Scientists believe this year's storms part of a cycle that started in 1995

Hurricane forecasters think that the world has shifted into a cycle of increased hurricane activity that may last well past the year 2025.

"We are a decade into the active phase of a natural 60-year or so cycle of hurricane activity," said Dr. Hugh E. Willoughby, of the International Hurricane Research Center in Miami. "This season is active, but not dramatically more so than others since 1995."

The last cycle of intense hurricane activity ran from about 1910-1960, peaking from the 1930s through the 1950s. The inactive phase that followed ended in 1995.

Since 1995, the National Oceanic and Atmospheric Administration hurricane index has averaged 139.6, about 50 percent above the 54-year average of 93.2 from 1950 through 2003. The increase occurred despite low indexes in 1997 and 2002. In those years, El Niño events occurred, an unusual warming of the ocean surface off the western coast of South America that suppress hurricane activity.

Hurricane researchers say warm sea water and several wind patterns converged to make the past two months so active -- but the real difference between this year and the past eight hurricane seasons was a ridge of high pressure over the East Coast and western Atlantic that did not allow hurricanes to make their natural curve to the north until they got as far west as Florida or a bit beyond.

In other years since 1995, the steering wind pattern was such that tropical weather was deflected north before reaching Florida, although storms hit North Carolina.

Most Atlantic hurricanes begin as thunderstorms off the west coast of Africa. The storms move west and intensify as they pick up energy from warm, tropical ocean waters. In a complicated chain of cause-and-effect, a slower thermohaline circulation affects wind patterns in ways that suppress hurricanes. Faster circulation causes winds that favor the formation of more hurricanes.

William Gray, an atmospheric scientist at Colorado State University said "When this thermohaline circulation, Atlantic Ocean circulation feature goes fast we have more major storms forming in the Atlantic," he said. "When it slows we have fewer ones. Now it was going slow the first decades of the 20th century, then it was fast from about the middle 20th century to the late 60s. From the late 60s to the middle 90s it was going very slow, with many fewer major storms. Now, since '95 this is the 10th year that we feel the thermohaline has been growing stronger and eight of these last 10 years have been very busy."

While global warming continues to be a factor in other climatic weather conditions (such as Antarctic Glaciers Melting Faster Due to Global Warming ) Professor Gray says hurricane activity follows a measurable pattern based on temperature changes and circulation patterns of air and water in the Atlantic Ocean. "What is happening now," he says, "is simply that pattern repeating itself, as it has done in the past, and will likely do in the future." Although NOAA scientists think that a greenhouse-gas induced warming may lead to a gradually increasing risk in the occurrence of highly destructive category-5 storms by the end of the 21st century.

Hurricane Wilma Video Clip

Further Reading: Atlantic basin hurricanes: Indices of climatic changes; Global Warming and Hurricanes; What is an El Niño?