Soot absorbs 80 per cent of the solar radiation it receives; directly warms the atmosphere

It is no longer just greenhouses gases and their ability to produce global warming that scientists worry about. Concern has been growing over the role played by soot.

Fine particles of soot result from the incomplete burning of fossil fuels and biomass. Soot is produced by diesel engines, the burning of coal, forest fires, burning of crop residues and when firewood and dung is used as household fuel.

Soot particles absorb 80 per cent of the solar radiation they receive and directly warm the atmosphere, said S.K. Satheesh of the Centre for Atmospheric and Oceanic Sciences at the Indian Institute of Science in Bangalore.

Cloud burn off

Absorption of sunlight by soot could heat the surrounding atmosphere to such an extent that clouds “burn off,” suggested Dr Satheesh in paper published in Nature in 2000 that was co-authored with V. Ramanathan of the Scripps Institution of Oceanography in California.

Soot in the atmosphere could be having as much as 60 per cent of the current global warming effect of carbon dioxide, observed Prof. Ramanathan and G. Carmichael in a review paper published in Nature Geoscience in 2008.

The increased levels of soot and other pollutants in the atmosphere were reducing monsoon rainfall over India, said Prof. Ramanathan and others in another paper in 2005. Droughts might double in frequency if the emissions continued unabated.

However, William Lau of the NASA Goddard Space Flight Centre in the U.S. and others have suggested that the soot from northern India along with dust from the deserts of western China, Afghanistan, Pakistan and the Middle East were producing an ’elevated heat pump’ over Tibet.

The effects

The rising hot air produced by enhanced heating drew in warm and moist air over the Indian subcontinent. Consequently, there could be an “advance of the rainy periods and subsequently an intensification of the Indian summer monsoon,” they remarked in a paper published in 2006.

More recently, there has been concern over soot hastening the melting of the Himalayan glaciers.

“Over areas of the Himalayas, the rate of warming is more than five times faster than warming globally, remarked Dr. Lau at a meeting of the American Geophysical Union in California earlier this month. The ‘elevated heat pump’ could be contributing as much or more to atmospheric warming in the Himalayas as greenhouse gases.

Besides, soot being deposited directly on the glaciers too seemed to be playing a part. Chinese and American scientists published this month the results of research that looked at ice cores from the Tibetan Plateau. “We find evidence that black soot aerosols deposited on Tibetan glaciers have been a significant contributing factor to observed rapid glacier retreat,” reported James Hansen, director of NASA’s Goddard Institute for Space Studies, and others in their paper in the Proceedings of the National Academy of Sciences of the U.S.A.

Cutting soot emission

In a recent article in the magazine Foreign Affairs, Jessica Seddon Wallack, director of the Centre for Development Finance at the Institute for Financial Management and Research in Chennai, and Prof. Ramanathan have argued that reducing soot and ozone precursors could rapidly slow the pace of global warming, thus giving efforts to cut carbon dioxide emissions time to get off the ground.

Emissions of soot and ozone precursors could be brought down significantly at relatively low cost with technologies that already existed. While carbon dioxide could remain in the atmosphere for centuries, soot stayed aloft only for days to weeks while ozone persisted for just weeks to months.

Reducing the emissions of these pollutants would quickly lower their concentration in the atmosphere and, in turn, their impact on global warming, they pointed out.

For U.S. dollars 15 billion, 500 million households could be provided with clean stoves, Prof. Ramanathan was quoted as saying in a recent media report. These families were currently using firewood, coal and dung as fuel and the switch would greatly reduce soot production.

Undesired result

Cutting soot levels in the atmosphere might produce the opposite effect – an increase in warming rather than a reduction, pointed out Dr. Satheesh,. He received the Bhatnagar Award this year.

Much of the warming of the atmosphere occurred when the earth’s surface became heated by radiation from the sun. Removing soot could increase the amount of sunlight reaching the surface, thereby leading to greater warming of the atmosphere.

One recent study showed evidence of such an effect in California where reduction in soot levels after about 1980 led to a statewide surface temperature increase.

By N Gopal Raj