The number of hurricanes in the most powerful categories -- like Katrina and Andrew -- has increased sharply over the past few decades, according to a new analysis sure to stir debate over whether global warming is worsening these deadly storms.
While studies have not found an overall increase in tropical storms worldwide, the number of storms reaching categories 4 and 5 grew from about 11 per year in the 1970s to 18 per year since 1990, according to a report in Friday's issue of the journal Science.
Peter Webster, of the Georgia Institute of Technology, said 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. Between 1970 and 2004 the average sea surface temperature in the tropics rose nearly 1 degree.
Co-author Greg Holland of the National Center for Atmospheric Research said the researchers can't say rising sea-surface temperatures caused Hurricane 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. The increase in storms they found is for category 4 and 5. Category 4 storms have wind speeds of 211kph to 249kph and Category 5 is for storms with sustained winds of 251kph and over.
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.''
There is a natural variability of the climate and some would interpret the changing number of storms to be part of that variability, Holland said. But the variability in the past has been over 10-year periods, and this is sustained over 30 years.
Webster added that sea surface temperatures ``are rising everywhere in the tropics and that is not connected to any natural variability we know.''
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 from 1975 to 1989. This increased to 25 in the 1990 to 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 Massachusetts Institute of Technology, reported in August in the journal Nature that hurricanes in both 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 other researchers were cautious.
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.
The study looked at storms worldwide, and ``for most of the world there was no way to determine objectively what the winds were in 1970,'' he said. The techniques used today were invented later and infrared satellite studies weren't available until the 1980s, Landsea said.