Fuel For Global Warming Debate

Hurricanes Category 4 And More 50% More Common Than In 1970s

(AP)  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 was an average of about 11 storms of the powerful category 4 and 5 range. Since 1990 that has climbed to an average of 18 per year worldwide, researchers led by Peter J. Webster at the Georgia Institute of Technology report in Friday's issue of the journal Science.

Katrina's sustained winds reached 175 mph, the National Oceanic and Atmospheric Administration's National Climatic Data Center said Thursday.

The climate 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 mph to 155 mph and Category 5 is for storms with sustained wind of 156 mph 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.