Dec. 28, 2000 The West Antarctic Ice Sheet’s contribution to global sea-level rise may be much slower today than it was in the past. New evidence indicates that the size of the ice sheet thousands of years ago has been overestimated and the ice sheet may not have been as big or as steady a source of sea-level rise as scientists thought.
Glaciologist Robert Bindschadler from NASA’s Goddard Space Flight Center (Greenbelt, Md.) will discuss the latest research results and changing views of the West Antarctic Ice Sheet’s history at the American Geophysical Union’s annual meeting in San Francisco on December 16.
"Our previous best estimates that the ice sheet is adding 1 millimeter per year to global sea level are almost certainly too high," says Bindschadler. This revised assessment is based on a synthesis of new data including past sea-level rise estimates presented at a workshop this fall on the West Antarctic Ice Sheet organized by Bindschadler.
Calculations of how much and how fast the ice sheet has thinned and retreated since the peak of the Earth’s last major ice age 20,000 years ago are based in large part on a recent reconstruction of how big the ice sheet was during that last glacial maximum. That reconstruction included a West Antarctic Ice Sheet three times as large as it is now. Currently, the ice sheet averages 2000 meters thick, covers an area the size of Mexico, and contains enough water to raise global sea level 5 meters.
But analysis of a 30-by-50-mile rise in the ice sheet near the Ross Ice Shelf called Siple Dome suggests that this feature was not overrun by a massive ice sheet in the past, which is what the reconstruction suggests. A team of glaciologists from the University of Washington led by Charles Raymond used an ice-penetrating radar to study the subsurface layering of Siple Dome.
Another line of evidence that throws the ice sheet's ancient bulk into question is the discovery that the ice sheet was still growing as recently as 8,000 years ago. The reconstruction assumed that the ice sheet reached its maximum growth 20,000 years ago and has only been in retreat since then.
According to a new reconstruction of historic sea-level around the world by W. R. Peltier of the University of Toronto, a major jump in sea level occurred before the West Antarctic Ice Sheet began its current retreat, but there is no sign of a subsequent rise large enough to account for melting of so much West Antarctic ice.
The question of how fast the ice sheet retreated still challenges scientists. Recent work, however, leads Bindschadler to conclude that the ice sheet experienced a rapid retreat phase some 7,000 years ago that was preceded and followed by a slower retreat that continues today. Bindschadler points to the geologic record of dated stages in the retreat of the ice sheet’s continental base as evidence that it has shrunk in fits and starts. Such episodic retreats may be controlled more by the varying depth of the underlying surface and water than by the changing climate.
"The portion of the West Antarctic Ice Sheet we have focused on for the past ten years appears to be in stage of near-zero retreat now," says Bindschadler, "but what it will do in the future is still uncertain.
"If you extend the new evidence and the new line of reasoning into the future, the behavior of the ice sheet is more difficult to predict. It suggests, however, that if the ice sheet loses its hold on the present shallow bed it is resting on, the final retreat could be very rapid."
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