New research shows that rising sea surface temperatures (SSTs) in hurricane "breeding grounds" of the Atlantic and Pacific Oceans seems to be basically due to human induced global warming, after effectuating dozens of computer models. "Our paper suggests that it's human-induced burning of fossil fuels that have altered the levels of greenhouse gases in the atmosphere that have led to this warming in regions where Atlantic and Pacific hurricanes form," said Californian climate scientist Benjamin Santer of Livermore's Program for Climate Model Diagnosis and Intercomparison.

That let them compare today's Earth with what the computer models indicated it would have been like if humans had never burned fossil fuels. Previous studies have shown a link between ocean water warming and increases in hurricane intensity, but they have focused on temperature changes over very large ocean areas - such as the entire Atlantic or Pacific basins.

The new approach targets SST changes in much smaller hurricane formation regions. Researches have shown an increase in the power of hurricanes, like Katrina, but also some skepticism about the reliability of computer models. "What we've seen in the past is small beans compared to what we'll see in the future," said Tom Wigley, climate scientist at the National Center for Atmospheric Research in Boulder, Colorado Wigley.

"The important conclusion is that the observed SST increases in these hurricane breeding grounds cannot be explained by natural processes alone. The best explanation for these changes has to include a large human influence."

In both regions, human-caused increases in greenhouse gases due to human burning of fossil fuels were found to be responsible for 67 % of the observed rise in the last 100 years of the SST. "We've used virtually all the world's climate models to study the causes of SST changes in hurricane formation regions," said Santer.

"In the real world, we're performing an uncontrolled experiment by burning fossil fuels and releasing greenhouse gases," he said.

"We don't have a convenient parallel Earth with no human influence on climate. This is why our study relied on computer models for estimates of how the climate of an 'undisturbed Earth' might have evolved. The bottom line is that natural processes alone simply cannot explain the observed SST increases in these hurricane breeding grounds. The best explanation for these changes has to include a large human influence."

Hurricanes are influenced by a variety of factors such as SST, wind, moisture and atmospheric stability. The increasing SSTs in the Atlantic and Pacific hurricane formation regions is likely to be the most important influence on hurricane strengthening. "The models that we've used to understand the causes of SST increases in these hurricane formation regions predict that the oceans are going to get a lot warmer over the 21st century," Santer said.

"Modeling with just natural variables produced "much smaller" temperature changes of around 1 percent or less," Wigley said.

"That causes some concern. In a post-Katrina world, we need to do the best job we possibly can to understand the complex influences on hurricane intensity, and how our actions are changing those influences."

William M. Gray of Colorado State University, a longtime hurricane expert who issues forecasts each year of the expected number of storms, 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."

Philip Klotzbach, of Colorado State, said "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".

While most studies reported increases in the most powerful storms, Klotzbach challenges this, reporting only a small increase and suggesting that may be due to improved observation technology.