The researchers found that the great Laurentide icesheet which smothered much of North America during the last Ice Age melted far swifter than realised, dumping billions of tonnes of water into the ocean.

The discovery raises worrying questions about the future stability of Greenland's icesheet, for the Laurentide melt occurred thanks to a spurt of warming that could be mirrored once more by the end of this century, they said.

"The word 'glacial' used to imply that something was very slow," said climate researcher Allegra LeGrande of New York's Columbia University.

"This new evidence from the past, paired with our model for predicting future climate, indicates that 'glacial' is anything but slow. Past icesheets responded quickly to a changing climate, hinting at the potential for a similar response in the future."

Their investigation, published online by the journal Nature Geoscience, centres on a key factor in the climate-change equation.

In February 2007, in the first volume of a landmark report, the Nobel-winning Intergovernmental Panel on Climate Change (IPCC) predicted the oceans would rise by between 18 and 59 centimetres (7.2 and 23.2 inches) by 2100.

The increase would depend on temperatures stoked by man-made greenhouse gases. The panel predicted warming of 1.8-4.0 degrees Celsius (3.25 and 7.2 degrees Fahrenheit) over the century.

But nine months later, in a precis for policymakers, the IPCC scrapped the 59-cm (23.2-inch) upper limit, admitting it did not know enough about meltwater runoff from Antarctic and Greenland, the world's two mighty stores of land ice.

Although scientists are confident Antarctica has so far escaped major damage from global warming, they are far less sure about Greenland, whose icesheet holds enough water to drive up sea levels by seven metres (22.75 feet).

Seeking help from the past, geologist Anders Carlson at the University of Wisconsin, led a team that delved into sediment left by the Laurentide Icesheet.

At its apogee some 20,000 years ago, the Laurentide was three kilometres (1.9 miles) thick and reached as far south as New York and Ohio today.

Then a big warming occurred, apparently caused by a slight orbital shift which increased radiation that the Earth received from the Sun.

Carlson's team looked for radioactive tags, left by organic debris in the sediment, as a telltale of when the icesheet retreated and vegetation began to sprout once more on the denuded surface.

Using this, they built up a map and a timetable for the Laurentide's retreat and compared this with coral records pointing to Earth's historic sea levels.

They calculate that the Laurentide had two bursts of very fast melting before finally disappearing about 6,500 years ago.

The first phase, around 9,000 years ago, drove up sea levels by around seven metres (22.75 feet), at 1.3 cm (half an inch) each year. The second, around 7,500 years ago, accounted for a rise of five metres (16.25 feet) at the rate of one cm (0.4 of an inch) annually.

By comparison, sea levels today are rising around 3.3 millimetres (0.12 of an inch) every year.

The researchers caution that Greenland is an island bathed in chill water, has a somewhat different geology from that of North America, and so the timetable of the Laurentide's breakup may not exactly apply to it.

Even so, the upper range of the IPCC's temperature estimates at century's end are in line with those of the naturally-induced warming that doomed the Laurentide, they said.

In addition, the Greenland ice sheet is far smaller than the Laurentide and thus lacks frigid bulk to help shield off warming.

"We have never seen an ice sheet retreat significantly or even disappear before, yet this may happen for the Greenland icesheet in the coming centuries to millennia," said Carlson.

In a commentary, also published in Nature Geoscience, Earth scientists Mark Siddall and Michael Kaplan said Greenland's glacial slab was entering into a temperature range at which it was becoming "particularly vulnerable."

"[The new] work suggests that future reductions of the Greenland ice sheet on the order of one metre (3.25 feet) per century are not out of the question," they said.

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