Scientists have worried for decades that the Antarctic ice sheet was shrinking, threatening a global rise in sea level. Now, satellite studies show that about 7.5 cubic miles of ice have eroded from a key area in just eight years.
Melting of that much ice doesn't mean that it is time to get into boats, said one researcher, but the finding may be a "yellow warning flag" that confirms long-term changes are under way in the ice fields covering the South Polar region.
The study, which appears Friday in the journal Science, involved altitude measurements of the West Antarctica Ice Sheet, the smaller of two major ice sheets. It covers 740,000 square miles of the frozen continent.
Based on satellite measurements, said Andrew Shepherd, a University College London geologist and first author of the study, it appears that since 1992 the ice sheet has lost ice principally through the speeded-up movement of the Pine Island Glacier, an ice stream that drains about a third of the ice sheet.
"The Pine Island Glacier is key," said Shepherd. "It is totally exposed to the sea, and people have identified it as the weak underbelly of the West Antarctica Ice Sheet."
Melting of the entire sheet theoretically could cause a global sea level rise of 25 to 45 feet, but Shepherd said that at the present rate of change it would take centuries for the Pine Island Glacier, which is only about 10 percent of the ice sheet, to affect sea level seriously.
Jane Ferrigno, a U.S. Geological Survey geologist and polar ice expert, said a speedup of the Pine Island Glacier, as reported by Shepherd and his co-authors, could foreshadow continuing changes of the West Antarctica Ice Sheet's ice levels.
The glacier "is moving faster than we thought," Ferrigno said. "This doesn't mean it could have an effect on coastal areas around the world within the next few decades, but this is a yellow warning flag. This is an area that should be watched carefully."
Shepherd said eight years of satellite data show a steady trend of ice-sheet shrinkage, with most of the decline coming in the Pine Island Glacier system, which drains an area about the size of the Mississippi River basin.
The Pine Island Glacier thinned by 30 to 36 feet during those eight years, and the glacier's grounding line the point where sea water undercuts the main stem of the glacier !51; has pushed inland by about three miles.
"The thinning is 10 times greater than the rate of snowfall in the basin," said Shepherd. "The speed of the glacier means that much more mass is going out (through melting and breaking off of icebergs) than is coming in."
Shepherd said if the present rate of change continues, the main stem of the Pine Island Glacier will be undercut by the sea and lifted up in about 600 years. When the glacier floats, i would cause a dramatic shift in sea level, he said.
Understanding how fast the Pine Island Glacier is moving and the effect of this motion on the total West Antarctica Ice Sheet "is of considerable importance" in predicting what will happen to the ice in Antarctica, Ferrigno said. She said the work by Shepherd and his co-authors adds new data for an area of the polar continent that was virtually unknown before.
Antarctica contains about 7.2 million cubic miles of ice, about 84 percent of all the glacial ice on Earth, according to the USGS. Melting all the Antarctica ice would cause a global sea level rise of about 240 feet. Such a rise would flood virtually all the world's coastal areas and drown many islands.
By PAUL RECER
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© 2001 CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.