BEIJING, China - A brown cloud of pollution caused by human activities, three kilometers thick and stretching from the Arabian Peninsula across Asia to the western Pacific Ocean, is darkening cities, speeding the melting of Himalayan glaciers and affecting human health, finds a new report from the United Nations Environment Programme.

Smoke and soot rises into the air over Mumbai. (Photo by Avinash Anand) Atmospheric brown clouds, formed by the burning of fossil fuels, biofuels, wood and plants, absorb sunlight and heat the air, experts write in the study released today in Beijing.

The clouds also mask the actual warming impact of climate change by anywhere between 20 and 80 percent because they include sulfates and other chemicals which reflect sunlight and cool the surface, according to the report.

Dimming of between 10 and 25 percent is occurring over Beijing and Shanghai, in China, Karachi, Pakistan, and New Delhi, India.

Professor Veerabhadran Ramanathan, head of the UNEP scientific panel carrying out the research, is based at the Scripps Institution of Oceanography in La Jolla, California.

The new report provides confirmation of the atmospheric brown clouds effects that Ramanathan's research first documented six years ago.

"Our preliminary assessment, published in 2002, triggered a great deal of awareness but also skepticism," he said. "That has often been the initial reaction to new, novel and far reaching, counter-intuitive scientific research."

Ramanathan called for an international response to the report's findings that "tackles the twin threats of greenhouse gases and brown clouds and the unsustainable development that underpins both."

"One of the most serious problems highlighted in the report is the documented retreat of the Hind Kush-Himalayan-Tibetan glaciers, which provide the headwaters for most Asian rivers, and thus have serious implications for the water and food security of Asia," he said.

The scientific team behind the report is drawn from universities and research centers in China, India, Japan, Korea, Singapore and Thailand as well as Europe and the United States.

The scientists found that the artificial lowering of temperature by atmospheric brown clouds is leading to sharp shifts in weather patterns, causing drying in northern China while increasing the risk of flooding in China's south.

Monsoon precipitation over India and South-East Asia has dropped up to seven percent since the 1950s, with the summer monsoon both weakening and shrinking.

The possible impact of atmospheric brown clouds could include elevated levels of ground-level ozone, which could result in crop losses of up to 40 percent in Asia.

Achim Steiner, UNEP's executive director, voiced hope that the report, "Atmospheric Brown Clouds: Regional Assessment Report with Focus on Asia" will serve as an early warning of the phenomenon, which he hopes will now be "firmly on the international community's radar."

He called on developed countries to help their poorer counterparts attain the technology needed to spur green economic growth.

"In doing so, they can not only lift the threat of climate change but also turn off the soot-stream that is feeding the formation of atmospheric brown clouds in many of the world's regions," Steiner said.

Atmospheric brown clouds start as indoor and outdoor air pollution consisting of particles called primary aerosols and pollutant gases, such as nitrogen oxides (NOx), carbon monoxide (CO), sulfur dioxide (SO2), ammonia (NH3), and hundreds of organic gases and acids.

"Widespread plumes resulting from the combustion of biofuels from indoors; biomass burning outdoors and fossil fuels, are found in all densely inhabited regions and oceanic regions downwind of populated continents," the report states.

The report points to 13 megacities as being "hotspots" for atmospheric brown clouds - Bangkok, Beijing, Cairo, Dhaka, Karachi, Kolkata, Lagos, Mumbai, New Delhi, Seoul, Shanghai, Shenzhen and Tehran.

Soot levels in these cities make up 10 percent of the total mass of all carbon particles in the atmosphere that result from human activities.

The clouds contain toxic aerosols, carcinogens and other harmful particles, which could result in more people suffering from respiratory disease and cardiovascular problems.

More research is needed to determine the precise role of the clouds on food production and farmers' livelihoods, the report states. More research is also required on the brown clouds that exist in parts of North America, Europe, Southern Africa and the Amazon Basin.

"The new research, by identifying some of the causal factors, offers hope for taking actions to slow down this disturbing phenomenon," said Professor Ramanathan. But he cautioned that "significant uncertainty remains in our understanding of the complexity of the regional effects of ABCs and more surprises may await us."

To read the report, "Atmospheric Brown Clouds: Regional Assessment Report With Focus on Asia," click here.