Experts hesitant to link hurricane season, warming

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.

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 consists of the Atlantic, the Caribbean, and the Gulf of Mexico. The always-mild Gulf Stream itself - which can give tropical storms an extra jolt of heat energy - has been warmer than normal.

The latest warm phase took hold in the mid-1990s. Right on schedule, the extraordinarily quiet hurricane period that began around 1970 ended abruptly in 1995. The last 11 seasons have had 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 U.S. taxpayers. Driven largely by hurricane damages, federal disaster expenses are off the charts.

The staggering bill is still being tallied, but Hurricane Katrina, blamed for more than 1,000 deaths and for displacing 400,000 people, will become the costliest disaster in the history of the Federal Emergency Management Agency.

Excluding the Sept. 11 attacks in 2001, FEMA has spent more than $60 billion on direct disaster aid since 1995 - more than $550 per household, and more than quadruple the total that it spent during the hurricane lull decades of the 1970s and 1980s combined.

So far in 2005, it has spent $35.1 billion, 2 1/2 times the total amount in the 1970s and '80s.