But a study published on Thursday in the British weekly journal Nature calculates that the rate of Greenland ice loss increased by 250 percent between May 2004 and April 2006 compared with the two years between April 2002 and April 2004.

Ice is now being lost at around 248 cubic kilometers (59.5 cu. miles) per year -- equivalent to a global sea level rise of about 0.5mm (0.02 inches) per year.

Taking other accelerating factors into account, such as major losses at two big glaciers in recent years, Greenland is contributing almost 0.7mm a year, said Tavi Murray, an environmentalist at Britain's Swansea University.

This is a significant rise compared to the 2001 estimates by the UN's Intergovernmental Panel on Climate Change (IPCC).

If averaged out for the 110-year span, those estimates would give Greenland a contribution of around 0.5mm (0.02 inches) per year of the planetary rise in sea level. But the nature of global warming means that the rise would occur especially towards the end of the century -- not near its start, as the new paper implies.

The new study says the central portion of the icesheet, at altitudes above 1,500m (4,875 feet), is thickening, thanks to increased snowfall.

But the margins of the icesheet, which are at lower altitudes and are thinner, are eroding fast, especially in the southeast and northeast of Greenland, where glaciers are spewing ice into the sea faster than before

The research, which used computer models and satellite measurements made by NASA's two GRACE satellites, was carried out by Isabella Velicogna and John Wahr of the University of Colorado.

The pair acknowledge that their work spans observations only four years, and climate science often needs to look at decades before drawing firm conclusions about longer trends.

However, it concurs with a separate study on Greenland that was published in August by the US journal Science.

It also comes less than a week after a paper, also published in Science, found that year-round sea ice in the Arctic shrank by one seventh between 2004 and 2005.

Loss of sea ice does not affect global sea levels. Ice that floats in the water displaces its own volume.

However ice that is on land, as an icesheet, glacier or permanent snowcap, adds to sea level when it melts and runs off.

Retreating ice cover also creates a vicious circle.

Ice, being white, reflects the Sun's rays. Less ice therefore means the sea warms, which in turn accelerates the shrinkage.

In addition, melting polar ice sends large volumes of dense water into the North Atlantic, which slows a conveyor belt of warm water that flows up from the tropics and gives northwestern Europe its balmy climate.

If this belt were ever stopped or braked, the western part of Europe could be plunged into a mini-Ice Age, according to some theories.

Greenland is second to Antarctica as a single source of land ice. If the Greenland icesheet melted entirely, that would boost sea levels by seven metres (22.75 feet), although this apocalyptic scenario is discounted unless global warming becomes unstoppable.

The IPCC estimated in 2001 that between 1990 and 2100, the mean global sea level would rise about 480 millimeters (19.2 inches) in a range from 90mm to 880mm (3.6 to 35.2 inches).

At the bottom of this range, Greenland would not contribute anything to the increase; at the top of the range, it would contribute around 90mm (3.6 inches) over the 110 years.

These ranges are based on how fast greenhouse gases, which trap solar heat, build in the atmosphere and help drive up Earth's surface temperature.

Related Links
Learn about Climate Science at TerraDaily.com
SpaceDaily
Search SpaceDaily
Subscribe To SpaceDaily Express