Hurricanes more powerful, study says
Researcher at MIT sees larger storms with stronger winds
By Scott Allen, Globe Staff | August 1, 2005
The destructive power of hurricanes in the North Atlantic and North Pacific has nearly doubled over the past 30 years, at least partly because of human-induced global warming, according to a controversial new study by a prominent climate researcher at the Massachusetts Institute of Technology.
Though the number of tropical cyclones worldwide has hovered at 90 a year for decades, MIT hurricane specialist Kerry Emanuel contends that the storms are growing larger and reaching higher maximum wind speeds than in the past. Focusing on the cyclones that have been most closely measured -- hurricanes striking the Eastern United States and typhoons in Southeast Asia -- Emanuel concluded that today's storms, on average, release far more energy than their predecessors did in the mid-1970s.
''There seems to be a clear correlation" between increasing strength and length of storms and a temperature increase of 0.5 degrees Celsius on the surface of the sea during the same period, said Emanuel, whose paper was published online yesterday by the journal Nature.
One of the nation's leading hurricane forecasters, William Gray of Colorado State University, said Emanuel is leaping to conclusions based on imprecise information about the strength of hurricanes, especially in decades past. He said Emanuel's formula for calculating the energy released by hurricanes obscures the fact that no one directly measured the winds in many of the storms, roughly estimating speeds from satellite images instead.
''It's a terrible paper, one of the worst I've ever looked at," said Gray, who does not believe that cyclone intensity worldwide is increasing. He also questioned Emanuel's contention that human actions, such as the burning of oil and other fuels, have caused the surface of the ocean to warm. Gray said the ocean-temperature increase is natural.
Suzana Camargo, a cyclone specialist at Columbia University in New York, said Emanuel's findings should be taken seriously, arguing that his conclusion about the growing power of hurricanes is similar to the increase in the energy of typhoons she measured when tropical Pacific temperatures rise by several degrees Celsius as a result of cyclical El Nino weather events. ''You don't have more typhoons; you have more intense ones. You have supertyphoons," she said.
Christopher Landsea, research meteorologist at the National Oceanic and Atmospheric Administration in Miami, said Emanuel deserves credit for taking on a tough issue where the violence of the storms often destroys the equipment intended to measure their ferocity. ''It's certainly the first paper that does systematically connect hurricane intensity . . . to ocean temperature rises that may be due to global warming," said Landsea, who, like Gray, is concerned about the lack of reliable measures of hurricane winds.
Mounting damage from hurricanes has been a major political issue in the United States, at least since Hurricane Andrew caused tens of billions of dollars in damage to Florida in 1992. After several relatively quiet decades, the number of hurricanes originating in the tropics between Africa and the Caribbean rose sharply in the mid-1990s, including two storms last year, Charley and Ivan, that rank among the worst in US history for total damage.
Though most analysts agree the uptick in the number of Eastern hurricanes is a result of natural factors, some environmentalists have argued that global warming intensifies the storms. Global temperatures have increased by roughly one degree Fahrenheit in the past century, at least partly as a result of carbon dioxide releases from the burning of fossil fuels. During the election last fall, a group called Scientists and Engineers for Change posted a billboard in Florida that read: ''Global warming = Worse hurricanes. George Bush just doesn't get it."
Until now, scientific arguments that global warming can intensify extreme weather have been based mainly on projections derived from models. Emanuel published a paper in 1987 suggesting that a warming of the ocean's surface waters would give hurricanes more fuel to churn faster. That theory is now supported by the main scientific panel tracking global warming, the Intergovernmental Panel on Climate Change.
However, detecting an actual increase in overall hurricane activity has proven elusive. Worldwide, the annual number of cyclones -- a term that includes typhoons and hurricanes -- has fluctuated consistently between 80 and 100 for decades. And scientists had generally agreed that the small rise in ocean temperatures so far is not enough to cause dramatic changes in hurricane intensity, so few researchers attempted to measure for a trend.
Emanuel said he, too, was surprised at his results, which found the total power of North Atlantic hurricanes has more than doubled since the mid-1970s, while western North Pacific cyclones have increased in intensity by 75 percent. He said his team reviewed all the wind speed measurements for hurricanes in both regions, making adjustments for the imprecision of past records, then feeding the numbers into his formula.
Colorado's Gray said he was appalled that Emanuel would take such shaky data on wind speeds, then feed them into a formula that puts such heavy weight on those numbers. You can get any result you want, he said.
Emanuel acknowledges that his technique has a large margin of error and that he would not have published the results if the increase in storm intensity hadn't so closely mirrored the rise in ocean temperatures over the past 30 years.
Even if Emanuel is proved right, the increasing damage from hurricanes may have more to do with where people choose to live than the power of the storms. Most researchers have generally agreed that the mounting damage is mostly a consequence of people building on the coast, effectively putting more people and property in harm's way.
The contention that storms are getting stronger ''could be true, but the significance of the trend he's identified is dwarfed by the damages from coastal development," Roger A. Pielke, director of the Center for Science and Technology Policy at the University of Colorado, said in an interview.
Emanuel noted that he agreed with Pielke a few months ago that global warming probably was not a major factor in hurricane activity. ''I wouldn't be so confident about that now," he said.