First, there was Opal (1995). Then Fran (1996), Floyd (1999) and Allison (2001).

But the string of deadly billion-dollar storms blowing in from the Atlantic was far from over.

In 2004, the quartet of Charley, Frances, Ivan and Jeanne ravaged Florida in rapid succession and set records for damage.

Then, somehow, it got worse.

With Katrina, the inundation of New Orleans and an Atlantic storm season so active it stretched forecasters beyond the normal alphabetical list of names into the Greek of Alpha, Beta and Gamma, 2005 racked up more storm deaths and destruction than the previous 10 years -- combined.

So although the close of this year's hurricane season on Nov. 30 may allow residents along the Southeast coast to heave a sigh of relief, it also points to a growing worry: What is going on with the weather?

With the surge in hurricanes since 1995, droves of people moving to affected coastal areas and the federal government spending billions in storm relief annually, the question has rarely seemed so urgent.

"People talk about what is 'normal' for hurricanes, but there's a new definition of 'normal' now," said Roger Pielke Jr., an environmental studies professor at the University of Colorado. "This is out of people's range of experience. But there will be far more activity and far more destruction."

Many storm researchers now agree that a decade or more of similarly rough seasons -- similar to the heightened storm activity that began in 1995 -- lies ahead.

But at the same time that the trio of Katrina, Rita and Wilma were battering Southeastern coasts, a controversy was brewing over the reasons for the rise in hurricane havoc. At issue: Is it merely a natural fluctuation or, more ominously, a product of global warming?

Until recently, a rough consensus in the small world of tropical cyclone specialists had held that there was no evidence of unnatural hurricane trends over the 20th century. Hurricane activity over the Atlantic has fluctuated naturally over decades going back as far as 1900, and it was unlikely that global warming could be having a significant impact, many researchers said.

But a pair of scientific papers published this year detected an unexpected spike in storm intensity over the past several decades, suggesting that global warming might already be having an effect. The research set off a passionate and sometimes personal debate in the small community of storm scientists.

"In the sense of the history of scientific ideas, we're either in the middle of a paradigm shift or a false paradigm shift," said Hugh Willoughby, the former director of hurricane research at the National Oceanic and Atmospheric Administration. "The situation would be deliciously ambiguous if there were not thousands of lives and billions of dollars on the table."

Besides adding weight to the argument that global warming could be having catastrophic effects, the findings spell more trouble for U.S. coastal areas vulnerable to fierce storms, where the population is rising fast. The risks are being borne by all U.S. taxpayers. Already, the federal government has been asked repeatedly for hurricane relief money.

"We have to decide as a society whether that's a problem," Pielke said. "Obviously, the benefits of living near the coast outweigh the costs because people are doing it. The question is: In the face of inevitable property damage and loss of life, how well do we prepare?

"Either way," he noted, "we are going to see many more years of intense hurricanes. Scientists on both sides agree on that."

The difference between the two scientific views is whether hurricane activity will simply fluctuate over time, as it apparently has done in the past, or whether global warming will inexorably ramp up the damage.

The opening skirmish in the debate began when Kerry Emanuel, a well-known researcher at the Massachusetts Institute of Technology, changed his mind. In July, he withdrew from a paper reflecting the consensus view that there was little evidence of a significant connection in the historical record. In an e-mail, he wrote to a co-author that "the problem for me is that I cannot sign on to a paper which makes statements I no longer believe are true."

"I see a large global warming signal in hurricanes," he wrote.

The next month, he published a paper in Nature considering 50 years of storm data and stating that indeed, hurricanes in the Atlantic and North Pacific were becoming more powerful. By a special measure of hurricane power he had defined for other research, they had roughly doubled in power over 30 years. Significantly, the increase tracked with the rise in sea surface temperatures.

There was more to come. In September, a group of scientists led by Peter Webster at Georgia Tech found that, worldwide, the number of the strongest hurricanes -- categories 4 and 5 -- has nearly doubled over the past 35 years. The authors aligned the finding with global warming and a rise in sea surface temperatures.

"Our work is consistent with the concept that there is a relationship between increasing sea surface temperature and hurricane intensity," Webster said at the time.

But he also noted that the findings did not perfectly fit with global warming. "It's difficult to explain," he said, why, when sea surface temperatures were rising the most in the past decade, the number of hurricanes and their longevity decreased.

Even with such cautions, however, reaction to the papers was immediate and powerful.

Some environmental groups touted the studies as a reason to immediately restrict carbon emissions in an effort to slow global warming. At the same time, other researchers launched tirades against the findings.

William Gray of Colorado State University, well known for his annual hurricane season forecasts, was at the forefront of the criticisms.

"If true, this is a very important finding that has great relevance as regards the globe's future climate and future hurricane destruction," he wrote in rebuttal to Emanuel's paper. "But the author's 'apparent' blockbuster results and his interpretation of his calculations are not realistic."

There are at least two fundamental problems with the studies, according to Gray and others.

The first is that Atlantic hurricane activity has long been known to undergo fluctuations over long periods. The flip-flops between active and inactive hurricane periods in the Atlantic are attributed to long-term trends in currents and salinity.

The 1950s and 1960s saw lots of hurricane activity, for example, whereas the '70s, '80s and early '90s did not. The uptick in hurricanes that began in 1995 had long been anticipated, though researchers hadn't been sure when exactly the change would come.

Critics said the rise in Atlantic hurricanes that the papers detected was probably the cause of this natural "oscillation," not global warming.

The second flaw in the papers, according to critics, is that both papers are based on hurricane records over decades, and those vary in quality, largely because the means of measuring hurricane intensity have changed over time.

In the northwest Pacific, for example, which sees a large share of hurricane-force storms, records of storm intensity have at times been based on flight observations and sometimes on satellite images.

"In a nutshell, the data sets they're using aren't reliable enough to answer those important questions," said NOAA hurricane researcher Christopher W. Landsea. "The data sets don't take into account the different ships, the different planes, the different satellite imagery. The data is affected by the different ways of measuring."

For all the skepticism from some quarters, the arguments have been persuasive elsewhere. Willoughby said he has been leaning toward the global warming explanation and that the two new papers helped.

Still, he said, it is too early to tell.

"Right now, I don't think there is a consensus answer," he said. "There are just a lot of smart people doing good work on a very important question."

A satellite image of the Gulf of Mexico shows Hurricane Rita, one of several powerful storms that wrought destruction along U.S. coasts this year.