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 "If we continue to use outdated technology to achieve industrialisation, this is only going to get worse," said Professor Ramanathan.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.
