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."
Evan's team used three sets of data on the maximum wind speed of the 10 tropical cyclones that occurred in the region between 1979 and 1996 and the 10 that developed between 1997 and 2010. The median maximum speed increased from 23 metres per second in the first period to 41 mps in the second. They used numerical modelling to develop their theory and unpick the underlying causes.
The study found wind sheer had dropped from an average of 11 mps in the period 1979-96 to eight mps in 1997-2010.
All the five strongest storms, which exceeded 50 mps, have occurred since 1998. In that year a major cyclone resulted in thousands of deaths and the inundation of numerous salt mines in India.
Other large cyclones since then have made landfall and caused loss of life and billions of pounds' worth of property damage in Pakistan, Iran and Oman.
"Storms that are more intense have a longer lifespan, so the probability that they are going to make landfall goes up," said Evan.
Dr Ryan Sriver of Pennsylvania State University, US, said that while the research shed "much-needed light" on the topic, the findings should be interpreted with caution. In an article published alongside the new research in the journal Nature, Sriver said: "This is a very small number of events to use as a basis for estimating climate trends."