Scientists said on Monday that they had found the first direct evidence linking the collapse of an ice shelf in Antarctica to global warming widely blamed on human activities.

A view of the breakup of the northern section of the Larsen B ice shelf is seen in an image taken from NASA's Moderate-resolution Imaging SpectroRadiometer satellite. Scientists said on Monday that they had found the first direct evidence linking the collapse of an ice shelf in Antarctica to global warming widely blamed on human activities. (Handout/Reuters)

Shifts in winds whipping around the southern Ocean, tied to human emissions of greenhouse gases, had warmed the Antarctic peninsula jutting up toward South America and contributed to the break-up of the Larsen B ice shelf in 2002, they said.

"This is the first time that anyone has been able to demonstrate a physical process directly linking the break-up of the Larsen Ice Shelf to human activity," said Gareth Marshall, lead author of the study at the British Antarctic Survey.

The chunk that collapsed into the Weddell Sea in 2002 was 3,250 sq kms (1,255 sq miles), bigger than Luxembourg or the U.S. state of Rhode Island.

Most climate experts say greenhouse gases, mainly from fossil fuels burned in power plants, factories and cars, are warming the globe and could bring more erosion, floods or rising seas. They are wary of linking individual events -- such as a heatwave or a storm -- to warming.

But the British and Belgian scientists, writing in the Journal of Climate, said there was evidence that global warming and a thinning of the ozone layer over Antarctica, caused by human chemicals, had strengthened winds blowing clockwise around Antarctica.

The Antarctic peninsula's chain of mountains, about 2,000 meters (6,500 ft) high, used to shield the Larsen ice shelf on its eastern side from the warmer winds.

"If the westerlies strengthen the number of times that the warm air gets over the mountain barrier increases quite dramatically," John King, a co-author of the study at the British Antarctic Survey, told Reuters.

WARMER SUMMERS

The average summer temperatures on the north-east of the Antarctic peninsula had been about 2.2 Celsius (35.96F) over the past 40 years.

But on summer days when winds swept over the mountains into the area the air could warm by 5.5 C (9.9 F). And on the warmest days, temperatures could reach about 10 C (50.00F).

King said temperature records in Antarctica went back only about 50 years but that there was evidence from sediments on the seabed -- which differ if covered by ice or open water -- that the Larsen ice shelf had been in place for 5,000 years.

"Further south on the main Antarctic continent temperatures are pretty stable," he said. "There is no clear direct evidence of human activity affecting the main area."

The collapse of the Larsen B ice shelf did not raise world sea levels because the ice was floating. A brimful glass of water with an ice cube jutting out will not spill if it melts because ice contracts as it melts.

But King said the removal of the floating ice barrier could accelerate the flow of land-based glaciers toward the sea, at least in the short term. That extra ice could raise sea levels.

© Copyright 2006 Reuters Ltd

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