Brown cloud might be intensifying storms over Indian Ocean

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A longstanding brown cloud of pollution over the Indian Ocean is causing cyclones to intensify in that region, according to a new study published this month in the journal Nature and involving researchers from multiple institutions, including the National Oceanic and Atmospheric Administration, or NOAA.

After the apparent recent increase in the number and intensity of hurricanes in the Atlantic Ocean and Gulf of Mexico, including the devastation of Hurricane Katrina in 2005, climate watchers everywhere have speculated whether these storms were made stronger by industrial or man-made emissions. This is reportedly the first study to indicate that human activity may, in fact, affect large storms.

Amato Evan, lead author on the study and a professor of environmental sciences at the University of Virginia, notes: “The thing that stands out to me, as someone who works in climate and tropical cyclones, is that human activity, things people do, can actually change these massive atmospheric phenomena. To me, this is kind of the first study that can unambiguously tie human activity to something as enormous and powerful as a tropical cyclone.”

The Atmospheric Brown Cloud, previously known as the Asian Brown Cloud, has been observed for decades and began forming prior to World War II. From space, it resembles a dense brown smog and hangs over the northern Indian subcontinent, the northern Bay of Bengal, and the northern Arabian Sea. One of the contributors to the paper, Veerabhadran Ramanathan of the Scripps Institution of Oceanography in La Jolla, has done some of the most important work on identifying the sources of this pollution, which is made up of particles like black carbon and sulfates and is a product of industrial development but also things as common as wood cookfires from an increasing population.

Cyclones naturally form over the warm waters of the Arabian Sea, but are often limited by wind shear -– the cacophony of short-distance winds moving at different speeds and different directions in the atmosphere. Wind shear can be thought of as turbulence and prevents the cyclones from organizing into powerful storms.

As the brown cloud shades the ocean (called “atmospheric dimming”), however, it affects surface temperatures, which lessen the effects of wind shear. As wind shear effects drop, the storm intensifies. The scientists looked at wind, temperature and satellite data from 1979 to present and correlated the increased pollution to increased storm wind speeds. According to a NOAA press release, five storms in the northern Indian Ocean since 1998 have had winds over 120 mph –- including category 5 Cyclone Gonu in 2007 — and have killed more than 3,500 people and caused over $6.5 billion in damage.

James Kossin, a climatologist at NOAA’s National Climatic Data Center in Ashville, N.C., and co-author on the study, cautions that it’s early to say smog causes bad storms. “It’s a hypothesis. It’s difficult to say with much certainty, and I think our main hurdle there is just a small sample of storms to look at. The results are very suggestive.”

“It gets into a tricky business when you want to start saying, ‘Here is the cost of that pollution associated with the tropical cyclones.’ That’s probably stretching it a bit far,” adds Evan. “But certainly it’s true in the Atlantic Ocean and it’s true anywhere: a bigger, stronger storm generally causes more damage.”

For Evan, the study has a significant upside: the pollutants that may be intensifying these cyclones are short-term, unlike greenhouse gases. They float into the lower or middle atmosphere and would clear out very quickly if emissions are cut.

“If emissions are reduced, we expect that this kind of trend would reverse on time scales of a few months,” Evans says. “It’s not like greenhouse gases, where we think we’re already in trouble. With these kinds of aerosols, if you just stopped all the emissions right now, the atmosphere would become much cleaner in a matter of weeks. And then the whole climate system, the ocean and the atmosphere, would essentially lose memory of those aerosols. It’s pretty dramatic.”

In an interesting side note, emails related to this study were among those listed in a recent FOIA request by the conservative American Traditions Institute as it investigates climate change science published by the former University of Virginia climate scientist Michael Mann. The ATI has close ties to energy interests that have opposed climate legislation, and the so-called ‘Climategate’ matter has been the subject of previous posts on this blog.

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-- Dean Kuipers