Tuesday, September 9, 2008

Greenland melt could see huge sea-level rises

HOW fast will our coastlines be swallowed up by rising sea levels? This week, an ice-age glacier lent support to the controversial view that sea levels could rise by 1 metre per century - and so drown land now occupied by 145 million people by 2100.

The Intergovernmental Panel on Climate Change (IPCC) last year forecast a minimum rise of 18 to 59 centimetres by 2100. And they admitted in the small print that the range did not include water from the ice sheets of Antarctica and Greenland. Together, they contain enough water to raise sea levels by an astonishing 70 metres though nobody knows how fast it will be released.

To get a better idea, Anders Carlson of the University of Wisconsin-Madison and colleagues looked at what had happened at the end of the last ice age, when the last fragments of the Laurentide ice sheet covered much of north-eastern Canada. The team studied beryllium isotope ratios in the ancient bedrock to determine the ice sheet's margins between 9000 and 6800 years ago - these ratios alter depending how much of the bedrock was exposed. They worked out that melting ice must have raised sea levels by 0.7 to 1.3 metres per century. Using one of the IPCC's latest climate models, they calculated the temperature change that led to these rises, and compared it to changes expected over Greenland by 2100 (Nature Geoscience, DOI: 10.1038/ngeo285).

"The forces that led to the demise of the Laurentide ice sheet are comparable to the forces we will experience this century," says Gavin Schmidt of the Goddard Institute in New York City, who collaborated with Carlson.

In an accompanying article, Mark Siddall of the University of Bristol, UK, and Michael Kaplan of the Lamont-Doherty Earth Observatory in New York say it is hard to say whether the two eras are analogous, but the study shows that rises of 1 metre per century "are not out of the question".

James Hansen, head of the Goddard Institute, warned that such a rise was possible. Others have not been so bold. "The problem is that nobody has come up with a credible way of estimating what the rate of disintegration will be," says Schmidt. "Hansen isn't saying rapid rates are inevitable. He is advising a cautious approach because we don't know what the upper bound is."

That's because the physics that govern the disintegration are poorly understood. No computer model can accurately mimic an ice sheet's demise. For instance, many glaciologists were surprised when the break-up of the Larsen B ice shelf unleashed the ice streams behind it. Before that happened, they thought such an event would not affect the rest of the Antarctic ice sheet.

"Three years ago Hansen's view would have been extreme," says Richard Hindmarsh of the British Antarctic Survey. "But our understanding of ice dynamics have improved and the theoretical possibility of [a 1 metre rise] has been demonstrated."

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From issue 2672 of New Scientist magazine, 03 September 2008, page 12

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