Global warming reducing hurricanes in U.S., report says

MIAMI — Intensifying one of the hottest debates in science, a new report concludes that global warming is diminishing the number of hurricanes that strike Florida and the rest of the United States.

The study found that the planet's oceans have been warming for more than a century. No surprise there, but this may be: Those warmer oceans are producing stronger crosswinds that tend to suppress the development and growth of hurricanes, the scientists said.

"We found a gentle decrease in the trend of U.S. landfalling hurricanes as global oceans warmed up," said Chunzai Wang, an oceanographer and climate scientist at the National Oceanic and Atmospheric Administration.

Some previous studies found that global warming was increasing the number and intensity of hurricanes, a conclusion that supported the conventional wisdom that warmer seas automatically turbocharge hurricane development.

The new study, by Wang and Sang-Ki Lee of the University of Miami, will be published today in the journal Geophysical Research Letters.

The study raises questions about how the insurance industry, which sets rates based on risk models, will respond to reports that appear to contradict each other.

Many other studies have used computerized statistical models to predict the future consequences of global warming, but Wang and Lee conducted an "observational" examination of records back to 1854.

They found that nearly every ocean on Earth has warmed since then, producing a variety of effects, including stronger crosswinds, a phenomenon called wind shear.

When they matched those findings with records of hurricanes that have struck the United States, they discovered a correlation that challenges some previous findings and predictions.

"The increased wind shear coincides with a weak but consistent downward trend in U.S. landfalling hurricanes, a reliable measure of hurricanes over the long term," the report said.

The researchers found that higher temperatures in the tropical Atlantic decrease wind shear, while higher temperatures in the tropical Pacific and Indian oceans increase shear, but the winds produced over the Pacific and Indian oceans are most important.

"Warmings in the tropical Pacific and Indian oceans win the competition and produce increased wind shear, which reduces U.S. landfalling hurricanes," the report concludes.

The study represents another salvo in the war between two camps of hurricane researchers.

Both groups agree global warming is occurring, but they differ about what effect — if any — it's having on hurricane development.

One side says hurricanes are forming more frequently than a century or more ago, maybe even twice as often, and are growing more powerful.

The other side acknowledges an upswing in hurricane activity. But those researchers say that when the peaks and valleys are smoothed out, hurricanes are forming at about the same rate — or possibly less frequently — than in the past.

Wang said additional study is required before firm predictions can be issued.