Mysterious brown haze covering world

In this satellite image from NASA, a band of brown haze appears to narrow somewhat as it crosses over South Korea, center, then spreads out over the Sea of Japan toward Japan, centre right. (image © PA Photos)

A massive mile-thick brown pollution haze has settled over vast areas of the planet, changing weather patterns and threatening health and crops, according to the UN.

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.

"All these have led to negative effects on water resources and crop yields," the report says.

Health problems associated with particulate pollution, such as cardiovascular and respiratory diseases, are linked to nearly 350,000 premature deaths in China and India every year, said Henning Rohde, a University of Stockholm scientist who worked on the study.

Soot levels in the air were reported to have risen alarmingly in 13 megacities: Bangkok, Beijing, Cairo, Dhaka, Karachi, Kolkata, Lagos, Mumbai, New Delhi, Seoul, Shanghai, Shenzhen and Tehran.

Brown clouds were also cited as dimming the light by as much as 25% in some places including Karachi, New Delhi, Shanghai and Beijing. The phenomenon complicates the climate change scenario, because the brown clouds also help cool the earth's surface and mask the impact of global warming by an average of 40%, according to the report.

Further reading

Photos: animals now extinct

Video: Himalayas could soon disappear

The earth's seven biggest mysteries

Natural wonders to see before they disappear

Would you dare visit these wild places?