The Indian Ocean Experiment, or INDOEX, revealed that the affected area includes most of the northern Indian Ocean, the Arabian Sea, and much of the Bay of Bengal - nearly 10m square km in total. The cloud rises three kilometres into the Earth's atmosphere, and consists of tiny particles of soot, chemicals and carbon monoxide - byproducts of burning fossil fuels. During the winter monsoons, the cloud blows out from Asia over the ocean, and during the summer wet season, it reverses direction to move backover the land. Scientists from the Scripps Institution of Oceanography in San Diego, which coordinated the project, say the pollution is unprecedented in the region's history, and will get worse as the population increases. Aerosol use, which has long been known to promote global warming, is still widespread in Asia. The tiny particles in the cloud reflect the sun's rays, cutting its heat by up to 10 per cent. In the ocean, less sunlight could threaten the survival of water-plants and plankton - key organisms in the world's food chain. Reduced sunlight also means that less water evaporates from the ocean - leading to lower rainfall and increasing the risk of drought. Scientists now want to discover how permanent the cloud is, and whether it is growing.