By Paul Rincon
BBC News science reporter
Earth could be headed for catastrophic sea level rise in the next few centuries if greenhouse gases continue to rise at present rates, experts say.
Greenland's glaciers have been sliding faster towards the sea
A study in the US journal Science suggests a threshold triggering a rise in sea level of several metres could be reached before the end of the century.
Scientists used an ancient period of warming to predict future changes.
Greenland could be as warm by 2100 as it was 130,000 years ago, when melting ice raised sea levels by 3-4m.
The implication is that Greenland would - eventually - melt by as much in response to present warming.
The findings come from two studies published in Science by Dr Jonathan Overpeck, of the University of Arizona in Tucson, and colleagues.
Their computer models show that, in addition to widespread melting of the Greenland ice sheet, this rate of warming could also lead to the collapse of about half of the West Antarctic ice sheet in 500 years.
Dr Overpeck's team used computer models to simulate the climate 130,000 years ago. Because Earth was tilted slightly more than today on its axis, more solar radiation hit the northern latitudes, driving warming there.
The researchers found that melting of the Greenland ice sheet could have raised sea levels by 2-3.5m. But they also concluded that the rest could have come from the West Antarctic ice sheet.
It was not as warm here, but much of the ice sheet remains below sea level. This, they believe, allowed warming ocean waters along with rising sea levels to destabilise it.
"The simulated climate warming agreed well with the observed climate warming," Dr Overpeck told the BBC News website. "So, we had a firm estimate of how much warmth was necessary to cause that much sea level rise."
The researchers then compared this with simulations of future warming to learn how much sea level rise would be expected in future.
They estimate peak rates of sea level rise exceeding 1m per century.
"These processes of rapid ice sheet retreat are already happening. It just takes a while to get metres of sea level rise. But our study says that if we warm the Earth by more than two times the equivalent of pre-industrial carbon dioxide levels, we could be entering the danger zone," said Dr Overpeck.
"The ice sheet retreat and sea level rise on the order of what happened 130,000 years ago is inevitable and irreversible."
Geoscientist Michael Oppenheimer of Princeton University, who is not an author on the new paper, told Science: "Palaeoclimate always has a large amount of uncertainty, [but] we should take this as a serious warning sign. You could lock in a dangerous warming during this century."
Other work in the journal Science shows "earthquakes" caused by sudden movement of Greenland's glaciers are rising.
Some of Greenland's glaciers, which are as large as Manhattan and as tall as the Empire State Building, can move 10m in less than a minute, according to Harvard University scientists. This jolt is sufficient to generate seismic waves.
Not only has the frequency of these events increased, but they appear to occur more frequently in late summer than other parts of the year.
When water accumulates at a glacier's base, it acts as a lubricant causing large blocks to lurch down valleys.
"Greenland's glaciers deliver large quantities of fresh water to the oceans, so the implications for climate change are serious. We believe further warming of the climate is likely to accelerate the behaviour we've documented," said co-author Meredith Nettles at the Lamont-Doherty Earth Observatory in New York, US.