Nature doesn't always know when to quit--and nothing says that quite like a hurricane. The atmospheric convulsion that was Hurricane Katrina had barely left the Gulf Coast before its sister Rita was spinning to life out in the Atlantic. In the three weeks between them, five other named storms had lived and died in the warm Atlantic waters without making the same headlines their ferocious sisters did. With more than two months left in the official hurricane season, only Stan, Tammy, Vince and Wilma are still available on the National Hurricane Center's annual list of 21 storm names. If the next few weeks go like the past few, those names will be used up too, and the storms that follow will be identified simply by Greek letters. Never in the 52 years we have been naming storms has there been a Hurricane Alpha.
If 2005 goes down as the worst hurricane season on record in the North Atlantic, it will join 2004 as one of the most violent ever. And these two seasons are part of a trend of increasingly powerful and deadly hurricanes that has been playing out for more than 10 years. Says climatologist Judy Curry, chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology: "The so-called once-in-a-lifetime storm isn't even once in a season anymore."
Head-snapping changes in the weather like this inevitably raise the question, Is global warming to blame? For years, environmentalists have warned that one of the first and most reliable signs of a climatological crash would be an upsurge in the most violent hurricanes, the kind that thrive in a suddenly warmer world. Scientists are quick to point out that changes in the weather and climate change are two different things. But now, after watching two Gulf Coast hurricanes reach Category 5 in the space of four weeks, even skeptical scientists are starting to wonder whether something serious might be going on.
"There is no doubt that climate is changing and humans are partly responsible," says Kevin Trenberth, head of the climate-analysis section at the National Center for Atmospheric Research (NCAR) in Boulder, Colo. "The odds have changed in favor of more intense storms and heavier rainfalls." Says NCAR meteorologist Greg Holland: "These are not small changes. We're talking about a very large change."
But do scientists really know for sure? Can man-made greenhouse gases really be blamed for the intensity of storms like Rita and Katrina? Or are there, as other experts insist, too many additional variables to say one way or the other?
That global warming ought to, in theory, exacerbate the problem of hurricanes is an easy conclusion to reach. Few scientists doubt that carbon dioxide and other greenhouse gases raise the temperature of Earth's atmosphere. Warmer air can easily translate into warmer oceans--and warm oceans are the jet fuel that drives the hurricane's turbine. When Katrina hit at the end of August, the Gulf of Mexico was a veritable hurricane refueling station, with water up to 5°F higher than normal. Rita too drew its killer strength from the Gulf, making its way past southern Florida as a Category 1 storm, then exploding into a Category 5 as it moved westward. "The Gulf is really warm this year, and it's just cooking those tropical storms," says Curry.