Coastal environments occupy one of the most dynamic interfaces on Earth, at the boundary between land and sea, and they support some of the most diverse and productive habitats. These habitats include natural ecosystems, in addition to important managed ecosystems, economic sectors, and major urban centers. The existence of many coastal ecosystems is dependent on the land-sea connection or arises directly from it (e.g., deltas and estuaries). Coastal ecosystems can encompass a wide range of environmental conditions over short distances, particularly of salinity (from fresh to hypersaline) and energy (from sheltered wetlands to energetic wave-washed shorelines). At a much coarser geographical scale, there is a spectrum of climate types—from tropical to polar—with concomitant broad-scale differences in biogeophysical processes and features. Coastal environments, settlements, and infrastructure are exposed to land-sourced and marine hazards such as storms (including tropical cyclones), associated waves and storm surges, tsunamis, river flooding, shoreline erosion, and influx of biohazards such as algal blooms and pollutants. All of these factors need to be recognized in assessing climate-change impacts in the coastal zone.

A summary of potential impacts appears in Box 6-3. Note, however, that owing to the great diversity of coastal environments; regional and local differences in projected relative sea level and climate changes; and differences in the resilience and adaptive capacity of ecosystems, sectors, and countries, the impacts summarized here will be highly variable in time and space and will not necessarily be negative in all situations.

Some natural features of the shore zone provide significant coastal protection, including coral reefs (the most extensive, massive, and effective coastal protection structures in the world); sand and gravel beaches, which function as wave energy sinks; and barrier beaches, which act as natural breakwaters. Coastal dunes form natural buffers and sand repositories, from which sand may be extracted during storms without major shoreline retreat; coastal vegetation often absorbs wind or wave energy, retarding shoreline erosion. Even the value of salt marsh as a sea defense (King and Lester, 1995) and mangroves as a sediment trap (Solomon and Forbes, 1999) have been recognized. These functions of natural coastal systems contribute to resilience, as discussed in Section 6.6.2.

Bijlsma et al. (1996) and the various regional reports in IPCC (1998) identify the areas of greatest sensitivity to accelerated sea-level rise. These areas comprised low-elevation coral atolls and reef islands, as well as low-lying deltaic, coastal plain, and barrier coasts, including sandy beaches, coastal wetlands, estuaries, and lagoons. To this list can be added coarse gravel beaches and barriers, especially if sediment-starved; cliffed coasts in unlithified deposits, particularly where the proportion of sand and gravel is limited; and ice-rich cliffed coasts in high latitudes. Bold and rock-dominated coasts are relatively less vulnerable but often include coastal reentrants with beaches, estuaries, or deltas, which may represent areas of localized vulnerability. On such coasts, wave runup and overtopping can be a factor that threatens infrastructure situated well above mean sea level (Forbes, 1996).

It is important to recognize that vulnerable coastal types in many parts of the world already are experiencing relative sea-level rise, from a combination of subsidence and the global component of sea-level rise identified to date. Submergence rates of 2.5 mm yr^-1 or more are not uncommon, and higher rates apply locally, such as in parts of China (Ren, 1994), the United States (Dean, 1990), Canada (Shaw et al., 1998a), and Argentina (Codignotto, 1997). Although this sea-level rise implies enhanced vulnerability, it also provides a basis for assessing coastal response to various rates of relative sea-level rise, where similar coastal types, boundary conditions, and system properties can be identified. Numerous studies along the U.S. Atlantic coast, where relative sea level is rising at rates of 2-4 mm yr^-1, have demonstrated common patterns of barrier beach retreat by washover and ephemeral inlet processes (Leatherman et al., 2000). More rapid retreat is recorded in delta-margin settings characterized by rapid subsidence (e.g., Stone and McBride, 1998).

In addition to submergence, seawater intrusion into freshwater aquifers in deltaic and nondeltaic areas is an increasing problem with rising sea level (Moore, 1999). This intrusion has been documented in diverse environments such as the arid Israeli coast, the humid Thailand coast, the Chinese Yangtze Delta, the Vietnamese Mekong Delta, and low-lying atolls (e.g., Melloul and Goldberg, 1997; Chen and Stanley, 1998; Singh and Gupta, 1999).

Although some low, sediment-starved, gravel barrier beaches show rapid retreat under rising sea level, this process is highly nonlinear and in some cases is more closely related to storm event frequency and severity (Forbes et al., 1997a). The response of coasts to storm-related sea-level variations around the North Sea has not been determined, although past increases in the winter means of high water levels of the order of 1-2 mm yr^-1 have taken place (Langenberg et al., 1999).

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