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Hurricane Intensity Grows, Study Finds

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The New York Times

Published: September 4, 2008

A new study finds that the strongest hurricanes and typhoons have become even stronger over the past 21/2 decades, adding grist to the contentious debate over whether global warming has made storms more destructive.

"I think we do see a climate signal here," said James B. Elsner, a professor of geography at Florida State University who is the lead author of the paper, being published in the current issue of the journal Nature.

The study, which also found that more typical, less powerful tropical storms had not become stronger over the 26-year period studied, is consistent with other researchers' hurricane models, Elsner said.

With oceans expected to continue warming, "one would expect more 4's and 5's," he said of Category 4 and Category 5 hurricanes, those with maximum sustained winds of at least 131 mph.

About 90 tropical storms form each year around the world. In the Atlantic, the stronger ones, with winds of at least 74 mph, are hurricanes; the equivalents in the Pacific and Indian oceans are typhoons. Ten named storms have formed in the Atlantic this hurricane season, which continues to the end of November.

Heat from the warming oceans will provide more energy to spin up hurricanes and typhoons, but the changing climate could also heighten conditions such as wind shear, which are winds blowing at different speeds and different directions at different altitudes, that tend to tear a storm apart.

Because of these environmental factors, most storms fall far short of their maximum possible intensity. Elsner, along with Thomas Jagger, a postdoctoral researcher at Florida State, and James Kossin, a research scientist at the University of Wisconsin-Madison, reasoned that warmer waters increased the possible intensity and that storms that develop in ideal conditions might have become stronger.

Having examined satellite data from 1981 through 2006, a period in which sea surface temperature rose to 83.3 degrees Fahrenheit from 82.8 degrees, they concluded that the highest wind speeds of the strongest storms averaged 156 mph in 2006, up from 140 mph in 1981.

Christopher Landsea, science and operations manager at the National Hurricane Center, who has been skeptical of the connection, said the statistical methodology in the new study was excellent.

Landsea questioned the underlying data, particularly corrections for data taken from the Indian Ocean before 1997, when there were fewer satellites.

He also said that the conclusions might have been skewed because the starting point of the data, 1981, coincided with a relatively quiet period of Atlantic hurricane activity, whereas the ending point, 2006, coincided with an active period that began about 1995.

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