Greenhouse emissions doubled the risk of a high temperature summer.
Man-made pollution during the past century doubled the chances of the heat wave that hit Europe last summer, say climatologists. It's the first time that a study has estimated how much human activity increased the risk of a specific weather event.
The sweltering temperatures of August 2003 left many people, particularly the elderly, struggling to cope. The heat wave caused many thousands of extra deaths, while forest fires ravaged large areas of land, causing $1.6 billion worth of damage in Portugal, for example.
A model climate
With information collected from ice cores and tree rings, scientists had already worked out that the summer of 2003 was probably the hottest in Europe for 500 years. Some saw this as evidence for man-made climate change, but until now no one had attempted a rigorous attribution of its causes.
Peter Stott of the Hadley Centre for Climate Prediction and Research in Reading, UK, turned to computer programs that could predict temperature trends across Europe from 1920 to the present, on the basis of certain starting conditions.
Along with researchers from the University of Oxford, he ran a range of climate models to predict what weather patterns would be expected across the continent with and without the human emissions that have global-warming effects, such as carbon dioxide.
The team found that climate simulations that incorporated man-made emissions predicted summer temperatures for the 1990s that were, on average, 0.5 °C warmer than the simulations without human contributions.
A half-degree rise in average expected temperature increases the probability that a given summer will be extremely hot. "The mean moves and the whole distribution moves with it," explains Stott.
He and his colleagues calculate that human influences doubled the likelihood of the 2003 heat wave. Their results appear in this week's Nature1.
According to Christoph Schär, who conducts climate research at the Federal Institute of Technology in Zurich, Switzerland, this is the first time that a study has worked out how global warming has affected the risk of a particular event.
It was only possible to do this, he explains, because the heat wave of 2003 was so extreme. Because it was so much warmer than previous summers, the team was able to pin down the causes with more certainty.
Scientists had not expected that this sort of opportunity would present itself so quickly. "We never thought that there could be a paper on this so soon," says Schär. In the future, he hopes that increased computing power will allow climatologists to build more detailed simulations, which will let them study the causes of more local weather events.
The result could make it easier for groups attempting to sue large emissions-producers for the damages caused by global warming. Last summer, for example, eight US states and New York City filed a lawsuit against five of the country's power companies for this reason. They are demanding cuts in emissions rather than compensation.
Defendants in such cases have complained that it is impossible to demonstrate actual damage from emissions such as carbon dioxide. But by clearly attributing the risks of a climate event to man-made pollution, the type of evidence produced in this latest study could in future be used to convince the courts (see ""The blame game":/news/2004/041129/full/432551a.html").
Meanwhile, Stott and his colleagues foresee more scorching times ahead. They predict that by the 2040s, more than half of Europe's summers will be warmer than that of 2003.
StottP., et al. Nature, 432. 610 - 614 (2004).