- The Washington Times - Tuesday, April 17, 2007


A new study raises the possibility that global warming makes it harder for hurricanes to form.

Researchers say global warming models predict an increase of vertical wind shear, or wind speed changes, over the tropical Atlantic and eastern Pacific oceans that can weaken hurricanes.

“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 researcher Gabriel A. Vecchi, of the National Oceanic and Atmospheric Administration’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,” Mr. Vecchi said.

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 that fuels the massive storms dissipates over a larger area.

The findings, by Mr. Vecchi and Brian J. Soden of the University of Miami, are reported in today’s issue of Geophysical Research Letters.

Mr. 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 Nino effect — a warming of the water in the tropical Pacific that can affect weather worldwide — subdued the Atlantic hurricane season.

Christopher W. Landsea of NOAA’s National Hurricane Center, called Mr. Vecchi’s study “a very important contribution to the understanding of how global warming is affecting hurricane activity.”

Mr. 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.”

But Kerry Emanuel, a hurricane expert at the Massachusetts Institute of Technology, said he thinks storms’ sensitivity to wind shear may be overestimated.

Mr. 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 percent increases hurricane power by 65 percent, whereas increasing shear by 10 percent decreases hurricane power by only 12 percent.

Mr. Vecchi and Mr. Soden used 18 complex computer climate models to anticipate the effects of warming in the years 2001 to 2020 and 2018 to 2100.

“What we can say is that the magnitude of the shear change is large enough that it cannot be ignored,” Mr. Vecchi said.

Any decrease in strength or frequency of storms caused by shear would apply only if all else were equal, Vecchi said, “but all else is not equal, since the shear increase is being driven by global warming.”

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