A drastic climate shift such as a thaw of Greenland's ice or death of the Amazon forest is more than 50 percent likely by the year 2200 in cases of strong global warming, according to a survey of experts.

The poll of 52 scientists, looking 100 years beyond most forecasts, also revealed worries that long-term warming would trigger radical changes such as the disintegration of the ice sheet in West Antarctica, raising world sea levels.

There's concern about the risks of massive changes in the climate system, said Elmar Kriegler of the Potsdam Institute for Climate Impact Research, lead author of the study in the U.S. journal Proceedings of the National Academy of Sciences.

Huge changes or tipping points, which might also include a slowdown of the warm Gulf Stream current that keeps Europe warm, are often dismissed as highly unlikely or scaremongering.

The survey issued late on Monday found that leading experts, when asked, reckoned there was a one in six chance of triggering at least one tipping point with a moderate temperature rise of between 2 and 4 Celsius (3.6-7.2 Fahrenheit) by 2200 from 2000.

But with a strong rise of between 4 and 8 Celsius by 2200, the chances of surpassing at least one of five tipping points reviewed rose to 56 percent.

The study shows that some of these events are not considered low probability, Kriegler told Reuters of the study, with colleagues in Germany and Britain.

He said the poll was relevant to government policymakers because any of the climate shifts examined would have huge 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, Hans Joachim Schellnhuber, director of the Potsdam Institute who was among the authors, said in a statement.

GREENLAND

Most likely of five tipping points was the onset by 2200 of a longer-term Greenland thaw that would make it largely ice free. Greenland contains enough water to raise world sea levels by 7 meters if it ever all melted.

Second most likely was a death of large tracts of the Amazon rainforest because of a drying trend, followed by the start of a disintegration of the West Antarctic ice sheet, which would raise seas by about 5 meters.

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

The survey was taken in late 2005 and early 2006, in parallel with much of the writing of the last U.N. Climate Panel report that said that a build-up of greenhouse gases from human activities was the main cause of warming.

That 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.

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