Thursday, August 16, 2007

'World on the edge' as risk looms of ice sheet collapse

The Scotsman: The world may be on the brink of a global warming "tipping point" that could see a 23ft rise in sea levels and doom future generations to catastrophic flooding, an expert has warned.

A temperature rise of just 1C would trigger the unstoppable collapse of the Greenland ice sheet, a vast layer of ice stretching 1,200 miles and covering 80 per cent of Greenland, said Professor Tim Lenton. Of this warming, 0.7C was already due to take place and was held up only by natural time lags in the climate system.

The Intergovernmental Panel on Climate Change has reported that the ice sheet will take at least 1,000 years to melt. But Prof Lenton's group at the University of East Anglia believes it could break up much sooner, within 300 years.

If this happened, sea levels would rise by 23ft, flooding coastal cities and communities and forcing millions of people to abandon their homes. Prof Lenton told New Scientist magazine: "We are close to being committed to a collapse of the Greenland ice sheet."

Other tipping points with unpredictable effects on world climate may already have been passed, say the scientists. A study by Prof Lenton's team identified eight dangerous tipping points that could be crossed this century. There might also be a domino effect, with several tipping points, each triggering the next.

One of the outcomes could be the collapse of a global ocean current system known as the thermohaline circulation, said the researchers. This would cause the Gulf Stream, which gives Britain its mild climate, to shut down, leaving the UK as cold as Canada.

Other tipping points could lead to the disintegration of the west Antarctic ice sheet, leading to another 23ft rise in sea levels, a die-back of the Amazon rainforest, the loss of northern coniferous forest, the "greening" of the Sahara and the collapse of the Indian summer monsoon.

A further effect might be to boost the El NiƱo southern oscillation, a "see-saw" climate cycle involving large changes in Pacific sea surface temperature that can have a major impact on weather systems.

Prof Lenton defines "tipping point" as the critical juncture at which a component of the Earth's climate system is switched to a different state by just a small change. He spelled out his warnings this week at a meeting on complexity in nature organised by the British Antarctic Survey in Cambridge.

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