SEATTLE -- A sheet of ice half the size of Alaska is on schedule to raise sea levels about 20 feet in 7,000 years, and there's nothing that can be done about it.
Scientists from the University of Washington and the University of Maine reported in Friday's edition of Science that the 360,000-square-mile West Antarctic ice sheet has been melting for 15,000 years and should be gone in 7,000.
A lingering effect of the Ice Age, the ice sheet's disintegration should raise sea levels nearly 1 centimeter a decade for a total of 6 meters, or nearly 20 feet.
The report comes a day after environmental groups started a multimillion-dollar national campaign to raise public awareness of global warming, which is blamed in many quarters on the greenhouse effect of man-made gases.
Global warming could hasten the melting, but it was not at work in what the researchers saw, said Howard Conway, a University of Washington research associate professor of geophysics and the report's principal investigator.
"The recent warming that's been happening in the world for the last 100 years, 150 years, we can't really see in the West Antarctic ice sheet yet," Conway said.
The rising sea levels will have a profound effect on coastal cities, particularly the millions of people clustered on the coast in Asia, Conway said.
The ice sheet, which sits on the edge of Antarctica along the Ross Sea, started shrinking at the close of the Ice Age.
Sheets in the Northern Hemisphere melted, raising the world sea level more than 300 feet.
Researchers speculate that changes in sea level and ocean circulation caused the West Antarctic ice to start disintegrating as the rising waters buoyed parts of it, straining its structure.
The University of Washington geophysicists measured the ice by sending radar signals into it from a snow machine hauled across Roosevelt Island, a dome of ice rising from the sea floor.
The signals reflected layers of volcanic ash and dust, which the researchers then modeled to determine the ice's age and retreat.
They figured the ice is caving into the sea at a rate of about 400 feet a year.
Geologists from the University of Maine analyzed algae from exposed beaches with carbon-14 dating, which estimates the age of organic materials.