Strongest Hurricanes May Be Getting Stronger...

A new study finds that the strongest of hurricanes and typhoons have become even stronger over the last two and a half decades, adding grist to the contentious debate over global warming and its ability to unleash more destructive storms.

At the same time, the study, published in Thursday’s issue of the journal Nature, finds that more typical, run-of-the-mill tropical storms have not become stronger over that same period.

“I think we do see a climate signal here,” said James B. Elsner, a professor of geography at Florida State University and lead author of the Nature paper. Dr. Elsner said the findings were consistent with hurricane models. With oceans expected to continue warming, “one would expect more 4’s and 5’s,” Dr. Elsner said of Category 4 and Category 5 storms with winds of at least 131 miles per hour.

About 90 tropical cyclone storms form each year. In the Atlantic, the stronger ones, with winds of at least 74 miles, are hurricanes; in the Pacific and Indian oceans, the exact equivalent are typhoons.

Heat from the warming oceans will provide more energy to spin up hurricanes and typhoons, but the changing climate could also heighten conditions like wind shear — winds blowing at different speeds and different speeds 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, but Dr. Elsner, along with Thomas H. Jagger, a postdoctoral researcher at Florida State, and James P. Kossin, a research scientist at the University of Wisconsin-Madison, reasoned that warmer waters increased the possible intensity, and storms that develop in ideal conditions might have gotten stronger.

Examining satellite data from 1981 to 2006, they concluded that the highest wind speeds for the strongest storms averaged 156 miles per hour in 2006, up from 140 miles per hour in 1981 as the sea surface temperature rose to 83.3 degrees Fahrenheit from 82.8 degrees. The increases in cyclone intensity were greatest in the Atlantic and Indian oceans.

Because the data comes from one set of satellites, the scientists avoided some of the calibration difficulties in earlier studies.

“This study offer definitive evidence that there are more of the very strongest hurricanes around the world, even though the total number of storms globally shows hardly any trend,” said Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology, who suggested in 2005 that global warming has already intensified cyclones.

Christopher W. Landsea, a science and operations manager at the National Hurricane Center in Miami who has been skeptical of the connection, said the statistical methodology in the new study was excellent, but questioned the underlying data, particularly corrections for data over the Indian Ocean prior to 1997 when there were fewer satellites observing the storms.

Dr. Landsea also said that the increase in Atlantic hurricanes arose from natural variations when decades of active hurricane seasons are followed by decades with few hurricanes. The current period of active hurricane seasons began around 1995.

“The paper has some elegantly calculated statistics, but these are generated on data that are not, in my opinion, reliable for examining how the strongest tropical cyclones have changed around the world,” Dr. Landsea said.

Dr. Thomas R. Knutson of the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton, said the 26 years of data is too short for long-term conclusions. “One is left with a very suggestive result and a very interesting result, but it’s not a definitive smoking gun for a greenhouse warming signal on hurricanes,” he said.