Dirty brown clouds created by millions of cooking fires in Asia contribute as much to global warming as greenhouse gas emissions and are a major factor in the melting of the Himalayan glaciers, scientists have announced. Experts had previously thought the clouds, which hang in a haze over the Indian Ocean, were actually deflecting sunlight and cooling the atmosphere.

The new findings will add weight to the argument that India and other developing nations need to use more renewable energy and find alternatives to wood-burning stoves. "All we are saying is that there is one other thing contributing to atmospheric warming and that is the brown cloud," said Veerabhadran Ramanathan, a senior scientist at the University of California who led the research into the clouds, which was carried out in the spring of 2006.

Mr Ramanathan and his colleagues dispatched unmanned measuring devices at different altitudes into the pollution, technically known as Atmospheric Brown Clouds, above the Maldives in the Indian Ocean. They were able to measure aerosol concentrations, soot levels and solar radiation and concluded that the pollution added to the heating of the atmosphere by about 50 per cent. They suggested that about half of recent global temperature increases could be the result of the pollution.

Mr Ramanathan, whose team's findings are contained in the current edition of Nature, said: "The conventional thinking is that brown clouds have masked as much as 50 per cent of the global warming by greenhouse gases through so-called global dimming. While this is true globally, this study reveals that over southern and eastern Asia, the soot particles in the brown clouds are intensifying the atmospheric warming trend caused by greenhouse gases by as much as 50 per cent."

In Asia there is widespread concern about the plight of Himalayan glaciers, some of which are retreating at a rate of 15 metres a year. Scientists fear that if they melt entirely, many rivers fed by the glaciers will disappear outside of the Monsoon season. Billions of people in Asia rely on water from rivers such as the Yangtze, the Ganges and the Indus.

Mr Ramanathan is working on a project in the foothills of the Himalayas to find alternatives to wood-burning stoves for more than 1,000 families. "If the pollution increases, the glacier retreat will be much worse than projected," he and his colleagues wrote. "It now depends on what energy path India, China and Asia take. The rapid melting of these glaciers, the third-largest ice mass on the planet, if it becomes widespread and continues for several more decades, will have unprecedented downstream effects on southern and eastern Asia."

Professor Syed Iqbal Hasnain, a glacier expert at the Centre For Policy Research in Delhi told the Associated Press that the brown clouds could be a factor in the melting of the glaciers. But he said more research was needed to understand the background to what was happening.

Achim Steiner, executive director of the United Nations Environment Programme that helped fund the project, said: "It is likely that in curbing greenhouse gases we can tackle the twin challenges of climate change and brown clouds and, in doing so, reap wider benefits, from reduced air pollution to improved agriculture yields."

(By Andrew Buncombe, The Independent, 03/08/2007)