Risks of global warming have been underestimated

'Today, we have to assume that the risks of negative impacts of climate change on humans and nature are larger than just a few years ago,' says Hans-Martin Fuessel from the Potsdam Institute of Climate Impact Research (PIK). Risks would increase drastically with only small increases in global mean temperature exceeding the 1990 level. Many ecosystems like tropical coral reefs prove to be much more susceptible to global warming and the rising concentration of carbon dioxide than assumed in the Third Assessment Report (TAR) by the IPCC in 2001. Extreme weather events as droughts, heat waves or tropical cyclones occur more frequently and cause larger damages than assessed at the beginning of this decade.

Total emissions and records of global temperature rise of the last years are close to the upper limits of IPCC-projections (Rahmstorf et al., 2007). 'If the associated risks are larger, the necessity is also larger to reduce the greenhouse gases emissions and to support affected regions to cope with the unavoidable consequences of climate change,' Fuessel concludes. This will also be a question of justice since many of the countries with the lowest emissions are most affected.

The new assessment by the authors around Joel Smith from Stratus Consulting in Colorado and Stephen Schneider from Stanford University in California is based on observations of global warming impacts and supported by a more thorough understanding of important processes in the climate system. Furthermore, since the publication of the TAR in 2001, the particularly affected regions, sectors and groups have been identified more precisely. Finally, there is growing evidence that - on multiple century time scales - even modest increases in global mean temperature above 1990 levels could commit the climate system to the risk of very large impacts like melting of the Greenland ice sheet, the authors report in the Proceedings of the National Academy of Sciences online early edition.

Three of the article's authors are also authors of the chapter in the TAR from 2001 where the five reasons for concern were first described. John Schellnhuber, director of PIK, has contributed decisively to the emergence of this idea and, also, to the form of depiction. 'The burning embers diagram has, as a translation, raised understanding of the complex of problems of climate change,' says Schellnhuber. The updated version points out that the European Union's 2 -target to limit global warming to two degrees Celsius is an absolute minimal requirement to protect climate. It would be a failure with severe consequences if the governments at the United Nations Climate Change Conference in Copenhagen did not acknowledge that insight, Schellnhuber says.

Reasons for concern:

Risk to unique and threatened systems: Coral reefs, endangered species, unique ecosystems, biodiversity hotspots, small island states, tropical glaciers, and indigenous communities could be damaged or lost irreversibly.

Risk of extreme weather events: Frequency, intensity, and consequences of extreme weather events like heat waves, floods, droughts, or tropical cyclones increase

Distribution of impacts: Some regions, countries, and populations face greater harm from climate change than others. The poorest countries that contributed least to climate change and do not have the means to adapt to the impacts are often more affected than the average.

Aggregate damages: Various impacts can be measured comprehensively with a single metric such as monetary damages, or the number of lives affected. So far, monetary aggregation is predominant in literature.

Risks of large scale discontinuities: Greenhouse gases emissions could push the Earth's climate system past critical thresholds so that important components may 'tip' into qualitatively different modes of operation. The melting of the Greenland ice sheet, the dieback of the Amazon rainforest, or a substantial reduction or collapse of the North Atlantic Meridional Overturning Circulation are examples.