Researchers refine assessment of tipping elements of the climate system

According to a study now published in Climatic Change, experts can not rule out that ice masses in the Amundsen Sea sector of Antarctica have already begun to destabilize. This is one of the results of a new assessment of the current state of six potentially unstable regions in the climate system with large direct impacts on Europe. The likelihood of climatic transitions of these elements generally increases as global mean temperature increases due to greenhouse gases emitted by human activity.

“We only provide a snapshot here, but it is in some ways sharper than those taken before”, says lead-author Anders Levermann of the Potsdam Institute for Climate Impact Research. For the first time, experts on the different potential tipping elements joined as coauthors for a common review of the state of knowledge about climatic transitions. Such assessment will, by definition, permanently evolve, says Levermann. Yet, understanding of the systems in question deepens. “Understanding these processes is crucial for future societal and economical decisions. From a risk assessment perspective, science – while pointing out uncertainties – has to provide stakeholders with information on likelihoods and potential effects of climatic transitions”, Levermann says. “Wait and see is no option.”

A partial collapse of the West Antarctic ice sheet for example could be equivalent to an additional 1.5 meters sea level rise, prior research showed. Most dykes in Europe may be elevated by only one meter. Beyond this region-specific threshold significant rebuilding would be necessary. However, the disintegration of this ice sheet might take hundreds of years. Nonetheless the effects could be significant. Apart from releasing water into the oceans, melting of the West Antarctic ice sheet would change the gravitational pull to the pole. Thereby it could lead to an even stronger sea level rise on European coastlines. All of this is incorporated in the scientists’ conclusions.

Arctic sea ice and Alpine mountain glaciers are estimated to be the most vulnerable to global warming of the short list presented in the study. Amongst the impacts of Arctic sea ice retreat is its influence on the North Atlantic atmospheric pressure system and thereby the Atlantic storm track in Europe. Alpine glacier shrinking is affecting water availability in the region as the seasonality of their run-off into rivers changes. With a 2 degrees Celsius warming, only minimal remains of Alpine glaciers will be left. Yet self-acceleration can not be safely detected for these two tipping elements. Also, for instance Arctic sea ice reduction might be reversed if global mean temperature sinks – even though such a scenario is not very likely.

The risks of a tipping point in Arctic ozone depletion are assessed to become insignificant when chlorine levels drop below 1980 levels, which will occur in 2060. High uncertainty is linked to the issue of the huge overturning in the Atlantic called the thermohaline circulation. Its potential collapse could be caused by freshwater inflows from Greenland ice sheet melting and changes in precipitation patterns. As insight in these changes is still limited, the likelihood of transition as well as the confidence in the assessment does not increase with temperature.

“Possible linkages of tipping elements make it even more advisable to use a risk management approach when dealing with global warming”, says Tim Lenton of the University of Exeter, UK, one of the authors. For example, a likely weakening of the thermohaline circulation in the Atlantic could lead to a warming of waters around Antarctica and shift the subpolar wind belt, inducing changes in ice sheet melting. “Those linkages are complex and are in urgent need of further exploration”, says Lenton.

Other tipping elements such as the Himalayan glaciers, Indian monsoon, or the thawing of Siberian permafrost, are beyond the scope of this study. They don’t have direct impact on Europe, the study says. However, an indirect impact is likely.

The term ‘tipping elements’ is defined as a strong response to small external perturbation. This might lead to the public misconception, some coauthors argue, that the change is always sudden and not reversible. While most tipping elements include such dynamic processes with self-amplification, they are not restricted to these. “The defining issue is the high sensitivity to changes in the background climate which poses a risk that society needs to be aware of”, Levermann says.

Article: Levermann, A., Bamber, J., Drijfhout, S., Ganopolski, A., Haeberli, W., Harris, N., Huss, M., Krüger, K., Lenton, T., Lindsay, R., Notz, D., Wadhams. P., Weber, S.: Potential climatic transitions with profound impact on Europe, Review of the current state of six ‘tipping elements of the climate system’. Climatic Change (2011) [DOI 10.1007/s10584-011-0126-5]

Weblink to article: http://www.springerlink.com/content/t435328g93882946/fulltext.pdf

Further Reading

Lenton, Timothy: Early warning of climate tipping points. Nature Climate Change (2011) [DOI: 10.1038/nclimate1143]

Weblink to article: http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate1143.html

E. Kriegler. J. Hall, H. Held, R. Dawson, H.-J. Schellnhuber (2009) Imprecise probability assessment of tipping points in the climate system. Proceedings of the National Academy of Sciences of the United States of America (PNAS), 106(13): 5041-5046, Link: http://www.pnas.org/content/early/2009/03/13/0809117106.abstract

T. Lenton, H. Held, E. Kriegler, J.W. Hall, H. Held, R. Dawson, H.-J. Schellnhuber (2008) Tipping element's in the earth's climate system. Proceedings of the National Academy of Sciences USA, 105: 1786-1793, Link: http://www.pnas.org/content/105/6/1786.full

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