WASHINGTON (Reuters) - Scientists say it's not easy to tell if global warming caused hurricanes Katrina and Rita but on Monday they forecast more unpredictable weather as Earth gets hotter.
Even skeptics agree that global warming is under way and that human activity is at least in part responsible. Climate experts also agree that this warming is likely to make the weather more extreme -- colder in some places, hotter in others, with droughts and severe rainstorms both more common.
"Global warming, I think, is playing a role in the hurricanes," said Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research in Boulder, Colorado.
"But a lot of what is going on is natural. What global warming may be doing is making them somewhat more intense," said Trenberth, a member of the U.N. Intergovernmental Panel on Climate Change.
James Elsner, professor of geography at Florida State University, agreed.
"Certainly this is an unusual season," he said in a telephone interview. "However, the question of attribution I don't think is very simple."
Katrina slammed into southern Louisiana and Mississippi on August 29, wiping out entire towns, triggering the devastating flooding of New Orleans and causing more than 1,000 deaths. Then on Saturday along came Rita, which briefly hit Category 5 strength with winds higher than 155 mph (249 kph) before dropping Category 3 by the time it hit the Texas-Louisiana coast.
"We have seen unusual seasons in the past and so we understand that we tend to see more strong storms when the Atlantic Ocean temperatures are warmer, which has been the case in the last 10 years or so," Elsner said.
"It was warm in the 1940s and '50s and we saw lots of strong storms during that period."
So far, 2005 has not been the busiest year for storms, even though there have been 17 named tropical storms in the Atlantic, Caribbean and Gulf of Mexico.
"That distinction belongs to the year 1933, in which there were 21 storms that reached tropical storm strength," said Eric Gross, an associate professor of history at Harding University in Searcy, Arkansas, who studies hurricanes and other natural disasters.
There were 19 tropical storms in 1995, Gross said in a statement.
In theory, warmer temperatures could bring more and fiercer hurricanes, experts agree. Hurricanes are fed by warm ocean surface temperatures and by higher amounts of water vapor.
Kerry Emanuel, a professor of meteorology at the Massachusetts Institute of Technology, published a study in the journal Nature last July that found big storms are 50 percent more intense and last 50 percent longer than those in the 1970s.
"My results suggest that future warming may lead to an upward trend in tropical cyclone destructive potential and -- taking into account an increasing coastal population -- a substantial increase in hurricane-related losses in the twenty-first century," he wrote.
Emanuel also has found that the IPCC-predicted rise in sea surface temperatures -- 3.6 degrees Fahrenheit (2 degrees Celsius) -- would raise a storm's intensity by 10 percent.
This temperature increase, Trenberth said, will add water vapor to fuel a hurricane's fury.
Even if storms are not yet affected by global warming, experts like Emanuel and Trenberth predict they will be.
"Global warming is remorselessly going on," Trenberth said. "This is something that when you take action, the benefits take place in 50 years and beyond. It is not something you can stop."