Mr. Emanuel, author of the book “Divine Wind: The History and Science of Hurricanes,” said that while the 1-degree rise in ocean temperature has been recorded globally, the correlation between SST and hurricane frequency does not appear in other parts of the world.
“Since 1981, the number and intensity of [Atlantic] hurricanes has almost doubled,” said Mr. Emanuel, based on his research published in a letter in the Aug. 4 issue of Nature, which surveyed 55 years of hurricane and ocean-temperature data in the Atlantic and Pacific. “In my mind, the jury is not out. The upswing since the 1980s is largely a global-warming signal. But if you polled my colleagues, I think you’d find they are divided on the issue.”
The Georgia Tech meteorologists used data collected around the world and arrived at similar conclusions.
“The best available data shows that the number of Category 4 and 5 hurricanes globally has almost doubled since 1970,” said Ms. Curry at Georgia Tech, in an e-mail response to questions. “In the North Atlantic, there has been a comparable increase in intensity, and also a 50 percent increase in the total number of North Atlantic hurricanes. This increase in hurricane activity has been linked to a 1-degree Fahrenheit increase in global tropical sea-surface temperature since 1970. This global temperature increase since 1970 is attributed to global warming.”
Asked about Georgia Tech’s findings regarding Category 4 and Category 5 hurricanes, Mr. Emanuel said that he had reviewed their data.
“I came up with the same result. I think they are right,” Mr. Emanuel said.
But Mr. Landsea of the National Hurricane Center vigorously disagreed with Mr. Emanuel in a rebuttal to his paper, published in Nature in December, saying, in effect, that Mr. Emanuel tortured the data until it confessed what he wanted to hear.
Regarding Georgia Tech, Mr. Landsea’s argument is that primitive measuring techniques, here and especially in Asia, where most of the major tropical storms occur, made for imperfect data, and inaccurate “data sets” generate incorrect conclusions.
“At the beginning of the [Georgia Tech] study, in 1970, there wasn’t even a tool for determining wind speed and how strong a hurricane was. The Dvorak Technique [for measuring wind speed] did not come into being until 1972,” he said, adding that it wasn’t perfected until 1984. “The incomplete data sets artificially causes the number of Category 4 and Category 5 hurricanes to go up.”
Asked about the increase in Atlantic hurricanes, he said: “I think that is real, but the largest component of that is the natural cycle,” he said.
According to NOAA hurricane records going back into the mid-1800s, hurricanes come in cycles. There have been quiet periods, with less hurricane activity, followed every 25 to 40 years by active periods, that last about 25 years. The current active period began in 1995 and is expected to last another 10 to 15 years.
Mr. Landsea agreed that the 1-degree rise in ocean temperature is largely a product of greenhouse gases — that is to say global warming — but he said it was not a primary factor in determining the size and intensity of recent hurricanes.