A year after Hurricane Katrina and other major storms battered the U.S. coast, the question of whether hurricanes are becoming more destructive because of global warming has become perhaps the most hotly contested question in the scientific debate over climate change.
Academics have published a flurry of papers either supporting or debunking the idea that warmer temperatures linked to human activity are fueling more intense storms. The issue remains unresolved, but it has acquired a political potency that has made both sides heavily invested in the outcome.
Paradoxically, the calm hurricane season in the Atlantic so far this year has only intensified the argument.
Both sides are using identical data but coming up with conflicting conclusions. There are several reasons.
Using different time periods to chart hurricane patterns can influence the results. Different academic backgrounds also affect how researchers interpret the data. Climate scientists tend to test hypotheses and examine the underlying causes of climate variability over time, which makes them more comfortable identifying broad climate trends. Hurricane forecasters tend to be more focused on predicting the intensity and paths of individual storms, and often focus on factors such as wind shear and water temperature that can cause a storm to shift within a matter of days or hours, so they tend to emphasize natural variability over long-term climate shifts.
Inevitably, the scientific debate has spilled into the policy arena. Former vice president Al Gore took up the issue in his recent film “An Inconvenient Truth,” suggesting that Katrina and other severe storms reflect a broader trend clearly traceable to global warming. Last week, environmentalist Lester Brown, president of the Earth Policy Institute, released a report that called the quarter of a million Katrina evacuees who will not return home “the world’s first climate refugees.”
On the other side, Myron Ebell, energy and global warming policy director at the Competitive Enterprise Institute, said these pronouncements amount to political opportunism. In contrast to activists who quickly attributed last year’s hurricanes to climate change, he said, his side is not ready to claim victory just because this year has brought fewer intense storms.
“I don’t think that says much one way or another about whether global warming causes hurricanes,” said Ebell, whose group receives funding from the fossil-fuel industry.
Scientists who doubt a link with global warming say this year’s average Atlantic hurricane season simply shows how variable weather can be. Christopher Landsea, who works in the National Oceanic and Atmospheric Administration’s Hurricane Research Division, published an opinion piece in the journal Science late last month in which he argued that data indicating that recent hurricanes have been more intense than those in the 1970s and ’80s may be based on flawed information. Measurement technologies were less sophisticated then and may have underestimated the strength of earlier storms, he said.
“We’re woefully underestimating how strong hurricanes were back then,” said Landsea, who wrote that five tropical cyclones that were originally classified as Category 3 would be rated as Category 4 today. “I’m sure it’s confusing to the general public, since you have different scientists saying different things. We’re all trying to figure out the same thing: What’s going on with our climate?”
In contrast to the Atlantic, the Pacific is experiencing a much more active than usual storm season this year. Earlier this month, Typhoon Saomai, the strongest to hit China in half a century, crashed into the country’s southeast coast and flattened tens of thousands of homes. It killed more than 300 people and prompted the evacuation of more than 1.5 million.
A number of factors might account for the fact that this year’s Atlantic season, which runs from June 1 to Nov. 1, has so far produced far fewer named storms than last year’s record-breaking season, and not a single hurricane. Sea surface temperatures are not as warm this year — the ocean needs to be at least 79 degrees Fahrenheit to sustain a hurricane — and the atmosphere is more stable because of clouds of Saharan dust that have swept across the Atlantic.
Studies supporting a link between global warming and storm intensity keep coming. The latest will be published this week by Florida State University geography professor James B. Elsner in the journal Geophysical Research Letters. Elsner found that average air temperatures during hurricane season predict the Atlantic Ocean’s surface temperatures, not vice versa, which he said means it is “much more likely the atmosphere is warming the ocean” and helping create more severe storms.
And Judith A. Curry, of Georgia Tech’s School of Earth and Atmospheric Sciences, who co-authored a paper last year suggesting that rising sea temperatures have been accompanied by more intense hurricanes, has challenged Landsea’s critique. She said Landsea and like-minded researchers have not “done the hard work” to reanalyze the entire historic hurricane database to determine whether it really is skewed. She does not go as far as Elsner, however, saying his paper identifies “an interesting statistical relationship” but does not physically explain how warmer air might be heating the Atlantic.
Curry’s work, in turn, has been challenged by Phil Klotzbach, a research associate at Colorado State University, who published a paper in May suggesting that, since 1986, there has been no global trend in hurricane intensity. Klotzbach’s paper, in Geophysical Research Letters, looked at a 20-year period rather than the 35-year period Curry and others examined, which explains how he reached different conclusions.
“At this point, we haven’t seen any significant correlation” between hurricanes and climate change, he said.
MIT professor Kerry Emmanuel — who helped spark the debate with a paper in the journal Nature a year ago suggesting that warmer sea surface temperatures had spawned more destructive storms — has made an effort to correct for measurement biases in his studies.
He is still criticized by researchers such as Landsea, but Emmanuel responded in an interview that the bias in the underlying data “isn’t very large.” He added that he and other researchers in Europe have found such a strong link between warming sea surface temperatures and more intense hurricanes that, “You literally have to argue that the correlation is an accident. That to me is improbable.”
Curry noted that the hurricane question has focused Americans on global warming far more than other climate-related developments, such as melting glaciers in Greenland. “Katrina was sort of the 9/11 of global warming,” she said in an interview. “It was a lot more real and immediate. It had more of a real socioeconomic impact in the way the melting of glaciers doesn’t.”
Many environmental groups have seized on the public’s concern, arguing that 2005’s brutal hurricane season highlights the dangers of global warming. The advocacy group Environmental Defense has a new Web site devoted to “Hurricanes and Climate Change,” including “11 Facts That Will Blow You Away.”
Meanwhile, William Hooke, who directs the American Meteorological Society’s policy program, said that whatever the answer turns out to be, “We ought not to lose sight of the fact that we’re doing a poor job of protecting ourselves against the hurricanes we have now.”