Will the Sea Level Change?
Warmer temperatures are expected to raise sea level by expanding ocean water, melting mountain glaciers, and melting parts of the Greenland Ice Sheet. Warmer temperatures also increase precipitation, as described below. Snowfall over Greenland and Antarctica is expected to increase by about 5 percent for every 1ºF warming in temperatures. Increased snowfall tends to cause sea level to drop if the snow does not melt during the following summer, because the only other place for the water to be is the ocean. (The amount of water in the atmosphere is less than the water it takes to raise the oceans one millimeter). Considering all of these factors, the IPCC estimates that sea level will rise 9 to 88 cm by the year 2100. A recent EPA study estimated that global sea level has a 50 percent chance of rising 45 cm (1-1/2 ft) by the year 2100, but a 1-in-100 chance of a rise of about 110 cm (over 3-1/2 ft). The following projection of sea level rise comes from the IPCC Third Assessment Report (2001)
Over the longer run, more substantial changes in sea level are possible. Some scientists believe that the West Antarctic Ice Sheet could slide into the oceans after a sustained warming, or if other factors raised sea level. The vulnerability of this ice sheet is poorly understood. It contains enough ice to raise sea level 6 meters (20 feet), and coastal scientists generally agree that sea level was 20 feet higher than today during the last interglacial period, which was only slightly warmer than today. While some scientists have suggested that there is fossil evidence on the polar ocean floor that this ice sheet collapsed during the last interglacial period, there is no scientific consensus on this question.
An EPA study solicited the opinions of eight U.S. glaciologists on the vulnerability of this ice sheet. All but one concluded that Antarctica is most likely to have a negligible contribution to sea level over the next century. Nevertheless, they all agreed that there is some risk that a catastrophic collapse of the ice sheet could occur over a couple of centuries if polar water temperatures warm by a few degrees. Most of the scientists estimated that such a risk had a probability of between 1 and 5 percent. Because of this risk, as well as the possibility of a larger than expected melting of the Greenland Ice Sheet, the EPA study estimated that there is a 1 percent chance that global sea level could rise by more than 4 meters (almost 14 feet) in the next two centuries.
Sea level rise along the US coast is likely to be somewhat greater than the global average. The EPA study includes a set of projections that coastal residents can use to calculate how much sea level will rise in specific communities. Along the coast of New York, which typifies the U.S. Coast, sea level is likely to rise 26 cm (10 inches) by 2050 and 55 cm (almost 2 feet) by 2100. There is also a 1 percent chance of a 55 cm rise by 2050, a 120 cm rise (4 ft) by 2100, and a 450 cm rise (15 feet) by the year 2200.