6.4. Coastal Systems
6.4.1. General Considerations
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 typesfrom tropical to polarwith
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.
Box 6-3. Potential Impacts of Climate Change
and Sea-Level Rise on Coastal Systems
Biophysical impacts can include the following:
- Increased coastal erosion
- Inhibition of primary production processes
- More extensive coastal inundation
- Higher storm-surge flooding
- Landward intrusion of seawater in estuaries and aquifers
- Changes in surface water quality and groundwater characteristics
- Changes in the distribution of pathogenic microorganisms
- Higher SSTs
- Reduced sea-ice cover.
Related socioeconomic impacts can include the following:
- Increased loss of property and coastal habitats
- Increased flood risk and potential loss of life
- Damage to coastal protection works and other infrastructure
- Increased disease risk
- Loss of renewable and subsistence resources
- Loss of tourism, recreation, and transportation functions
- Loss of nonmonetary cultural resources and values
- Impacts on agriculture and aquaculture through decline in soil and water quality.
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).