Increased flood risk linked to global warming

Likelihood of extreme rainfall may have been doubled by rising greenhouse-gas levels

Climate change may be hitting home. Rises in global average temperature are remote from most people's experience, but two studies in this week's Nature conclude that climate warming is already causing extreme weather events that affect the lives of millions. The research directly links rising greenhouse-gas levels with the growing intensity of rain and snow in the Northern Hemisphere, and the increased risk of flooding in the United Kingdom.

Insurers will take note, as will those developing policies for adapting to climate change. "This has immense importance not just as a further justification for emissions reduction, but also for adaptation planning," says Michael Oppenheimer, a climate-policy researcher at Princeton University in New Jersey, who was not involved in the studies.

There is no doubt that humans are altering the climate, but the implications for regional weather are less clear. No computer simulation can conclusively attribute a given snowstorm or flood to global warming. But with a combination of climate models, weather observations and a good dose of probability theory, scientists may be able to determine how climate warming changes the odds. An earlier study3, for example, found that global warming has at least doubled the likelihood of extreme events such as the 2003 European heatwave.

More-localized weather extremes have been harder to attribute to climate change until now. "Climate models have improved a lot since ten years ago, when we basically couldn't say anything about rainfall," says Gabriele Hegerl, a climate researcher at the University of Edinburgh, UK. In the first of the latest studies1, Hegerl and her colleagues compared data from weather stations in the Northern Hemisphere with precipitation simulations from eight climate models (see page 378). "We can now say with some confidence that the increased rainfall intensity in the latter half of the twentieth century cannot be explained by our estimates of internal climate variability," she says.

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