March 17, 2009

Climate “˜Tipping Point’ Seen Likely In Next 200 Years

A poll of experts finds that a dramatic climate shift such as the death of the Amazon forest or the disappearance of Greenland's ice is more than 50 percent likely during the next 200 years under the worst case global warming scenarios.

The survey of 52 scientists also revealed concerns that long-term global warming would spur drastic changes such as the disintegration of the ice sheet in West Antarctica, something that would raise world sea levels.

"There's concern about the risks of massive changes in the climate system," Elmar Kriegler of the Potsdam Institute for Climate Impact Research, the study's lead author, told Reuters.

These "tipping points" might also include a slowdown of the warm Gulf Stream current that keeps much of Europe warm. 

Such huge changes are often dismissed as highly improbable or even as scaremongering, but experts participating in the survey believe there is a one in six chance of triggering at least one of these events with a moderate temperature increase of 2 to 4 degrees Celsius (3.6-7.2 Fahrenheit) by 2200.

However, a significant temperature rise of 4 to 8 degrees Celsius by 2200 puts chances of surpassing at least one of five tipping points at 56 percent.

"The study shows that some of these events are not considered low probability," Kriegler told Reuters, adding that the survey was pertinent to policymakers because any of these tipping points would have enormous economic impacts.

"The results of the survey provide further evidence for the need of ambitious climate protection in order to minimize the risks of far-reaching consequences for our entire planet," said Hans Joachim Schellnhuber, director of the Potsdam Institute who was among the study's authors, in a statement.

Of the five tipping points examined in the study, the highest probability event was the onset by 2200 of a Greenland thaw that would make it essentially ice free, raising world sea levels by 7 meters.

The second most likely event was a death of large tracts of the Amazon rainforest brought about by a drying trend, followed by the disintegration of the West Antarctic ice sheet, which would raise sea levels by roughly 5 meters.

The other two tipping points -- a shift toward a constant El Nino warming of the Pacific Ocean and a breakdown of the system of Atlantic currents including the Gulf Stream -- were considered much less likely.

The poll was taken in late 2005 and early 2006, at the same time a U.N. Climate Panel report said that an accumulation of greenhouse gases from human activity was the primary cause of global warming.

However, the U.N. report focused only on the coming century and said that "abrupt climate changes...are not considered likely to occur in the 21st century."

The current study was published in the U.S. journal Proceedings of the National Academy of Sciences.


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