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.
The Midwest, for instance, suffered three weeks of intense rain in May and June, with more than five inches falling on some days. That brought a reprise of the area's 1993 flooding, which was thought to be a once-in-500-years event. The proximate cause was the western part of the jet stream dipping toward the Gulf of Mexico, then rising toward Iowa—funneling moisture from the gulf to the Midwest, says meteorologist Bill Gallus of (the very soggy) Iowa State University. The puzzle, he says, is why the trough kept reforming in the west, creating a rain-carrying conveyor belt that, like a nightmarish version of a Charlie Chaplin movie, wouldn't turn off. One clue is that global warming has caused the jet stream to shift north. That has brought, and will continue to bring, more tropical storms to the nation's north, and may push around the jet stream in other ways as well.
Global warming has left its clearest fingerprint on heat waves. Since the record scorcher of 1998, the average annual temperatures in the United States in six of the past 10 years have been among the hottest 10 percent on record. Climatologists predict that days so hot they now arrive only once every 20 years will, by midcentury, hit the continental United States once every three years. Scientists also discern a greenhouse fingerprint in downpours, which in the continental United States have increased 20 percent over the past century. In a warmer world, air holds more water vapor, so when cloud conditions are right for that vapor to form droplets, more precipitation falls. Man-made climate change is also causing more droughts on top of those that occur naturally: attribution studies trace droughts such as that gripping the Southwest to higher sea-surface temperatures, especially in the Pacific. Those can fluctuate naturally, as they did when they caused the severe droughts of the 1930s and 1950s. But they are also rising due to global warming, causing a complicated cascade of changes in air circulation that shuts down rainfall.
