Drone aircraft flying in loose formation through the brown cloud that forms over the Indian subcontinent have revealed that the nearly two-mile thick haze exacerbates atmospheric warming by 50 percent. This means the soot in the cloud more than cancels out any cooling from its fellow aerosols—and causes as much warming as carbon dioxide (CO2), the leading greenhouse gas.
Atmospheric physicist Veerabhadran Ramanathan of the Scripps Institution of Oceanography at the University of San Diego and his colleagues flew 18 missions with three unmanned aircraft from a base on the island of Hanimaadhoo in the Indian Ocean. Launched via radio control (model plane enthusiasts take note), the aircraft (each weighing more than 60 pounds and with wingspans of nearly six feet) followed programmed routes.
They flew in vertical formation: one drone directly above the brown cloud, one below and one in the thick of it. The plane above measured how much solar radiation bounces back while the one below measured the amount that penetrated. "The difference between the two tells how much sunlight is getting trapped in the atmosphere," Ramanathan says. The craft inside the cloud measured how water vapor and other cloud components were affected. But the key is, he says, "they all had to sample the same column of real estate."
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The researchers found that "this brown cloud was as much as 50 percent of the background heating," he says. "We are sometimes thinking that aerosols might save us from global warming" by reflecting sunlight and dimming Earth below, but "our study shows us not necessarily. While they may be cooling the globe, they are causing mischief in other places."
The primary culprit seems to be the black carbon in soot, which soaks up any sunlight it can, thereby warming whatever it touches. And the dominant source for all this black carbon is cooking fires, Ramanathan says. All these cooking fires are, in effect, drying the region, both by contributing to the melting of glaciers that feed Asia's major rivers as well as by decreasing the evaporation that drives rainfall. Aerosols across the board, from black carbon to sulfates, appear to be increasing across Asia as it industrializes.
But the problem can be solved by swapping other fuels and methods for the wood in cooking fires. "The aerosol lifetime is two weeks," Ramanathan says. "If the world pays attention and puts resources to it, we will see an effect immediately. I'm talking weeks, at most a few months, not decades or centuries."
That contrasts with solutions for CO2 emissions, which will require much longer periods to show effects. Because the brown cloud appears to be at least as important, eliminating it could buy time to implement more far-reaching solutions before catastrophic glacial melt and other climate change impacts occur, Ramanathan argues.
Ramanathan and colleagues plan to demonstrate this on a small scale over the next few years in the Himalayas, over a 12-square-mile area in the foothills. "We want to create a black carbon hole," he says.
In addition, the unmanned drones provide a new and relatively cheap method for measuring such atmospheric effects, such as powerful storms or the absorption of sunlight in regular clouds. "To address that you need many aircraft to look at the cloud from all sides," he notes—and only unmanned, autonomously navigated drones will do.
Previous efforts, such as the one that identified the Asian brown cloud in the 1990s, cost millions of dollars to keep conventional aircraft airborne and could only capture data over relatively short time spans. "It's going to be a revolution," he says, "in the way we can look at the planet."