"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."
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