Oct. 8, 1999 An immense expanse of Antarctic ice that has been receding steadily for 10,000 years poses the most immediate threat of a large sea level rise because of its potential instability, a new study indicates.
The West Antarctic Ice Sheet - about 360,000 square miles, or roughly the size of Texas and Colorado combined - rests on the Antarctic land mass below sea level, which makes it particularly susceptible to rising sea level. Its complete collapse would raise global sea level 15 to 20 feet, enough to flood many low-lying coastal regions.
The new study shows that the ice sheet's complete disintegration in the next 7,000 years could be inevitable, said Howard Conway, a University of Washington research associate professor of geophysics, who is the lead author for a paper describing the research in the Oct. 8 issue of Science.
While human-caused climate change could hasten the ice sheet's demise, it might be that there is nothing humans can do to slow or reverse the trend, Conway said.
"Collapse appears to be part of an ongoing natural cycle, probably caused by rising sea level initiated by the melting of the Northern Hemisphere ice sheets at the end of the last ice age," he said. "But the process could easily speed up if we continue to contribute to warming the atmosphere and oceans."
UW geophysics professor Edwin Waddington; Anthony Gades, a UW geophysics research associate; University of Maine geological sciences professor George Denton; and Brenda Hall, a UM post-doctoral researcher in geological sciences, also took part in the study.
Using evidence gathered from raised beaches and radar imaging of subsurface ice structures to reconstruct historic changes, the scientists found the ice sheet has both thinned and decreased in area since the last glacial maximum 20,000 years ago.
Ice covering the region once was as much as a half-mile thick in places. Land previously weighed down by the dense ice has elevated since being freed from its burden. The timing of deglaciation was determined by carbon-14 dating of samples found on raised beaches that are now up to 90 feet above present sea level.
Other evidence comes from Roosevelt Island, an ice island in the Ross Sea. Floating ice now surrounds it, but reconstructions suggest that ice in the area of Roosevelt Island was about 1,600 feet thicker and was grounded during the last ice age.
The researchers found that the grounding line (the boundary between floating ice and grounded ice) has receded about 800 miles since the ice age and has withdrawn an average of about 400 feet per year for the last 7,600 years. That average is similar to the current rate, and there is no indication the retreat is slowing, Conway said. If the grounding line continues to withdraw at that rate, complete disintegration of the ice sheet will take about 7,000 years.
Other scientists have found evidence suggesting the West Antarctic Ice Sheet might have disintegrated in the past. Fragments of tiny algae called diatoms have been recovered from cores drilled through the ice and into the land beneath. It is believed diatoms require open water to build their colonies, which suggests the region once was free of ice, perhaps as recently as 130,000 years ago between the last two ice ages, Conway said.
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