They call it the Asian Brown Cloud. Anyone who has flown over South Asia has seen it – a vast blanket of smog that covers much of the region.

It is also what colours those sunsets at the Taj Mahal. Now a group of scientists has carried out the first detailed study of the phenomenon and arrived at a troubling conclusion.

They say that it is causing Himalayan glaciers to melt, with potentially devastating consequences for more than two billion people in India, China, Bangladesh and other downstream countries.

In a study published yesterday by Nature, the British journal, they say that black soot particles in the cloud are absorbing the Sun’s heat and pushing up temperatures at the same altitude as most Himalayan glaciers.

Scientists have already observed that two thirds of the 46,000 glaciers in the Himalayas are shrinking, leading to increasingly severe floods downstream and, eventually, to widespread drought. Greenhouse gases were previously thought to be the main cause of the problem, which threatens the sources of Asia’s nine main rivers – including the Indus, the Ganges and the Yangtze.

But the research team from the Scripps Institution of Oceanography in California says that the Asian Brown Cloud – made up of gases and suspended particles known as aerosols – is just as much to blame. “My one hope is that this finding will intensify the focus of Asian scientists and policy makers on the glacier issue,” Veerabhadran Ramanathan, who led the research, told The Times. “These glaciers are the source for major river systems, so at least two billion people are directly involved in this.”

The cloud is an enormous plume of smoke from factories, power plants and wood or dung fires that stretches across the Indian subcontinent, into SouthEast Asia.

Professor Ramanathan’s team examined it using three unmanned aircraft similar to those used by the US military, but fitted with fifteen instruments to measure temperature, humidity and aerosol levels. The drones were launched from the Maldives island of Hanimadhoo and carried out 18 missions over the Indian Ocean in March 2006, flying simultaneously through the cloud at different altitudes. They found that the cloud amplified the effects of solar heating on the surrounding air by 50 per cent.

The professor said that some aerosols in the cloud reflected sunlight, cooling the earth beneath in a process known as “global dimming” that is also worrying climate change experts.

Others absorbed heat radiation from the Sun because of their dark colour.

When he put his data into a computer model for climate change, it estimated that Himalayan temperatures had risen 0.25C (0.45F) a decade since 1950 – twice the average rate of global warming. “If we continue to use outdated technology to achieve industrialisation, this is only going to get worse,” said Professor Ramanathan. “But there is some good news.” Unlike greenhouse gases, which can stay in the atmosphere for 200 years, aerosols drop to the ground after two to three weeks.

Asian countries can therefore tackle the problem relatively quickly if they find alternatives to fuels such as coal, diesel, wood and dung, which account for the majority of aerosols in the air.

The United Nations Environment Programme, which supported the research, urged all governments to take similar steps since brown clouds have now been observed in Africa, America and the Middle East.

“The main cause of climate change is the build-up of greenhouse gases from the burning of fossil fuels,” said Achim Steiner, the United Nations under-secretary-general and the executive director of UNEP. “But brown clouds, whose environmental and economic impacts are beginning to be unravelled by scientists, are complicating and aggravating their effects.”

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