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How warming helps Antarctic ice.

Climate warming in the last half century has destroyed several of the floating glaciers--called ice shelves--that skirt the coastline of Antarctica. The demise of these smaller shelves has raised questions about the vulnerability of Antarctica's two largest shelves, each nearly the size of Texas.

A new study may cool some of that concern. Analysis of water currents beneath the giant Filchner-Ronne ice shelf suggests that warming could thicken the floating ice rather than melt it, reports Keith W. Nicholls of the British Antarctic Survey in Cambridge. His findings appear in the July 31 Nature.

To study the Filchner-Ronne shelf, the British survey drilled holes through the 800-meter-thick ice and lowered instruments into the ocean beneath. The survey's measurements of temperature and currents revealed an apparent paradox: The water flowing under the shelf is warmer in winter than in summer.

This anomaly occurs because of sea ice formation near the front of the shelf in winter. The freezing process locks up freshwater in ice and leaves behind extra-salty water, which sinks to the bottom even though it is slightly warmer than the surrounding water. The salty current flows under the shelf and melts the ice. Sea ice doesn't grow during summer, so the salty current under the ice slackens and melting slows.

If wintertime warming in the future reduces the formation of sea ice, it will weaken the salty current beneath the ice shelf and inhibit melting, suggests Nicholls. As long as the pattern of currents remains similar to today's, "the response of the ice shelf to a warming of the climate will be for it to thicken, rather than threatening its longevity," he says.

Even in this scenario, other processes could threaten the ice shelf, says Richard B. Alley of Pennsylvania State University in State College. Melted water on the surface could weaken the shelf by draining into crevasses. "What this really points out is how blastedly complex this system is and how much we have to learn," says Alley.
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Title Annotation:researcher Keith W. Nichols found evidence that global warming has thickened the Filchner-Ronne ice shelf rather than caused it to thin, as previously supposed
Author:Monastersky, Richard
Publication:Science News
Article Type:Brief Article
Date:Aug 30, 1997
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