Warming ocean temperatures appear to be fueling stronger, more intense hurricanes around the world, a new study suggests.

The number of storms that reach Category Four and Five-the most powerful, damaging hurricanes-has nearly doubled over the past 35 years, the study finds. However, the frequency and duration of hurricanes overall have stayed about the same.

Category Four hurricanes have sustained winds from 131 to 155 miles an hour (211 to 249 kilometers an hour). Category Five hurricanes-like Katrina at its peak in the Gulf of Mexico-have sustained winds of 156 miles an hour (251 kilometers an hour) or more.

The study finds that the increase in hurricane intensity coincides with a rise in sea surface temperatures around the world of about 1°F (0.5°C) between 1970 and 2004.

Writing in tomorrow's issue of the journal Science, the study's authors stop shy of pinning the increase in hurricane intensity on global warming. To do so would require a longer historical period of study and a better understanding of hurricane dynamics, they say.

But in an interview with National Geographic News, the study's lead author, Peter Webster, said, "I'm prepared to make an attribution to global warming."

If the increases in hurricane strength and sea surface temperatures were part of a natural cycle, as some scientists believe, then there would be decreases in other regions to compensate for them. But the increases found in the study are both worldwide.

"There's a plus and minus with oscillations," said Webster, an atmospheric scientist at the Georgia Institute of Technology in Atlanta. "What we see is a universal increase [in temperature] and a universal change in hurricane intensity."

Scientists know that warm ocean waters fuel hurricanes. Webster says it therefore follows that the more fuel there is, the bigger the storms will be. But why the frequency and duration of hurricanes aren't also rising is poorly understood.

"The relationship between sea surface temperature and intensity is not one that has surprised us," Webster said in a telephone briefing with reporters. "The other factors mentioned for hurricanes are more awkward."

Results Questioned

Chris Landsea, a meteorologist with NOAA's National Hurricane Center in Miami, said that the increase in hurricane strength that Webster's team has observed is questionable.

"I've got real concerns about whether this is a real change or whether it's an artifact of the data," he said. "I'm pretty skeptical that it's a real change."

For one, Landsea said, scientists mostly use satellite data to measure hurricane intensity, a technique that has improved dramatically over the past 30 years. As a result, the measurements are likely skewed lower in the earliest years of the period studied.

He added that even though the researchers found an increase in the number of hurricanes reaching Category Four and Five, the maximum wind speed recorded did not creep up, as would be expected if the hurricanes were really getting stronger.

"That's a huge inconsistency in the study," he said. "Something is either wrong here, or there was no real change in Category Four or Five [storms.]"

Lastly, the lowest up-tick in hurricane intensity-five percent-was found in the Atlantic Ocean, where hurricane data is most complete. Landsea said if the global data were better, the increase in intensity might be lower, if it exists at all.

Landsea is currently applying modern methods to historical Atlantic hurricanes to reassess their intensity. Before scientists can say with confidence that hurricanes are getting stronger worldwide, the reassessment must be done for all oceans, he said.

Webster and his colleagues note in Science that satellite techniques used to measure storm intensity have changed over the years. But the method for measuring maximum wind speeds, which they used for the study, has not.

In addition, the North Atlantic and Eastern Pacific hurricane data has been calibrated with aircraft reconnaissance. Where only satellite data is available, the authors say, the data is consistent with the measurements verified by aircraft.

Study Agreement

Michael Mann is the director of the Earth System Science Center at Pennsylvania State University in University Park. He said the new findings are robust and consistent with models of how the global climate responds to warmer oceans.

According to Mann, the models predict that the number of intense hurricanes-though not necessarily the total number of hurricanes-will increase with rising sea surface temperatures.

"So the observations Webster and colleagues have analyzed here and the trends they find are fully in keeping with theoretical climate model predictions," he said.

Mann added that the lower up-tick in intensity in Atlantic hurricanes, as compared to those in other oceans, is likely the result of the El Niño weather phenomenon interfering with hurricane formation.

El Niño, which is driven by a periodic warming of the Pacific Ocean, disrupts the high-altitude wind patterns favorable to the formation of hurricanes in the Atlantic Ocean, he explained.

Some scientists speculate that el Niño events will become longer and more frequent in response to global warming.

Under such a scenario, Mann said, warmer sea surface temperatures (which are favorable to hurricane formation) and el Niño (which tends to inhibit it) will cancel each other out.

"If we go to other [ocean] basins, those two factors are not working against each other," Mann said. "There the long-term trends are more clear."

Be Prepared

According to Mann, Webster's study suggests that unless there's a dramatic reduction in fossil-fuel use, the trend of more intense hurricanes will continue well into the future.

Webster said he hopes that whether people believe in global warming or not, they'll take the study seriously. This is especially true for residents of New Orleans as they begin thinking about rebuilding.

"We can't make the assumption that Katrina was a once in a lifetime event," he said. "So if you are going to rebuild New Orleans, at least rebuild it properly."

Commentary:
What has the US been doing about Global Warming? Nothing since the GOP took control of Congress. When you have the time search this site for Global Warming and look at what's happening. The experts say there will be no polar icecaps within 100 years. Senator Clinton toured the arctic to see what was happening. Greenland isn't supposed to be green, but it is. Glaciers are melting etc. The evidence if overwhelming that Global Warming is taking place. But the GOP and Bush must pander to big oil. Big oil that's making a fortune while they're in power.