Human activity has at least doubled the risk of heat waves like the one in 2003 that killed thousands in Europe, researchers conclude in a study being published today in the scientific journal Nature.
The three British authors -- Peter A. Stott of the University of Reading, and D.A. Stone and M.R. Allen of Oxford University -- used two computer models to assess the likelihood that a summer like that in 2003, which was Europe's hottest in centuries and was blamed for at least 35,000 deaths, would have occurred without human influences. They concluded, at a confidence level of more than 90 percent, that human activity doubled, if not quadrupled, the chances of "a heat wave exceeding this threshold magnitude."
The report is the first attempt to calculate the extent to which human activity has affected the chances of a specific weather event occurring.
The researchers did not point to a single cause of the heat wave, such as specific emissions of greenhouse gases, but attributed it to a combination of human emissions and natural temperature variations.
Based on current trends, the scientists calculated that by the 2040s, half of Europe's summers are likely to be as warm as the one in 2003.
Many environmentalists argue that governments must curb greenhouse gases linked to climate change to prevent warming-related disasters, but some academics and Bush administration officials say there is not enough science yet to justify such an economically disruptive move.
"This is significant because it is the first time scientists have demonstrated the human fingerprint on a particular weather event with the kind of certainty that would stand up in court," said Annie Petsonk, Environmental Defense's international counsel. She added that, just as investors reevaluated whether to put their money into tobacco companies after evidence emerged linking cigarettes to cancer, "this will begin to cause investors to look at the potential global-warming liability that companies that refuse to limit their greenhouse gas emissions will face."
Myron Ebell, the Competitive Enterprise Institute's director of global warming and international environmental policy, questioned the wisdom of basing scientific conclusions on computer models.
"Modeling is not science," said Ebell, who noted that this past summer, both Europe and the East Coast boasted unusually cool temperatures. "This is a very small-potatoes paper based on modeling that can't be proved or disproved" for the next 50 years, he said.