A polluted Hong Kong island skyline is seen in September 2008. High to very high pollution levels were recorded in various districts in the territory at the time. (Bobby Yip/Reuters)

A brown haze of soot, particles and chemicals that hangs over parts of Asia is darkening cities, melting glaciers in the Himalayas and making weather systems more extreme, the United Nations said Thursday.

Scientists who have studied the thick brown clouds, which they estimate to be more than three kilometres thick, said the haze stretches from the Arabian Peninsula to China and the Western Pacific Ocean. It is officially known as atmospheric brown clouds.

The scientists, who come from China, India, Europe and the U.S., said in a new report commissioned by the UN Environment Program that the brown clouds are aggravating the impact of climate change caused by greenhouse gases in some regions.

They said they are issuing the warning now about the brown haze because it is a "serious and significant" environmental challenge facing the planet that poses a threat to human health and food production.

"Imagine for a moment a three-kilometer-thick band of soot, particles, a cocktail of chemicals that stretches from the Arabic Peninsula to Asia," Achim Steiner, UN undersecretary general and executive director of the program, said during a news conference on the findings.

"All of this points to an even greater and urgent need to look at emissions across the planet, because this is where the stories are linked in terms of greenhouse emissions and particle emissions and the impact that they're having on our global climate," he said.

The brown clouds have darkened 13 cities in Asia, including Beijing, Shanghai, Bangkok, Cairo, Mumbai, New Delhi and Tehran, "dimming" sunlight in some places by as much as 25 per cent.

The brown clouds, produced by the burning of fossil fuels, wood and plants, form particles like black carbon and soot that absorb sunlight and warm the air, enhancing the greenhouse effect.

Mask for warming impact

Scientists, however, said the brown clouds also "mask" the warming impacts of climate change by an average of 40 per cent because they contain particles that reflect sunlight and cool the earth's surface.

According to the report, the phenomenon has been studied closely in Asia, but it is not unique to the region, with brown clouds seen over parts of North America, Europe, southern Africa and the Amazon Basin.

The scientists said the brown clouds are having a negative impact on air quality and agricultural production in Asia with risks to human health increasing. Health problems associated with the brown clouds include cardiovascular and respiratory diseases.

Veerabhadran Ramanathan, head of the scientific panel that is carrying out the research and a professor at the Scripps Institution of Oceanography at the University of California in San Diego, said the huge cloud masses can cross continents in the space of three to four days.

It's not a regional issue, but a global one, he said.

"The main message is that it's a global problem. This is not a problem where we point fingers at our neighbours. Everyone is in someone else's backyard," Ramanathan said.

He said one of most serious problems noted in the report is melting of the Hindu Kush-Himalaya-Tibetan glaciers, which provide the head-waters for the major river systems including the Ganges, Brahmaputra, Mekong and Yangtze rivers.

The Ganges basin, for example, is home to more than 400 million people and holds 40 per cent of India's irrigated croplands.

He said the melting has "serious implications for the water and food security of Asia."

The Chinese Academy of Sciences estimates that the glaciers have shrunk five per cent since the 1950s and the volume of China's nearly 47,000 glaciers has fallen by 3,000 square kilometres over the past quarter century.

The brown clouds also have helped decrease the monsoon season in India. The weather extremes caused by the clouds may have also helped to reduce production of crops such as rice, wheat and soybean, according to the report.

Ramanathan said he hopes the report "triggers" an international response to the problems of greenhouse gases and brown clouds and the "unsustainable development" that underlies them both.

"The new research, by identifying some of the causal factors, offers hope for taking actions to slow down this disturbing phenomenon," he said.

With files from the Associated Press