Compared to spectacular geophysical phenomena such as earthquakes, volcanoes, and hurricanes, the contemporary rate of long-term sea level rise appears benign. For most populated regions of the world, sea level is increasing at a rate of only a few millimeters per year. There may be an additional component of rise from vertical land movement that locally alters the rate from the global value, but usually by no more than a factor of 2 or 3. Local sea level actually falls at some sites in tectonically active regions of the earth (e.g., in Japan and Alaska) or areas in the Baltic deeply covered by ice sheets at the last glacial maximum 21,000 years ago. However, these are the exceptions. The situation in the conterminous United States is more typical, where the local sea level is rising virtually everywhere at rates averaging about 2-3 mm/yr, with rates up to 6 mm/yr or more in some specific areas.
What makes rising water level so important to humanity is its potential to alter ecosystems and habitability in coastal regions, where an ever-increasing percentage of the population is located. In the United States, a majority of the populace live in the 25 states having coastlines on the Gulf of Mexico and the Atlantic and Pacific Oceans. It is further estimated that about one-half of this population lives within 50 miles of a shoreline (NRC, 1990). Considering the entire planet, Cohen et al. (1997) estimated that in 1994 about 2.1 billion people (37% of the world's population) lived within 100 km of a coast. For many persons, such as those living in the Bay of Bengal, sea level rise is a life-and-death issue because as sea level rises, vulnerability to severe storms also increases.
Nicholls and Leatherman (1994) summarize the physical effects of sea level rise in five categories. These are erosion of beaches and bluffs, increased flooding and storm damage, inundation of low-lying areas, salt intrusion into aquifers and surface waters, and higher water tables. The first of these impacts is the most familiar. Beach erosion is ubiquitous in the United States and is also a problem worldwide. Along the U.S. east coast about 80-90% of the beaches are eroding due to long- and short-term processes (Galgano, 1998;
Sea Level Rise
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Galgano et al., 1998). Bruun (1962) long ago proposed that long-term erosion of sandy beaches was a consequence of sea level rise. His model predicts that beaches will erode by 50-200 times the rate of increase of sea level, a far greater effect than simple inundation. Verification of the Bruun theory was achieved in a small wave tank by Schwarz (1965,1967) and for areas in Lake Michigan (Hands, 1983) where the tides are negligible and very large variations of water level occur in relatively short times. But validation of the concept for open ocean coastlines with their complex interplay of geological, meteorological, and oceanographic factors has remained elusive and controversial (Pilkey and Davis, 1987). However, Zhang (1998) and Leatherman etal. (2000) have demonstrated a relationship between sea level rise and beach erosion by analyzing a new large database of historical shoreline positions and sea level rise computed from tide gauge measurements. It has been found that open-ocean sandy beaches on the U.S. east coast not affected by inlets or engineering modifications erode at a rate that averages about 150 times the rate of sea level rise. There is no reason to believe that this result is not a general one for sandy beaches everywhere. As an example of the importance of erosion generated by sea level rise, consider the popular middle Atlantic beach resort of Ocean City, Maryland. The rate of sea level rise in the 20th century in this area has been about 3.5 mm/yr, which translates into a beach erosion rate of about 5 m per decade. Since the beach has a nominal width of only a few tens of meters, sand replenishment (often known as beach nourishment) is frequently required to provide a beach wide enough for recreational use and for protection of buildings from storm-generated waves and surges. If the rate of rise of sea level increases as a consequence of global warming, the already-expensive ($82 million to date for this community) beach nourishment program will become an even larger financial burden.
Rising sea levels have other important effects besides erosion. Low-lying coastal plains are vulnerable to inundation and suffer serious consequences of salt intrusion into aquifers. Some islands in the Chesapeake Bay that once supported farming and logging activities have become so contaminated with salt that agricultural activities are now impossible. Other islands in the Bay have disappeared (Kearney and Stevenson, 1991). The roughly 10,000 km of coastline in the Bay is eroding rapidly, creating serious economic, social, and political problems for the region. Rising sea levels also threaten coastal ecosystems when marshes drown because they cannot build upward (vertically accrete) fast enough. Marshes develop ponds as they drown, and ultimately can disappear entirely, as is happening now to the important wildlife refuge at Blackwater, Maryland.
Developing countries and island nations are especially at risk from rising sea levels. As a rule, they lack the resources for mitigation activity or relocation of populations to less threatened areas as sea level increases. The very existence of some island nations is in jeopardy. In terms of sheer numbers of affected persons, the greatest problem lies in populated river deltas. About
100 million people live within one meter of present-day mean sea level (Nicholls and Leatherman, 1995). As habitable land in the affected locations dwindles, neighboring areas could experience significant political stress from environmental refugees. These few examples of the effect of sea level rise give only a glimpse of the threat of increasing sea levels to the environment. A more detailed view is the subject of Chapter 8.
In the earth science community there is an increasing consensus that the overall global rate of sea level rise during the last approximately 100 years has been nearly 2 mm/yr (Warrick et al., 1996), sharply higher than the average rate during the last several millennia. This result is based on analyses of tide gauge data taken since the 19th century, historical land records, archeological data, and geological evidence from the late Holocene period. Possibly half of the apparent increase of global sea level during the past 100 years can be accounted for by thermal expansion of the oceans and melting of small glaciers (Warrick et al., 1996). The remainder remains unaccounted for; the obvious candidates are the Antarctic and Greenland ice sheets, but their ice mass balance is too poorly known for a definitive conclusion (Warrick et al., 1996; Bindschadler, 1998; Wingham et al., 1998). It is even possible that an additional rise of nearly 1 mm/yr during the last 50 years has been averted by net above-ground storage of water in large and small reservoirs, as discussed in Chapter 5. In any case, although the present (i.e., the past 100 years) rate of rise and its interpretation are subject to a certain amount of disagreement, it is a fact that sea level is rising in most coastal regions just when rapid coastal development is taking place. If global warming and an increased rate of sea level rise occur in the next century (Warrick et al., 1996), present-day problems attributable to sea level increase will be exacerbated.
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