Rising sea levels: predictions and plans.

Rising sea levels: Predictions and plans

Global warming as a result of the "greenhouse' effect will cause the water in the world's oceans to expand, raising the average sea level by 4 to 8 centimeters in the next 40 years, according to British researchers working with computer models of the ocean.

These estimates of the "thermal' expansion of the ocean, reported in the Nov. 12 NATURE, "are the most sophisticated calculations published yet,' says Tom M.L. Wigley, who worked with Sarah C.B. Raper at the University of East Anglia's Climatic Research Unit in Norwich.

Over the past decade, scientists have started to explore the implications of a warmer climate as they continue to observe carbon dioxide and methane accumulating in the atmosphere. Like the glass of a greenhouse, these and other gases are thought to trap heat radiated by the earth, thereby warming the planet. Indeed, some findings indicate that global temperatures are already rising.

Wigley and Raper worked with an ocean model that represents the transfer of heat between a warming atmosphere and the ocean. As input for the model, they adopted a standard estimation that temperatures would rise 0.6 to 1| C by the year 2025.

Such a rise not only would cause the thermal expansion of the oceans, but also would hasten the melting of glaciers and possibly even the icecaps that cover Greenland and the Antarctic. Though meltwater from ice would swell the oceans more than thermal expansion, it is more difficult to forecast how a temperature rise would affect melting, says Wigley. A 1983 report by the National Research Council (NRC), considering all factors, estimated that sea levels will rise 70 centimeters by the year 2075.

A new NRC report released in September explores the engineering implications of such a rise. While large increases in sea level would obviously flood lowlying areas, even small increases would cause significant damage, says Robert G. Dean of the University of Florida at Gainesville, who headed the committee drafting the report.

For every 1-centimeter rise in sea level, 1 meter of sandy beach on the outer coast would erode under the incessant pounding of waves, according to the report. For a 10-centimeter rise in the ocean level, the boundary between saltwater and freshwater at the mouths of rivers would advance 1 kilometer into the river, and saltwater would similarly advance into underground deposits of freshwater. Such a saline intrusion causes concern for drinking-water supplies and for the animal habitats in estuaries.

While the average sea level is now rising by a fraction of a centimeter per year, scientists expect the rate to increase in the future. Says Dean, "There's really no cause for immediate alarm, but it's also not a time for complacency. The next three or four decades should be used to plan for the rise.'

The NRC report recommends that decision-making models be used to help coastal planners determine whether to abandon or to fortify facilities and areas threatened by the encroaching ocean. Many power plants built in low areas are designed with a limited lifetime, and so could be abandoned. But airports built on landfills could require dikes and pumping stations.

According to the report, scientists studying the rise will need more monitoring stations that record not only the sea level rise but also the rate of beach erosion and other secondary effects. This is especially true in the relatively unmonitored Southern Hemisphere.

Though sea levels on the average will rise due to global warming, other, unrelated processes will also affect these levels and may even offset the rise in certain areas. For example, the earth's crust under Scandinavia is still reacting to the retreat of the Ice Age glaciers. Without the weight of that ice, the crust is rebounding and rising, thereby lowering the sea level along that coast.