Study: Global warming not to blame for rise in hurricanes

Cyclone Nargis recently made international headlines for dealing death and destruction to Burma.

Yet unlike the immediate aftermath of Hurricane Katrina, global warming received little blame for the intensification of Nargis. That's possibly because, in the nearly three years since New Orleans flooded, the science of hurricanes and climate has matured.

A new scientific paper authored by prominent National Oceanic and Atmospheric Administration scientists, published this weekend, reinforces the changing landscape.

The paper, which simulated Atlantic hurricane activity during warming 21st century conditions, found 27 percent fewer tropical storms, and 18 percent fewer hurricanes. The strongest hurricanes, the researchers found, had slightly higher wind speeds.

"This does not support the idea that we've already seen a large positive trend in hurricane activity emerging from greenhouse gases," said lead author Tom Knutson, a Princeton, N.J.-based research meteorologist for NOAA. "In fact, it points in the other direction."

Published online today in Nature Geoscience, the paper is notable for finding such a striking reduction in Atlantic activity. Earlier research has suggested a small reduction in total storms, or little change in a warming world.

By suggesting a marked decrease in activity, the new work bolsters the views of Chris Landsea, science and operations officer at the National Hurricane Center, who has argued that the apparent recent increase in Atlantic storm counts is due solely to better observational tools — satellites and the like — which blanket coverage of the Atlantic hurricane basin.

"After taking into account the changes in monitoring, the number of storms we're seeing now is on par with previous busy periods in the Atlantic," Landsea said.

This doesn't mean there's necessarily an emerging consensus on global warming's impact on hurricanes. While most research has suggested that the number of storms may not increase, and some like Knutson's has indicated a decrease in storm counts, there are troubling signs that the stronger storms' intensities may spike.

That's what concerns scientists like Judith Curry, chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology.

Some simulations, she said, have found that while the average intensity of storms may not increase substantially, the number of Category 4 and 5 hurricanes, the most powerful ones, does rise.

Some observational evidence may bear this out. Since 1935 there have been four seasons in the Atlantic basin with two or more Category 5 storms. Two of those seasons have occurred in the last three years, 2005 (Emily, Katrina, Rita and Wilma) and 2007 (Dean and Felix).

"There is strong agreement that intensity will increase in a warmer climate, although there are arguments about the magnitude of the increase," Curry said.

The hurricane-climate debate took off during the 2005 Atlantic hurricane season, which set a record of 28 tropical storms and hurricanes.

At the time several influential published papers suggested global warming was already having a measurable impact on hurricane activity.

"We now have a much more diverse scientific literature, by far," said author Chris Mooney, whose book Storm World chronicles the furious debate within the scientific community that has ensued since 2005 about hurricanes and climate.

But in addition to the scientific backdrop, Mooney said other reasons may exist to explain global warming's general absence from the post-Nargis debate.

"There's a different bad guy, not global warming but the junta," he said of Burma's ruling body. "And there's the risk of looking craven and opportunistic if you go off about global warming right now."

For casual observers following the 2005 hurricane season, there were considerable reasons to think a warmer world would produce more storms.

Hurricane forecasters say tropical storms will form when sea surface temperatures reach 80 degrees. In a warmer world, then, periods of favorable conditions for storm formation should last longer. So why doesn't the new study, and other simulations, identify such a trend?

"The response of Atlantic tropical cyclones to increased greenhouse gases depends on more than the warming Atlantic Ocean," said Gabriel Vecchi, a NOAA scientist who co-authored the new paper with Knutson.

"In our experiments, the reduction in Atlantic storms seems to be, at least partly, related to a projected increase in wind shear in the tropical Atlantic as we look out into the 21st century."

Nearly all scientists agree the hurricane-climate debate remains unsettled because computer modeling of hurricane activity for the next century remains problematic.

For past simulations, researchers have generally used just one of the climate models employed by the Intergovernmental Panel on Climate Change, or IPCC, to forecast future global temperatures. But each of these models returns different solutions for wind shear changes over the Atlantic Ocean.

A strength of the new study by Knutson is that it uses an average of all the computer models used by the IPCC, a technique that generally leads to better forecasts.

But until the climate models all agree on different variables, such as future wind shear, knowing what future hurricane seasons will bring is impossible, scientists say.

eric.berger@chron.com