The number of Atlantic hurricanes that form each year has doubled over the past century and global warming is largely to blame, according a new study.
The increase occurred in two major steps of about 50 percent each, one in the 1930s and the second since 1995.
WASHINGTON — The debate over whether global warming affects hurricanes may be running into some unexpected turbulence. Many researchers believe warming is causing the storms to get stronger, while others aren't so sure. Now, a new study raises the possibility that global warming might even make it harder for hurricanes to form.
The findings, by Gabriel A. Vecchi of the National Oceanic and Atmospheric Administration (NOAA) and Brian J. Soden of the University of Miami, are reported in Wednesday's issue of Geophysical Research Letters.
Vecchi and Soden used 18 complex computer climate models to anticipate the effects of warming in the years 2001-2020 and 2018-2100.
Included in the results were an increase in vertical wind shear over the tropical Atlantic and eastern Pacific oceans.
Vertical wind shear is a difference in wind speed or direction at different altitudes. When a hurricane encounters vertical wind shear the hurricane can weaken when the heat of rising air dissipates over a larger area.
On the other hand, warm water provides the energy that drives hurricanes, so warmer conditions should make the storms stronger.
"We don't know whether the change in shear will cancel out the increased potential from warming oceans, but the shear increase would tend to make the Atlantic and East Pacific less favorable to hurricanes," said Vecchi, of NOAA's Geophysical Fluid Dynamics Laboratory in Princeton, N.J.
"Which one of the two — warming oceans or increasing shear — will be the dominant factor? Will they cancel out? We and others are currently exploring those very questions, and we hope to have a better grasp on that answer in the near future," Vecchi said.
"What we can say is that the magnitude of the shear change is large enough that it cannot be ignored," he added.
Any decrease in strength or frequency of storms caused by shear would apply only if all else was equal, Vecchi said, "but all else is not equal, since the shear increase is being driven by global warming."
Soden, of Miami's Rosenstiel School for Marine and Atmospheric Science, added: "This study does not in any way undermine the widespread consensus in the scientific community about the reality of global warming."
The massive destruction caused by Hurricanes Katrina and Rita in 2005 focused attention on tropical cyclones — as these storms are also known — and some well-known researchers suggested the warming seas were fueling stronger storms.
Last year an El Niño — a warming of the water in the tropical Pacific that can affect weather worldwide — dampened the Atlantic hurricane season.
Now, just weeks before the traditional June 1 start of the hurricane season, forecasters and residents of hurricane-threatened regions nervously wait to see what this summer will bring.
The government's hurricane season forecast has yet to be issued, but a top storm researcher has predicted a very active 2007 Atlantic hurricane season. Colorado State University's William Gray expects at least nine hurricanes, with a good chance one will hit the U.S. coast.
While Vecchi and Soden's research indicates increased wind shear in the Atlantic and eastern Pacific, their models did not find the same thing elsewhere.
The models projected that the west and central Pacific should become more favorable to development of the storms, called typhoons in those areas.
Kerry Emanuel, a hurricane expert at the Massachusetts Institute of Technology, said he thinks storms' sensitivity to wind shear may be overestimated.
Emanuel, who was not involved in this research, said he published a study last year that calculated that increasing the potential intensity of a storm via warming by 10% increases hurricane power by 65%, whereas increasing shear by 10% decreases hurricane power by only 12%.
On the other hand, Christopher W. Landsea of NOAA's National Hurricane Center, called Vecchi's study "a very important contribution to the understanding of how global warming is affecting hurricane activity."
Landsea, who was not part of the research, said he believes it "provides evidence that the busy period we've seen in the Atlantic hurricanes since 1995 is due to natural cycles, rather than manmade causes."
The research was funded by NOAA and NASA.