Jonathan Bamber, an ice sheet expert at the University of Bristol, told the conference that previous studies had misjudged the so-called Greenland tipping point, at which the ice sheet is certain to melt completely. "We're talking about the point at which it is 100% doomed. It seems quite an important number to get right." Such catastrophic melting would produce enough water to raise world sea levels by more than 6m.

"We found that the threshold is about double what was previously published," Bamber told the Copenhagen Climate Congress, a special three-day summit aimed at updating the latest climate science ahead of global political negotiations in December over a successor to the Kyoto treaty. It would take an average global temperature rise of 6C to push Greenland into irreversible melting, the new study found.

Previous estimates, including those in the recent reports from the Intergovernmental Panel on Climate Change, said the critical threshold was about 3C - which many climate scientists expect to be reached in the coming decades.

"The threshold temperature has been substantially underestimated in previous studies. Our results have profound implications for predictions of sea level rise from Greenland over the coming century," the scientists said.

There is time to make sure the earth doesn't exceed that threshold. 6C - that's a lot.