Friday, June 11, 1999 Published at 17:20 GMT 18:20 UK
Pollution cloud threatens Asia
A thick brown cloud of pollution the size of the United States has formed over the Indian Ocean, contributing to acid rain and cutting the amount of sunlight reaching the water.
Scientists sponsored by the US National Science Foundation spent six weeks earlier this year tracking the cloud with aircraft, ships, balloons and satellites.
The cloud rises three kilometres into the Earth's atmosphere, and consists of tiny particles of soot, chemicals and carbon monoxide - by-products 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 back over the land.
Levels of sunlight reduced
Tiny particles in the cloud reflect the sun's rays, cutting its heat by up to 10%.
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.