Coastal systems and low-lying areas
Since the TAR, our understanding of the implications of climate change for coastal systems and low-lying areas (henceforth referred to as ‘coasts’) has increased substantially, and six important policy-relevant messages emerge.
Coasts are experiencing the adverse consequences of hazards related to climate and sea level (very high confidence).
Coasts are highly vulnerable to extreme events, such as storms, which impose substantial costs on coastal societies [6.2.1, 6.2.2, 6.5.2]. Annually, about 120 million people are exposed to tropical cyclone hazards. These killed 250,000 people from 1980 to 2000 [6.5.2]. Throughout the 20th century, the global rise of sea level contributed to increased coastal inundation, erosion and ecosystem losses, but the precise role of sea-level rise is difficult to determine due to considerable regional and local variation due to other factors [6.2.5, 6.4.1]. Late 20th century effects of rising temperature include loss of sea ice, thawing of permafrost and associated coastal retreat at high latitudes, and more frequent coral bleaching and mortality at low latitudes [6.2.5].
Coasts are very likely to be exposed to increasing risks in future decades due to many compounding climate-change factors (very high confidence).
Anticipated climate-related changes include: an accelerated rise in sea level of 0.2 to 0.6 m or more by 2100; further rise in sea surface temperatures of 1 to 3°C; more intense tropical and extra-tropical cyclones; generally larger extreme wave and storm surges; altered precipitation/runoff; and ocean acidification [WG1 AR4 Chapter 10; 6.3.2]. These phenomena will vary considerably at regional and local scales, but the impacts are virtually certain to be overwhelmingly negative [6.4, 6.5.3]. Coastal wetland ecosystems, such as salt marshes and mangroves, are very likely threatened where they are sediment-starved or constrained on their landward margin [6.4.1]. The degradation of coastal ecosystems, especially wetlands and coral reefs, has serious implications for the well-being of societies dependent on coastal ecosystems for goods and services [6.4.2, 6.5.3]. Increased flooding and the degradation of freshwater, fisheries and other resources could impact hundreds of millions of people, and socio-economic costs for coasts are virtually certain to escalate as a result of climate change [6.4.2, 6.5.3].
The impact of climate change on coasts is exacerbated by increasing human-induced pressures (very high confidence).
Utilisation of the coast increased dramatically during the 20th century and this trend is virtually certain to continue through the 21st century. Under the SRES scenarios, the coastal population could grow from 1.2 billion people (in 1990) to between 1.8 billion and 5.2 billion people by the 2080s, depending on future trends in coastward migration [6.3.1]. Hundreds of millions of people and major 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 [6.3]. Three key hotspots of societal vulnerability are: (i) deltas (see Figure TS.8), especially the seven Asian megadeltas with a collective population already exceeding 200 million; (ii) low-lying coastal urban areas, especially those prone to subsidence; and (iii) small islands, especially coral atolls [6.4.3].
Adaptation for the coasts of developing countries is virtually certain to be more challenging than for coasts of developed countries (high confidence).
Developing countries already experience the most severe impacts from present coastal hazards [6.5.2]. This is virtually certain to continue under climate change, even allowing for optimum adaptation, with Asia and Africa most exposed [6.4.2, B6.6, F6.4, 6.5.3]. Developing countries have a more limited adaptive capacity due to their development status, with the most vulnerable areas being concentrated in exposed or sensitive settings such as small islands or deltas [6.4.3]. Adaptation in developing countries will be most challenging in these vulnerable ‘hotspots’ [6.4.3].
Adaptation costs for vulnerable coasts are much less than the costs of inaction (high confidence).
Adaptation costs for climate change are virtually certain to be much lower than damage costs without adaptation for most developed coasts, even considering only property losses and human deaths [6.6.2, 6.6.3]. 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, it is virtually certain that the full benefits of adaptation are even larger [6.5.2, 6.6.2]. Without action, the highest sea-level scenarios combined with other climate change (e.g., increased storm intensity) are about as likely as not to make some low-lying islands and other low-lying areas (e.g., in deltas and megadeltas) uninhabitable by 2100 [6.6.3]. Effective adaptation to climate change can be integrated with wider coastal management, reducing implementation costs among other benefits [6.6.1.3].
The unavoidability of sea-level rise, even in the longer term, frequently conflicts with present-day human development patterns and trends (high confidence).
Sea-level rise has substantial inertia and will continue beyond 2100 for many centuries [WG1 AR4 Chapter 10]. Breakdown of the West Antarctic and/or Greenland ice sheets would make this long-term rise significantly larger. For Greenland, the temperature threshold for breakdown is estimated to be about 1.1 to 3.8°C above today’s global average temperature. This is likely to happen by 2100 under the A1B scenario [WG1 AR4 Chapter 10]. This questions both the long-term viability of many coastal settlements and infrastructure (e.g., nuclear power stations) across the globe and the current trend of increasing human use of the coastal zone, including a significant coastward migration. This issue presents a challenge for long-term coastal spatial planning. Stabilisation of climate is likely to reduce the risks of ice sheet breakdown, and reduce but not stop sea-level rise due to thermal expansion [B6.6]. Hence, since the IPCC Third Assessment it has become virtually certain 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 [6.6.5, 6.7].