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Warmth doth stretch Antarctica's tongues.

It seems as obvious as an ice cube in a glass of water. When greenhouse gases in the atmosphere warm the Earth's climate, Antartica's glaciers should melt around the edges and shrink in size.

But glacial ice does not behave so simply. New evidence suggests that during warmer times in the past, Antartica's icy fringes have stuck out even farther than they do today, according to Eugene W. Domack from Hamilton College in Clinton, N.Y., and colleagues from the University of Arizona and the Geological Survey of Japan. "What this says is that under warmer conditions, the Antarctic glaciers would expand rather than melt and recede," says Domack.

The team reached this conclusion after studying sediments from three sites near outlet glaciers, which carry ice from East Antarctica's interior into the sea. The three sites all sit offshore from ice shelves or smaller ice tongues, aprons of glacial ice that extend into the ocean.

Domack and his co-workers used radiocarbon techniques to date changes in the sediments corresponding to ice advances or retreats. Certain layers of the sediments contain pebbles and ground-up rocks -- recording a time when the ice reached far out over the ocean. In contrast, other layers hold the remains of marine algae from a time when the ice edge had retreated, leaving these sites covered by open water.

In the November GEOLOGY, the researchers report that ice shelves and tongues had reached their greatest extent between 7,000 and 4,000 years ago -- a time when global temperatures were about 1[degrees]C or 2[degrees]C warmer than they are today. Since then, the ice ed ge has retreated at these three sites.

Domack thinks that the warmer climate 7,000 years ago caused the extension of outlet glaciers. That theory dovetails with recent research on snowfall in the Antarctic, which suggests that a modest hike in air temperatures could increase the ice volume on that continent. As air temperatures rise, the atmosphere can hold more water vapor; hence, more snow would fall over Antarctica. Additional snow falling on the outlet glaciers could cause them to extend farther out over the water, says Domack.

The same does not hold true for all glaciers, however. Ice in warmer locations lies closer to the melting point than does Antarctic ice. So any global warming should cause these less stable glaciers to retreat, Domack says.

Mark F. Meier, a glaciologist with the University of Colorado at Boulder, believes Domack and his colleagues must collect more evidence from other sites around Antarctica to prove their case. "[Their theory] may be right, but I'd hate to go to court with it," he says.

The behavior of outlet glaciers can ripple far beyond Antarctica. If these ice shelves and tongues extend in the future, they can raise global sea levels, says Meier. Conversely, increased precipitationi in the interior of Antarctica pulls moisture out of the ocean, slowing the rise in sea level. Because scientists do not know which of these two processes will outweigh the other in Antarctica, they remain unsure how quickly global sea levels will rise over the next century, says Terence J. Hughes of the University of Maine, Orono.
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Title Annotation:greenhouse effect and glacial ice
Author:Monastersky, Richard
Publication:Science News
Date:Nov 16, 1991
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