Moreover, the researchers did not find any increase in the maximum wind speed of the strongest global storms, Landsea said. Climate models suggest warm seas should also increase the intensity of the very strongest storms, meaning the researchers should have found some effect.

Landsea said it's unlikely global warming would already be increasing hurricane intensity. He cited the work of Thomas Knutson, a meteorologist with the National Oceanic and Atmospheric Administration, who studies the long-term effects of warming seas on hurricanes.

If global oceans warm between 2 and 5 degrees Fahrenheit in the next century, Knutson found, major storms would see a 5 percent to 12 percent increase in wind speeds. Such an effect would simply be too small to measure today.

"I'm not saying global warming won't have an effect," Landsea said. "Yes, hurricanes may become stronger, but it's going to pretty darn tiny, and it's a long way into the future. I wouldn't expect to see any global warming signal in the hurricane record for decades."

Gregory Holland, a scientist at the National Center for Atmospheric Research and another author of the new study, defended his group's research. The data is reliable, he said, and the trend is clear.

"There is no doubt there is a substantial increase here in the number of category-4 and category-5 storms," Holland said.

The research group studied storms on a worldwide scale because, in individual basins, there are substantial, decades-long ups and down in the number of storms. The Atlantic has experienced such a trend since about 1995, when the number and intensity of storms has increased, following a widely accepted trend known as the Atlantic Multidecadal Oscillation.

Between 1970 and 1994 there were an average of 0.8 major storms per year. The decade ending last year saw an average of 2.3 major storms a year, nearly a three-to-one increase.

A researcher at Colorado State University's hurricane forecasting program, Phil Klotzbach, said these periodic oscillations, more than any global warming trend, may explain the data in the new study.