In autumn 2000 devastating floods swept through England and Wales, inundating homes, swamping roads and rail lines, and requiring the evacuation of more than 11,000 people. That fall was the wettest in the region since records began in 1766, and the subsequent flooding caused billions of dollars in damage. Now climatologists suggest that climate change doubled the odds of such catastrophic flooding in 2000.

"Greenhouse gas emissions due to human activity have affected the odds of floods in England and Wales," says physicist Pardeep Pall of the Swiss Federal Institute of Technology (ETH Zurich), who led the research published February 17 in Nature. "The odds of a flood occurring in the autumn of 2000 likely increased by double or more." (Scientific American is part of the Nature Publishing Group.)

Such inundations are becoming more common, according to the International Federation of Red Cross and Red Crescent Societies, among other disaster statistics keepers. In fact, according to reinsurer Munich Re, extreme floods have tripled globally since 1980. The reason may well be climate change caused by increasing concentrations of atmospheric greenhouse gases—now roughly 390 parts per million, up from 280 ppm in the 1700s.

Warmer atmospheric air means more water vapor, which is itself a greenhouse gas, exacerbating the problem. What goes up, must come down and, more and more, that water vapor is coming down in extreme precipitation events—defined in North America as more than 100 millimeters of rainfall (or the equivalent in snow or freezing rain) falling in 24 hours—according to new research also published February 17 in Nature that examines such events in the Northern Hemisphere.

"We expect a widespread increase in heavy precipitation due to greenhouse gas warming leading to a moister atmosphere," explains climatologist Gabriele Hegerl of the University of Edinburgh in Scotland. That's because water vapor increases by roughly 7 percent for every degree Celsius of warming in the lowest level of the atmosphere—or, more simply put, warmer air means warmer water, which means more of it in the form of vapor. That vapor still coalesces into clouds, raindrops and snowflakes, which is why basic physics suggests that more water vapor results in more rain. That's exactly what the study found for the first time. "Our study shows that the heaviest [precipitation] events have increased in magnitude, meaning that rare events are becoming less rare," notes climatologist Francis Zwiers of the University of Victoria in British Columbia.

Hegerl, Zwiers and their climatologist colleagues at Environment Canada examined daily records from more than 6,000 weather stations around the globe of rainfall, snowfall and other precipitation stretching from 1951 to 1999. In each year of that period, they determined how extreme precipitation had been. By compiling the information from all these years and comparing it with the precipitation patterns predicted by computer models of the climate, the scientists noted a similar pattern emerging in the real-world data. What's more this pattern could not be explained by natural climate fluctuations, suggesting that human-induced climate change is the culprit behind an increase in downpours and blizzards in the last 50 years of the 20th century—at least in the Northern Hemisphere.

"There are characteristic patterns of increase and decrease, for example, in response to an El Nino event," which is a cyclical climate event marked by warming waters in the western Pacific Ocean that has global impacts, Zwiers says. "That's not the kind of change we saw." Instead a human fingerprint emerged from the data pattern. More worryingly, the actual increase in rain, snow and sleet were larger than predicted by the computer models.

The study marks the first time that human influence on the climate has been demonstrated in the water cycle, and outside the bounds of typical physical responses such as warming deep ocean and sea surface temperatures or diminishing sea ice and snow cover extent.

To attribute any specific extreme weather event—such as the downpours that caused flooding in Pakistan or Australia, for example—requires running such computer models thousands of times to detect any possible human impact amidst all the natural influences on a given day's weather. "It is a reasonable question: is human influence anything to do with this nasty bit of weather we're having?" explains physicist Myles Allen of the University of Oxford, who helped oversee the English flooding study. "Answering it isn't easy."

So, the U.K. team also called on tens of thousands of volunteers who ran a climate model thousands of times on their personal computers in the background as part of the Web site. After six years of running such simulations, the verdict is in: Increasing greenhouse gas concentrations as a result of burning fossil fuels and cutting down forests increased the risks of flooding in two out of three model runs by more than 90 percent.

The bad news is that such record-breaking downpours, blizzards and sleet storms are likely to continue to get worse as atmospheric greenhouse gas concentrations continue to rise, causing global temperatures to continue to warm and making the atmosphere more and more humid. "The human influence on the climate system has the effect of intensifying precipitation extremes," Zwiers notes. "It is therefore reasonable to expect that precipitation extremes will continue to intensify," although how much is still a mystery, largely thanks to an unclear understanding of the atmospheric impact of how tiny flecks of pollution in the atmosphere—known as aerosols to scientists and comprising materials ranging from soot to sulfur dioxide.

As for whether the world's recent extreme weather was made worse by human influence, that answer likely won't be available for years—and only if a research effort similar to the one that analyzed the 2000 U.K. floods is undertaken. "The human impact in this is still an open question," Zwiers says.

But the U.K. Met Office (national weather service), the U.S.'s National Center for Atmospheric Research and other partners around the globe aim to change that in the future by developing regular assessments—much like present evaluations of global average temperatures along with building from the U.K. flooding risk modeling efforts—to determine how much a given season's extreme weather could be attributed to human influence. "We will develop that science further so that we can provide regular and scientifically robust evidence on how the odds of these phenomena are changing," says climate modeler Peter Stott of the U.K. Met Office.

Already, it is becoming clear that burning fossil fuels and clearing forests are having an impact on the atmosphere, which is rebounding to the detriment of the humans behind those activities. "One of the problems people find with climate change is it's a victimless crime…nobody's particularly affected by small changes in global average temperatures," Allen notes. "Extreme weather is what actually hurts people."