Global warming caused by humans is largely responsible for heating hurricane-forming regions of the Atlantic and Pacific oceans, probably increasing the intensity of the storms, scientists reported yesterday.
The scientists used 22 computer models to simulate how the world's climate works and to help answer a key question: Are hurricanes becoming more intense because of natural influences, or man-made ones?
The scientists, reporting in the Proceedings of the National Academy of Sciences, said there was an 84 percent chance that human-induced climate change was responsible for most of the ocean warming . Oceans have warmed by about 1 degree in the past century, and natural influences alone could not account for that, they said.
Hurricanes draw their strength from warm seawater, and even small changes in temperature can give a storm much more energy, increasing its fury.
The findings, if borne out by further research, could mean that hurricanes with the strength of Katrina and Rita may become more common .
``Natural variability doesn't cut it for the observed ocean temperatures," said Ben Santer of the Lawrence Livermore National Laboratory and the lead author of the report that looked at two key hurricane breeding grounds in the North Atlantic and Pacific oceans. ``The study suggests we are responsible."
Most climate scientists agree that hurricanes have become more active since 1995, especially in the North Atlantic. Some researchers say that the increased activity is part of a natural cycle, similar to one that peaked in the 1950s, and like that one, will last about 30 to 40 years.
But during the last year, a number of scientists have published research suggesting that hurricanes, especially in the North Atlantic, have gained unprecedented intensity since the 1970s. They said the magnitude of this increase in hurricane strength does not fit with the pattern of natural fluctuations, and thus attribute it to the release of heat-trapping gases, chiefly carbon dioxide, from power plants and cars, which warms the atmosphere and in turn the oceans.
Climate scientists not involved in the study said the analysis was thoughtful and rigorous -- although some warned about placing too much faith in the reliability of complex computer models. There is little doubt that humans are contributing to the warming of the oceans, they said, but more research is needed to better grasp how much.
Scientists who question the link between global warming and hurricane intensity said the study did not get at the real issue: whether hurricanes have really become more intense.
Hurricane measurements were less precise decades ago, said Christopher Landsea, a scientist at the National Hurricane Center, possibly resulting in underestimations of the strength of some storms. Furthermore, he added, the hurricane intensities that are being recorded today far surpass what would be expected from the ocean warming that has been observed, suggesting that the measurements may be flawed.
``That is a huge discrepancy," said Landsea. ``It is a well-done study -- but it didn't get at those issues."
Finding out why ocean temperatures are rising is vital to understanding threats to crowded coastlines.
Ideally, scientists would compare sea-surface temperatures today with readings from hundreds or even thousands of years ago to understand what natural climate cycles can do to ocean temperatures.
But ocean temperature records only go back about 150 years, and those data are incomplete.
So Santer and his group tried an approach that is a mainstay of scientists who study global warming. They used mathematical models -- sets of equations that describe the climate's behavior -- to create a virtual world without human influence.
They were trying to answer two questions: Is there a natural cycle contributing to a rise in sea-surface temperatures? And to what degree are humans contributing to the ocean warming?
No single model has been agreed upon to explain the complicated workings of the entire climate system; instead, scientists have devised 22 models, each using its own equations. Santer's team looked at all the outcomes to increase their confidence in the results.
The group ran 80 simulations on superfast computers to see what ocean temperature changes would occur over hundreds of years under different scenarios, from volcano eruptions that can temporarily cool sea temperatures to solar events that can heat the seas. Then they compared the results with actual ocean temperatures.
Overall, the group saw no clear natural reason or cycle in the North Atlantic and Pacific that could explain the warming of oceans over the past century. Instead, they concluded that human-caused climate change is the primary factor .
``The work that we've done closes the loop," that humans are warming the oceans, said Tom Wigley of the National Center for Atmospheric Research, who is a co-author of the study.
It's unclear what policy implications might come out of the study, although it will probably add to growing pressure from environmentalists and some legislators to pass laws limiting the release of carbon dioxide, a potent greenhouse gas, from power plants.
Beth Daley can be reached at email@example.com