Hurricane link to climate change is hazy

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Research may show why storms in different regions respond differently to global warming.

Will destructive hurricanes such as Katrina become more common in a warmer world? Two recent studies suggest that they will. But the question has split the research community (see Nature 435, 1008–1009; 200510.1038/4351008b), and some say that the studies highlight how little is known about the physics of hurricane formation.

The media, and some politicians, have been quick to blame global warming for the disastrous Atlantic hurricane seasons of 2004 and 2005, including Katrina. The two new studies make no such direct link, but do suggest that the intensity — although not the frequency — of tropical cyclones is increasing sharply, perhaps as a result of global warming.

The first study, published in August (Nature 436, 686–688; 2005), shows that cyclones have become more destructive over the past 30 years, with storms being both longer lived and more intense. The second, published last week (Science 309, 1844–1846; 2005), concludes that today's cyclones are stronger, although less common, than 35 years ago.

“The potential for more events like Katrina is on the rise,” says Greg Holland, a hurricane expert at the National Center for Atmospheric Research in Boulder, Colorado, and an author of the Science paper. “You can never be sure, but it seems to be consistent with global change.”

A brace of studies indicates that cyclones have increased in strength over the past 30 years. Credit: J. DESCLOITRES, MODIS LAND RAPID RESPONSE TEAM/NASA/GSFC

But not everyone is convinced. “Hurricanes tend to go in decadal cycles,” points out Chris Landsea, a meteorologist at the US National Hurricane Center in Miami, Florida. “Making out the tiny global-warming signal amid these large natural swings is hardly possible at this point.”

And although the new studies are persuasive, they are not definitive. For instance, the Science paper concludes that peak wind speeds have remained constant since 1970. But this finding seems to be inconsistent with the trend towards stronger storms, says Kerry Emanuel, a researcher at the Massachusetts Institute of Technology and author of the Nature paper.

Many of the arguments arise because there are insufficient data. “There seems to be a monumental lack of understanding of the role hurricanes play in the climate system,” says Peter Webster, an atmospheric researcher at the Georgia Institute of Technology in Atlanta and an author of the Science paper.

One uncertainty, he says, is how changes in sea surface temperature affect the formation of tropical cyclones. Hurricanes in the Atlantic Ocean, for example, seem to respond to such changes quite differently from typhoons in the Pacific. But just 12% of cyclones form in the Atlantic, the only region where reconnaissance aircraft fly regularly to make precise measurements of hurricane winds.

Research is now under way to address the issue. As a follow-up to the Science paper, the authors have started to investigate how oceans interact with the atmosphere in different parts of the world. The goal, says Judy Curry of the Georgia Institute of Technology, is to determine how and why natural fluctuations favour different hurricane patterns in different ocean basins.

Understanding these complex mechanisms, she says, should help researchers better quantify how much a storm trend is due to global warming — or not.

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Schiermeier, Q. Hurricane link to climate change is hazy. Nature 437, 461 (2005) doi:10.1038/437461a

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