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Sea Level and Climate

Sea Level :Introduction
Global sea level and the Earth's climate are closely linked. The Earth's climate has warmed about 1°C (1.8°F) during the last 100 years. As the climate has warmed following the end of a recent cold period known as the "Little Ice Age" in the 19th century, sea level has been rising about 1 to 2 millimeters per year due to the reduction in volume of ice caps, ice fields, and mountain glaciers in addition to the thermal expansion of ocean water. If present trends continue, including an increase in global temperatures caused by increased greenhouse-gas emissions, many of the world's mountain glaciers will disappear. For example, at the current rate of melting, all glaciers will be gone from Glacier National Park, Montana, by the middle of the next century (fig. 1). In Iceland, about 11 percent of the island is covered by glaciers (mostly ice caps). If warming continues, Iceland's glaciers will decrease by 40 percent by 2100 and virtually disappear by 2200. Most of the current global land ice mass is located in the Antarctic and Greenland ice sheets (table 1). Complete melting of these ice sheets could lead to a sea-level rise of about 80 meters, whereas melting of all other glaciers could lead to a sea-level rise of only one-half meter.

Glacial-Interglacial Cycles
Climate-related sea-level changes of the last century are very minor compared with the large changes in sea level that occur as climate oscillates between the cold and warm intervals that are part of the Earth's natural cycle of long-term climate change.

During cold-climate intervals, known as glacial epochs or ice ages, sea level falls because of a shift in the global hydrologic cycle: water is evaporated from the oceans and stored on the continents as large ice sheets and expanded ice caps, ice fields, and mountain glaciers. Global sea level was about 125 meters below today's sea level at the last glacial maximum about 20,000 years ago (Fairbanks, 1989). As the climate warmed, sea level rose because the melting North American, Eurasian, South American, Greenland, and Antarctic ice sheets returned their stored water to the world's oceans. During the warmest intervals, called interglacial epochs, sea level is at its highest. Today we are living in the most recent interglacial, an interval that started about 10,000 years ago and is called the Holocene Epoch by geologists.

Sea levels during several previous interglacials were about 3 to as much as 20 meters higher than current sea level. The evidence comes from two different but complementary types of studies. Old shoreline features provide one line of evidence. Wave-cut terraces and beach deposits from regions as separate as the Caribbean and the North Slope of Alaska suggest higher sea levels during past interglacial times. A second line of evidence comes from sediments cored from below the existing Greenland and West Antarctic ice sheets. The fossils and chemical signals in the sediment cores indicate that both major ice sheets were greatly reduced from their current size or even completely melted one or more times in the recent geologic past. The precise timing and details of past sea-level history are still being debated, but there is clear evidence for past sea levels significantly higher than current sea level.

Potential Sea-Level Rise
If Earth's climate continues to warm, then the volume of present-day ice sheets will decrease. Melting of the current Greenland ice sheet would result in a sea-level rise of about 6.5 meters; melting of the West Antarctic ice sheet would result in a sea-level rise of about 8 meters (table 1). The West Antarctic ice sheet is especially vulnerable, because much of it is grounded below sea level. Small changes in global sea level or a rise in ocean temperatures could cause a breakup of the two buttressing ice shelves (Ronne/Filchner and Ross). The resulting surge of the West Antarctic ice sheet would lead to a rapid rise in global sea level.

Reduction of the West Antarctic and Greenland ice sheets similar to past reductions would cause sea level to rise 10 or more meters. A sea-level rise of 10 meters would flood about 25 percent of the U.S. population, with the major impact being mostly on the people and infrastructures in the Gulf and East Coast States (figure below).

Researchers at the U.S. Geological Survey and elsewhere are investigating the magnitude and timing of sea-level changes during previous interglacial intervals. Better documentation and understanding of these past changes will improve our ability to estimate the potential for future large-scale changes in sea level.

Impacts of sea level rise
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. According to another example given by Day and Templett [1989] that discusses the problem of land loss in the Mississippi delta, with 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.