Global Warming Is a Cause of This Year’s Extreme Weather

It's almost a point of pride with climatologists. Whenever someplace is hit with a heat wave, drought, killer storm or other extreme weather, scientists trip over themselves to absolve global warming. No particular weather event, goes the mantra, can be blamed on something so general. Extreme weather occurred before humans began loading up the atmosphere with heat-trapping greenhouse gases such as carbon dioxide. So this storm or that heat wave could be the result of the same natural forces that prevailed 100 years ago—random movements of air masses, unlucky confluences of high- and low-pressure systems—rather than global warming.

This pretense has worn thin. The frequency of downpours and heat waves, as well as the power of hurricanes, has increased so dramatically that "100-year storms" are striking some areas once every 15 years, and other once rare events keep returning like a bad penny. As a result, some climatologists now say global warming is to blame. Rising temperatures boost the probability of extreme weather, says Tom Karl, director of the National Climatic Data Center and lead author of a new report from the Bush administration's Climate Change Science Program; that can "lead to the type of events we are seeing in the Midwest." There, three weeks of downpours have caused rivers to treat their banks as no more than mild suggestions. Think of it this way: if once we experienced one Noachian downpour every 20 years, and now we suffer five, four are likely man-made.

It's been easier to connect global warming to rising temperatures than to extreme weather events—and even the former hasn't been easy. Only in this decade have "attribution" studies managed to finger greenhouse gases as the chief cause of the rising mercury, rather than a hotter sun or cyclical changes. (The last two produce a different pattern of climate change than man-made warming does.) Now the same "whatdunit?" techniques are being applied to droughts, downpours, heat waves and powerful hurricanes. "We can look at climate-model simulations and likely attribute [specific extreme weather] to human activity," says Gerry Meehl of the National Center for Atmospheric Research.

http://www.newsweek.com/id/143787