Scientists have discovered that the global warming gas CO2 has an accomplice - clouds of soot in the atmosphere known as brown clouds. These clouds, which are over three kilometres thick, are produced by industry, traffic and fires and contain a mixture of carbon particles and oxides of nitrogen and sulphur. Scientists had thought that these clouds mainly cool the planet by reflecting sunlight back into space, perhaps mitigating the effects of the greenhouse effect by up to 50%, but their effect on the local environment wasn't known. To find out Vee Ramanathan, from the University of California at San Diego, sandwiched one of these clouds in southern India between three unmanned aircraft which were equipped with instruments to measure temperature and light intensity. The readings showed that the soot particles were capturing heat from the sun and transferring it to the atmosphere, warming things up locally. "We found that the brown cloud enhanced solar heating by around 50%" says Ramanathan. "And the effect is considerable; "the warming from brown clouds is about the same as the warming from recent rises in greenhouse gases." This means that the heating effect may be sufficient to explain the retreat of Himalayan glaciers in recent years. But even brown clouds have a silver lining it seems, because they last for only two weeks before dissipating. So if steps are taken to prevent their formation it might be possible to reduce the rate of Himalayan melting.