CLIMATE CHANGE is usually associated with increasing levels of greenhouse gases, such as carbon dioxide, trapping heat and thereby causing global warming. But there is growing concern that human activity might also be altering the climate, perhaps in unpredictable ways, by releasing a large amount of soot and other pollutants into the atmosphere in the form of tiny particles known as aerosols.

Aerosols can, however, be produced naturally too. Winds, for instance, whip vast quantities of dust and sea salt into the air. Likewise, sulphur-containing compounds known as sulphates can arise by natural processes and plants release organic materials. Nor are aerosols in the atmosphere invariably a bad thing. Clouds would not form if there were no fine particles in the atmosphere around which water vapour could condense into droplets.

Unlike greenhouse gases that persist and thereby accumulate in the atmosphere, aerosols are short-lived. These fine particles may stay aloft in the atmosphere for only one to three weeks. Aerosols are not evenly distributed around the globe. Although they are often concentrated near places where they are generated, they can also be transported by winds for considerable distances, sometimes across continents. Moreover, the type of aerosols found in the atmosphere and the quantities in which they are present change from time to time.

Long before aerosols became the hot area for international research that they are today, scientists at the Indian Space Research Organisation had watched the ebb and flow of these particles. Although their observations were carried out from just a few stations on the ground, it was clear there were major seasonal variations in the concentrations of natural aerosols and anthropogenic ones that arise from human activity.

It was the Indian Ocean Experiment (INDOEX) undertaken jointly by scientists and institutions from the United States, Europe, and India in the late 1990s that highlighted the extent of the problem posed by human pollutants. INDOEX found a thick haze over the Indian subcontinent and the surrounding oceans. Soot (which gave the cloud a distinctive brown tinge), sulphates from the burning of coal, nitrates from vehicular emissions, and fly ash formed the bulk of the haze, and were mixed with natural aerosols such as sea salt and dust.

Aerosols influence the climate in a number of ways. Many of these fine particles scatter sunlight and stop some of it from reaching the earth's surface. Increased levels of water-absorbing particles, such as sea salt and natural sulphates, lead to more cloud droplets forming; the greater cloudiness reflects more sunlight back into space and again less light reaches the ground. These days, scientists are paying more attention to soot. That is because soot particles don't just block sunlight but absorb it, thereby heating up the lower atmosphere and literally burning off clouds that might have formed.

Human-produced aerosols over South Asia and the northern Indian Ocean have dimmed sunlight by as much as seven per cent between 1930 and 2000, while enhancing the solar heating of the lower atmosphere by about 30 per cent to 50 per cent, according to work published by V. Ramanathan and his group at the Scripps Institution of Oceanography in California.

It is possible that soot could produce an even greater impact than previously thought, says S.K. Satheesh of the Centre for Atmospheric and Oceanic Sciences at the Indian Institute of Science. Dust and soot, for instance, need not remain as separate particles. Instead, the tiny soot particles can stick to and coat grains of dust. Such composite particles might triple the dimming of sunlight and perhaps double the cloud burn-off than if dust and soot had remained separate. A similar effect might also be produced by sulphate particles coated over with soot.

The Indo-Gangetic plain is one of the most polluted regions in the world. Measurements made during December 2004 at Kanpur found soot concentrations that were comparable to those at big cities in the country, according to S.N. Tripathi and his team at IIT Kanpur. Samples collected from an aircraft showed soot concentrations at a height of two km that were higher than that at ground level in many European industrial areas. The same was true for Hyderabad as well, suggesting that the problem was not restricted to the Indo-Gangetic plain.

India and China, the two most populous countries and now experiencing rapid economic growth, are major soot producers. Dr. Ramanathan and his colleagues believe that most of the soot in the air above South Asia comes from the burning of fossil fuel. On the other hand, Chandra Venkataraman of IIT Mumbai and fellow scientists argued in a paper published last year that the burning of wood, agriculture waste, animal dung, and other forms of biomass was the "largest source" of soot emissions in India.

But there are seasonal variations in soot levels and even dramatic year-to-year changes. Dr. Tripathi found that, for as yet unknown reasons, soot levels in Kanpur during December 2005 were far below those recorded a year earlier. Likewise, the pollution is said to have been particularly bad during the early months of 1999 when many key INDOEX observations were made.

Soot concentrations over the Indian subcontinent and the surrounding oceans are high principally from November to February, according to Dr. Satheesh. Thereafter, till the monsoon sets in, dust from the deserts of Rajasthan and West Asia predominate. Besides, natural aerosols account for nearly three-quarters of all aerosols present globally, he adds.

While natural aerosols were important in determining the current climate, "when we are talking about climate change, then the focus is on man-made [aerosols]," Dr. Ramanathan told The Hindu when he was in India recently.

Many climate models suggest global warming would increase rainfall over India. But when the effect of aerosols was introduced into a climate model, "we found that the monsoon rainfall in this model was decreasing," he said. Records showed that India's monsoon rainfall had, in fact, gradually decreased over the last 50 years. In a paper published last year, Dr. Ramanathan and his colleagues suggested that the frequency of droughts in India could increase in the coming decades if pollution continued unabated.

Effect on the monsoon

According to Dr. Ramanathan, what appeared to be happening was that pollution dimmed sunlight over the Arabian Sea and consequently that part of the ocean was preferentially cooling compared to the southern Indian Ocean. Since rain systems gravitate towards warmer oceans, the reduced temperature gradient between those two parts of the ocean slowed monsoon circulation and thereby decreased rainfall.

In a press release issued by the Scripps Institution earlier this year, Dr. Ramanathan was quoted as saying: "The greenhouse gases are pushing in one direction, warming the ocean and trying to make more rain, and the aerosols are pushing in another direction for cooler oceans and less rain." Some years the aerosols might win and in other years the greenhouse effect might get the upper hand. The tussle between the two could produce greater year-to-year variability in monsoon rainfall over South Asia that could be difficult to cope with.

The role of natural aerosols, which are dominant over South Asia and the neighbouring oceans during the pre-monsoon period, cannot be ignored when looking at climate impact, objects J. Srinivasan of the Centre for Atmospheric and Oceanic Sciences at the Indian Institute of Science. Recent work at the U.K. Met Office had shown that incorporation of dust aerosols into a climate model made for more accurate simulation of the African monsoon.

When the effect of carbon dioxide on climate change is studied, the levels of the gas generated naturally as well as by humans were both taken into account. "We must adopt the same approach when looking at how aerosols will influence the climate," he told The Hindu . The Indian monsoon rainfall was known to increase and decrease in cycles that were sometimes several decades long, he says. There was little evidence that the decline in rainfall over the last 50 years was not part of such a cycle.

"At present, our understanding of the role of aerosols in determining climate and the capabilities of climate models to simulate the Indian monsoon are both quite poor. Without improvements on both fronts, it is difficult to confidently predict how changes in aerosol levels will influence something as complex as the monsoon," according to Dr. Srinivasan.

How aerosols influenced regional climate was a new field and it would take time to resolve this issue, agrees Dr. Ramanathan. It had taken over a century to evolve a consensus on greenhouse gases and global warming. Although hopefully it would not take that long to find definitive answers to the impact of aerosols, a lot more studies needed to be carried out, he said.

Then again, it is not necessary to wait for a scientific consensus to emerge before seeking ways to reduce soot and other human-produced aerosols. Cutting pollution would, after all, immediately benefit people's health. But bringing about such reduction in countries such as India and China means finding alternate and affordable ways to meet the energy needs of a vibrant economy as well as of a large population, many of them quite poor. That is not going to be easy.