Hurricane Warning

The buildup of human-induced greenhouse gases in the atmosphere appears to be the primary driving force behind warmer oceans that fuel more powerful hurricanes, according to a new study by 19 top climate scientists from the United States and Europe.

Previous studies had already suggested a connection between warming ocean temperatures and stronger hurricanes. This study provides a new and important link needed to show that global warming, not natural cycles, is responsible, according to the authors and other hurricane researchers.

"Clearly, this is a result of the increasing concentrations of greenhouse gas emissions in the atmosphere," said co-author Tom Wigley, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo. "The work that we've done kind of closes the loop here."

The study used 22 sophisticated computer climate models to examine the tropical regions of the Atlantic and Pacific oceans, where hurricanes, also called tropical cyclones, are born. In those areas, the temperatures have risen an average of between a half degree Fahrenheit and 1.2 degrees Fahrenheit, over the last century.

Humans are very likely to blame for at least 67 percent that warming, the report said

The report provides further evidence, said scientists, that people are changing the way Earth's climate responds to an ever-increasing amount of carbon dioxide and other greenhouse gases put there by the burning of fossil fuels.

The paper appears today in the online edition of the Proceedings of the National Academy of Sciences. The research was led by Benjamin Santer of the Lawrence Livermore National Laboratory and funded by the U.S. Department of Energy.

'Hindcasting'

For the new study, scientists first tested the ability of the computer models to accurately simulate real-world climate conditions where hurricanes form.

After running computer simulations of a period of time that's already passed -- called "hindcasting" -- the researchers compared the simulation to what actually happened to see how well the computer performed.

"If the models can simulate the observations, that tells us about the credibility and skill of the models, and that's important," Wigley said. He noted the models performed "exceptionally well," accurately predicting long-term climate trends, natural weather patterns, and even the effect of volcanic eruptions.

Having established their confidence in the computer models, researchers then used them to examine what was making the oceans warmer in those hurricane-forming regions.

In more than 80 simulations, they systematically introduced the varied factors that might influence hurricane formation into the simulation -- including greenhouse gases, ozone and even solar activity.

The key was being able to run the simulation by selectively turning on and off those influences to see how they individually influenced the climate, Wigley said. By taking only the greenhouse gas buildup out of the simulation, for example, scientists were able to see how Earth's climate might have evolved without the Industrial Revolution.

The models revealed that while natural cycles including volcanoes do have some influence on sea surface temperatures, the chief culprit is global warming.

"Greenhouse gases really are the dominant cause of the forcing of the climate system by human influences," Wigley said.

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