Global Warming: The Culprit?

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Local hot spots like this are not the same as global climate change, but they do appear to be part of a larger trend. Since 1970, mean ocean surface temperatures worldwide have risen about 1°F. Those numbers have moved in lockstep with global air temperatures, which have also inched up a degree. The warmest year ever recorded was 1998, with 2002, 2003 and 2004 close behind it.

So that ought to mean a lot more hurricanes, right? Actually, no--which is one of the reasons it's so hard to pin these trends down. The past 10 stormy years in the North Atlantic were preceded by many very quiet ones--all occurring at the same time that global temperatures were marching upward. Worldwide, there's a sort of equilibrium. When the number of storms in the North Atlantic increases, there is usually a corresponding fall in the number of storms in, say, the North Pacific. Over the course of a year, the variations tend to cancel one another out. "Globally," says atmospheric scientist Kerry Emanuel of the Massachusetts Institute of Technology, "we do not see any increase at all in the frequency of hurricanes."

But frequency is not the same as intensity, and two recent studies demonstrate that difference. Two weeks ago, a team of scientists that included Curry and Holland published a study in the journal Science that surveyed global hurricane frequency and intensity over the past 35 years. On the whole, they found, the number of Category 1, 2 and 3 storms has fallen slightly, while the number of Categories 4 and 5 storms--the most powerful ones--has climbed dramatically. In the 1970s, there were an average of 10 Category 4 and 5 hurricanes a year worldwide. Since 1990, the annual number has nearly doubled, to 18. Overall, the big storms have grown from just 20% of the global total to 35%. "We have a sustained increase [in hurricane intensity] over 30 years all over the globe," says Holland.

Emanuel came at the same question differently but got the same results. In a study published in the journal Nature last month, he surveyed roughly 4,800 hurricanes in the North Atlantic and North Pacific over the past 56 years. While he too found no increase in the total number of hurricanes, he found that their power--measured by wind speed and duration--had jumped 50% since the mid-1970s. "The storms are getting stronger," Emanuel says, "and they're lasting longer."

Several factors help feed the trend. For example, when ocean temperatures rise, so does the amount of water vapor in the air. A moister atmosphere helps fuel storms by giving them more to spit out in the form of rain and by helping drive the convection that gives them their lethal spin. Warm oceans produce higher levels of vapor than cool oceans--at a rate of about 1.3% more per decade since 1988, according to one study--and nothing gets that process going better than greenhouse-heated air. "Water vapor increases the rainfall intensity," says Trenberth. "During Katrina, rainfall exceeded 12 inches near New Orleans."