Tropical storms getting stronger, say scientists

4.09.08 09:27

Scientists have found the clearest evidence yet that warming oceans are whipping up more powerful cyclone storms, such as Hurricane Gustav.

The strongest tropical cyclones appear to be getting stronger, a study of satellite data spanning 25 years has shown.

With the exception of the South Pacific Ocean, the same pattern was seen all round the world.

The greatest changes occurred in the North Atlantic and northern Indian oceans, both of which have been lashed by violent storms this year.

Hurricane Gustav, which caused the mass-evacuation of the US city of New Orleans, is an example of a powerful north Atlantic cyclone.

In May the "very severe" cyclone Nargis, which developed in the Bay of Bengal, killed up to 50,000 people and left millions homeless in Burma. It was the worst natural disaster in the region since the 2004 Indian ocean tsunami.

Powerful rotating storms are known as hurricanes in the Atlantic, typhoons in the Pacific and cyclones in Asia. All come under the general heading of "tropical cyclones".

A recent trend of increasing sea surface temperatures in the tropical Atlantic had already been linked to strengthening hurricanes in the region.

The new study led by Dr James Elsner, from Florida State University in Tallahassee, US, and reported in the journal Nature, looked at maximum wind speeds in an archive of satellite data.

A significant upward trend was seen for the strongest cyclones, which was linked to rises in sea surface temperature.

Cyclones were ranked in "percentile" bands from one to 100 according to their strength. A one degree centigrade sea surface warming caused the wind speed of those at the 80th percentile - 20 places from the top - to increase by 1.9 metres per second. But the same temperature rise caused those in 10th position (90th percentile) to increase intensity by 6.5 metres per second.

Generally a 1C rise in temperature raised the frequency of strong cyclones from an average of 13 to 17 a year, an increase of 31%.

The scientists wrote: "Our results are qualitatively consistent with the hypothesis that as the seas warm, the ocean has more energy to convert to tropical cyclone wind..

"The results clearly show that the strongest tropical cyclones are getting stronger."

By: John von Radowitz