Hurricanes Have Grown More Intense Since 1970, Researchers Say, and Global Warming Is a Prime Suspect
By RICHARD MONASTERSKY
The number of intense hurricanes developing around the globe has climbed markedly in the past 35 years, according to researchers who mined storm data from the tropical ocean basins and are publishing their findings in a paper in today's issue of Science.
While the total number of tropical cyclones has waxed and waned over the decades with no overall change, the proportion that reach the status of Category 4 and 5 hurricanes -- the strongest storms -- has climbed from 20 percent to 35 percent since 1970, according to Peter J. Webster, a professor of earth and atmospheric sciences at the Georgia Institute of Technology, and his colleagues.
The researchers say they cannot tell whether the increase is part of a natural cycle or an indication that global warming has altered the behavior of storms. But they note that the temperature of the sea surface has climbed in all tropical ocean regions over the same period and that such changes, in theory, could spawn stronger storms.
Judith A. Curry, chairwoman of Georgia Tech's School of Earth and Atmospheric Sciences, says that when researchers see sea-surface temperatures and hurricane intensity "relentlessly rising," it gives them confidence that "these two things are connected and that there is probably a substantial contribution from greenhouse warming and it's not just natural variability."
"This could be a very long-period cycle, and it could go down in the next 30 years," says another author of the paper, Greg J. Holland, director of the Mesoscale and Microscale Meteorology Division at the National Center for Atmospheric Research. "Unfortunately I don't think that."
The new findings are consistent with a study, published last month in Nature by Kerry A. Emanuel, an atmospheric scientist at the Massachusetts Institute of Technology (The Chronicle, September 8). Mr. Emanuel reported that the total power released by tropical storms in the North Atlantic and North Pacific Oceans had increased substantially in the past 30 years, a trend that dovetails with the changes expected to occur because of global warming. Forecasts using computer climate models have suggested that warming of the tropical oceans could spur the growth of more strong hurricanes, such as Katrina, which was a Category 4 storm when it made landfall.
More Katrinas
Mr. Webster and his colleagues say the random nature of hurricanes makes it impossible to discern whether the long-term change they have identified has influenced any one storm. But the trend they detected suggests there are more Katrina-type storms now than in the past. "Who knows?" he asks. "Had this trend not been there, then Katrina may have been a Category 2 or a Category 3 and done a lot less damage."
Some scientists have criticized Mr. Emanuel's report on hurricane power, and the new paper is also likely to generate a scientific storm. Christopher W. Landsea, science-and-operations officer at the National Hurricane Center, in Miami, says the new study relies on incomplete storm data that were collected using different techniques over the years. He questions the veracity of the reported increase in intense storms. "My conclusion is that it's an artifact of the database," he says.
Mr. Webster and his colleagues analyzed the number, intensity, and duration of tropical storms. They chose to limit their study to the period of satellite storm observations, which began in 1970. For each storm, meteorologists used the satellite images to estimate wind speeds by examining the cloud patterns in the storm and characteristics of the cyclone's eye.
Mr. Landsea says that the techniques and the satellites have changed with time, making it difficult to identify any trend over the decades in storm wind speeds. "They didn't take into account the changes in methodology," he says of Mr. Webster and his colleagues.
Unlike other ocean basins, however, the Atlantic has good wind measurements taken by aircraft that flew through the center of the hurricanes. Mr. Landsea notes that the Atlantic data show the smallest change over time. The most intense storms went from being 20 percent of the total in the late 1970s and 1980s to being 25 percent of the total from 1990 through 2004. That change, he says, is consistent with a cyclical pattern in hurricane activity seen in the Atlantic since the late 1920s.
But Mr. Holland responds that his team took into account the changes in the data. He also notes that storms have grown more intense in all ocean basins, including the well-studied Atlantic and the eastern North Pacific, where meteorologists have some aircraft measurements to check against the satellite data. "We've done the best we can," he says. "If it is a factor of the database, it is a very, very remote chance that is the case."