Study: More Hurricanes in Strongest Classes

WASHINGTON - The number of hurricanes reaching the most powerful Categories 4 and 5 has nearly doubled worldwide since 1970, researchers reported Friday in a study that arrived as Americans watched the Gulf Coast dig out from Katrina.

The study, published in the journal Science, found that while the total number of tropical storms and hurricanes has stayed roughly flat, the proportion spinning up into the strongest classes jumped about 80 percent — from roughly 10 a year to about 18 a year.

"The trend is not in how many storms form, but in how strong they get," said the study's lead author, a climatologist at the Georgia Institute of Technology. "And the regions with the biggest increases are the North Atlantic and the Indian and Pacific oceans."

The rise parallels a roughly 1-degree Fahrenheit increase in tropical ocean temperatures over the same period. Hurricanes draw their energy from warm surface water, and warmer seas can feed higher wind speeds.

The findings, published online weeks early because of Katrina, stoked a scientific dispute. Researchers who doubt a warming link say better satellites now catch strong storms that would have been missed 35 years ago, and that Atlantic hurricane activity naturally cycles over decades.

"You cannot take a 35-year record and separate natural cycles from greenhouse warming," said a skeptic at Colorado State University, the center of seasonal hurricane forecasting. "We have had hyperactive eras before."

Supporters counter that the increases showed up in every ocean basin — a global signal they say is hard to explain as a regional cycle — and note that sea-surface warmth is only part of the story; winds aloft matter too.

All sides caution against pinning any single storm on climate change. "Katrina was not caused by global warming," the lead author said. "But the question going forward is whether Katrinas become more common."

The debate has a policy edge: the administration has resisted mandatory limits on greenhouse gases, while several senators have cited the storm in pushing climate legislation.