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temperature or salinity of the upper ocean layers cause expansion or contraction of the water volume. These relatively short-term changes in sea level may persist for a few days, several months, or even several years, and their magnitude may be as much as 5 to 15 cm (2 to 6 inches).

Climate-Related Sea Level Change

Climate-related contributions to sea level change can be associated either with variations in the actual mass of water in the ocean basins or with thermal expansion (due to changing density and thus variations of temperature and salinity).

The mass of water at or near the earth's surface is practically constant for periods of 10,000 years or less. What matters for sea level is the partitioning of this mass of water among the major hydrologic reservoirs. The four major reservoirs are the oceans (1,370 million km3), ice (30 million km3), surface waters (8 to 19 million km3), and atmospheric moisture (0.01 million km3) (National Research Council, 1990). The melting of the northern continental ice sheets between 15,000 and 7,000 years before the present probably accounted for most of the rise of the sea to current levels.

Some have suggested that greenhouse warming could lead to disintegration of the West Antarctic Ice Sheet, most of which is grounded below sea level. If climate becomes warmer, and warmer ocean water intrudes under the ice sheet, the release of ice from the sheet would accelerate. Estimates suggest that several hundred years would be required to achieve this amount of warming (Bryan et al., 1988; Meier, 1990). The current estimated effect on sea level of the West Antarctic Ice Sheet is -0.6 ± 0.6 mm (-0.02 ± 0.02 inches) per year, or a net decrease. Glaciers other than the West Antarctic and Greenland ice sheets have been estimated to have contributed about 0.46 ± 0.26 mm (0.017 ± 0.01 inches) per year to sea level rise since 1900 (Meier, 1990).

Differences in water temperature, or in a combination of temperature and salinity, account very well for seasonal and interannual variations in sea level (National Research Council, 1990). This thermal expansion is not large enough, however, to account for the changes over tens of thousands of years. Warming the entire ocean from 0°C (32°F) to the current global average temperature of about 15°C (59°F) would involve thermal expansion of only about 10 m (about 30 feet).

Evidence of Sea Level Rise over the Last 100 Years

Several studies of various periods during the last 100 years are in general agreement that mean sea level is rising (see the following reviews: Aubrey, 1985; Barnett, 1985; Robin, 1986). Estimates range from about 0.5 to 3.0