The "brown cloud" is pollution from burning wood and fossil fuels

US researchers used unmanned aircraft to measure the effects of the "brown clouds" on the surrounding area.

Writing in Nature, they said the tiny particles increased the solar heating of the lower atmosphere by about 50%.

The warming could be enough to explain the retreat of glaciers in the Himalayas, the scientists proposed.

The clouds contain a mixture of light absorbing aerosols and light scattering aerosols, which cause the atmosphere to warm and the surface of the Earth to cool.

The main sources of the pollutants came from wood burning and fossil fuels, the team added.

The unmanned aircraft flew 18 missions through the "brown cloud"

Aerosols, also known as particulates, cool the land or sea below because they filter out light from the Sun.

While this process, known as "global dimming", is fairly well understood, the effect aerosols have on the surrounding atmosphere is still unclear.

The scientists, from the University of California San Diego and the Nasa Langley Research Center, said there remained a degree of uncertainty because, until now, estimates had largely been derived from computer models.

Solar heating

For their study, the team of researchers used three unmanned aircraft, fitted with miniaturised instruments that were able to measure aerosol concentrations, soot amounts and the flow of energy from the Sun.

The crafts flew over the polluted region of the Indian Ocean at varying heights between 500m (1,640ft) and 3,000m (9,840ft).

"During 18 flight missions, the three unmanned aerial vehicles were flown with a separation of tens of metres or less and less than 10 seconds (apart), which made it possible to measure the atmospheric solar heating rates directly," they wrote.

"We found that atmospheric brown clouds enhanced lower atmospheric solar heating by about 50%.

"[The pollution] contributes as much as the recent increase in anthropogenic greenhouse gases to regional lower atmospheric warming trends," they suggested.

"We propose that the combined warming trend of 0.25 Kelvin per decade may be sufficient to account for the observed retreat of the Himalayan glaciers."

Seasonal glacier and snow melt from the mountain range feeds rivers that supplies water to about 40% of the world's population.

The United Nations Environment Programme (Unep), in its latest Snow and Ice Outlook report, said the ice sheets in the region could retreat by up to 81% by the end of the century.