Study: Warmer oceans strengthen hurricanes

This satellite image release by NOAA shows Tropical Depression Gustav west-northwest of Texarkana, Ark. on Wednesday.

As the oceans warm, the strongest tropical storms get even stronger, climate scientists reported Wednesday.

A study led by Florida State University researcher James B. Elsner suggests that the strongest storms have gotten stronger over the past quarter century, partly thanks to rising ocean temperatures. The study found that, over the same time, the intensity of weaker tropical storms has not increased. The study will be published in the Sept. 4 edition of the journal Nature.

Mr. Elsner and his team were attempting to verify the "heat-engine" theory of cyclone intensity. Proposed by MIT researcher Kerry Emmanuel in 2005, this theory holds that hurricanes are driven by the "intake" of warm air near the ocean surface and the "exhaust" of colder air above the storm.

"As seas warm, the ocean has more energy that can be converted to tropical cyclone wind," said Elsner in an FSU press release, though he qualified his findings. "Our results do not prove the heat-engine theory. We just show that the data are quite consistent with it."

Using data derived from US, European, and Japanese satellites, Elsner and his team looked at the strongest tropical storms – a category that includes hurricanes, cyclones, and typhoons – over the past 26 years. They found that wind speeds for the strongest tropical storms increased from an average of 140 mph in 1981 to 156 mph in 2006. At the same time ocean temperatures, averaged over the all regions where tropical cyclones form, increased from 82.8 degrees to 83.3 degrees.

The study found that storms are getting particularly stronger in the North Atlantic and Indian Oceans.

"It's almost like a survival-of-the-fittest argument," Elsner told Reuters. The news agency explained why he thinks that only the strongest storms seem to thrive:

The heat engine theory suggests all storms should strengthen as the ocean's surface gets hotter, but in reality, few tropical cyclones achieve their full maximum potential intensity.
A cyclone's intensity can be cut by other factors, such as where they form, how close they are to land, El Nino patterns and solar activity, the researchers said.
Strong storms seem able to overcome these factors and gather more fuel from warming waters, Elsner said.

Other scientists have doubts about Ensler's data set. The New York Times quotes Christopher Landsea, a science and operations manager at the National Hurricane Center in Miami and a vocal opponent [see update below] of the supposed connection between global warming and hurricane intensity, who says that there were not enough satellites over the Indian Ocean before 1997 to draw reliable information. Mr. Landsea believes that the recent increase in hurricane activity is due to natural variations.

The paper also quotes Thomas Knutson, a scientist with the National Oceanic and Atmospheric Administration who in 2004 suggested [PDF] that carbon dioxide leads to stronger hurricanes. Mr. Knutson said that 26 years of data is not enough time to draw long-term conclusions. “One is left with a very suggestive result and a very interesting result," he told the Times,"but it’s not a definitive smoking gun for a greenhouse warming signal on hurricanes.”

Update: My colleague Pete Spotts, who is far more knowledgeable about this area than I am, takes issue with my labelling of Christopher Landsea as a "vocal opponent" of the idea that global warming intensifies hurricanes. Landsea frequently says that he remains unconvinced that there is enough data to say that hurricane activity in recent decades has been influenced by global earming, but, as Pete pointed out to me in an email, that's not the same thing as saying there is no connection. "[A]s far as I know," Pete writes, "Landsea has never rejected the hypothesis that in the future, global warming could affect tropical cyclones."

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