The latest published attempt to forecast increases in sea level suggests that the water will continue rising at least as fast as it has over the last 100 years, during which the world's oceans rose four to six inches. But the forecast has also identified a major uncertainty that could mean sea level will rise considerably faster.
The sea level is rising slowly because the atmosphere is warming, probably as a result of the greenhouse effect. The world's average air temperature is now about one degree Fahrenheit warmer than a century ago.
At least two results of the warming are known to affect sea level: melting of small glaciers and thermal expansion of sea water.
Water, like any substance, expands as it warms because its atoms vibrate through a larger space and keep each other farther apart. A third possible result of the warming remains uncertain. It is not clear whether significant melting has occurred in the massive ice sheets blanketing Greenland and Antarctica.
Between 1985 and 2025, rising atmospheric concentrations of carbon dioxide and other Earth-insulating "greenhouse gases" are expected to boost global temperatures another one to two degrees, raising it in 40 years more than in all of the last 100.
Two climatologists who analyzed the situation said this expected warming should cause the sea level to rise at the pace of the last century because of thermal expansion alone. If the big ice sheets also begin to melt faster, their contribution in forthcoming decades could lead to even faster flooding of coastal regions.
The analysis was published in the Nov. 12 issue of Nature by T.M.L. Wigley of the National Center for Atmospheric Research in Boulder, Colo., and S.C.B. Raper of Britain's University of East Anglia.