The Greenland ice sheet is all but doomed to melt away to nothing, according to a new modelling study. If it does melt, global sea levels will rise by seven metres, flooding most of the world's coastal regions.
Jonathan Gregory, a climatologist at the University of Reading, UK, says global warming could start runaway melting on Greenland within 50 years, and it will "probably be irreversible this side of a new ice age". The only good news is that it a total meltdown is likely to take at least 1000 years.
Greenland has the world's second largest ice cap, a remnant of the last ice age. It is 3000 metres high and contains 2.85 million cubic kilometres of ice.
Some researchers question Gregory's predictions. Ice loss may depend as much on the complex dynamics of ice flows as on temperatures. And Greenland meltwater would make the North Atlantic less salty, perhaps triggering a collapse of the Gulf Stream. That could cool the climate over Greenland and, perhaps, halt the melting.
But Gregory warns that, if his calculations are correct, the "the Greenland ice sheet is likely to be eliminated unless much more substantial reductions in [carbon dioxide] emissions are made than those envisaged" so far by scientists or politicians.
At present, snowfall onto the ice cap is balanced by meltwater and icebergs draining away into the Atlantic Ocean. But Gregory and co-author Philippe Huybrechts, a glaciologist at the Free University in Brussels, Belgium, calculate that if the island warms by an annual average of 3 degrees Celsius, melting will exceed snowfall and the ice sheet will begin to disappear.
Once under way, the melting will be almost impossible to stop, argues Gregory. As the ice melts, the cap's surface will sink to lower altitudes, warming the surface further, reducing snowfall and accelerating melting.
"Even if global climate returned to pre-industrial conditions, the ice sheet might not regenerate," says Gregory. NASA scientist Bill Krabill estimates that Greenland may already be losing ice at the rate of about 50 cubic kilometres a year.
Gregory and Huybrechts first warned of the possible meltdown on Greenland in 1999. But their latest analysis, published on Wednesday, significantly strengthens the case.
Gregory has analysed 35 different predictions of climate change from seven global climate models. All but one forecast that the threshold for runaway melting on Greenland will be exceeded, in some cases as early as 2035.
He says that if the world warms above this threshold, melting will accelerate. If warming stabilises at 3 degrees Celsius, the ice sheet could survive for several thousand years. But if temperatures rise by 8 degrees Celsius, which several scenarios predict, then it would disappear in 1000 years.
Journal reference: Nature (vol 428, p 616)