The peak of a busy hurricane season in the Atlantic Ocean basin is just past, and already the number of major tropical cyclones is above average.  The latest strong storm to strike land was Hurricane Ike, which left thousands without power and unable to return to the Gulf coast homes in the U.S.  The situation is worse in the Caribbean, especially in Haiti which has been devastated by a series of storms this year. VOA science correspondent Paul Sisco reports on the latest in a series of studies which find that hurricanes are growing in intensity and global warming might be to blame.

Flood waters from Hurricane Ike inundate the town of Clear Lake Shores, Texas, 13 Sep 2008

Hurricane Ike sweeps ashore in Texas, a storm hurricane experts called a monster became the third major hurricane in the Atlantic Ocean basin this year.

Scientists from Florida State University and the University of Wisconsin report in the journal Nature the most powerful tropical storms are becoming more intense. They analyzed 15 years of satellite and other data from nearly 200 tropical storms.

Three years ago, another study found a near doubling of the number of the strongest hurricanes.  Author, Judith Curry at the Georgia Institute of Technology, linked the increased intensity of the storms to warmer sea surface temperatures caused, in part, by human-induced global warming. "It's the greater intensity, higher wind speeds, specifically, number category four and five hurricanes," she said.

Judith Curry

She sees the trend continuing. "I believe that over the next 25 years we are going to see unprecedented activity over the North Atlantic," Curry said.

Not everyone agrees, but the findings are fueling the global warming debate.

"There probably is a global warming signature in the storms but it has been very difficult to find so far," says Jeff Halverson, who is a hurricane expert at the University of Maryland.  He explains how hurricanes form.

"Very quickly, warm water is the fuel for hurricanes," Halverson said. "The water evaporates.  The water vapor condenses -- releases heat into the storm that's the flow of energy out of the ocean into the atmosphere. The warmer the water, the more water vapor, the faster the engine runs because water vapor is the fuel.  You can think of ocean temp [temperature] as the octane rating of the fuel."

Chris Landsea is with the National Hurricane Center. "There's a very very tiny influence of global warming on hurricanes in my opinion,"  Landsea said.

He argues stronger storms are the result of climatic variability and natural weather cycles.  Halverson says he agrees there is a cyclic pattern but, of the new study, he adds.

"It is the best data set we have so far and yes, believe it or not, there appears to be a trend, an upward trend in the intensity of storms that may be due to global warming," Landsea said.

The underlying causes of the phenomenon are open to debate.  However, the data strongly suggests wind speeds will increase in the strongest tropical storms for the next several years.