Tue Jan 15, 2008 6:39pm EST
By Alister Doyle, Environment Correspondent
(Reuters) - Climate change has caused the greatest thaw of Greenland's ice in half a century, perhaps heralding a wider meltdown that would quicken a rise in world sea levels, scientists said on Tuesday.
"We attribute significantly increased Greenland summer warmth and ice melt since 1990 to global warming," a group of researchers wrote in the Journal of Climate, adding to recent evidence of faster Antarctic and Arctic thaws.
"The Greenland ice sheet is likely to be highly susceptible to ongoing global warming," they said. Greenland contains enough ice to raise world sea levels by 7 meters (23 ft), a process that would take centuries if it were to start.
Melt water from Greenland -- excluding ice losses from glaciers slipping into the sea -- totaled 453 cubic kms (110 cubic miles) in 1998, the most ahead of 2003, 2006, 1995 and 2002 in detailed records stretching back to the 1950s.
Preliminary data showed that 2007 would rank second or third highest and confirm the last decade as the biggest melt, said Edward Hanna of England's University of Sheffield who led the study with colleagues in Belgium, the United States and Denmark.
So far, the water runoff has been largely offset by rising snowfalls in Greenland that may also be a side-effect of climate change. Even freezing air can hold more moisture, and so deliver more snow, if it gets slightly less chilly.
DEMISE OF ICE
But continued warming could threaten an irreversible meltdown. The report noted that typical climate models pointed to a warming for Greenland of 4-5 degrees Celsius (7.2 to 9 Fahrenheit) by 2100.
"The ice probably wouldn't grow back under current conditions," Hanna said.
"If you have an extra 3-5 degrees Celsius warming ... then you can reach a point of no return ... bringing the eventual demise of the ice sheet. That could take probably 1,000 or 2,000 years," he said.
On Monday, a climate researcher said that Antarctica lost billions of tonnes of ice over the last decade, contributing more to rising sea levels around the world.
The U.N. Climate Panel, which blames global warming mainly on human emissions of greenhouse gases from burning fossil fuels, projects a rise in sea levels of between 18 cms and 59 cms (7 and 23 inches) by 2100.
The panel assumes that the little-understood rate of ice flow from Greenland and Antarctica will not change from 1993-2003, when their mass losses accounted for less than half of annual sea level gains of 3.1 millimeters (0.12 inch).
Hanna said that there was also a warm period around 1940 in Greenland -- but that warming was triggered by natural variations in the Arctic climate, perhaps shifts in ocean currents. This time, the Greenland warming fits a far broader trend across the planet.
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(Editing by Peter Millership.)