THE Antarctic ice sheet, which holds 70 per cent of the world’s fresh water, has thinned significantly in the past four years, the first observations from a pair of satellites show.

The frozen continent is shedding about 36 cubic miles (152 cu km) of ice every year — enough to supply Los Angeles with water for 36 years — according to research suggesting that sea levels could rise more rapidly than predicted.

Scientists had expected that over the coming century global warming would increase the size of the Antarctic ice sheet, as higher temperatures brought increased snowfall, but the new data suggest that it is losing mass.

The findings, from the satellites of the Gravity Recovery and Climate Experiment (Grace) begun in 2002, are a concern because the ice sheet would increase sea levels by as much as 45m (150ft) were it all released. The West Antarctic ice sheet, where the bulk of the melting is taking place, holds water that would raise the sea level by more than 6m (20ft). The East Antarctic sheet, which is eight times larger, appears to be more stable.

The results indicate that rising temperatures are having a major impact on both ice caps: two weeks ago, a separate study found that the amount of ice dumped into the ocean by glaciers in Greenland had doubled over five years.

The Grace experiment is the first to involve a comprehensive survey of the Antarctic ice sheet, the first results of which are published today in the journal Science. It uses two satellites to detect minute changes in the Earth’s gravity field, which is influenced by the amount of ice locked in the polar cap.

It found that the volume of ice being lost from the ice sheet, which is up to 2,000m thick, is raising global sea levels by 0.4mm a year. Melting ice from Greenland is contributing another 0.5mm to sea level rise.

“This is the first study to indicate the total mass balance of the Antarctic ice sheet is in significant decline,” said Isabella Velicogna, of the University of Colorado at Boulder, who led the research.

“The overall balance of the Antarctic ice is dependent on regional changes in the interior and those in coastal areas. The changes we are seeing are probably a good indicator of the changing climactic conditions there.”

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