Just what's going on in Antarctica?
� Barry James International Herald Tribune��
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� Friday, June 7, 2002
Warmer and cooler at the same time
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PARIS
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Like the Delphic oracle, the Antarctic sends out conflicting signals about global climate change. Part of it is warming significantly, but much has been cooling for some time.
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These signals are picked up and amplified both by those who argue that the world is headed for disaster as the climate heats up faster than at any time in recorded history, and others who argue that scientific warnings, notably by the Intergovernmental Panel on Climate Change, of major sea level rises over the next century are overly pessimistic.
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"The picture is complex rather than conflicting," says John King, principal investigator of Antarctic climate processes at the British Antarctic Survey. "There is a tendency to want to oversimplify things and to treat the whole of the Antarctic as one unit. The fact is that it is a large area geographically and it is hardly surprising that different things are happening in different places."
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Much of the world's climate research is focused on the poles, because they are sensitive to even minor changes, and because they provide clues to changes going back tens of thousands of years.
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Evidence of atmospheric change is clearer in the Arctic - where scientists have found that permafrost zones have melted, the extent and thickness of sea ice have decreased and glaciers have receded - than in the Antarctic.
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The U.S. government's National Science Foundation recently announced a major five-year study of environmental changes in the Arctic. Some scientists believe that even a small amount of warming in the northern polar region could trigger an abrupt change that could alter the Gulf Stream, which conveys surface water across the Atlantic, and turn northern Europe into a much colder place in a matter of decades.
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Warming also is clearly having an impact in the Antarctic Peninsula, jutting about 1,500 kilometers (950 miles) toward South America and surrounding waters, but scientists do not yet know whether this is a temporary, recurring phenomenon or evidence of permanent change.
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Sir Ernest Shackleton was bedeviled by unseasonably warm temperatures and melting ice as he tried to escape entrapment in the floes during his ill-fated 1914-1916 expedition to cross the entire continent. And air temperature over much of the peninsula region has warmed an average of one degree centigrade (1.8 degrees Fahrenheit) in the past 40 years. In Signy Island, about 700 kilometers northeast of the peninsula, however, British researchers have found that lake waters have warmed by a degree in as little as 15 years, indicating that the change may be speeding up.
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Earlier this year, satellites observed the rapid collapse of much of the enormous floating Larsen ice shelf that had existed on the peninsula since the last Ice Age, 12,000 years ago. The likeliest cause of that, geologists said, is that surface ice melted and penetrated through the shelf, weakening it to the point where it broke up catastrophically. With summer temperatures in the peninsula substantially above freezing, "it is almost certain that warming precipitated the collapse," King said.
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"What is well established is that most of the Antarctic Peninsula has been warming significantly over the past 50 years or so for which we have records," he said. "Over the rest of the continent, the signals are a lot less clear - at some stations you will see a slight warming, and at others you will see a cooling trend, but none of the observations would pass the test that we would use for statistical significance. Indeed, for the vast majority of the continent we don't have the observations that would enable us to say whether it is warming or cooling."
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Scientists have been keeping an eye on the Larsen shelf since 1995, when a large chunk broke away. The recent collapse of a section of ice 200 meters thick and 3,240 square kilometers in surface area - equivalent to 650 feet thick by 1,250 square miles - has left the shelf with only about 40 percent of its original size, and what remains is losing stability. David Vaughan, a glacier expert at the British Antarctic Survey, has called the speed of the collapse "staggering." Other ice shelves are closer to breaking up than scientists previously thought. Although these events are dramatic, they do not affect ocean levels, because the shelves jut out from the continental land mass and in effect float on the water. It would be a different matter if the vast ice sheets actually on the continent also started to melt. Containing most of the world's fresh water, the sheets are up to 5 kilometers thick.
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Many questions have been raised about the stability of the West Antarctic sheet, containing about 13 percent of the continent's ice, which is anchored to rock that is below sea level. If this ice were to melt precipitously, it could raise the world's average ocean level by about 5 meters. Using engineering risk-analysis methods, British and Norwegian scientists concluded last year, however, that there was only a 5 percent chance of major sea-level rise due to disintegration over a period of a few hundred years of the ice covering West Antarctica.
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Satellite and other evidence shows that the ice sheets are actually getting thicker, and that the Antarctic Dry Valleys, the continent's largest ice-free area, have cooled somewhat. The National Science Foundation says records show a decline in seasonally averaged surface air temperatures of 0.7 degrees centigrade per decade, but has no explanation for this fall. Antarctica is the only continent where such cooling has been observed. The Antarctic ice cap contains an estimated 30 million cubic kilometers of glacial ice, and if all this were to melt, the average sea level around the world would rise by 60 meters. This is not going to happen any century soon, but Antarctica was once part of a temperate zone attached to Australia and South America, over which dinosaurs roamed 65 million years ago.
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To put together a picture of climate changes in the past, a European scientific team backed by 10 nations recently drilled more nearly 3 kilometers on the Dome Concordia, high on the East Antarctic plateau, to reach layers of ice that fell from snow that fell more than 500,000 years ago. Ice core samples now being studied at European laboratories cover four glacial and interglacial periods, supporting evidence of several cycles of climate change.
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"The climate 10,000 years ago was typified by rapid variations," King said. "It was nonglacial for a few decades and then it went back to glacial."
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He said the collapse of the Larsen ice shelf may herald an end to a period of stability that has prevailed throughout recordable history.
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"It is important to explain this stability as well as the changes," he said. "And we have a long way to go in explaining why things are changing rapidly in the Antarctic Peninsula but not so much in the rest of the continent. What we will be trying to do is to improve the way in which Antarctic processes fit into the global climate model."�
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Copyright © 2002 The International Herald Tribune
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