Glacial melting can be sudden, as past evidence shows


Scientists say the fast-track melting of the Greenland icesheet is possible

Kangerlussuaq Icesheet, Greenland

Icesheets, like this one near Kangerlussuaq Greenland, may melt more rapidly, as past ones did, according to new research.

Credit: Wikimedia/L. Chang

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Scientists say they could no longer rule out a fast-track melting of the Greenland icesheet - a prospect, doomsayers have warned, that would see much of the world's coastline drowned by rising seas.

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.

"The word 'glacial' used to imply that something was very slow," says 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.

Past research

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 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 over the century.

But nine months later, in a summary for policymakers, the IPCC scrapped the 59-cm 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.

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 peak some 20,000 years ago, the Laurentide was three kilometres 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.

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