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This week's issue of Nature features the first study to demonstrate a link between greenhouse gas emissions and an extreme weather event — in this case, the unusually high temperatures in Europe during the summer of 2003 that led to 14,000 deaths more than the seasonal average in France alone.

In their paper, Peter Stott of the UK Hadley Centre for Climate Prediction and Research, and colleagues say they "estimate that it is very likely that human influence has at least doubled the risk" of extreme weather events, such as the European heat wave.

While it is impossible to attribute a single meteorological event, such as a heat wave or a flood, to changes in the atmosphere, it is possible, using available computer models, to estimate whether human activities increase the risk of such events taking place.

Stott and colleagues did just that. They used a computer model to compare the likelihood of the 2003 heat wave taking place with and without human activities, which in the model included greenhouse gas emissions.

The researchers conclude that human activities at least doubled the chance of the heat wave taking place, and say they are 90 per cent sure this statement is accurate.

Previously, other scientists had made similar but unquantified estimates. In a SciDev.Net editorial earlier this year, Saleemul Huq, director of the Climate Change Programme at the International Institute for Environment and Development, wrote, "It may be impossible to attribute [the 2004 floods in Bangladesh] to climate change".

"But what can be said with certainty is that such events will occur with increasing frequency in the future, due to changes in the global climate system caused by greenhouse gas emissions attributable to human activities."

Scientists are a long way from being able to determine exactly how far human activities were responsible for increasing the risk of severe floods in Bangladesh in 2004. However, says Stott, in theory the same methods that he and his colleagues used in their study could be applied to any weather event, anywhere in the world.

PRECIS, a climate model based on the Hadley Centre's one, but able to be run on a personal computer, is just one of tools already being used in countries including Bhutan, Brazil and India to gain a better understanding of regional climate systems.

Eventually, such information could lead to risk studies of regional weather events, like the one done by Stott and his colleagues.

In accompanying article in Nature, Myles Allen, who supervised Stott's study, and UK lawyer Richard Lord write about the legal implications of the team's findings.

"Although these ideas may seem far-fetched now," they write, "we could one day see Californian farmers suing member states of the European Union for authorising emissions that threatened the security of their water supplies."

Stott agrees that following the same reasoning, farmers in Egypt might also one day sue European states for increasing the salinity of the water they use to water their crops.

In fact, a group of environmental lawyers is already urging UNESCO to investigate the effects of climate change on three World Heritage Sites: the Belize Barrier Reef, and two national parks, one in the Andean and the other in the Himalayan mountain ranges.

Pro Public, the environmental group leading the Himalayan campaign points out that in Nepal, it is not only the park that is endangered, but also the communities living beneath the swelling glacier lakes (see Melting glaciers threaten communities in South Asia).

"The Nepalese are not responsible for what's happening there," said Prakash Sharma, executive director of Pro Public in an interview with BBC Online. "We might be a little bit responsible but I think [it is] the global phenomenon of climate change."

Link to paper in Nature by Stott et al

Link to article by Allen and Lord in Nature

Link to BBC Online news story

References: Nature 432, 551 (2004) / Nature 432, 610 (2004)

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