The incredible 2005 hurricane season, one of the deadliest and by far the most expensive on record, has inflamed worries about global warming.

Is this only the beginning of an era of more powerful storms? Has warming pushed the North Atlantic past a dangerous threshold?

Some of the country's most respected hurricane experts say it is impossible to indict global warming as the driving agent behind Katrina and the season's record numbers of storms.

But changes in North Atlantic circulation almost certainly were a factor, scientists say. And, in this case, those changes are having an effect on the budgets of every household in the nation.

A team of hurricane researchers has documented that tropical storms follow 25- to 40-year cycles in which busy periods alternate with lulls. These cycles are part of the Atlantic Multidecadal Oscillation (AMO).

During the active phase, sea surface temperatures in the Atlantic Basin are above average, and in recent years they have been near all-time highs. The basin includes the Atlantic, the Caribbean, the Gulf of Mexico, and the Gulf Stream - which can give tropical storms an extra jolt of heat energy.

The latest warm phase took hold in the mid-1990s. Right on schedule, an extraordinarily quiet hurricane period that began around 1970 ended abruptly in 1995. The last 11 seasons have wrought a level of activity unmatched in the era of reliable records.

What drives the oscillation?

While hard data are not available, computer models tie the oscillation to changes in the North Atlantic's circulation, according to experts at the Atlantic Oceanic and Meteorological Laboratory in Miami.

These changes are far more subtle than the ones that could disrupt climate, but they are significant nonetheless.

The changing cycles also are believed to be responsible for droughts in the Southwest and Midwest, including the Dust Bowl disaster of the 1930s, which coincided with a warm phase.

The big question for researchers is whether the latest warm phase and the ferocity of the 2005 hurricane season got a charge from global warming.

They don't know. What they do know is that the cycle is far from over.

"This could last another 20 years," says Alberto Mestas-Nuñez, a hurricane researcher in Miami.

That is a chilling forecast, both for owners of coastal property and for taxpayers. Driven largely by hurricane damages, federal disaster expenses are off the charts.

Hurricane Katrina alone, blamed for hundreds of deaths and the biggest relocation in history, will cost every househould at least $100 in disaster expenses.

The Federal Emergency Management Agency has spent about $30 billion in direct disaster aid since 1995 - more than triple what it spent in the 40-year history of the program.