Half of Greenland and vast areas of Antarctica are destined to melt if global warming continues at the same pace until the end of the century, scientists warned on Thursday.
Their research shows that the loss of so much ice will trigger dramatic rises in sea levels, ultimately swamping low-lying islands and coastal regions and threatening the flood defenses of cities such as London.
The last time so much ice was lost from the poles -- in a period between ice ages 129,000 years ago -- global sea levels rose between 4m and 6m.
Experts believe many coastal regions would suffer long before sea levels rose significantly, because even a minor rise will make storm surges more devastating and increase the risk of flooding.
The warning comes from climate scientists who combined historical records of Arctic and Antarctic ice melting with advanced computer models capable of predicting future environmental conditions.
They found that if nothing is done to put the brakes on climate change, Greenland, the west Antarctic ice sheet and other expanses of polar ice will be warmed beyond a "tipping point" after which their melting is inevitable.
Jonathan Overpeck, director of the Institute for the Study of Planet Earth at the University of Arizona, and Bette Otto-Bliesner, at the US National Center for Atmospheric Research in Colorado, looked back in history to find a time when sea levels were higher than they are today.
They identified a period when glacial melting increased sea levels by several meters.
They used a computer model of the environment to work out how warm the world must have been to trigger the mass melting and concluded that the Earth was between 3oC and 5oC warmer than it is today.
Using the same model, the scientists spun the clock forward to see when climate change is likely to make the world as warm again.
"We showed that that level of warming will come later in this century unless we act on carbon emissions," Overpeck said.
"An Arctic warming of 3oC to 5oC is enough to cause 4m to 6m of sea level rise," he said.
If temperatures do rise as the scientists predict, the ice at the poles will not be lost immediately. Enough ice is likely to melt within the next 100 years to raise sea levels by a meter, but ultimately the fresh water pouring into the North Atlantic would slow down the Gulf stream by a quarter.
"These ice sheets have melted before and sea levels rose. The warmth needed isn't that much above present conditions," said Otto-Bliesner.
The major concern is that unless climate change slows down significantly, the eventual loss of polar ice and subsequent 6m rise in sea levels will become unavoidable.
"There has been an increasing number of observations from the ice sheets suggesting they are responding faster to climate change than anticipated," Overpeck said.
"Now along come our results showing these kinds of changes occurred in the past and lead to large ice sheet retreat and sea level rise," he said.
A 1m rise in the sea level would see the Maldives disappear, make most of Bangladesh uninhabitable and put cities such as New Orleans "out of business," according to Overpeck.
The research was published in two papers in the US journal Science on Thursday.