Using a trick of sunlight itself, tiny metallic disks could be levitated to the stratosphere where they would shade Earth's surface and counteract the effects of global warming, a new paper proposes. But even the scientist who dreamed up the idea says the little saucers should be used only as a last resort, if efforts to stem global warming by limiting the build-up of heat-trapping green house gases fails.The idea is a novel type of geoengineering—the concept of tinkering with the atmosphere to reduce the effects of global warming. Spelled out online today in the Proceedings of the National Academy of Sciences, it takes advantage of a natural phenomenon called photophoresis, in which the movement of a particle in a gas is affect by light shining on the particle and warming it. For example, suppose a disk-shaped particle is warmer than the surrounding air and its top consists of a different material than its bottom. Differences in the way the two materials react with light cause gas molecules to push the object upward. In nature, photophoresis makes particles like silicate dust migrate up and down in the atmosphere, and it explains how "solar mills" like the one in the illustration spin.Now, physicist David Keith of the University of Calgary in Canada proposes using the effect to control the climate. He envisions cranking out scads of 20-nanometer-wide nanodisks whose tops are made of aluminum and whose bottoms are made of barium titanate. Because the barium titanate more readily transmits heat and energy to impinging air molecules than the aluminum, the push on the bottom of the disk would be greater than on the top when it is warmed by sunlight. So that pressure would push it upward to a height of 40 to 50 kilometers, just out of the stratosphere. In addition, because barium titanate can be electrically polarized, electric fields in the atmosphere would stabilize the disks and keep them from flipping over. The particles would sink slowly during the night but rise during the day. And the aluminum tops, protected with a thin coating, would reflect sunlight back into space, cooling the planet. “I've invented a flying saucer,” Keith jokes.Previously, scientists have proposed a variety of other ways of cooling the planet using various techniques of blocking the sun. The most widely discussed is to mimic the cooling effect of volcanic eruptions of sulfur dioxide, a gas which in the atmosphere is converted into droplets of sulfuric acid, which scatter sunlight away from Earth's surface. But Keith says that the nanodisks technique might have fewer risks. For example, the sulfur dioxide particles can contribute to the destruction of the ozone layer. In contrast, the disks would block the sun above the stratosphere and avoid that danger.Keith emphasizes that the scheme should be used only in an emergency, as its possible side effects—sun-blocking can alter rain patterns, for example—could be worse than the effects of warming. In the meantime, he says, some laboratory testing—and with proper oversight, possible outdoor tests—could help spell out the dangers and costs.But volcano expert Alan Robock of Rutgers University in New Brunswick, New Jersey, says that even such testing is going too far. "It's irresponsible to advocate in situ atmospheric testing" before issues such as international governance or environmental risks are spelled out, he says. Ken Caldeira, a geochemist at the Carnegie Institution of Washington in Stanford, California, also urges caution. "I have no idea whether such particles could really be cost-effectively manufactured and deployed," he says. "This study illustrates how we are at the infancy of thinking about the ways in which we might seek to diminish the impacts of excess greenhouse gases in the atmosphere."
Keith himself says that it's crucial to cut emissions of greenhouse gases, such as carbon dioxide, which cause global warming in the first place. The little disks should be the last line of protection, he says. "Seat belts reduce the risk of being injured in accidents," he says "But having a seat belt doesn't mean you should drive drunk at 100 miles an hour."
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