ONE of our worst fears about global warming has been confirmed. Greenland's ice is melting faster than ever. The process could reach a point of no return before the end of the century, raising the sea to catastrophic levels. Hopes that increased snowfall on Antarctica would mitigate the problem have also been dashed.

Greenland hosts the second-largest icecap on Earth, holding 10 per cent of the global ice mass. If the whole thing melted, global sea level would rise by 6.5 metres. Earlier this year Eric Rignot of the California Institute of Technology in Pasadena and his team used satellite radar interferometry to determine that Greenland's glaciers were melting more and more each year, with about 220 cubic kilometres lost in 2005.

Now Jianli Chen and colleagues at the University of Texas at Austin have confirmed the scale of the problem using independent data from the twin satellites that make up the Gravity Recovery and Climate Experiment (GRACE). These satellites monitor shifts in water and ice over a particular spot on Earth every 30 days by measuring slight changes in the gravitational field. Chen's team devised ways to home in on areas the size of Greenland and found that the massive island is losing about 239 cubic kilometres of ice per year, mainly from the eastern parts (Science, DOI: 10.1126/science.1129007).

The ice loss corresponds to 0.6 millimetres of sea level rise per year. "That's a very big number," says Chen. It's about 20 per cent of the total sea level rise that is occurring each year. Large outflows from east Greenland could also disrupt the North Atlantic current that warms much of northern Europe, says team member Byron Tapley.

Though Chen's findings agree remarkably well with Rignot's, not everyone is convinced. "Based upon our analysis of satellite data, including ICESat and GRACE, we believe the melting rate is only one-third of what they have reported," says Jay Zwally of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

That doesn't mean Zwally isn't worried. His own previous work showed no significant loss of ice prior to 2002, so he agrees that melting is accelerating. "There's no question that Greenland has changed in the last five years from being close to balanced to being imbalanced," Zwally says.

While it would take millennia for all of Greenland's ice to melt at the rate calculated by Chen, positive feedback mechanisms could accelerate the process. For instance, melt water is lubricating the bases of the glaciers, speeding up their calving rate. So a threshold exists beyond which the melting of the whole sheet is irreversible. That could happen within the century, says Zwally.

John Church of the Antarctic Climate and Ecosystems Cooperative Research Centre in Hobart, Australia, agrees. "It is possible that by the end of this century the Greenland ice will have melted to a sufficient extent to cross the threshold."

Worse still, one of the possible safety nets appears not to be working as expected. Climate models predicted that as the world warms, Antarctica might mitigate sea-level rise by accumulating 15 to 20 per cent more snow, since warmer air can hold more moisture. However, Andrew Monaghan of Ohio State University in Columbus and his team studied ice cores from Antarctica and found that no such increase has occurred over the last 50 years (Science, vol 313, p 827).

Rising seas from Greenland's melt alone would be catastrophic. Huge parts of the world would be inundated, especially low-lying areas such as Bangladesh and the Nile delta. Great tracts of Florida would go underwater, and London and Manhattan would be threatened, along with the world's coastal infrastructure of ports, power plants and refineries.

"The world can't afford it," says climate change scientist John Harte of the University of California, Berkeley. "Even the most outrageously high estimates of what it would cost to do something about this are way, way less than what it would cost if it happened. It's frightening."

From issue 2565 of New Scientist magazine, 19 August 2006, page 8-9