Coasts will face increasing risks from climate change through this century. Impacts include more frequent flooding, erosion, ecosystem loss, and damage from storms. These risks disproportionately threaten dense, low-lying coastal populations. While adaptation is challenging, a combination of protection, accommodation, and retreat strategies can reduce risks. However, sea level rise will continue for centuries, potentially questioning the viability of some coastal settlements without mitigation to limit long-term rise.

1. Chapter 6:
Coastal Systems and Low-lying
Areas
Coordinating Lead Authors
Robert J. Nicholls (UK) and Poh Poh Wong (Singapore)
Lead Authors
Virginia Burkett (USA), Jorge Codignotto (Argentina), John Hay
(New Zealand), Roger McLean (Australia), Sachooda Ragoonaden
(Mauritius), Colin D. Woodroffe (Australia)
Contributing Authors
Pamela Abuodha (Kenya), Julie Arblaster (USA/Australia), Barbara Brown (UK), Don Forbes (Canada), Jim Hall
(UK), Sari Kovats (UK), Jason Lowe (UK), Kathy McInnes (Australia), Susanne Moser (USA), Susanne Rupp-
Armstrong (UK), Yoshiki Saito (Japan), Richard Tol (Ireland)
Review Editors
Job Dronkers (Netherlands), Geoff Love (Australia), Jin-Eong Ong (Malaysia)

2. The coastal system

3. Main conclusions
• Coasts are experiencing the adverse consequences of hazards
related to climate and sea level [*** D].
• Coasts will be exposed to increasing risks over coming decades
due to many compounding climate-change factors [*** D].
• The impact of climate change on coasts is exacerbated by
increasing human-induced pressures [*** C].
• Adaptation for the coasts of developing countries will be more
challenging than for coasts of developed countries, due to
constraints on adaptive capacity [** D].
• Adaptation costs for vulnerable coasts are much less than the
costs of inaction [** N].
• The unavoidability of sea-level rise even in the longer-term
frequently conflicts with present-day human development
patterns and trends [** N].

4. Observed impacts
due to climate variability and change
• Storms already impose substantial costs on coastal
societies.
• Annually, about 120 million people are exposed to
tropical cyclone hazards which had killed 250,000
people from 1980 to 2000.
• Through the 20th Century, global rise of sea level
contributed to increased coastal inundation, erosion
and ecosystem losses, but with considerable local
and regional variation due to other factors.
• Late 20th Century effects of rising temperature
include loss of sea ice, thawing of permafrost and
associated coastal retreat, and more frequent coral
bleaching and mortality.

5. Tropical cyclone tracks
Map of the cumulative tracks of all tropical cyclones during the 1985–2005 time
period. The Pacific Ocean west of the International Date Line sees more tropical
cyclones than any other basin, while there is almost no activity in the Atlantic
Ocean south of the Equator.

6. Mississippi delta – post-Katrina
Areas in red were converted to open water during the hurricane.
Yellow lines on index map of Louisiana show tracks of Hurricane
Katrina on right and Hurricane Rita on left

7. Recent coral bleaching
events

8. Climate risks will increase
through the 21st Century
Climate Driver (trend) Main Physical and Ecosystem Effects on Coastal Systems
Increased CO2 fertilisation; Decreased seawater pH (or ‘ocean acidification’) negatively
CO2 concentration (↑) impacting coral reefs and other pH sensitive organisms.
Increased stratification/changed circulation; Reduced incidence of sea ice at higher
Sea surface latitudes; Increased coral bleaching and mortality; Poleward species migration; Increased
temperature (SST) (↑, algal blooms.
R)
Inundation, flood and storm damage; Erosion; Saltwater Intrusion; Rising water tables/
Sea level (↑, R) impeded drainage; Wetland loss (and change).
Increased extreme water levels and wave heights; Increased episodic erosion, storm
Intensity (↑, R) damage, risk of flooding and defence failure;
Storm
Frequency (?, R) Altered surges and storm waves and hence risk of storm damage and flooding.
Track (?,R)
Wave climate Altered wave conditions, including swell; Altered patterns of erosion and accretion; Re-
orientation of beach planform.
(?, R)
Run-off (R) Altered flood risk in coastal lowlands; Altered water quality/salinity; Altered fluvial sediment
supply; Altered circulation and nutrient supply.
These phenomena will vary considerably at regional and local scales, but
the impacts are virtually certain to be overwhelmingly negative.

9. Extreme sea-level simulations
Cairns, Australia
Current climate
2050 climate
Cairns road network

10. Major impact types
• Corals are threatened with increased bleaching and mortality
due to rising sea surface temperatures.
• Coastal wetland ecosystems, such as salt marshes and
mangroves, are especially threatened where they are sediment-
starved or constrained on their landward margin.
• Degradation of coastal ecosystems, especially wetlands and
coral reefs, has serious implications for the well-being of
societies dependent on the coastal ecosystems for goods and
services.
• Increased flooding and the degradation of freshwater, fisheries
and other resources could impact hundreds of millions of people
and socio-economic costs will escalate as a result of climate
change for coasts.

11. Coasts and people
Population and economic density in the coastal zone is greater than
other areas of the earth’s surface.

12. Climate change in the coastal
zone is an additional stress
• Human utilisation of the coast increased dramatically during
the 20th century
• This trend is virtually certain to continue through the 21st
century and coastal population could grow from 1.2 billion
people (in 1990) to 1.8 to 5.2 billion people by the 2080s.
• Increasing numbers of people and assets at risk at the coast
are subject to additional stresses by land-use and hydrological
changes in catchments, including dams that reduce sediment
supply to the coast.

13. Threatened deltas
Relative vulnerability of coastal deltas as indicated by the indicative
population potentially displaced by current sea-level trends to 2050
(Extreme > 1 million; high 1 million to 50,000; medium 50,000 to 5,000)

14. Threatened areas
• Populated deltas (especially Asian megadeltas), low-
lying coastal urban areas, and atolls are key societal
hotspots of coastal vulnerability, occurring where the
stresses on natural systems coincide with low human
adaptive capacity and high exposure.
• Regionally, south, south-east and east Asia, Africa
and small islands are most vulnerable.
• Climate change therefore reinforces the desirability of
managing coasts in an integrated manner.

15. Integrated assessment of
sea-level rise

16. Adaptation is challenging,
especially in developed countries
• While physical exposure can significantly influence the
vulnerability for both human populations and natural systems, a
lack of adaptive capacity is often the most important factor that
creates a hotspot of human vulnerability.
• Adaptive capacity is largely dependent upon development
status.
• Hence, while developing nations may have the political or
societal will to protect or relocate people who live in low-lying
coastal zones, their vulnerability is much greater than a
developed nation in an identical coastal setting.
• Vulnerability will also vary between developing countries, while
developed countries are not insulated from the adverse
consequences of extreme events.

17. Many adaptation options are
available
Coastal Adaptation Objectives Adaptation Responses Examples
Adaptation (Klein and Tol, 1997) (after Cooper et al., 2002;
(IPCC CZMS, DEFRA, 2001)
1990)
Advance the line Land claim; polders
Protect Increased robustness Hold the line Dyke; beach nourishment
Accommodate Increased flexibility 'Flood proof' buildings
Floating agricultural systems
Retreat the line Managed realignment
Retreat Enhanced adaptability Limited intervention Ad hoc seawall
No intervention Monitoring only
Reversing maladaptive Sustainable adaptation Wetland restoration
trends
Improved awareness and Community-focussed Flood hazard mapping; flood
preparedness adaptation warnings

18. Inaction will cost more than
adaptation
• Adaptation costs for climate change are much lower than
damage costs without adaptation for most developed coasts,
even considering only property losses and human deaths.
• As post-event impacts on coastal businesses, people, housing,
public and private social institutions, natural resources, and the
environment generally go unrecognised in disaster cost
accounting, the full benefits of adaptation are even larger.
• Without adaptation, the high-end sea-level scenarios combined
with other climate change (e.g., increased storm intensity) are
as likely as not to make some islands and low-lying areas
uninhabitable by 2100, so effective adaptation is urgently
required.

19. Sea-level rise will continue
beyond 2100
• Sea-level rise has substantial
inertia and will continue beyond
2100 for many centuries.
• Irreversible breakdown of the
West Antarctica and/or
Greenland ice sheets, if
triggered by rising temperature, 6000
Land area (10^3 km^2)
would make this long-term rise 5000
Population (millions)
GDP MER (US$ billions)
significantly larger, ultimately GDP PPP (US$ billions)
Count (see legend)
questioning the viability of
4000
many coastal settlements across 3000
the globe. 2000
1000
0
0 2 4 6 8 10
Height above high water (m)

20. Implications of long-term
change
• Settlement patterns also have substantial inertia, and
this issue presents a challenge for long-term coastal
spatial planning.
• Stabilisation of climate could reduce the risks of ice
sheet breakdown, and reduce but not stop sea-level
rise due to thermal expansion.
• Hence, it is now more apparent than the TAR that
the most appropriate response to sea-level rise for
coastal areas is a combination of adaptation to deal
with the inevitable rise, and mitigation to limit the
long-term rise to a manageable level.

21. Conclusions
• Climate change and sea-level rise present
significant challenges for coastal areas.
• While our knowledge needs to be expanded a
range of response options are available.
• A combination of adaptation and mitigation
would seem most appropriate.