September 24, 2005

The Gulf Coast: A Victim of Global Warming?

North Atlantic hurricanes are growing worse, but an MIT climatologist says it would be 'absurd' to attribute Katrina or Rita to long-term climate change.

By Wade Roush

As evacuees galvanized by Katrina fled the Gulf Coast in advance of Hurricane Rita this week, only to sit idling for hours on clogged freeways, they had plenty of time to wonder who or what was to blame for the storm-tossed mess. But while many have attributed the size and violence of Katrina and Rita to global warming, MIT climatologist Kerry Emanuel says it would be wrong to blame the devastation caused by any individual hurricane on long-term climate change.

There are troubling signs in the meteorological record of a link between global warming and hurricane intensity, says Emanuel, a professor in MIT's Department of Earth, Atmospheric and Planetary Sciences. But the best available science suggests that the now-scattered populations of the Texas, Louisiana, Mississippi, and Alabama coasts are the victims of mere happenstance.

There are simply too few examples of catastrophic hurricanes hitting U.S. shores to make out any statistical trend, says Emanuel. "It would be absurd to attribute the Katrina disaster to global warming," Emanuel wrote on his website this month.

What Emanuel does believe is that the average power of many tropical cyclones -- the blanket terms scientists use for hurricanes, typhoons, and cyclones -- has risen sharply over the past several decades, at least in the North Atlantic and North Pacific. Moreover, the increase is closely tied to changes in the surface temperatures of the oceans where tropical cyclones are born. In other words, when the sea surface temperature rises, the energy of the cyclones above that surface also rises -- and at an even faster rate.

And lately, ocean temperatures have been rising more than they've been falling. Emanuel's examination of North Atlantic and North Pacific storm records over the last 50 years show a marked increase in the average intensity of tropical cyclones.

"It's a big effect," says Emanuel. "It's gone up 50 to 80 percent over the last three decades or so."

Emanuel published his findings in the August 4 issue of the journal Nature and has been at the center of his own storm of debate ever since. Politicans and pundits have latched onto his work as proof that global warming is starting to have an impact. And Emanuel's fellow scientists are also asking tough question. Despite the appearance of corroborating results from a team of researchers at Georgia Tech and the National Center for Atmospheric Research in the September 16 issue of Science, some researchers have said they are surprised by the magnitude of the intensity increases, which don't show up in existing climate-change theory or computer models.