Global warming is increasing the frequency of the most intense hurricanes, insist scientists who have analysed data from six oceans.
The study comes in answer to critics, mostly scientists involved in hurricane prediction, of previous research. They argued that local factors such as the difference in wind direction at various altitudes (known as wind shear) would swamp any effect of warmer oceans.
Hurricanes such as Katrina, which devastated New Orleans last summer, start as clusters of thunderstorms over the tropical oceans. Evaporating water rises and cools, condensing as clouds, and the change from vapour to droplets releases energy that heats the air and drives it still higher. If enough of these storms form in close proximity they can create a column of humid air which sucks air in at the base and releases it at the boundary between the atmosphere and stratosphere.