Human-made greenhouse gases -- including pollution from power plants and car tailpipes -- have warmed ocean surfaces where hurricanes spawn, according to a study by 19 climate researchers published in the journal Proceedings of the National Academy of Sciences.
Some scientists say that might explain an increase in the average strength of hurricanes between 1995 and now.
Not all researchers agree. Some question whether hurricanes are abnormally strong right now and whether global warming could be the cause.
The truth, once settled, will be hugely important to North Carolina. In the past 150 years, only Florida, Texas and Louisiana have endured more major hurricanes.
Ryan Boyles, North Carolina's acting climatologist, said this state already faces serious risks from hurricanes. Policymakers have long had reason to look more critically at development trends that are increasing populations in flood plains and in coastal areas, where hurricane danger looms largest.
In the latest finding, researchers say an increase of 1 degree Fahrenheit in sea surface temperature globally, much of it since 1970, can be explained only by pollution.
Analysis by 22 computer climate models crunching huge amounts of data ruled out natural causes as the sole explanation. Natural causes include complicated El Nino weather patterns, weather-altering volcano eruptions, even changes in the energy emitted by the sun.
"There is no way the observed changes could be related just to natural variables," said Tom Wigley, a National Center for Atmospheric Research researcher and a co-author of the journal, published by the National Academy of Sciences.
The study was prompted by conclusions over the past two years by other scientists. They determined that increases in sea surface temperatures are pumping up hurricane intensity, said Wigley and his co-author, Ben Santer, a researcher at the federal Lawrence Livermore National Laboratory.
Peter Webster of the Georgia Institute of Technology has observed that in the Atlantic Ocean, the average number of named storms was higher between 1995 and 2005 than in a previous decade of high hurricane activity, from 1945 to 1955. Category 4 and 5 hurricanes rose to an average of nearly three a year, compared to less than one a year in 1945-55.
"That's the best data we have," Webster said.
Not all of those powerful storms, luckily, have struck the United States.
The vast majority of climate scientists agree that the Earth's temperature is rising and that man-made pollution is partly to blame. Gases such as carbon dioxide, released by smokestacks and cars, trap heat in the atmosphere.
The full implications of a warming planet -- on land and on sea, globally and locally -- are still under study. Meteorologist and researcher Chris Landsea of the National Hurricane Center in Miami is not convinced that the warming of ocean surfaces detected to date has spawned fiercer hurricanes.
In addition, an analysis Landsea and others published last year concluded that some hurricanes in the past may have been more powerful than recognized. Observational tools, especially satellite imaging, capture detail invisible to earlier observers.
Landsea has said a different sort of pattern might be at work -- alternation between a period of lower intensity hurricanes for 25 to 40 years, then a period of higher activity for a similar length of time. During the past century, he said, the '40s, '50s and '60s were intense hurricane periods, while the '70s '80s, and '90s were fairly quiet, with the recent intensity picking up the past 11 years.
The new results linking water temperature to greenhouse gases don't answer those criticisms. "I don't think this really advances the debate," Landsea said.
But scientists linking storm intensity to rising ocean surface temperatures say they are winning this argument. About one dozen peer-reviewed research articles published since 2005 detected ties, they note.
"This is a complex problem that you can't just think about or solve with brainstorming," Wigley said. "You need to have mathematically complex models."
As scientists debate, the public awaits clarity. In the meantime, the weather forecast isn't great.
The National Weather Service predicts a high likelihood of an above-normal 2006 Atlantic hurricane season, with three to four major hurricanes. So far in this year's hurricane season {June 1 through Nov. 30) there have been five named tropical storms and two hurricanes, according to the National Hurricane Center. One -- Hurricane Ernesto -- hit Florida and North Carolina, although it caused relatively little damage.