast and associated changes in erosion or accretion.
Natural modes of climate variability, which can affect severe storm
behavior and wind and wave climate, may also undergo anthropogenic
changes in the future. Ocean and atmospheric temperature change can
affect species distribution with impacts on coastal biodiversity. Carbon
dioxide (CO
2
) uptake in the ocean increases ocean acidity and reduces
the saturation state of carbonate minerals, essential for shell and
skeletal formation in many coastal species. Changes in freshwater
input can alter coastal ocean salinity concentrations. Past and future
changes to these physical drivers are discussed in this section (see also
Table 5-1).
5.3.3.1. Severe Storms
Severe storms such as tropical and extratropical cyclones (ETCs) can
generate storm surges over coastal seas. The severity of these depends
on the storm track, regional bathymetry, nearshore hydrodynamics, and
the contribution from waves. Globally there is low confidence regarding
changes in tropical cyclone activity over the 20th century owing to
changes in observational capabilities, although it is virtually certain that
there has been an increase in the frequency and intensity of the
strongest tropical cyclones in the North Atlantic since the 1970s (WGI
AR5 Section 2.6). In the future, it is likely that the frequency of tropical
cyclones globally will either decrease or remain unchanged, but there
will be a likely increase in global mean tropical cyclone precipitation
rates and maximum wind speed (WGI AR5 Section 14.6).
ETCs occur throughout the mid-latitudes of both hemispheres, and their
development is linked to large-scale circulation patterns. Assessment of
changes in these circulation features reveals a widening of the tropical
belt, poleward shift of storm tracks and jet streams, and contraction of
the polar vortex; this leads to the assessment that it is likely that, in a
zonal mean sense, circulation features have moved poleward (WGI
AR5 Sections 2.7.5 to 2.7.8) but there is low confidence regarding
regional changes in intensity of ETCs (e.g., Seneviratne et al., 2012).
With regard to future changes, a small poleward shift is likely in the
Southern Hemisphere but changes in the Northern Hemisphere are
basin specific and of lower confidence (WGI AR5 Section 14.6.3).
G
lobally, it is unlikely that the number of ETCs will fall by more than a
few percent due to anthropogenic climate change (high confidence;
WGI AR5 Section 14.6.3).
5.3.3.2. Extreme Sea Levels
Extreme sea levels are those that arise from combinations of factors
including astronomical tides, storm surges, wind waves and swell, and
interannual variability in sea levels. Storm surges are caused by the
falling atmospheric pressures and surface wind stress associated with
storms such as tropical and ETCs and therefore may change if storms
are affected by climate change. To date, however, observed trends in
extreme sea levels are mainly consistent with mean sea level (MSL)
trends (e.g., Marcos et al., 2009; Haigh et al., 2010; Menendez and
Woodworth, 2010; Losada et al., 2013) indicating that MSL trends rather
than changes in weather patterns are responsible.
Assuming that sea level extremes follow a simple extreme value
distribution (i.e., a Gumbel distribution), and accounting for the
uncertainty in projections of future sea level rise, Hunter (2012) has
developed a technique for estimating a sea level allowance, that is, the
minimum height that structures would need to be raised in a future
period so that the number of exceedances of that height remains the
same as under present climate conditions (Figure 5-2). Such an allowance
can be factored into adaptive responses to rising sea levels. It should be
noted, however, that extreme sea level distributions might not follow a
simple Gumbel distribution (e.g., Tebaldi et al., 2012) owing to different
factors influencing extreme levels that may not be measured by tide
gauges (e.g., Hoeke et al., 2013).
Regarding future changes to storm surges, hydrodynamic models forced
by climate models have been used in several extratropical regional
studies such as the northeast Atlantic (e.g., Debenard and Roed, 2008;
Wang et al., 2008; Sterl et al., 2009) and southern Australia (Colberg
and McInnes, 2012). These studies show strong regional variability and
sensitivity to the choice of Global Climate Model (GCM) or Regional
Climate Model (RCM). The effect of future tropical cyclone changes on
storm surges has also been investigated in a number of regions using
a range of different methods. These include methods to stochastically
generate and/or perturb cyclones within background environmental
conditions that represent historical (e.g., Harper et al., 2009) and GCM-
represented future conditions (e.g., Mousavi et al., 2011; Lin et al.,
2012). Regional studies include Australia’s tropical east coast (Harper
et al., 2009), Louisiana (Smith et al., 2010), Gulf of Mexico (Mousavi et
al., 2011), India (Unnikrishnan et al., 2011), and New York (Lin et al., 2012),
and the details of the methods and findings vary considerably between
the studies. While some studies indicate for some regions increase to
extreme sea levels due to changes in storms, others indicate the opposite.
In general, the small number of regional storm surge studies together
with the different atmospheric forcing factors and modeling approaches
means that there is low confidence in projections of storm surges due
to changes in storm characteristics. However, observed upward trends
in MSL together with projected increases for 2100 and beyond indicate
that coastal systems and low-lying areas will increasingly experience
extreme sea levels and their adverse impacts (high confidence) (see also
WGI AR5 Section 13.7).
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Coastal Systems and Low-Lying Areas Chapter 5
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5.3.3.3. Winds and Waves
Changes in wind climate affect large-scale wave climate. Winds also
influence longshore current regimes and hence upwelling systems
(Narayan et al., 2010; Miranda et al., 2012; see also Sections 6.3.3,
6.3.5). Energy dissipation via wave breaking contributes to longshore
and cross-shore currents, elevated coastal sea levels through wave set-
up, and run-up and beach erosion. Changes to wind and wave climate
therefore can affect sediment dynamics and shoreline processes (e.g.,
Aargaard et al., 2004; Reguero et al., 2013), and extreme winds and
waves are a threat to coastal populations. The coastal impacts of wave
climate change are also a function of wave direction and period as well
as the coastline itself, which can influence shoaling and refraction. Long
period swell, which dominates the wave energy field, poses a significant
danger to coastal and offshore structures and shipping (e.g., Semedo
et al., 2011) and can causes significant flooding of coastlines with steep
shelf margins (Hoeke et al., 2013).
There is low confidence in trends calculated from measurements of
mean and extreme winds and their causes due to the limited length of
records and uncertainties associated with different wind measurement
techniques (Seneviratne et al., 2012). However, there is increasing
evidence for a strengthening wind stress field in the Southern Ocean
since the early 1980s from atmospheric reanalyses, satellite observations,
and island station data (WGI AR5 Section 3.4.5). Positive trends in wave
height have been detected in the Northeast Atlantic over the 1958–2002
period based on reanalyses and ship observations and in the Southern
Ocean between 1985 and 2008 based on satellite data (medium
confidence) (WGI AR5 Section 3.4.6; see Table 5-2).
Projected changes in mean and extreme winds and waves were assigned
low confidence (Seneviratne et al., 2012) owing to limited studies.
Although there has been an increase in studies addressing future wave
climate change (Hemer et al., 2013), generally low confidence remains
in projected wave climate change (except for medium confidence over
the Southern Ocean), and this is due to uncertainties in future winds,
particularly those associated with storms (see WGI AR5 Section 13.7).
5.3.3.4. Sea Surface Temperature
Sea surface temperature (SST) has significantly warmed during the past
30 years along more than 70% of the world’s coastlines, with highly
heterogeneous rates of change both spatially and seasonally (Lima and
Wethey, 2012). The average rate is 0.18 ± 0.16°C per decade and the
average change in seasonal timing was -3.3 ± 4.4 days per decade.
These values are larger than in the global ocean where the average of
change is 0.11 [0.09 to 0.13]°C per decade in the upper 75 m of the
ocean during the 1971–2010 period (WGI AR5 Section 3.2.2) and the
seasonal shift is -2.3 days per decade (Lima and Wethey, 2012). Extreme
0
.2
0
.3
0
.4
0
.5
0
.6
0
.7
0
.8
°120W °60W
°60S
°30S
°30N
°60N
°60E °120E °180W °0
°0
Allowance (m)
Figure 5-2 | The estimated increase in height (m) that flood protection structures would need to be raised in the 2081–2100 period to preserve the same frequency of
exceedances that was experienced for the 1986–2005 period, shown for 182 tide gauge locations and assuming regionally varying relative sea level rise projections under an
Representative Concentration Pathway 4.5 (RCP4.5) scenario (adapted from Hunter et al., 2013).
372
Chapter 5 Coastal Systems and Low-Lying Areas
5
e
vents have also been reported. For example, the record high ocean
temperatures along the western Australian coast during the austral
summer of 2010/2011, with nearshore temperatures peaking at about
5°C above average, were unprecedented (Pearce and Feng, 2013). In
summary, positive trends in coastal SSTs are seen on the majority of
coastlines, and the rate of rise along coastlines is higher on average
than the oceans (high confidence). Based on projected temperature
increases there is high confidence that positive coastal SST trends will
continue.
5.3.3.5. Ocean Acidification
Anthropogenic ocean acidification refers to the changes in the carbonate
chemistry primarily due to the uptake of atmospheric CO
2
(Box CC-OA).
Seawater pH exhibits a much larger spatial and temporal variability in
coastal waters compared to open ocean owing to the variable contribution
of processes other than CO
2
uptake (Duarte et al., 2013a) such as upwelling
intensity (Feely et al., 2008; Box CC-UP), deposition of atmospheric
nitrogen and sulfur (Doney et al., 2007), carbonate chemistry of riverine
waters (Salisbury et al., 2008; Aufdenkampe et al., 2011), as well as
inputs of nutrients and organic matter (Borges, 2011; Cai et al., 2011).
For example, pH (NBS scale) ranges from 6 to 9 in 24 estuaries (Borges
and Abril, 2011) and short-term (hours to weeks) changes of up to 0.5
pH units are not unusual in coastal ecosystems (Hofmann et al., 2011).
Few high-quality ocean acidification time series exceed 5 years in the
coastal ocean (Wootton et al., 2008; Provoost et al., 2010; Waldbusser
et al., 2010). Some exhibit considerable differences compared to open
ocean stations, illustrating that anthropogenic ocean acidification can
be lessened or enhanced by processes such as primary production,
respiration, and calcification (Borges and Gypens, 2010; Kleypas et al.,
2011).
Under the IS92a CO
2
emission scenario, the global pH (total scale) of
coastal waters has been projected to decrease from about 8.16 in the
year 1850 to 7.83 in 2100 (Lerman et al., 2011) but with considerable
spatial variability. For example, using the same CO
2
emission scenario,
Cai et al. (2011) projected an overall decline of pH in the Northern Gulf
of Mexico of 0.74 over the same period, a value that is much greater
than that of the open ocean (Box CC-OA).
To summarize, seawater pH exhibits considerable temporal and spatial
variability in coastal areas compared to open ocean owing to additional
natural and human influences (very high confidence). Coastal acidification
is projected to continue but with large and uncertain regional and local
variations (high confidence).
5.3.3.6. Freshwater Input
Changes in river runoff arise from changes in climate drivers such as
precipitation, complex interactions between changing levels of CO
2
,
plant physiology, and, consequently, evapotranspiration (e.g., Gedney
et al., 2006; Betts et al., 2007) as well as human drivers such as land
use change, water withdrawal, dam building, and other engineered
modifications to waterways (see more detailed discussion in Chapter 3).
A
n assessment of runoff trends in 925 of the world’s largest ocean-
reaching rivers, which account for about 73% of global total runoff,
indicates that from 1948–2004 statistically significant trends were
present in only one-third of the top 200 rivers and, of these, two-thirds
exhibited downward trends and one-third upward trends (Dai et al.,
2009). While precipitation changes dominate freshwater flows, decreasing
trends in river discharges may be further enhanced as a result of human
pressures (Dai et al., 2009; Section 3.2.3).
Average annual runoff is generally projected to increase at high latitudes
and in the wet tropics and to decrease in most dry tropical regions
(Section 3.4.5). Shifts to earlier peak flows are also projected in areas
affected by snowmelt (Adam et al, 2009). However, there are some
regions where there is considerable uncertainty in the magnitude and
direction of change, specifically South Asia and large parts of South
America. Both the patterns of change and the uncertainty are largely
driven by projected changes in precipitation.
To summarize, there is medium confidence (limited evidence, high
agreement) in a net declining trend in freshwater input globally, although
large regional variability exists. Trends are dominated by precipitation
changes although human pressures on water supply may enhance
downward trends (medium confidence). Uncertainty in future changes
in runoff is linked to precipitation uncertainty. Runoff is generally
projected to increase in high latitudes with earlier peak flows and in
the wet tropics and decrease in other tropical regions, however, with
large uncertainty (medium confidence).
5.3.4. Human-Related Drivers
Coastal systems are subject to a wide range of human-related or
anthropogenic drivers (e.g., Crain et al., 2009) that interact with climate-
related drivers and confound efforts to attribute impacts to climate
change. Some of the major terrestrially based human drivers that
directly or indirectly cause changes are briefly reviewed. Related drivers
in the marine environment are discussed in Sections 6.4 and 30.6.
5.3.4.1. Socioeconomic Development
Socioeconomic development (SED) drives coastal impacts in several
ways. SED influences the number of people and the value of assets
exposed to coastal hazards. Since AR4, a number of studies have
estimated the influence of future sea level rise and associated hazards
on coastal population and assets. Although these estimates are subject
to uncertainties associated with global elevation and population data
sets (Lichter et al., 2011; Mondal and Tatem, 2012), all the studies
indicate high and growing exposure of low-lying coastal areas. The Low
Elevation Coastal Zone (LECZ) constitutes 2% of the world’s land area
but contains 10% of the world’s population (600 million) and 13% of
the worlds urban population (360 million), based on year 2000
estimates (McGranahan et al., 2007). About 65% of the world’s cities
with populations of greater than 5 million are located in the LECZ
(McGranahan et al., 2007). The global population exposed to the 1-in-
100-year extreme sea level (i.e., the sea level that has a 1% chance of
being exceeded every year) has increased by 95% from 1970 to 2010,
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Coastal Systems and Low-Lying Areas Chapter 5
5
w
ith about 270 million people and US$13 trillion worth of assets being
exposed to the 1-in-100-year extreme sea level in 2010 (Jongman et al.,
2012). In 2002, about US$1.9 trillion worth of assets below the 1-in-
100-year extreme sea level were concentrated in the following 10 port
cities: Miami (USA), New York-Newark (USA), New Orleans (USA), Osaka-
Kobe (Japan), Tokyo (Japan), Amsterdam (Netherlands), Rotterdam
(Netherlands), Nagoya (Japan), Virginia Beach (USA), and Guangzhou
(China) (Hanson et al., 2011). Compared to other regions, Asia exhibits
the greatest exposure in terms of population and assets (Jongman et
al., 2012).
For many locations, population and assets exposure is growing faster
than the national average trends owing to coastward migration, coastal
industrialization, and urbanization (e.g., McGranahan et al., 2007; Seto,
2011; Smith, 2011; see also Chapter 8; high confidence). Coastal net
migration has largely taken place in flood- and cyclone-prone areas,
which poses a challenge for adaptation (de Sherbinin et al., 2011). These
processes and associated land use changes are driven by a combination
of many social, economic, and institutional factors including taxes,
subsidies, insurance schemes, aesthetic and recreational attractiveness
of the coast, and increased mobility (Bagstad et al., 2007; Palmer et
al., 2011). In China, the country with the largest exposed population,
urbanization and land reclamation are the major drivers of coastal land
use change (Zhu et al., 2012). Although coastal migration is expected
to continue in the coming decades, it is difficult to capture this process
in global scenarios, as the drivers of migration and urbanization are
complex and variable (Black et al., 2011).
SED also influences the capacity to adapt. Poor people living in urban
informal settlements, of which there are about 1 billion worldwide, are
particularly vulnerable to weather and climate impacts (de Sherbinin et
al., 2011; Handmer et al., 2012). The top five nations classified by
population in coastal low-lying areas are developing and newly
industrialized countries: Bangladesh, China, Vietnam, India, and Indonesia
(McGranahan et al., 2007; Bollman et al., 2010; Jongman et al., 2012).
SED and associated land reclamation are also major drivers of the
destruction of coastal wetlands, which also makes human settlements
more vulnerable because wetlands act as natural buffers reducing wave
and storm impacts on the coast (e.g., Crain et al., 2009; Shepard et al.,
2011; Arkema et al., 2013; Duarte et al., 2013b). Finally, socioeconomic
development is expected to exacerbate further a number of human
pressures on coastal systems related to nutrient loads, hypoxia, and
sediment delivery, which is discussed in the following subsections.
5.3.4.2. Nutrients
Increased river nutrient (nitrogen, phosphorus) loads to coasts in many
regions are observed, and simulated by regional and global models
(Alexander et al., 2008; Seitzinger et al., 2010). Anthropogenic global loads
of dissolved inorganic nutrients (DIN, DIP) are two to three times larger
than those of natural sources (Seitzinger et al., 2010), causing coastal
ecosystem degradation (Sections 5.3.4.3, 5.4.2.6). Large variations exist
in magnitude and relative sources of nutrient loads. Anthropogenic
sources are related primarily to fertilizer use in agriculture and fossil
fuel emissions (NO
x
) (Galloway et al., 2004; Bouwman et al., 2009).
Future trends depend on measures available to optimize nutrient use
i
n crop production and minimize loss to rivers from agriculture (crop,
livestock), sewage, and NO
x
emissions. In scenarios with little emphasis
on nutrient management, global nutrient discharge increases (DIN
29%, DIP 64%) between 2000 and 2050 (Seitzinger et al., 2010). With
ambitious nutrient management, global DIN loads decrease slightly and
DIP increases (35%). Climate change is projected to change water
runoff (Chapter 3) that influences river nutrient loads. Studies of climate
change effects related to increased watershed nutrient sources are
needed. In summary, nutrient loads have increased in many world regions
(high confidence); future increases will depend largely on nutrient
management practices (medium confidence).
5.3.4.3. Hypoxia
The presence of excessive nutrients in coastal waters, which causes
eutrophication and the subsequent decomposition of organic matter, is
the primary cause of decreased oxygen concentration (hypoxia).
Globally, upwelling of low oxygen waters (e.g., Grantham et al., 2004) and
ocean warming, which decreases the solubility of oxygen in seawater
(Shaffer et al., 2009), are secondary drivers but can be locally important.
The oxygen decline rate is greater in coastal waters than in the open
ocean (Gilbert et al., 2010). Hypoxia poses a serious threat to marine
life, which is exacerbated when combined with elevated temperature
(Vaquer-Sunyer and Duarte, 2011; see also Section 6.3.3). The number
of so-called dead zones” has approximately doubled each decade
since 1960 (Diaz and Rosenberg, 2008). Fishery catches from these
areas are generally lower than predicted from nutrient loading alone
(Breitburg et al., 2009). Although non-climate anthropogenic factors are
responsible for virtually all hypoxia in estuaries and inner continental
shelves, climate drivers such as ocean warming, altered hydrological
cycles, and coastal current shifts and changes in upwellings may interact
with eutrophication in the next decades (Rabalais et al., 2010; Meire et
al., 2013; high confidence).
5.3.4.4. Sediment Delivery
Human activities in drainage basins and coastal plains have impacted
the coastal zone by changing the delivery of sediment to the coast.
Sediment trapping behind dams, water diversion for irrigation, and sand
and gravel mining in river channels all contribute to decrease sediment
delivery, whereas soil erosion due to land use changes helps increase
it (Syvitski, 2008; Walling, 2006). It is estimated that the global discharge
of riverine sediment was 16 to 19 Gt yr
–1
in the 1950s before widespread
dam construction (e.g., Syvitski et al., 2005; Milliman and Farnsworth,
2011) and it has decreased to 12 to 13 Gt yr
–1
(Syvitski and Kettner, 2011).
Out of 145 major rivers with mostly more than 25 years of record, only
seven showed evidence of an increase in sediment flux while 68 showed
significant downward trends (Walling and Fang, 2003). The number of
dams has increased continuously and their distribution has expanded
globally. As of early 2011, the world has an estimated 16.7 million
reservoirs larger than 0.01 ha (Lehner et al., 2011). Globally, 34 rivers
with drainage basins of 19 million km
2
in total show a 75% reduction
in sediment discharge over the past 50 years (Milliman and Farnsworth,
2011). Reservoir trapping of sediments is estimated globally as 3.6 Gt
yr
–1
to more than 5 Gt yr
–1
(Syvitski et al., 2005; Milliman and Farnsworth,
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Chapter 5 Coastal Systems and Low-Lying Areas
5
2
011; Walling, 2012). Human pressure is the main driver of the observed
declining trend in sediment delivery to the coast (high agreement).
5.4. Impacts, Vulnerabilities, and Risks
5.4.1. Introduction
This subsection briefly introduces the diverse approaches and methods
applied in the literature on coastal impact, vulnerability, and risk. The
following subsections then assess this literature related to coastal
natural systems (Section 5.4.2) and coastal human systems (Section
5.4.3). Much of this literature focuses on RSLR and extreme sea level
events as the main drivers. The main biophysical impacts of this driver
are increasing flood damage, dry-land loss due to submergence and
erosion, wetland loss and change, saltwater intrusion into surface and
ground water, and rising water tables and impeded drainage (Table 5-3).
Impacts and risks are assessed using a wide variety of approaches from
the local to global scale. Sea level rise exposure approaches are applied
at all scales to assess values exposed to sea level rise (e.g., people,
assets, ecosystems, or geomorphological units). Submergence exposure
approaches assess exposure to permanent inundation under a given
sea level rise (e.g., Dasgupta et al., 2009; Boateng, 2012) whereas flood
exposure approaches assess exposure to temporary inundation during
a coastal flood event by combining the extreme water level of the flood
event with a given level of sea level rise (e.g., Dasgupta et al., 2011;
Kebede and Nicholls, 2012).
Indicator-based approaches are also used at all scales to aggregate data
on the current state of the coastal systems into vulnerability indices
(
Gornitz, 1991; Hinkel, 2011), based on either biophysical exposure or
hazard variables (e.g., Bosom and Jimenez, 2011; Yin et al., 2012),
socioeconomic variables representing a social group’s capacity to adapt
(e.g., Cinner et al., 2012), or both kinds of variables (e.g., Bjarnadottir
et al., 2011; Li and Li, 2011; Yoo et al., 2011).
At local scales (<100 km coastal length), process-based models are
applied to assess flooding, erosion, and wetland impacts. Approaches
include assessments of flood damage of single extreme water level
events using numerical inundation models (e.g., Lewis et al., 2011; Xia
et al., 2011). Erosion impacts are assessed using either numerical
morphodynamic models (e.g., Jiménez et al., 2009; Ranasinghe et al.,
2012) or simple geometric profile relationships such as the Bruun Rule
(Bruun, 1962). For ecosystem impacts ecological landscape simulation
models are used to predict habitat change due to sea level rise and
other factors (e.g., Costanza et al., 1990).
At regional to global scales, numerical process-based models are not
available for assessing the impacts of RSLR and extreme sea level events
due to data and computational limits. Global scale assessments of coastal
impacts have been conducted with the models Climate Framework for
Uncertainty, Negotiation and Distribution (FUND) and Dynamic and
Interactive Coastal Vulnerability Assessment (DIVA). FUND is an integrated
assessment model with a coastal impact component that includes
country-level cost functions for dry-land loss, wetland loss, forced
migration, and dike construction (Tol, 2002). DIVA is a dedicated coastal
impact model employing subnational coastal data (Vafeidis et al., 2008)
and considering additional impacts such as coastal flooding and erosion
as well as adaptation in terms of protection via dikes and nourishment
(Hinkel and Klein, 2009). DIVA assesses coastal flood risk based on
hydrologically connected elevation and extreme water level distributions
Frequently Asked Questions
FAQ 5.1 | How does climate change affect coastal marine ecosystems?
The major climate-related drivers on marine coastal ecosystems are sea level rise, ocean warming, and ocean
acidification.
Rising sea level impacts marine ecosystems by drowning some plants and animals as well as by inducing changes of
parameters such as available light, salinity, and temperature. The impact of sea level is related mostly to the capacity
of animals (e.g., corals) and plants (e.g., mangroves) to keep up with the vertical rise of the sea. Mangroves and
coastal wetlands can be sensitive to these shifts and could leak some of their stored compounds, adding to the
atmospheric supply of these greenhouse gases.
Warmer temperatures have direct impacts on species adjusted to specific and sometimes narrow temperature
ranges. They raise the metabolism of species exposed to the higher temperatures and can be fatal to those already
living at the upper end of their temperature range. Warmer temperatures cause coral bleaching, which weakens
those animals and makes them vulnerable to mortality. The geographical distribution of many species of marine
plants and animals shifts towards the poles in response to warmer temperatures.
When atmospheric carbon dioxide is absorbed into the ocean, it reacts to produce carbonic acid, which increases
the acidity of seawater and diminishes the amount of a key building block (carbonate) used by marine ‘calcifiers’
such as shellfish and corals to make their shells and skeletons and may ultimately weaken or dissolve them. Ocean
acidification has a number of other impacts, many of which are still poorly understood.
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Coastal Systems and Low-Lying Areas Chapter 5
5
(Hinkel et al., 2013) and erosion based on a combination of the Bruun
Rule and a simplified version of the Aggregated Scale Morphological
Interaction between a Tidal inlet and the Adjacent coast (ASMITA)
model for tidal basins (Nicholls et al., 2011). The results of these models
are discussed in Sections 5.4.3.1 and 5.5.5.
For impacts on natural systems, the key climate-related drivers considered
are temperature, ocean acidification, and sea level. A variety of approaches
are applied including field observations of ecosystem features (e.g.,
biodiversity, reproduction) and functioning (e.g., calcification, primary
production), remote sensing (e.g., extent of coral bleaching, surface area
of vegetated habitats), and perturbation experiments in the laboratory
and in the field.
5.4.2. Natural Systems
Coastal ecosystems are experiencing large cumulative impacts related
to human activities (Halpern et al., 2008) arising from both land- and
ocean-based anthropogenic drivers. Anthropogenic drivers associated
with global climate change are distributed widely and are an important
component of cumulative impacts experienced by coastal ecosystems.
There is no wetland, mangrove, estuary, rocky shore, or coral reef that
is not exhibiting some degree of impact. Overexploitation and habitat
destruction are often the primary causes of historical changes in coastal
systems leading to declines in diversity, structure, and functioning (Lotze
et al., 2006). Further, extreme climate events generate changes to both the
mean and the variance of climatic variables over ecological time scales.
5.4.2.1. Beaches, Barriers, and Sand Dunes
Beaches, barriers, and sand dunes are about half as common as rocky
coasts (Bird, 2000; Davis and FitzGerald, 2004) and often exhibit distinct
and seasonal changes. Owing to their aesthetic qualities, they are highly
valued for recreation and residences.
5.4.2.1.1. Observed impacts
Globally, beaches and dunes have in general undergone net erosion
over the past century or longer (e.g., for an overview, see Bird, 2000).
A number of studies have investigated shoreline change by comparing
historical maps and imagery, available since about the mid-19th century
with more recent maps and imagery to quantify combined climate and
non-climate changes. For example, along the U.S. Mid-Atlantic and New
England coasts the long-term rate of erosion, based on 21,184 transects
equally spaced along more than 1000 km of coast, is 0.5 ± 0.09 m yr
–1
,
with 65% of transects showing net erosion (Hapke et al., 2011). A
similar study by Webb and Kench (2010) in the central Pacific utilized
historical aerial photographs and satellite images to show physical
changes in 27 islets located in four atolls over a 19- to 61-year period.
The analysis highlighted the dynamic nature of sea level rise response
in the recent past, with physical changes in shoreline progradation and
displacement influencing whether the island area increased (46%),
remained stable (46%), or decreased (14%).
Attributing shoreline changes to climate change is still difficult owing
to the multiple natural and anthropogenic drivers contributing to
coastal erosion. For example, rotation of pocket beaches (i.e., where one
end of the beach accretes while the other erodes and then the pattern
reverses) in southeast Australia is closely related to interannual changes
in swell direction (Harley et al., 2010). Additional processes, unrelated
to climate change, that contribute to coastal change include dams
capturing fluvial sand (e.g., in Morocco; Chaibi and Sedrati, 2009).
Statistically linking sea level rise to observed magnitudes of beach
erosion has had some success, although the coastal sea level change signal
is often small when compared to other processes (e.g., Leatherman et
al., 2000a,b; Sallenger et al., 2000; Zhang et al., 2004). A Bayesian
network incorporating a variety of factors affecting coastal change,
including RSLR, has been successful in hindcasting shoreline change,
and can be used to evaluate the probability of future shoreline change
(Gutierrez et al., 2011).
While some coastal systems may be able to undergo landward retreat
under rising sea levels, others will experience coastal squeeze, which
occurs when an eroding shoreline approaches hard, immobile structures
such as seawalls or resistant natural cliffs. In these