The combined heating effect of greenhouse gases and the brown clouds is necessary and sufficient to account for the retreat of Himalayan glaciers observed over the past 50 years, the researchers conclude.
Led by Scripps atmospheric chemistry professor Veerabhadran Ramanathan, the team describes their findings in a paper to be published in the August 2 edition of the journal 'Nature.'
Not entirely made up of water vapor like regular clouds, brown clouds contain soot, sulfates, nitrates, hundreds of organic compounds, and fly ash from urban, industrial and agricultural sources.
'The conventional thinking is that brown clouds have masked as much as 50 percent of the global warming by greenhouse gases through the so-called global dimming,' said Dr. Ramanathan.
'While this is true globally,' he said, '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 percent.'
The Himalayan glaciers feed the major Asian rivers - the Yangtze, the Ganges and the Indus - that supply water to billions of people in China, India and across southeast Asia.
'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,' the authors warn.
'Ramanathan and colleagues, for the first time ever, used small and inexpensive unmanned aircraft and their miniaturized instruments as a creative means of simultaneously sampling of clouds, aerosols and radiative fluxes in polluted environments, from within and from all sides of the clouds,' said Jay Fein, program director in the National Science Foundation's Division of Atmospheric Sciences.
The aircraft, flying in stacked formations over the Maldives, an island nation in the Indian Ocean, measured the brown clouds from different altitudes, creating a profile of soot concentrations and light absorption unprecedented in its level of vertical detail.
The flights took place in March 2006 during the region's dry season when air masses, loaded with industrial and vehicle emissions and pollution from biomass burning, travel south from the continent to the Indian Ocean.
'These measurements, combined with routine environmental observations and a state-of-the science model, led to these remarkable results,' said Fein.
When the researchers fed both greenhouse gas and brown cloud data into computer climate models, they found that the region's atmosphere has warmed 0.25 degrees Celsius (0.5 degrees F) per decade since 1950 at altitudes ranging from two to five kilometers (6,500 to 16,500 feet) above sea level - the same altitude where the Himalayan glaciers lie.
The analysis showed that the brown cloud effect is necessary to explain temperature changes that have been observed in the region over the last 50 years.
It also indicates that south Asia's warming trend is more pronounced at higher altitudes than closer to sea level.
Achim Steiner, executive director of the UN Environment Programme which helped support the research, said, 'The main cause of climate change is the buildup of greenhouse gases from the burning of fossil fuels. But brown clouds, whose environmental and economic impacts are beginning to be unraveled by scientists, are complicating and in some cases aggravating their effects.'
Steiner hopes the Scripps' research will spur the international community to take urgent action to limit global warming, in particular at the next crucial UN climate change convention in Indonesia this December. This conference is expected to negotiate a global successor agreement to the Kyoto Protocol, which expires in 2012.
By burning less fossil fuels, South Asia may be able to arrest the glaciers' retreat and reduce regional air pollution at the same time.
Steiner 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 agricultural yields.'