A 'brown cloud' of pollution over the Indian Ocean resulting from human activities has led to stronger tropical cyclones in the Arabian Sea, according to an international team of scientists. The findings appear in a paper published this month in the journal Nature titled 'Arabian Sea tropical cyclones intensified by emissions of black carbon and other aerosols'. The research was conducted by scientists from NOAA's National Climatic Data Centre (NCDC), the Gwangju Institute of Science and Technology, Gwangju, South Korea and the Scripps Institution of Oceanography in California.
"We're showing that pollution from human activity as simple as burning wood or driving a vehicle with a diesel engine can actually change these massive atmospheric phenomena in a significant way," said study lead author Amato Evan, an assistant professor in the Department of Environmental Sciences at the University of Virginia. "It underscores the importance of getting a handle on emissions in the region."
The Arabian Sea is typically warm enough to allow tropical cyclones to form and develop. However a phenomenon known as wind shear (caused by winds moving at different speeds and directions at different levels in the atmosphere), has restricted the strength to which these storms generally grow. The study has shown that during the past 30 years, increased concentrations of airborne particles in South Asia have altered the pattern of the sun's heating of the ocean. This in turn has changed the regional wind patterns and weakened the wind shear effect, making conditions more favourable for generating more intense tropical storms.
The scientists used both observations and models to demonstrate the relationship between decreasing wind shear and the growth of the Atmospheric Brown Cloud, a thick layer of pollution over the North Indian Ocean caused by human emissions of aerosols like black carbon and sulfates. The team then linked the reduced wind shear to an increase in the number of highly intense storms with winds over 120 mph, including five storms since 1998 that have killed more than 3,500 people and caused damages of more than $6.5 billion (£4.1 billion).
It is another example of just how complex the different environmental connections are, and how important understanding them continues to be. As James Kossin climatologist at NCDC and co-author on the paper put it, "The research shows that pollution can threaten humans in unexpected ways. In this case, by reducing wind shear in the Arabian Sea and making conditions more favourable for tropical cyclones to intensify."