Is global warming making hurricanes more ferocious? New research suggests the answer is yes. Scientists call the findings both surprising and "alarming" because they suggest global warming is influencing storms now rather than in the distant future.
However, the research doesn't suggest global warming is generating more hurricanes and typhoons.
The analysis by climatologist Kerry Emanuel of the Massachusetts Institute of Technology shows for the first time that major storms spinning in both the Atlantic and the Pacific since the 1970s have increased in duration and intensity by about 50 percent.
Palm trees wave and the sea is whipped up by winds caused by Hurricane Emily in the seaside town of La Pesca in northeastern Mexico July 20, 2005. Hurricane Emily slammed into northeastern Mexico and the U.S. border area on July 20, forcing thousands of people to run for cover and threatening dangerous mudslides. Photo by Andrew Winning/Reuters
These trends are closely linked to increases in the average temperatures of the ocean surface and also correspond to increases in global average atmospheric temperatures during the same period.
"When I look at these results at face value, they are rather alarming," said research meteorologist Tom Knutson. "These are very big changes."
Knutson, who wasn't involved in the study, works in the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, N.J.
Emanuel reached his conclusions by analyzing data collected from actual storms rather than using computer models to predict future storm behavior.
Before this study, most researchers believed global warming's contribution to powerful hurricanes was too slight to accurately measure. Most forecasts don't have climate change making a real difference in tropical storms until 2050 or later.
But some scientists questioned Emanuel's methods. For example, the MIT researcher did not consider wind speed information from some powerful storms in the 1950s and 1960s because the details of those storms are inconsistent.
Researchers are using new methods to analyze those storms and others going back as far as 1851. If early storms turn out to be more powerful than originally thought, Emmanuel's findings on global warming's influence on recent tropical storms might not hold up, they said.
"I'm not convinced that it's happening," said Christopher W. Landsea, another research meteorologist with NOAA, who works at a different lab, the Atlantic Oceanographic & Meteorological Laboratory in Miami. Landsea is a director of the historical hurricane reanalysis.
"His conclusions are contingent on a very large bias removal that is large or larger than the global warming signal itself," Landsea said.
Details of Emanuel's study appear Sunday in the online version of the journal Nature.
Theories and computer simulations indicate that global warming should generate an increase in storm intensity, in part because warmer temperatures would heat up the surface of the oceans. Especially in the Atlantic and Caribbean basins, pools of warming seawater provide energy for storms as they swirl and grow over the open oceans.
Emanuel analyzed records of storm measurements made by aircraft and satellites since the 1950s. He found the amount of energy released in these storms in both the North Atlantic and the North Pacific oceans has increased, especially since the mid-1970s.
In the Atlantic, the sea surface temperatures show a pronounced upward trend. The same is true in the North Pacific, though the data there is more variable, he said.
"This is the first time I have been convinced we are seeing a signal in the actual hurricane data," Emanuel said in an e-mail exchange.
"The total energy dissipated by hurricanes turns out to be well correlated with tropical sea surface temperatures," he said. "The large upswing in the past decade is unprecedented and probably reflects the effects of global warming."
This year marked the first time on record that the Atlantic spawned four named storms by early July, as well as the earliest category 4 storm on record. Hurricanes are ranked on an intensity scale of 1 to 5.
In the past decade, the southeastern United States and the Caribbean basin have been pummeled by the most active hurricane cycle on record. Forecasters expect the stormy trend to continue for another 20 years or more.
Even without global warming, hurricane cycles tend to be a consequence of natural salinity and temperature changes in the Atlantic's deep current circulation that shift back and forth every 40 to 60 years.
Since the 1970s, hurricanes have caused more property damage and casualties. Researchers disagree over whether this destructiveness is a consequence of the storms' growing intensity or the population boom along vulnerable coastlines.
"The damage and casualties produced by more intense storms could increase considerably in the future," Emanuel said.