To shed light on this question, scientists at the University of Wisconsin and Columbia University's Center for Climate Systems Research analyzed the disappearance of the Laurentide Ice Sheet, the last ice sheet to melt completely in the Northern Hemisphere and the closest example of what can be expected to happen to the Greenland Ice Sheet in the next century.

Their findings show that sea level rise as a result of ice sheet melt can happen very rapidly. The study will be published online this week in Nature Geoscience.

"We have never seen an ice sheet retreat significantly or even disappear before, yet this may happen for the Greenland Ice Sheet in the coming centuries to millennia," said Anders Carlson, the study's lead author and assistant professor of geology and geophysics at the University of Wisconsin-Madison.

"What we don't know is the rate of melting of the Greenland Ice Sheet. The geologic data we compiled on the retreat history of the Laurentide Ice Sheet, however, gives us a window into how fast these large blocks of ice can melt and raise sea level."

There are two challenges to determining the rate of melt for the Greenland Ice Sheet-a terrestrial ice mass covering more than 1.7 million km�. The current rate of sea level rise is about 3 mm/year.

In its Fourth Assessment Report, the Intergovernmental Panel on Climate Change (IPCC) indicated up to 59 cm of sea level rise, and stated that, if the observed contributions from the Greenland and Antarctic Ice Sheets between 1992 and 2003 were to increase in direct parallel with global average temperature change, the upper ranges of sea-level rise would increase by 10 to 20 cm.

This prediction, however, is based on data collected in a very short period of time-mostly from the last decade-and is not enough to give a clearer idea about what might happen to the Greenland Ice Sheet.

The second challenge is that ice sheet modeling is still in its infancy, owing in part to the lack of observations of ice sheet decay, and therefore cannot accurately depict projected melt.

To overcome these challenges, this study took a different approach to examining the potential for future changes to Greenland by exploring the last example of an ice sheet disappearance 9,000 years ago.

Analyzing geologic data and computer models, the team of researchers used terrestrial and marine records to reconstruct the demise of the Laurentide Ice Sheet, a land-based ice mass that covered much of North America, until its ultimate disappearance at around 6,500 years ago.

The ice sheet, which once covered most of Canada and the upper reaches of the United States, had two intervals of rapid melting, the first around 9,000 years ago, and the second 7,500 years ago.

The researchers estimate that around the time of the first melting phase, the retreating ice sheet led to about approximately 7 meters of sea level rise at about 1.3 cm a year.

The second phase accounts for around 5 meters of sea level rise at about 1.0 cm a year. These rates are comparable to evidence for global sea level rise for this interval derived from coral records.

"I was surprised to see that the model-in agreement with Anders' data-showed the Laurentide Ice Sheet disappearing at 2.7 m/year," said Allegra LeGrande, who led the computer modeling portion of this study and is a postdoctoral research scientist at the NASA Goddard Institute for Space Studies and the Center for Climate Systems Research at Columbia University.

"This finding shows the potential for ice to disappear quickly, given the right push."

The simulations of the Laurentide rapid melting episode show that the driving factors for the thinning of the ice sheet were increased solar radiation caused by a change in the earth's orbit which increased summer temperatures. Similar temperature increases may occur over Greenland by the end of this century.

IPCC predictions for changes in sea level for the next century are mainly based on the expansion of the oceans through warming, accounting less for contributions from ice sheet melt.

This analysis of the Laurentide Ice sheet finds that the ice sheet 9,000 years ago was under similar pressure to melt as the Greenland Ice Sheet will be by the year 2100, implying a greater potential for mass loss on Greenland and resulting sea level rise.

(Although this finding should not be extrapolated for an absolute prediction in sea level rise over the next ten years.)

"The word 'glacial' used to imply that something was very slow," said LeGrande. "This new evidence compiled from the past paired with our model for predicting future climate indicates that 'glacial' is anything but slow. Past ice sheets responded quickly to a changing climate, hinting at the potential for a similar response in the future."

In an accompanying News and Views letter in Nature Geoscience, Mark Siddall, a researcher at Columbia's Lamont-Doherty Earth Observatory, writes "Carlson and colleagues... show that the decay of the Laurentide ice sheet in the early Holocene was extremely fast during the periods they consider ... Their work suggests that, in principle, future melt rates on the order of one metre per century are certainly not out of the question."

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