The Courier-Mail
By: Richard Ingham
Scientists yesterday said they could no longer rule out a fast-track melting of the Greenland icesheet -- a prospect, once the preserve of doomsayers, 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, they said.
"The word `glacial' used to imply that something was very slow,'' climate researcher Allegra LeGrande, of New York's Columbia University, said.
"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 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 predicted the oceans would rise by between 18cm and 59cm by 2100.
The increase would depend on temperatures stoked by artificial greenhouse gases. The panel predicted warming of 1.8-4C during the century.
But nine months later, in a precis for policymakers, the IPCC scrapped the 59cm upper limit, admitting it did not know enough about meltwater run-off 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 7m.
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.
In its heyday about 20,000 years ago, the Laurentide was 3km 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.
Professor 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 bare 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 the Laurentide had two bursts of very fast melting before disappearing about 6500 years ago.
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 break-up 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 icesheet is far smaller than the Laurentide and lacks frigid bulk to shield off warming.
"We have never seen an icesheet retreat significantly or even disappear before, yet this may happen for the Greenland icesheet,'' Professor Carlson said.