Because hurricanes form over warm ocean water, it is easy to assume that the recent rise in their number and ferocity is because of global warming.

That is not the case, scientists say. Instead, the severity of hurricane seasons changes with cycles of temperatures of several decades in the Atlantic Ocean. The recent onslaught "is very much natural," said William Gray, a professor of atmospheric science at Colorado State University who issues forecasts for the hurricane season.

From 1970 to 1994, the Atlantic was relatively quiet, with 38 major hurricanes, but no more than three in any year and none at all in three of those years. Cooler water in the North Atlantic strengthened wind shear, which tends to tear storms apart before they turn into hurricanes.

In 1995, hurricane patterns reverted to the active mode of the 1950s and'60s. From 1995 to 2003, 32 major hurricanes, with sustained winds of 111 mph or greater, stormed across the Atlantic. It was chance, Gray said, that only three struck the United States at full strength.

Historically, the rate has been 1 in 3.

Then last year, three major hurricanes, half of the six that formed during the season, hit the United States. A fourth, Frances, weakened before striking Florida.

"We were very lucky in that eight-year period, and the luck just ran out," Gray said. "We always thought it would run out."

Global warming eventually may intensify hurricanes somewhat, though different climate models disagree.


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In an article this month in the journal Nature, Kerry A. Emanuel, a hurricane expert at the Massachusetts Institute of Technology, wrote that global warming already might have had some effect. The total power dissipated by tropical cyclones in the North Atlantic and North Pacific — including typhoons in the Pacific and Indian Oceans — increased 70 to 80 percent in the last 30 years, he wrote.

Even that seemingly large jump is not what has been pushing the hurricanes of the last two years, Emanuel said: "What we see in the Atlantic is mostly the natural swing."