John Fogerty once crooned "Who'll stop the rain?" Not humanity, apparently, as new research shows that human-caused climate change has significantly increased the chances of extreme rain- and snowfall around the world, along with the deadly floods that follow.
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| Pakistan floods 2011 |
This is according to two new studies published Wednesday in the British journal Nature.
While other studies have suggested that global warming may be partly responsible for an increase in heavy precipitation, what's new in this study is the formal finding that human influence has "likely made intense precipitation stronger, on average, over the second half of the 20th century," says study co-author Francis Zwiers of the University of Victoria in British Columbia.
"The observed change cannot be explained by natural fluctuations of the climate system alone," he says.
One of the studies reported that that the most significant rain and snow events were 7 percent wetter in the 1990s than they had been in the 1950s.
Scientists based their findings on rainfall data from 1951 to 1999 in Northern Hemisphere land areas, including North America, Eurasia and India.
The scientists took all the information that showed an increase in extreme rain and snow events from the 1950s through the 1990s, and ran dozens of computer models numerous times. They put in the effects of greenhouse gases -- which come from the burning of fossil fuels -- and then ran numerous models without those factors.
Only when the greenhouse gases are factored in did the models show a similar increase to what actually happened. Essentially, the computer runs show climate change is the only way to explain what's happening.
The other study dealt with the floods that swamped the U.K. in fall 2000 and determined that climate change made them over twice as likely to occur.
Why would global warming lead to more precipitation? According to study co-author Myles Allen of the University of Oxford in England, warmer air holds more water.
Senior scientist Kevin Trenberth of the National Center for Atmospheric Research in Boulder, who was not part of either study, expands on this: "The water holding capacity of the atmosphere goes up with higher temperatures (and higher sea-surface temperatures), and so there is simply more moisture lurking around waiting to be caught up in any storm."
The effects of greenhouse gases on precipitation appear to be global: "Extreme precipitation is expected to increase almost everywhere in a warmer world, even though we expect reductions in mean precipitation in some locations and increases in others," reports Zwiers.
Overall, according to Zwiers, computer models suggest that the northern high latitudes will see the largest percentage increases in mean annual precipitation, and that the tropics will see the largest percentage increases in extreme precipitation.
"Damaging weather events have always happened since well before humans had any substantial influence on climate," says Allen. "This research allows us to quantify how rising greenhouse gas levels may be loading the dice in favor of certain events, such as the U.K. floods of 2000, and against other events."
However, climate scientist Jerry North of Texas A&M University, while praising the work, said he worried that the studies were making too firm a connection based on weather data that could be poor in some locations. More
