By Ann O'Neill
CNN

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Charley, Frances, Ivan and Jeanne tore up parts of Florida last year. After tweaking Florida, Katrina and Rita are wreaking havoc this year along the Gulf Coast from Alabama to Texas.

But don't rush to blame it on global warming, experts warn.

Max Mayfield, director of the National Hurricane Center in Miami, told a Senate subcommittee on Tuesday that we're in a period of heightened hurricane activity that could last another decade or two.( See scientists collect data -- 1:33)

"The increased activity since 1995 is due to natural fluctuations (and) cycles of hurricane activity driven by the Atlantic Ocean itself along with the atmosphere above it and not enhanced substantially by global warming," he testified.

Mayfield's colleague at the National Hurricane Center, meteorologist Chris Landsea, said two recent studies about global warming and hurricanes raise more questions than they answer. He added that the impact of global warming is "minimal for the forseeable future."

Landsea said the studies indicate global warming could increase hurricane wind speeds and rainfall by about 5 percent --100 years from now. But, he added, more study is needed, looking back at historical data and making it more compatible with modern reporting techniques.

The debate over global warming
Brenda Ekwurzel, climate scientist of the Union of Concerned Scientist National Climate Education Program, told CNN that while global warming might not be causing hurricanes, it already is making them more intense.

"We would never point to a single weather event and blame global warming," she said. "While hurricanes have bedeviled the Gulf Coast region for years, global warming is making matters worse."

Ekwurzel points to recent studies indicating that carbon dioxide is raising ocean temperatures.

"And those warmer oceans are converting low-grade storms into powerful hurricanes," she said. "In short, the warm oceans are like fuel to a hurricane. It's like throwing gasoline on a fire."

But not all hurricane experts are willing to make the link between global warming and hurricanes. At least not yet.

They say the string of major storms that have struck the southeastern United States over the past two seasons signal a return to normal.

Return to normal
"From 1970 to 1995, there weren't that many hurricanes, and the ones we had were nice, well-mannered, housebroken hurricanes that stayed out to sea and didn't make a mess," said Hugh Willoughby, a hurricane researcher at Florida International University in Miami.

"The only thing I can say," he added, "is this run of good luck we had is ending."

"This year you can just say nature is averaging out its climatology," said Colorado State University's famed hurricane predictor, William Gray. (See video of the science of the storm --3:55)

Katrina and Rita are what Gray calls "Bahama busters," storms that form off the Bahamas rather than near the coast of Africa. They explode after feeding on the warm waters of the Gulf of Mexico.

The past century saw 18 "Bahama busters," Gray said.

Even Katrina's and Rita's back-to-back pounding of the Gulf Coast has a precedent. In 1915, Gray said, New Orleans and Houston areas were hit by Category 4 storms six weeks apart.

"You can't blame that on global warming," he observed.

Gray first sounded the alarm in 1995, noting that the surface waters in the north Atlantic Ocean had warmed slightly. 1995 saw 11 hurricanes and eight tropical storms, the highest tally since 1933.

By 1997, Gray's annual forecasts warned of "a new era" of hurricanes.

He put forth the theory that many climatologists, including Mayfield and Willoughby, now embrace -- that hurricanes are driven by cycles of rising water temperature and salinity that affect the speed of currents in the Atlantic.

Warm currents drive hurricanes
The technical name for the engine driving the hurricane cycles is the Atlantic Multidecadal Oscillation, or AMO for short. It can cause droughts in the West and hatch hurricanes in the East.

"This cycle has been repeating back to the Ice Age," Willoughby said. "It's related to changes in the ocean currents that move heat northward. If it's fast, we get a lot of hurricanes."

Studies show the AMO was cool -- and the currents slower -- from 1900 to 1925, warm from 1926 to 1969, cool from 1970 to 1994 and warm since 1995.

And so, to a generation of Americans with little experience with hurricanes, it seems like these monsters are coming out of nowhere.

Gray and Willoughby are among the skeptics who doubt global warming can be blamed for the trend of the past few years. They are joined by the hurricane trackers at the National Hurricane Center.

"We're just entering a busy time here," said Chris Lauer, a meteorologist at the center.

"You see a few decades of slower activity, followed by a few decades of higher oscillation," he said. "Our position is the recent increase in hurricane activity is not caused by global warming."

Researchers at the Georgia Institute of Technology in Atlanta, Georgia, suggested earlier this month that more than nature and coincidence might be driving the storms.

More study needed
In the September's issue of the journal Science, Peter Webster and Judith Curry documented a 60 percent global jump in major hurricanes with winds of 131 mph or more and a 1-degree increase in the tropical ocean surface temperature.

But Webster warned on Georgia Tech's Web site that more study was needed before blaming global warming.

"We need a longer data record of hurricane statistics," he said, "and we need to understand more about the role hurricanes play in regulating the heat balance and circulation in the atmosphere and oceans."

Willoughby said he is keeping an open mind about the role of global warming but believes it won't be a factor for at least another 100 years.

"The answer I give everybody, because it has all been so politicized, is I don't know," he said.

Gray was more direct. "There are all these medicine men out there who want to capitalize on general ignorance on this subject," he said.

"With all the problems in the world, we shouldn't be dealing with this."

Willoughby believes the debate over hurricanes and global warming is healthy. "It's good for the science," he said.