Article By Nic Fleming , for the Guardian
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guardian.co.uk,
Wednesday 2 November 2011

Powerful tropical cyclones that cause large numbers of deaths and massive property damage around the Arabian Sea are becoming increasingly common as a result of pollution, scientists say.

Analysis of the intensity of storms between 1979 and 2010 suggests that thick layers of haze have created atmospheric conditions that intensify cyclones and increase the chances they will reach land.

There has been a sixfold increase in fine aerosol emissions locally since the 1930s from forest fires, domestic heating and diesel use, creating a layer of pollution 3km thick over the Indian Ocean.

It has long been known that high wind shear – a large difference between the speed of winds near the sea surface and those higher up in the atmosphere – can prevent storms from forming or prevent existing ones from intensifying. Dr Amato Evan of the University of Virginia in Charlottesville, US, and his colleagues suggest that the giant pollution cloud reduces the surface temperature of the Arabian Sea, which in turn reduces wind shear and triggers the formation of more powerful cyclones.

"We have found a clear connection between human activity and changes in atmospheric conditions that create favourable conditions for the formation of large tropical cyclones," said Evan. "In my mind that's a big step forward. Every analysis we did pointed in the same direction. We only have 30 years of data, but we have used data from multiple different sources, so I'm confident our results are robust."

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