Friday, July 11, 2008

Scientists surprised by midwinter collapse of massive ice shelf

Andrew Darby Hobart 
The Age, July 12, 2008

EVEN the depths of winter are proving unable to halt the climate change-induced collapse of an Antarctic ice shelf.

When the Wilkins shelf began a runaway disintegration at the end of last summer, scientists thought it unlikely the collapse would continue through the pole's coldest months.

But satellite images show losses growing in recent days, so that at last sight, only a thin and fractured ice bridge held the bulk of the giant shelf in place. Its loss would put the rest of the 14,500- square-kilometre ice shelf at risk, the European Space Agency said.

The British Antarctic Survey's David Vaughan said the rate of break-up showed scientists were too conservative in the early 1990s when they predicted Wilkins would be lost in 30 years.

"The truth is it's going more quickly than we guessed," Dr Vaughan said.

The Wilkins is the largest of seven shelves on the Antarctic Peninsula to surrender their ice because of increased temperatures. Across the peninsula air temperature has risen an average of 2.5 degrees in 50 years, the greatest rise in the world.

The best-known ice shelf collapse so far was of Larsen B on the peninsula's east coast. Like all floating ice shelves, its 3250- square-kilometre loss did not raise sea level, but did unleash land-bound glacial ice behind.

A meeting of the international Scientific Committee on Antarctic Research in St Petersburg this week heard that 87% of the glaciers on the Antarctic Peninsula were in retreat.

The Wilkins is also located much further south towards the great West Antarctic ice sheet than Larsen B. The loss of the West sheet would raise global sea level by five metres, and the St Petersburg meeting heard that one key glacier there is moving 40% faster than in the 1970s.

Scientists have been closely watching the unstable Wilkins for the past decade, and the initial loss of 570 square kilometres in a few days last February raised the alarm again.

US National Snow and Ice Data Centre research scientist Walter Meier forecast then that other weakened areas of the Wilkins would go, but said: "Things are freezing up, so that will probably be it for the year."

Commenting on the winter collapse, Australian glaciologist Neil Young said: "I wouldn't have predicted this. But these days I expect to be surprised."

ESA scientists said their images of a further 160 square kilometres breaking off in May were the first ever to document the phenomenon in winter. By this week, the images showed that 1350 square kilometres of the Wilkins had shattered into smaller floating ice pieces.

Dr Young, of the Antarctic Climate and Ecosystems CRC in Hobart, said summer melt water trickling down through the ice could be a cause, but also pointed to reduced sea ice and a later onset of ice cover.

"These are subtle differences in temperature," he said, "but the volumes of water are so great there is a large transport of heat."

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