Researchers have yet to reach a consensus on how much and how quickly melting of the Greenland Ice Sheet will contribute to sea level rise. 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.

Greenland ice sheet
Photo by Hannes Grobe, Alfred Wegener Institute for Polar and Marine Research
Greenland ice sheet (click image to enlarge)

Image credit: Hannes Grobe, Alfred Wegener Institute for Polar and Marine Research

"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."

The team of researchers examined 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 scientists 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.

"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."

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