Vast areas of Asia, the Middle East, southern Africa and the Amazon Basin, are affected by the smog-like plumes, caused mainly by the burning of fossil fuels and firewood, are known as "atmospheric brown clouds".

When mixed with emissions of carbon dioxide and other gases blamed for warming the earth's atmosphere like a greenhouse, they are the newest threat to the global environment, according to a report commissioned by the UN Environment Programme.

"All of this points to an even greater and urgent need to look at emissions across the planet," said Achim Steiner, head of the UNEP.

Brown clouds are caused by an unhealthy mix of particles, ozone and other chemicals that come from cars, coal-fired power plants, burning fields and wood-burning stoves. First identified by the report's lead researcher in 1990, the clouds were depicted in the report as being more widespread and causing more environmental damage than previously known.

Perhaps most widely recognised as the haze this past summer over Beijing's Olympics, the clouds have been found to be more than a mile thick around glaciers in the Himalaya and Hindu Kush mountain ranges.

They hide the sun and absorb radiation, leading to new worries not only about global climate change but also about extreme weather conditions.