If the worst global warming scenarios come true, we may have to turn to engineering to save the planet. Along those lines, one astronomer has come up with a radical plan to cool Earth: launch trillions of feather-light discs into space, where they would form a vast cloud that would block the sun's rays.
This isn't the first time a "sunshade" has been proposed. In 1989, James Early at the Lawrence Livermore National Laboratory suggested building a giant glass shield--2000 kilometers in diameter--that would be rocketed to the so-called inner Lagrange point, where objects stay fixed between the sun and Earth. The massive size of this object, however, would have entailed first colonizing the moon, then using materials found there to build the shield and then launch it.
Instead, University of Arizona Steward Observatory optics expert Roger Angel proposes using screens just 0.6 meters across, weighing about a gram each. These discs would be manufactured on Earth using very thin, transparent material that doesn't reflect the sun, but instead refracts it, so as to avoid having the sun's radiation push them out of orbit. The discs would also have three 0.1-meter-long protruding electronic "ears" with a solar power source so they could adjust their position, making them essentially tiny spacecraft.
A stack of 800,000 of these "flyers" would be loaded into a vehicle, sent up to the Lagrange point, then fed into space by a robotic arm. To cancel the warming from a doubling of CO2 levels, the shield would have to block 1.8% of the sun's heat input, which would require a cloud of flyers 100,000 kilometers long, Angel says. About 16 trillion flyers would have to be deployed, which could be done with 20 launchers that would each send up a stack every 5 minutes for 10 years. The cost, if done using electromagnetic propulsion, a technology in development that uses superconducting magnets rather than chemical fuel to jolt a rocket into space: a few trillion dollars over 25 years, he estimates in an analysis that appears online this week in the Proceedings of the National Academy of Sciences.
"It's more elegant than previous versions," say theoretical physicist and futurist Freeman Dyson of the Institute for Advanced Studies in Princeton, New Jersey. Others are skeptical of sunblocking solutions in general, include those involving spewing sulfur aerosols into the atmosphere (Science, 20 October, p. 401). One worry is that such schemes may make people complacent about the need to curb greenhouse gas emissions. Angel disagrees: "This is in no sense a way to say we should continue with business as usual," he says. "But if there's nothing you can do except shade the planet, you may want to think about it."