WASHINGTON -- The world's leading experts on the West Antarctic Ice Sheet met recently for the first time to discuss the latest findings on the stability of the Earth's most precariously perched ice sheet. On Sept. 13-18, 1998, at the University of Maine, Orono, scientists reported on all aspects of the ice sheet, from deep ice cores offering clues of past climates to satellite views of flowing ice streams.
For the past decade, scientists have scrambled over the vast, remote West Antarctic Ice Sheet hoping to discover if or when the giant ice mass will slide into the ocean and send sea level swiftly upward, submerging coastal communities. Rapid changes in sea level have happened in the past, according to Robert A. Bindschadler, a glaciologist at NASA's Goddard Space Flight Center. But the important question for scientists is what is going to happen in the future.
"For the last 4,000 to 5,000 years there haven't been any major jumps in sea level. Now about half the world's population lives near the coast. We've set ourselves up to be vulnerable," said conference organizer Bindschadler. Sea level is currently Increasing very slowly -- just a few millimeters each year -- but if the ice streams moving toward the sea start speeding up, sea level could rise several inches a year. According to Bindschadler, the entire West Antarctic Ice Sheet has enough mass to raise sea level more than 15 feet.
The West Antarctic Ice Sheet lies mainly in the South Pacific ocean and contains over 3 million cubic kilometers of ice covering about ten percent of the total Antarctic area. It is Earth's only ice sheet that could possibly collapse and slide into the ocean, said Bindschadler, because it sits atop slippery marine sediments below sea level.
Charles R. Bentley, a University of Wisconsin glaciologist and one of the researchers speaking at the conference, said it is important to keep in mind how long such a collapse may take. If the collapse takes place over 5,000 years, the change in sea level would only be about 1 millimeter per year, about half as fast as the current rise. "That sort of thing could hardly be considered a disaster," he said. But, Bentley added, if it takes only 100 years, sea levels could rise 50 millimeters (2 inches) per year.
To understand the behavior of the West Antarctic Ice Sheet, scientists must understand how the ice moves. Ice streams -- currents of fast flowing ice -- are the ice sheet's superhighways, swiftly transporting land into the sea. Ice streams are different than glaciers in that they are bordered by ice instead of rock. The ice streams flow 10 to 100 times faster than the ice surrounding them, with speeds between 1500-3000 feet per year. Ice streams average about 50 miles across and 62 miles long. Each stream dumps 3.5-7 cubic miles of ice into the ocean each year.
Measuring the flow of these vast ice streams from the ground can be treacherous, so many researchers rely on satellite observations. Bindschadler compared satellite images of ice streams taken over the last 35 years, which included recently declassified U. S. intelligence data. He found that ice streams once thought to flow at a constant rate are actually slowing down, making the question much more complicated of whether the West Antarctic Ice Sheet, as a whole, is wasting away.
Ted A. Scambos, a University of Colorado glaciologist, and Mark A. Fahnestock of the University of Maryland found the same slow-down in more recent satellite images. "We wanted these things to be like normal glaciers," said Scambos who added that the movement of mountain glaciers is fairly easy to understand. "It's becoming clear that's not the case. What you have is a system of ice streams and runaway glaciers that switch on and off and change position and mass flux." So if the rivers of ice are speeding up and slowing down and in some cases even stopping, how much ice is lost to the sea becomes difficult to determine.
Eric R. Rignot, a glaciologist at NASA's Jet Propulsion Laboratory, used satellite radar data to study the Pine Island Glacier. Satellites can help determine where glaciers and ice streams leave the land and hit the ocean because the floating ice bobs on the tides, straining the ice where it Is attached to land, Rignot found that the point where the Pins Island Glacier hits the ocean is retreating very quickly, moving back over three miles in just a few years. Rignot, who recently published his findings in Science, said that Pine Island Glacier's swift retreat is like a fire alarm. "It provides some very strong incentive to study this area. If that's where the action takes place, we better not miss it," he said.
The Chapman Conference on the West Antarctic Ice Sheet was sponsored and organized by the American Geophysical Union and co-sponsored by the International Glaciological Society.
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