Many fine reviews of the impact of sea level rise on coastal environments are available. Bird [1993], Warrick et al., [1993], and Nicholls and Leatherman [1994] emphasize and document the serious consequences of even a few mm per year increase of sea level. An excellent educational video presentation on the subject entitled Vanishing Lands, produced by the Laboratory for Coastal Research at the University of Maryland (College Park), vividly presents the impact of rising sea level in the Chesapeake Bay region of the United States.
Nicholls and Leatherman [op. cit.] summarize the physical effects of sea level rise into 5 categories. These are inundation of low-lying areas, erosion of beaches and bluffs, salt intrusion into aquifers and surface waters, higher water tables, and increased flooding and storm damage. All of these effects have important impacts, but I shall consider only the first two in this review because they have had and are continuing to have very dramatic impacts on coastal regions worldwide.
A rise of a few mm per year by the sea, although not threatening spectacular inundation of the sort described earlier, is still extremely important. Direct land loss of low lying areas can rapidly (decadal to centennial periods) damage or destroy coastal ecosystems. A good illustration of this occurs in the Chesapeake Bay. The average rate of relative sea level rise (RSLR) in this area has been approximately 3.5 mm per year during the twentieth century. This is about twice the global value of sea level rise. Regional subsidence, discussed below, is responsible for the increment over the global value. Downs et al. [1994] describe the widespread loss of the Bay's wetlands due to this increase of sea level. As just one example of the rapid and inexorable change that can occur, one third of the area of the Blackwater Wildlife Refuge area near Cambridge, Maryland was lost by inundation between 1938 and 1979.
This problem of land loss due to relative sea level rise is
pervasive in coastal areas, and is aggravated by high levels of
local subsidence. Another example is given by Day and
Templett [1989]. They discuss the problem of land loss in the
Mississippi delta, which has the largest rate of loss in the United
States. There has been a net loss of wetlands of up to 100 km
per year in the delta area during this century, and the situation
will be aggravated if sea level rise increases by the amounts
predicted to accompany global warming in the future.
In addition to inundation, long-term sea level rise can cause erosion and shoreline retreat by creating a sediment budget deficit. For a recent and thorough discussion of this effect, see Bird, [1993]. Simplifying considerably, for a sandy beach region the nearby seafloor profile takes on a shape primarily dependent on sand grain size, and secondarily on the energy of the incoming waves. In particular, the higher the energy of the waves, the greater the depth at which the wave action will disturb the depth profile, and the further offshore this limiting effect will occur. Thus the critical parameter for the nearshore depth profile is the ratio of the distance L (the active profile width) offshore at which the waves have an effect on the bottom, divided by the water depth D (depth of closure) at that distance. The dimensionless ratio L/D varies from about 50-200, with 100 being a typical value.
When sea level rises, the nearshore bottom profile changes in an attempt to gain a new equilibrium and restore the ratio L/D. This equilibrium is achieved by erosion of the shore by the amount of the sea level rise multiplied by the ratio L/D. (This relation of erosion, sea level rise, active profile width, and depth of closure is widely known as the Bruun rule [ Bird, 1993], after its developer.) Since L/D is usually of order 100, the extent of horizontal beach erosion can be typically two orders of magnitude greater than the amount of the sea level rise. Thus while the up to 4 mm per year long term rise in the middle Atlantic region of the U.S. does not seem like much, a horizontal beach loss of 100 times this amount each year (that is, 4 meters in a decade) is significant indeed. Consider Virginia Beach, Virginia. This beach is now only 30-50 meters wide even after a major beach nourishment project.
It is clear from the above discussion that developing countries with large populations in or near deltas and other low-lying areas are especially vulnerable to future sea level rise. Nicholls and Leatherman [1994] have made estimates of the social and economic costs of sea level rise for many such nations. Their projections show that scores of millions of persons will be affected, and that direct economic losses and mitigation activities will pose serious financial burdens.