Global warming will produce fewer hurricanes


Global warming will produce fewer Atlantic hurricanes, according to a study published today in the journal Nature Geoscience by a U.S. government meteorologist.

Global warming will not increase the number of Atlantic hurricanes

Global warming will produce fewer hurricanes

May 18, 2008

Global warming will produce fewer Atlantic hurricanes, according to a study published today in the journal Nature Geoscience by a U.S. government meteorologist.

Devising a regional climate model of the Atlantic basin that reproduces the observed rise in hurricane counts between 1980 and 2006, Tom Knutson of the National Oceanic and Atmospheric Administration’s fluid dynamics lab in Princeton, N.J. and colleagues conclude that warming oceans due to rising atmospheric greenhouse-gas concentrations will not increase the frequency of tropical storms and hurricanes.

“Our model supports the hypothesis that the primary driver of the recent increase in Atlantic hurricane numbers was the warming of the tropical Atlantic relative to the other tropical basins, rather than local increases in tropical Atlantic SSTs alone,” the authors write.

The two top images capture Katrina with an intact eyewall at 5:45 p.m. Eastern Daylight Time on Sunday, August 28, 2005 as it moved over warm water in the Gulf of Mexico. By 5:45 a.m. on Monday, the weakened eyewall is being further disrupted by interaction with the land surface. GEMPAK images by Jeff Weber, UCAR; data from water vapor and infrared bands of NOAA GOES-E satellite.

“The decrease in Atlantic tropical storm and hurricane frequency simulated by our model—in response to a pronounced twenty-first-century greenhouse warming—is markedly different from the large increase in future hurricane activity that might be projected using statistical analysis based only on the recent covariability of hurricanes and local [sea surface temperatures] SSTs,” they conclude.

Decline in the number of storms, but an increase in intensity

The simulations project an 18 percent decline in the number of hurricanes and a 27 percent fall in the number of tropical storms in the Atlantic by end of the century. The number of hurricanes making landfall in the Western Atlantic will decrease by 30 percent due to “enhanced vertical wind shear”.

While storms are expected to become rarer, the model forecasts that they will intensify: rainfall within 30 miles of storm will climb by 37 percent and wind strength will increase by 2 percent.

Controversial results

“As in previous studies, there is no indication that the region of Atlantic tropical storm formation expands with greenhouse warming,” write Knutson and colleagues. “Storms typically develop in the control simulation, and in observations, over (SSTs) in excess of a ‘threshold’ temperature of about 26 °C. In the warmer climate, given the relatively homogeneous tropical warming, the ‘threshold’ temperature for storm formation increases in the model by the same amount as the mean tropical SSTs… Consequently, no pronounced expansion of the tropical storm formation region is simulated.”

Other research has suggested that climate change is already, and will continue, increasing the frequency of hurricanes. Studies by Kerry Emanuel of MIT,
Peter Webster and Judith Curry of Georgia Tech, School of Earth and Atmospheric Sciences, and Greg Holland of the National Center for Atmospheric Research have linked warming oceans to a doubling in the number of Category 4 and 5 hurricanes worldwide over the past 35 years.

Still, other work supports Knutson’s findings. A 2007 study, published in Geophysical Research Letters, suggested that an increase in vertical wind shear in the tropical Atlantic and East Pacific Oceans could inhibit the formation and intensification of hurricanes. William Gray, a meteorologist at Colorado State University, has long argued that the recent upswing in hurricanes is likely a result of naturally occurring multi-decadal Atlantic Ocean circulation variations, not global warming.

“There have been similar past periods (1940s-1950s) when the Atlantic was just as active as in recent years. For instance, when we compare Atlantic basin hurricane numbers over the 15-year period (1990-2004) with an earlier 15-year period (1950-1964), we see no difference in hurricane frequency or intensity even though the global surface temperatures were cooler and there was a general global cooling during 1950-1964 as compared with global warming during 1990-2004,” he said. “We have no plausible physical reasons for believing that Atlantic hurricane frequency or intensity will change significantly if global ocean temperatures continue to rise.”

Chris Landsea, a hurricane expert with NOAA, agreed, telling the Associated Press that the new study is “consistent with what’s being said all along.”

“I think global warming is a big concern, but when it comes to hurricanes the evidence for changes is pretty darn tiny,” Landsea was quoted as saying.

The Atlantic hurricane season begins June 1. Researchers at Colorado State University forecast 15 named storms and 8 hurricanes for the 2008 season. They say there is a 69 percent chance that at least one major hurricane (category 3,4 or 5 storm) will hit the U.S.

THOMAS R. KNUTSON, JOSEPH J. SIRUTIS, STEPHEN T. GARNER, GABRIEL A. VECCHI AND ISAAC M. HELD (2008). Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions. Nature Geoscience. 18 May 2008.

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