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Glacial melting can be sudden, as past evidence shows

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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Telltale signs

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, at 1.3 cm each year. The second, around 7,500 years ago, accounted for a rise of 5 m at the rate of 1 cm annually.

By comparison, sea levels today are rising around 3.3 mm every year.

Unknowns

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 be the same.

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 say.

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," says Carlson.

In a commentary, also published in Nature Geoscience, Earth scientists Mark Siddall and Michael Kaplan say 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 per century are not out of the question," they write.

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