Scientists say the number of violent cyclones around the world has doubled over the past 35 years, and storms such as the one that devastated New Orleans could become more common. Many experts blame violent storm activity on global warming.

Researchers at the Georgia Institute of Technology say Katrina appears to be part of an increasing trend of category four and five storms, the most violent cyclones to hit land. In the Pacific, such storms are called typhoons.

Category four storms have sustained winds of up to 250 kilometers, while category five storms exceed that.

Greg Holland of the National Center for Atmospheric Research and colleagues at Georgia Tech's School of Earth and Atmospheric Sciences report an average of 10 category four and five hurricanes per year globally during the 1970s. Since 1990, that number has nearly doubled to 18.

"Indeed, if you're taking a proportional sense, they've gone from being around 20 percent of the total to being around 35 percent of the total," said Greg Holland.

Based on satellite data, researchers found the total number of hurricanes has inexplicably decreased while their intensity and duration have increased.

The largest increases in the number of category four and five storms has occurred in the North Pacific, the Southwest Pacific and the North and South Indian Oceans. In the North Atlantic, scientists have found hurricanes have become more numerous and longer lasting, particularly since 1995.

Kevin Trenberth is head of the climate analysis section at the National Center for Atmospheric Research in Boulder, Colorado and has done related studies.

Mr. Trenberth says cyclones are becoming more intense because of increases in sea surface temperatures as a result of global warming. He says hurricanes and typhoons form when sea surface temperatures are above 26 degrees celsius. Mr. Trenberth says that produces water vapor, which is fuel for violent storms.

"As that moisture condenses and forms precipitation, rainfall, then it releases that heat back into the atmosphere," said Kevin Trenberth, "And so there are tremendous rainfalls associated with these storms. In the case of Katrina, for instance, just north of New Orleans, there was over 12 inches [30 centimeters] of rainfall and over a swath of over 100 miles [160 kilometers] all around New Orleans there was over eight inches [20 centimeters] of rainfall. And so, this contributes substantially to the flooding."

The authors of the study declined to say the increase in the number of violent hurricanes caused by global warming is the result of human activity.

Whether it is or it isn't, Mr. Trenberth says climate problems may be long term.

"Even if we were somehow able to wave our magic wand and no longer put any more carbon dioxide into the atmosphere, then we would still have global warming continuing for sometime into the future," he said. "And the best estimates from our climate models are that there would be a further increase of about one degree temperature in the global mean temperatures simply because of the carbon dioxide that's already in the atmosphere, because it has a long life time. That stays with us for maybe a hundred years."

The study on hurricane intensity is published in the September 16th issue of Science.