WASHINGTON -- An Antarctic ice sheet the size of Texas and Colorado combined is melting and could disappear in 7,000 years, possibly raising worldwide sea levels by 16 feet.
Based on geologic measurements that date when rocks first become free of ice, researchers have found that the West Antarctic Ice Sheet started retreating about 10,000 years ago, said John O. Stone, first author of a study appearing Friday in the journal Science.
"There was a gradual and continuous melting," said Stone, a professor of geology at the University of Washington, Seattle. Over thousands of years, he said, the ice has retreated at the rate of about 2 inches a year in a steady pattern that shows no sign of slowing.
If the sheet does melt entirely, he said, the global sea level could rise by as much as 16 feet, enough to drown some islands and coastal areas.
"If this kind of melting rate were to persist for 7,000 years, the rate of change is one that humans can accustom themselves to," said Stone. "The real problem is that there are places in the world" where a 4-inch rise over a few decades "would be a quite serious concern because of storm surges and tides," he said.
"Our measurements suggest a steady rate of melting, but we couldn't rule out short, rapid events," Stone added.
Stone said the study cannot prove or disprove that the melting is being affected by global warming, a gradual increase in temperatures that some believe is accelerated by the burning of fossil fuels. Instead, he said, the researchers have measured what is apparently a natural cycle of ice buildup and melting that may have been going on periodically for millions of years.
Robert P. Ackert Jr. of the Woods Hole Oceanographic Institute said the study establishes a baseline trend of natural melting against which any added melting cause by human influences on the climate can be measured.
"While mankind may not be able to stop this trend," Ackert said warming caused as a result of human activity "could conceivably accelerate it. I would not interpret this data to mean we no longer need to be concerned about this issue."
Stone and Gregory A. Balco of the University of Washington, along with researchers from three other institutions, measured the chemical isotopes in rocks collected on the side of mountains of western Marie Byrd Land in the area of the West Antarctic Ice Sheet.
At the height of the last glacial age, ice covered these mountains. As the ice melted and the glaciers retreated, boulders and rocks on the mountainsides were uncovered. When this happened, the rocks began being hit by cosmic rays from deep space, altering the isotopic chemistry of the rocks. By measuring these isotopes, the researchers could determine when the rock became free of ice.
"The clock starts ticking when the ice melts away," Stone said.
Matching the isotopic date with the altitude of where the rock was found gives the researchers a gauge of how fast the ice is melting.
Stone said that the measurements show there was no stuttering in the melting rate of the ice.
"We see no evidence that it has stopped," he said. "The pattern we see is very steady and continuous. ... We've had 10,000 years of climate much like it is today and the ice sheet has been shrinking continuously during that time."
Stone said the researchers were startled to find that the ice sheet in Antarctica began to shrink at about the time that most of the ice mountains formed during the last ice age already had disappeared from northern Europe and North America.
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