Climate 'Tipping Points' Weighed for Likelihood

March 16, 2009 -- The risk of Earth's climate hitting a dangerous inflection point in the next two centuries is about as likely as a coin flipping on heads, according to a survey of 52 climate experts from around the world.

Elmar Kriegler of the Potsdam Institute for Climate Impact Research and a team of researchers conducted the poll to assess how concerned climate scientists were about the likelihood that human-induced climate change will lead to five 'tipping point' scenarios. From the melting of Greenland's ice sheet to permanently warm El Nino conditions in the Pacific Ocean, any of the five holds potentially devastating consequences for human civilization.

"These events are really massive changes to components of the Earth system," Kreigler said. "But what we found is scientists do not consider them to be low-probability events."

The respondents gauged what they thought the probabilities of the five events occurring were under low, medium, and high warming conditions from the year 2000 until 2200. Melting of the Greenland ice sheet was pegged as the most likely climate catastrophe under high warming (an increase of four to eight degrees Centigrade or 7.2 to 14.4 degrees Fahrenheit), with scientists turning in predictions of at least 60 percent that the island would be ice-free.

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But such speculation is inherently uncertain, and estimates varied widely. The likelihood of a permanent El Nino centered around 50 percent, as did the possibility of a catastrophic die-off of the Amazon's tropical rain forest.

Disintegration of the western Antarctic ice sheet, which could raise global sea level by four to six meters (13 to 20 feet) was thought to be between 35 percent and 85 percent likely.

If carbon emissions are greatly reduced in the next few decades, much of the warming could be avoided. Even in the middle scenario of an additional two to four degrees C (3.6 to 7.2 degrees F) warming above 2000 levels, damaging climate change could still take place.

Kriegler argued that lowering the risks of dangerous climate consequences to safe levels means keeping warming below two degrees C. That doesn't leave much wiggle room, though -- even if we stopped all greenhouse gas emissions today, we're already committed to between 0.5 and one degree C (0.9-1.8 degrees F) of warming.

"I don't like the term 'tipping points,'" Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Colo., said. "When you start to look in detail at any of these processes, all these questions of uncertainty come up, but there are positive feedbacks that come into play with additional warming that could make changes more rapid than we'd expect."

"The broad brush view of this work is a correct one," he added. "The general sense most of us have now of climate change is a disquieting one."

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