May 18 (Bloomberg) -- The number of hurricanes and tropical storms forming over the Atlantic may drop this century because of global warming, the National Oceanic and Atmospheric Administration said in a study that contradicts prior research.

The scientists took a climate model that mirrors the increase in Atlantic storms in recent years, and then plugged in forecasts for future warming. The results, published online yesterday in Nature Geoscience, showed a decline in the numbers of both hurricanes and tropical storms.

Today's study challenges theories that climate change will boost storminess. Research last year led by Greg Holland, a climate scientist at the National Center for Atmospheric Research in Boulder, Colorado, linked last century's increase in hurricane activity to warmer sea surface temperatures.

"This study does not support the notion that increasing greenhouse gases are causing a large increase in Atlantic hurricane or tropical storm frequency,'' the paper's lead author, Thomas Knutson, a NOAA scientist, said May 16 in a conference call with reporters. ``Rather for future climate conditions we simulate a reduction.''

The scientists also found that the storms that do occur will be about 1 to 2 percent more intense, Knutson said, adding that ``intensity is really not the strong point of this particular model.'' The frequency of the most intense hurricanes rose, as did rainfall, results that confirm Holland's findings.

Tropical cyclones, including hurricanes, with sustained winds of at least 74 miles (119 kilometers) an hour, as well as tropical storms with 39 mile-an-hour winds, get their energy from the oceans. Typically, warmer seas fuel more storms. Their number varies from year to year, depending on global climate patterns linked to temperature changes in the Pacific Ocean.

Hurricane Katrina

In 2005, 27 named Atlantic storms formed, eclipsing 1933's record of 21. The 15 hurricanes of 2005 topped the 12 from 1969. One of them, Hurricane Katrina, devastated New Orleans and killed more than 1,800 along the U.S. Gulf Coast. That prompted concerns about the increasing numbers and intensity of Atlantic storms.

"This model is capable of predicting the increase in hurricane activity that we've seen in recent decades and yet project a decrease in the future,'' Isaac Held, a co-author of the paper, said on the call. ``That definitely implies that we can't extrapolate from the past 25 years into the future.''

Over the past century, a more than 0.7-degree Celsius (1.3- degree Fahrenheit) rise in sea surface temperatures led to a more than doubling of tropical cyclones in the Atlantic, Holland and co-researcher Peter Webster of the Georgia Institute of Technology in Atlanta wrote in a paper last year in the U.K. journal Philosophical Transactions of the Royal Society A.

The latest research will help improve understanding and prediction of what may happen to hurricane activity as temperatures rise, Holland said in a May 16 e-mailed response to questions. At the same time, he warned of ``deficiencies'' in some of the study methods.

`Considerable Deficiencies'

``The results are largely determined by the larger-scale model, which has considerable deficiencies for hurricane simulation,'' Holland said, referring to one of the models used by Knutson's team. ``This means that the estimates of numbers are considerably uncertain, especially for future climate.''

Knutson and colleagues plugged data from each of the hurricane seasons from 1980 to 2006 into their model. They then altered atmospheric and temperature data to reflect possible scenarios for conditions at the end of this century published last year by the United Nations Intergovernmental Panel on Climate Change (IPCC).

Under the altered conditions -- including sea surface temperatures 1.72 degrees Celsius warmer than now -- they found that tropical storm numbers declined by 27 percent and hurricanes by 18 percent. Storm systems with winds of at least 100 miles an hour more than doubled.

Warming Atlantic

The reasons for the reductions are being investigated, the scientists wrote. Knutson said that the IPCC projections used involve a ``relatively uniform warming'' across all tropical oceans. Recent years have been characterized by a warming of the tropical Atlantic relative to the other oceans, he said.

In the warmer climate projections, rainfall around systems increased by more than a third within 50 kilometers of the storm center, according to the scientists.

The findings on rainfall and intensity agree with the forecast made last year by the IPCC that it is ``likely'' future hurricanes will become more intense with heavier rain and greater peak wind speeds. The panel at the time said there is less certainty in forecasts relating to the frequency of storms.

"My current research leads me to fully agree with Knutson et al. on the rainfall and intensity changes,'' Holland said in his e-mail. ``Careful and objective studies such as this are welcomed and I strongly encourage more work on this important topic.''

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