Antarctica is a key to sea level rise, which threatens coastal areas around the world.. It has enough ice to raise seas by 57 meters (190 feet) if it ever all melted, meaning that even a tiny thaw at the fringes is a concern.
Until now, the exact cause of the collapse of the Larsen-B ice shelf, a floating mass of ice bigger than Luxembourg at the end of glaciers in the Antarctic Peninsula, had been unknown. Some experts suggested it was thinned by sea water from below.
Writing in the journal Science, a team of scientists blamed rising air temperatures, saying that melt water and rain in the brief Antarctic summer had flowed into deep cracks.
Water expands when it turns to ice, and the re-freezing meltwater in the Larsen-B shelf - perhaps 200 meters thick - led to a build-up of huge pressures that shattered the ice in 2002.
A rival theory had been that warmer sea water had destabilized ice where the shelf was grounded on the seabed. Studying the seabed, however, the scientists found evidence that water had flowed freely under the ice for the past 12,000 years.
"This implies that the 2002 Larsen-B Ice Shelf collapse likely was a response to surface warming," they wrote. Since 2002, several other shelves have broken up around the Antarctic Peninsula, which is below South America.
WARNING SIGN
The Larsen-B captured the public imagination and even featured in a Hollywood disaster movie about climate change, "The Day After Tomorrow", showing a huge crevasse appearing through a scientific base on the ice.
"Hollywood underplayed that one," said Eugene Domack, an author of the study at the University of South Florida. "It fractured into thousands of icebergs, not just one huge crevasse."
Loss of floating ice shelves does not directly affect sea levels but can accelerate the slide of glaciers from land into the sea, raising levels. Thursday's study was by scientists in Italy, the United States, Portugal, Germany, Canada and Britain.
Domack told Reuters the findings could help scientists spot other ice at risk of breaking up. Pools of summer meltwater on the surface of ice shelves - visible from space - could be an early warning sign, he said.
The northern part of the Larsen-C ice shelf, further south and four times the size of the Larsen-B shelf, has been showing signs of instability, he said.
Scientists have linked warmer air over the Antarctic Peninsula to climate change and to a thinning of the ozone hole that shields life from cancer-causing solar rays, driven by man-made chemicals.
A U.N. report on Wednesday said that the ozone layer is showing its first signs of recovery after years of depletion, in a rare piece of good news about the environment.
(Editing by Alison Williams)
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