A series of studies over the past year or so have shown an increase in the power of hurricanes in the Atlantic and Pacific oceans, a strengthening that storm experts say is tied to rising sea-surface temperatures.
And most of that temperature increase can be blamed on global warming caused by human activities such as automobile and industrial pollution, scientists report in Wednesday's issue of Proceedings of the National Academy of Sciences.
"The work that we've done kind of closes the loop here," said Tom Wigley of the National Center for Atmospheric Research in Boulder, Colo., a co-author of the paper.
"The important conclusion is that the observed (sea-surface temperature) increases in these hurricane breeding grounds cannot be explained by natural processes alone," said Wigley. "The best explanation for these changes has to include a large human influence."
Benjamin Santer of Lawrence Livermore National Laboratory, Livermore, Calif., plus Wigley and their team studied the relationship of climate and hurricanes using 22 different climate models at 15 institutions around the world.
Climate models are complex sets of mathematical equations that high-speed computers use to simulate weather and climate and to forecast changes. The researchers used them to run 80 different simulations analyzing the response of sea-surface temperatures to a variety of factors and then compared the results from the independent models.
While previous studies have looked at entire oceans, this work focused on the smaller areas of the Atlantic and Pacific where tropical storms form.
This study builds a connection between the theoretical foundation of global warming and changes that are being observed in those areas where hurricanes are born, said Robert Corell of the American Meteorological Society, who moderated a briefing on the work.
While they reported the connection between rising ocean temperatures and increasing storm power, the researchers declined to predict future changes.
Asked if they would recommend changes in public policy, Greg Holland of the National Center for Atmospheric Research said, "It is important to note that we're not policymakers. Our role is to present the best possible conclusions from the available evidence."
Kerry Emanuel, a hurricane expert at Massachusetts Institute of Technology, added that regardless of global warming, the United States does need to address problems in dealing with hurricanes ranging from insurance to disaster response.
Not so sure of the findings was William M. Gray of Colorado State University, a longtime hurricane expert who issues forecasts each year of the expected number of storms.
Gray said the models do not deal with all necessary ocean processes and called the report "a desperate attempt to keep the bandwagon going. They've kept it going with global warming and now they want to keep it going with hurricanes."
"I am very sure over the test of time it will not hold up," said Gray, who was not part of the research team.
Philip Klotzbach, also of Colorado State, said that "sea-surface temperatures have certainly warmed over the past century, and that there is probably a human-induced component.
"To me, the big challenge is still determining what percentage is natural and what percentage is caused by humans. This paper sheds some light on that question; however, there is still a considerable amount of uncertainty," he said.
Christopher Landsea, a meteorologist at the National Oceanic and Atmospheric Administration's Hurricane Research Division in Miami, praised the new paper as very well thought out.
But, he said, while the paper discusses sea-surface temperature increases, it does not address the sensitivity of hurricanes to ocean temperature changes or questions about hurricane records in prior years.
While studies by Emanuel in Nature and Peter J. Webster of the Georgia Institute of Technology in Science reported increases in the most powerful storms, Klotzbach challenged those findings in Geophysical Research Letters, reporting only a small increase and suggesting that may be due to improved observation technology.
Santer's research was funded by the Department of Energy.