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See Inside May 2011

Climate Change Will Bring More Extreme Precipitation and Floods




Mark Humphrey, AP Photo

In the past year floods have submerged cities as far apart as Nashville, Tenn., and Nowshera, Pakistan. An epic heat wave touched off peat fires in Moscow that wreathed the capital in smoke. A drought in northeastern China ruined the wheat crop. Blizzards left the U.S. buried in snow—and collapsed the roof of a football stadium. “It is a reasonable question: Is human influence on climate anything to do with this nasty bit of weather we’re having?” physicist Myles Allen of the University of Oxford said in a recent press briefing.

It hasn’t been an easy question to answer. But now, after years of research, scientists have begun to detect a human fingerprint in many extreme weather patterns. In a study written up in February in Nature (a sister publication of Scientific American), researchers examined daily records of rainfall, snowfall and sleet from more than 6,000 weather stations between 1951 and 1999.

They found a rise in cases of extreme precipitation, such as rainstorms that deliver 100 millimeters of rainfall or more in 24 hours. The uptick could not be explained by natural climate fluctuations; instead it more closely matched what the patterns that computer models of climate predict for increasing concentrations of greenhouse gases. Humanity, in other words, has likely loaded the weather dice in favor of severe storms.

The study suggests that record-breaking downpours, blizzards and sleet storms will continue—though by how much and how soon remain a mystery. The U.K.’s Met Office, the U.S. National Center for Atmospheric Research and other partners aim to bridge that knowledge gap by making regular assessments—much like present evaluations of global average temperatures—of how much a given season’s extreme weather is from human influence.

Linking a particular weather event to human-­induced climate change remains problematic.
“We shouldn’t expect that human influence should be a factor in all of these events,” says climatologist Francis Zwiers of the University of Victoria in ­British ­Columbia, who led the research published in ­Nature. Still, we don’t get off scot-free.

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