A new space-based study of Antarctica shows its ice sheet is shrinking. Researchers used satellites to plot changes in the Earth's gravity in the Antarctic during the period 2002-2005. Writing in the journal Science, they conclude that the continent is losing 152 cubic km of ice each year, with most loss in the west. In recent years scientists have found other evidence that West Antarctic ice is melting, which could contribute to sea level rise. In his contribution to a recent report on climate change, the director of the British Antarctic Survey, Chris Rapley, described the West Antarctic ice sheet as "a giant awakened". But gathering comprehensive data on both the Antarctic and Greenland ice sheets has not been straightforward, and studies have produced apparently contradictory results, with some indicating a loss of ice and others that sheets are thickening. The new study, conducted by scientists at the University of Colorado at Boulder, uses a technique which has not been tried before: measuring gravity over Antarctica. Data comes from a pair of satellites together known as Grace which orbit the Earth in tandem, measuring changes in its gravitational field. When they fly over regions where there is lots of material below, such as mountain ranges, or where crustal rocks are more dense, they will register an increase in the Earth's gravity - tiny, but measurable. "Their orbits have a very large inclination, of 89 degrees in fact, so you get very good coverage over the Antarctic," said Isabella Velicogna, one of the Colorado team. "We see the entire ice sheet and measure the mass change of the entire ice sheet; then we look at the east and west separately," she told the BBC News website. Overall, Dr Velicogna's group found an annual decrease in ice sheet mass of 152 cubic km. There is a clear loss in the west, whereas the mass of the East Antarctic sheet appears to be constant.
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