A giant iceberg that snapped away from Greenland last week is a signal that global warming is causing the island's continent-sized ice cap to melt faster than expected, scientists say.
The 250-square-kilometre (100-square-mile) chunk, four times the size of Manhattan, broke away from the Petermann ice shelf on Greenland's northwestern tip.
The breakoff -- the largest in the Arctic in half a century -- points to Greenland's worrying potential to stoke sea levels in the coming decades and centuries, climate experts say.
"It is a warning sign that we are seeing changes," said University of Colorado glaciologist Konrad Steffen, who is overseeing the Greenland section of a major report by the UN Intergovernmental Panel for Climate Change (IPCC), due in 2013.
"The ice sheet is continuing to lose volume at an accelerated rate," he said by phone.
"We are now at 350 cubic kilometres (84 cubic miles) ice loss per year. That's more than twice the ice in all the glaciers in the Alps."
If it melted completely, Greenland's ice sheet could boost the global water mark by at least five metres (17.5 feet).
None of the world's several dozen top specialists believes this scenario to be likely over the next two or three centuries.
But many of these experts have recently shifted from a common view that the ice sheet is largely stable, sharply revising their estimates of how much could melt by 2100.
By some calculations, Greenland's runoff could boost average sea levels in 2100 by half a metre (one and a half feet) to a metre (3.25 feet) -- enough, at the upper end of the range, to lay waste to low-lying coastal cities and drive hundreds of millions of people toward higher ground.
Ice shelves are created by glaciers that run off onto the coast, creating a tongue of land that spreads out onto the surface of the sea.