5 february 2008

Tipping elements in the Earth's climate system

Rainforest
There is uncertainty about he prospects for areas such as the rainforest. Credit: iStockphoto/Bryan Faust.

 A team of climate experts has compiled a shortlist of nine areas in the world that are in danger of passing critical thresholds or ‘tipping points’ due to climate change. They use the term ‘tipping point’ to mean that the climate system could kick-start abrupt and potentially irreversible changes for sub-systems of the Earth system. The international team, which includes Professor John Schellnhuber of the James Martin 21st Century School at Oxford University’s Environmental Change Institute, publishes its study this week in the online Early Edition of the Proceedings of the National Academy of Sciences.

The study warns that the Arctic sea ice and the Greenland ice sheet are regarded as the most vulnerable areas. The West Antarctic ice sheet is probably less sensitive to climate change, but there is a lot of uncertainty about projections of its future behaviour. This also applies to the Amazon rainforest and Boreal forests, the El Niño phenomenon, and the West African monsoon.

Professor Schellnhuber said: ‘This is the first systematic analysis of the tipping elements issue. We have developed a mathematical formalism for describing tipping elements and we have reviewed the complete pertinent literature. We have also identified the tipping elements in the Earth’s climate systems with regard to their relevance for climate policy. One could look at this paper as a “mini-IPCC-report” focusing on tipping elements.’

The scientists warn that there may be no Artic sea-ice during summer within a few decades. As Artic sea-ice melts, it exposes a much darker ocean surface, which absorbs more radiation than white sea-ice so that the warming increases. This causes more rapid melting in summer and decreases ice formation in winter. Over the last 16 years ice cover during summer declined markedly. The critical threshold global mean warming may be between 0.5 to 2 degrees Celsius, but could already have passed. The study also says that the Greenland ice-sheet could disappear within 300 years, causing the sea level to rise by up to seven metres, if there was local warming of more than three degrees.

Given the scale of potentially dramatic impacts from tipping elements the researchers say stronger mitigation is needed and adaptation will have to go beyond the current incremental approaches. In addition the researchers are recommending that a rigorous study of potential tipping elements in human socio-economic systems should be carried out. The study was led by the University of East Anglia in collaboration with Oxford University and the Potsdam Institute for Climate Impact Research in Germany.