The Odds of Tipping

Should average global temperature increase by more than 4 degrees Celsius, one or several parts of the climate system could tip to a new state. Experts' estimates of the probability of tipping vary, and it also remains uncertain by how much global temperature will increase in the future.

But – as the authors report in the Proceedings of the National Academy of Sciences online early edition – these uncertainties do not imply that far-reaching events caused by global warming are unlikely.

An international team of researchers lead by Elmar Kriegler of the Potsdam Institute for Climate Impact Research (PIK) elicited the opinions of 52 climate scientists about the sensitivity of five so-called tipping elements. Tipping elements are parts of the climate system which, through human interference, can change quickly and irreversibly. In the current study the sensitivity of the following tipping elements is evaluated: Atlantic thermohaline circulation, El Niño phenomenon, Amazon rainforest, Greenland, and West Antarctic ice sheets.

43 experts estimated upper and lower bounds for the probability of those elements undergoing dramatic changes, given three different global warming scenarios: a warming by less than 2°C, by 2 – 4°C, or by an extreme of 4 – 8°C until 2200. “Strong global warming of more than 4°C by the year 2200 so far does appear to be a clear possibility”, Kriegler says.

The analysis of the survey is now published in the online version of the US American “Proceedings of the National Academy of Sciences”. If temperatures were to increase by 2 – 4°C then – so the scientists estimate – at least one element will tip with a one in six chance. If global temperatures were to increase even further then this probability increases to more than one in two (56%). In such a warming scenario the majority of respondents consider the probability of a complete melting of the Greenland ice sheet and a large-scale die-back of the Amazon rainforest to be particularly high. “The results show that the estimated probabilities increase strongly parallel to the progressive scenarios of future warming” Kriegler summarises the expert survey.

The authors write that expert elicitations have occasionally been criticised for not contributing new scientific information as long as they are not backed by new data, modelling or theories. However, in the context of risk analysis such surveys have proven to be a useful tool to summarise expert knowledge for decision makers. “We do not prescribe society specific climate policy measures,” says Hans Joachim Schellnhuber, director of PIK, and coauthor of the article. “But 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.”

Reorganization of the Atlantic Thermohaline Circulation: Most of the experts believe that the ocean circulation will remain stable under small future warming. However, if global temperatures were to rise by more than 4°C experts see a significant increase in the probability of a collapse of the existing system of Atlantic circulation.

Increased occurrence of the El-Niño phenomenon and large-scale die-off of the Amazon rainforest: Even if intense warming should occur some experts do not expect any change of the El-Niño phenomenon. Others estimate that El-Niño will become more frequent. The continued existence of the Amazon rainforest depends on these potential changes, because the El-Niño phenomenon causes drought in the Amazon region. In case of increased recurrences of El-Niño models indicate that large parts of the rainforest will die off. Most experts assume a one in two chance for a large-scale die-back of the rainforest in case of warming by more than 4°C.

Complete melting of the Greenland Ice Sheet: Even for a warming by less than 2°C some of the respondents see the risk of a complete melting of the Greenland ice sheet. For a strong increase of global temperature by more than 4°C almost all experts expect a melting of the ice sheet with a high probability of more than one in two.

Disintegration of the West Antarctic ice sheet: There are significant uncertainties in our knowledge about the response of the ice sheet to further warming. Therefore, experts differ in their estimates of the probability of its disintegration. Higher probability estimates may have been motivated by recent findings of acceleration of inland glaciers following the disintegration of ice shelfs on the West Antarctic peninsula.

Article: Elmar Kriegler, Jim W. Hall, Hermann Held, Richard Dawson, and Hans Joachim Schellnhuber (2009). Imprecise probability assessment of tipping points in the climate system. Proceedings of the National Academy of Sciences, Online Early Edition

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