THE quantity of ice dumped into the ocean by Greenland’s glaciers has doubled over the past five years, suggesting that global warming will cause sea levels to swell more quickly than predicted.

A significant study of the glaciers that drain the Greenland ice sheet — the world’s largest — has revealed dramatic increases in their speed of advance, linked to rising temperatures in the region.

The findings indicate that the contribution of Greenland’s melting ice to rising sea levels has been underestimated, and will increase in a warming world. They offer the first signs that the melting of the ice sheet, which covers an area slightly smaller than that of Mexico, could take place more rapidly than thought.

Although the sheet will still take hundreds, if not thousands, of years to disappear, it contains enough water to increase global sea levels by 7 metres (23ft). It covers an area of 1.7million sq km (660,000 sq miles), and is up to 3 km (1.9 miles) thick.

Although melting sea ice will not directly affect sea levels, the vast quantities of ice in the land-based caps of Greenland and Antarctica would have catastrophic effects were they to melt. The sheets gain mass through snowfall, and lose mass as ice melts, evaporates or is drained to the sea by glacier flow. When glaciers reach the ocean, they dump ice into it in the form of calving icebergs.

Scientists predict that a 2.7C (4.9F) increase in temperatures from present levels could destabilise the Greenland and West Antarctic ice sheets irreversibly, condemning the world to inexorable sea-level rises. The Intergovernmental Panel on Climate Change estimates that temperatures will rise by 1.4C to 5.8C by the year 2100.

In the new research, Eric Rignot, of Nasa’s Jet Propulsion Laboratory, in Pasadena, California, and Pannir Kanagaratnam, of the University of Kansas, used satellite data to monitor the speeds of glaciers that drain the Greenland ice sheet, from 1996 to 2005. They recorded large increases in the pace of advance, first in the south of Greenland and, after 2000, from the north. Several of the largest glaciers have doubled in velocity over the period, now advancing at 12 kilometres (7.5 miles) a year.

The result has been a steady increase in the amount of ice carried from the Greenland ice sheet into the sea, from about 90 cu km annually in 1996 to 224 cu km last year, meaning that Greenland is contributing half a millimetre a year to the global sea-level rise, from a total annual increase of 6mm.

The increasing speed of glacier flow appears to be linked to rising temperatures in Greenland, where temperature in the southeast has increased by 3C in 20 years. “Climate warming can work in different ways, but if you warm up the ice sheet, the glacier will flow faster,” said Dr Rignot, who presented the results yesterday at the American Association for the Advancement of Science conference in St Louis. His paper will be published today in the journal Science.

Julian Dowdeswell, of the British Antarctic Survey, said the findings suggested that estimates of sea-level rise from the Greenland ice sheet were probably too low.

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