Hurricanes 'becoming stronger'
2005-09-16 07:14
Washington - 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 four and five 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 J Webster, of the Georgia Institute of Technology, said it's the warm water vapour 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 Centre 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 five storm at sea and was category four when it made landfall.
Category four storms have wind speeds of 211kph to 249kph and Category five is for storms with sustained wind of 251kph and over.
Co-author Judith Curry of Georgia Tech said they're confident that the measured increase in sea surface temperatures is associated with global warming, adding that the increase in category four and five 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.
Sea surface temperatures 'are rising'
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 four and five 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.
Climatologist Kerry Emanuel reported in August in 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 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.
On the net:
www.sciencemag.org
- AP