Giant pollution cloud hovers over Asia.

Scientists investigating the effects of pollutant particles on climate last March discovered a dense brown haze covering 9 million square kilometers - an area the size of the United States - over the Indian Ocean. The cloud, thought to have been blown out to sea during the winter monsoons, covers the Arabian Sea to the west of the Indian coast, the Bay of Bengal to the east, and the equatorial Indian Ocean to about 5 degrees south of the equator. It is about 3 kilometers thick, and contains soot, sulphates, nitrates, organic particles, fly ash, mineral dust, and high concentrations of gases such as carbon monoxide and sulfur dioxide.

The research team that made the discovery included scientists from the United States, Europe, India, and the Maldives, who had come to the region to work on the Indian Ocean Experiment (INDOEX), a study of the climatic effects of aerosols - small pollutant particles. V. Ramanathan, of the Scripps Institution of Oceanography, and a co-scientist in the project said, "what really shinned us (was) how pervasive these aerosols were and how they could survive at such long distances from where they originated."

The discovery has intensified debate about the impacts of human activity on climate change. The particles in the cloud are much darker than those found over North America and Europe, so the effects on climate may also be different. It is uncertain whether the overall result is one of warming or cooling. Aerosols, when mixed with clouds, act as reflective surfaces and have a cooling effect, but the soot particles in the cloud increase the absorption of sunlight, producing a warming effect. The recent discovery that ships crossing the Indian Ocean are a significant source of sulfur particles (which are converted into cooling aerosols), has further complicated the question.

One concern raised by the cloud is that if the amount of light reaching the ocean surface is reduced by aerosols, photosynthesis in the ocean could be disrupted, ultimately causing dramatic changes in the food chain. Another concern is that monsoon winds are likely to blow the cloud back over the Indian sub-continent, where it may cause acid rain, which would harm both terrestrial and aquatic life.

In any case, analysts regard this cloud as an indicator of the increasingly severe air pollution hazards faced by Asian countries. The combination of coal and firewood burning, vehicular emissions, and the widespread use of diesel fuel in these nations contributes to high levels of particulate emissions - the most serious air pollutant worldwide, according to the World Health Organization. It causes the death of 460,000 people each year. The high density of the cloud has been linked both to substantial pollution in South and East Asian cities, and the increasing incidence of respiratory disorders in those cities. For more information, visit www-indoex.ucsd.edu.