(AFP) – Sep 3, 2008
PARIS (AFP) — Global warming is likely to boost the power of the strongest tropical cyclones, a study released on Wednesday says.
An additional one degree Celsius (1.8 Fahrenheit) in sea temperatures in tropical regions where cyclones breed could lead to a nearly one-third rise in the number of the most powerful storms, it says.
"As the seas warm, the ocean has more energy to convert to tropical cyclone wind," say authors of the paper, released by the London weekly Nature.
Previous research, based on observations over the past 30 years, has already suggested that hurricanes -- as cyclones in the Atlantic are known -- have become more intense as a result of warmer seas.
But the observational record for the Atlantic is more detailed and goes back farther than for storms in the Indian Ocean, known as cyclones, or those in the Pacific, which are called typhoons.
Seeking to fill in the blanks, a trio of US scientists crunched satellite data for the period of 1981-2006 in all of these storm basins.
They totted up the number of storms and the maximum wind speeds attained during each event, and compared this with sea-surface temperatures.
Over the 25 years under scrutiny, they saw no increase in the overall number of storms.
But there was a significant increase in wind speed among the most powerful storms, or those in roughly the top quarter for intensity.
This trend -- the stronger the cyclone the greater the change -- occurred in all storm basins, except for the South Pacific.
The apparent reason is that the South Pacific is already the warmest sea region, and thus has seen less of a relative increase in temperature compared to the Atlantic, Indian Ocean and North Pacific.
No such increase was seen in the wind speed among storms of lesser intensity.
The study, headed by James Elsner of Florida State University, supports the so-called "heat-engine" theory, based on the idea that warmer seas provide more of the raw thermal fuel that drives cyclones.
The paper calculates that a 1 C (1.8 F) rise leads to an increase from 13 to 17, or 31 percent, in the annual number of cyclones that fall into the "strong" category.
In its Fourth Assessment Report, the UN's Intergovernmental Panel on Climate Change (IPCC), gathering top scientists, said last year that from 1906 to 2005, global surface temperatures had risen by 0.74 C (1.33 F), with the rise concentrated overwhelmingly in the final decades of the century.
They predicted that by 2100, there would be warming of between 1.8 and 4.0 C (3.24 and 7.2 F)
However, these figures cannot be directly applied to the oceans because of differences in sea depth, the impact of ocean circulation and the long time it takes for the huge ocean mass to warm.
The new study adds several caveats: it says the ranges of uncertainty are still large and notes that its calculations do not factor in changes in solar radiation or the state of the El Nino system, a big factor in the turnover of ocean warmth.
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