Hurricane expert reconsiders global warming's impact

One of the most influential scientists behind the theory that global warming has intensified recent hurricane activity says he will reconsider his stand.

The hurricane expert, Kerry Emanuel of the Massachusetts Institute of Technology, unveiled a novel technique for predicting future hurricane activity this week. The new work suggests that, even in a dramatically warming world, hurricane frequency and intensity may not substantially rise during the next two centuries.

The research, appearing in the March issue of Bulletin of the American Meteorological Society, is all the more remarkable coming from Emanuel, a highly visible leader in his field and long an ardent proponent of a link between global warming and much stronger hurricanes.

His changing views could influence other scientists.

"The results surprised me," Emanuel said of his work, adding that global warming may still play a role in raising the intensity of hurricanes. What that role is, however, remains far from certain.

Emanuel's work uses a new method of computer modeling that did a reasonable job of simulating past hurricane fluctuations. He, therefore, believes the models may have predictive value for future activity.

During and after the 2004 and 2005 hurricane seasons, which were replete with mega-storms and U.S. landfalls, scientists dived into the question of whether rising ocean temperatures, attributed primarily to global warming, were causing stronger storms.

Among the first to publish was Emanuel, who — just three weeks before Hurricane Katrina's landfall — published a paper in Nature that concluded a key measurement of the power dissipated by a storm during its lifetime had risen dramatically since the mid-1970s.

In the future, he argued, incredibly active hurricane years such as 2005 would become the norm rather than flukes.

Other factors likely

This view, amplified by environmentalists and others concerned about global warming, helped establish in the public's mind that "super" hurricanes were one of climate change's most critical threats. A satellite image of a hurricane emanating from a smokestack featured prominently in promotions for Al Gore's An Inconvenient Truth.

"Kerry had the good fortune, or maybe the bad fortune, to publish when the world's attention was focused on hurricanes in 2005," Roger Pielke Jr., who studies science and policy at the University of Colorado, said of Emanuel. "Kerry's work was seized upon in the debate."

After the 2005 hurricane season, a series of other papers were published that appeared to show, among other things, that the most intense hurricanes were becoming more frequent.

What has not been as broadly disseminated, say Pielke and some hurricane scientists, is that other research papers have emerged that suggest global warming has yet to leave an imprint on hurricane activity. One of them, published late last year in Nature, found that warming seas may not increase hurricane intensity.

That paper's co-author, Gabriel Vecchi, a research scientist with the National Oceanic and Atmospheric Administration, said Emanuel's new work highlights the great uncertainty that remains in hurricane science.

"While his results don't rule out the possibility that global warming has contributed to the recent increase in activity in the Atlantic, they suggest that other factors — possibly in addition to global warming — are likely to have been substantial contributors to the observed increase in activity," Vecchi said.

Scientists wrangling with the hurricane-global warming question have faced two primary difficulties. The first is that the hurricane record before 1970 is not entirely reliable, making it nearly impossible to assess with precision whether hurricane activity has increased during the last century.

The second problem comes through the use of computer models to predict hurricane activity. Most climate models, which simulate global atmospheric conditions for centuries to come, cannot detect individual tropical systems.

Emanuel's new research attempts to get around that by inserting "seeds" of tropical systems throughout the climate models and seeing which develop into tropical storms and hurricanes. The "seeds," bits of computer code, tend to develop when simulated atmospheric conditions, such as low wind shear, are ripe for hurricane formation.

'A lot of work to do'

In the new paper, Emanuel and his co-authors project activity nearly two centuries hence, finding an overall drop in the number of hurricanes around the world, while the intensity of storms in some regions does rise.

For example, with Atlantic hurricanes, two of the seven model simulations Emanuel ran suggested that the overall intensity of storms would decline. Five models suggested a modest increase.

"The take-home message is that we've got a lot of work to do," Emanuel said. "There's still a lot of uncertainty in this problem. The bulk of the evidence is that hurricane power will go up, but in some places it will go down."

The issue probably will not be resolved until better computer models are developed, said Judith Curry, of the Georgia Institute of Technology, a leading hurricane and climate scholar.

By publishing his new paper, and by the virtue of his high profile, Emanuel could be a catalyst for further agreement in the field of hurricanes and global warming, Curry said.

The generally emerging view, she said, seems to be that global warming may cause some increase in intensity, that this increase will develop slowly over time, and that it likely will lead to a few more Category 4 and Category 5 storms. How many? When? No one yet knows.