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ENVIRONMENTAL IMPACT

Number of strongest storms seen rising

WASHINGTON -- Powerful hurricanes like Katrina, the most destructive such storm ever to hit the United States, are becoming more common, according to a new study sure to fuel debate over whether global warming is to blame.

In the 1970s there were about 11 storms on average of the powerful category 4 and 5 range. Since 1990 that has climbed to 18 per year on average worldwide, researchers led by Peter J. Webster at the Georgia Institute of Technology report in today's issue of the journal Science.

Katrina's sustained winds reached 175 miles per hour, the National Oceanic and Atmospheric Administration's National Climatic Data Center said.

The center's monthly analysis termed Katrina the nation's most destructive hurricane ever.

While Katrina's wind slowed a bit near shore and it was not as powerful as 1969's Hurricane Camille, its damage was much more widespread with hurricane-force winds extending miles from the storm's center.

The increase in powerful hurricanes coincides with a rise of nearly 1 degree Fahrenheit in the tropical sea surface temperature around the world.

It's the warm water vapor from the oceans that drives tropical storms, and as the water gets warmer the amount of evaporation increases, providing more fuel for the tempests.

Co-author Greg Holland of the National Center for Atmospheric Research said the researchers can't say rising sea-surface temperatures caused a specific storm, such as Katrina. But their study shows the potential for more Katrina-like events to occur, he said.

Katrina was a category 5 storm at sea and was category 4 when it made landfall. Category 4 storms have wind speeds of 131 miles per hour to 155 miles per hour and Category 5 is for storms with sustained wind of 156 miles per hour and over.

There was no increase in the total number of tropical storms worldwide; the change was in how many of them grew into the most dangerous categories.

Co-author Judith Curry of Georgia Tech said the team is confident that the measured increase in sea surface temperatures is associated with global warming, adding that the increase in category 4 and 5 storms ''certainly has an element that global warming is contributing to."

Some interpret the changing number of storms to be part of natural variability, Holland said. But the variability in the past has been over 10-year periods, and this is sustained over 30 years.

In their analysis of hurricanes, known as typhoons or cyclones in other parts of the world, the researchers counted 16 category 4 and 5 storms in the Atlantic-Caribbean-Gulf of Mexico in 1975-1989. This increased to 25 in the 1990-2004 period.

In the eastern Pacific the increase was from 36 to 49 storms and it went from 85 to 116 in the western Pacific. In the southwest Pacific the increase was from 10 to 22 powerful storms, while the total went from one to seven in the north Indian Ocean and from 23 to 50 in the south Indian Ocean.

Kerry Emanuel, a climatologist at the Massachusetts Institute of Technology, reported in August in the journal Nature that hurricanes in the Atlantic and Pacific have increased in duration and intensity since the 1970s.

While the new study looks at the problem differently, ''we are clearly seeing the same signal in the data," Emanuel said.

But Christopher Landsea, a meteorologist at the National Oceanic and Atmospheric Administration's Hurricane Research Division in Miami, questioned the data showing an increase in major storms, saying the estimates of the wind speed in storms in the 1970s may not be accurate.

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