The vast Greenland ice sheet could begin to melt more rapidly than expected towards the end of the century, accelerating the rise in sea levels as a result of global warming, scientists warned yesterday.
Water running off the ice sheet could triple the current rate of sea level rise to around 9mm a year, leading to a global rise of almost 1 metre per century, the researchers found.
Sea levels are already on the rise as a result of increasing temperatures, because the oceans expand as they warm up, but until now scientists have had a poor understanding of how quickly ice sheets such as those in Greenland and Antarctica will begin to disappear.
There are signs that the Greenland ice sheet, which covers 1.7 million square kilometres of land, has already begun to melt faster than expected. The reason is thought to be surface water on the ice sheet trickling down through fissures to the underlying bedrock, making the ice sheet less stable, and the loss of buttressing ice shelves along the coastline.
Climate scientists are uncertain how susceptible ice sheets are to global warming, largely because they have never witnessed one disappear, so researchers led by Anders Carlson at the University of Wisconsin-Madison decided to look back to the end of the last ice age for clues.
Around 20,000 years ago, when the last ice age was at its peak, a giant mass of frozen water called the Laurentide ice sheet covered much of what is now North America. The ice sheet, which was three miles thick in some places, had almost completely melted 6,500 years ago as the world warmed as part of its natural cycle. At the time, surface air temperatures were similar to those that climate scientists predict for 2100.
The researchers used evidence in the geological record and computer simulations to reconstruct the demise of the Laurentide ice sheet, which was the last ice sheet to completely disappear in the northern hemisphere.
They dated boulders and fossilised organisms left on fresh ground as the ice sheet retreated, and found that it went through two periods of rapid melting. Computer simulations revealed that around 9,000 years ago, water melting off the ice sheet caused sea levels to rise by about 7 metres at a rate of around 1.3cm a year. The second stage of rapid melting began 7,500 years ago, when sea levels rose by 5 metres at a rate of around 0.7cm a year.
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The reconstruction suggests that the Greenland ice sheet may melt in a similar fashion. "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 millenia," said Carlson, whose study appears in the journal Nature Geoscience.
"We're not talking about something catastrophic, but we could see a much bigger response in terms of sea level from the Greenland ice sheet over the next 100 years than what is currently predicted," Carlson added.
The most recent report from the UN's Intergovernmental Panel on Climate Change predicts that sea levels will have risen by around 10cm at most by 2100, but according to Carlson's analysis, rapid melting of the Greenland ice sheet could cause much greater rises.
"For planning purposes, we should see the IPCC projections as conservative," he said. "We think this is a very low estimate of what the Greenland ice sheet will contribute to sea level."
In an accompanying article, Mark Sidall at Bristol University describes how a 1 metre rise in sea level would submerge an estimated 2.2m square kilometres of land, largely in Asia, and displace around 145 million people at a global cost of $944 billion.
He points out, though, that while the Laurentide ice sheet completely vanished at the end of the last ice age, the Greenland ice sheet remains, suggesting it is more resistant to warming. "To what extent this dynamic response of the Laurentide ice sheet to past temperature change can be considered analagous to present and future reduction of the Greenland ice sheet remains unresolved," he writes. "But [the researchers'] work suggests that future reductions of the Greenland ice sheet on the order of 1 metre per century are not out of the question."