National Geographic : 2009 Aug
PHOTO: ALBERTO GARCIA, CORBIS THE BIG IDEA | GEOENGINEERING that way, because volcanoes already do it. After Mount Pinatubo erupted in the Philippines in 1991, launching ten million tons of sulfur into the stratosphere and spreading a sun-dimming haze around the planet, the average temperature dropped by about a degree Fahrenheit for a year. With carefully designed particles, geoengineers might make do with a fraction of that tonnage--- though because they fall out of the stratosphere, the particles would have to be delivered continu- ally, year after year. Still, says Caldeira, the sulfate scheme would be "essentially free compared with the other costs of mitigating climate change." Not so the idea suggested by Roger Angel, an eminent astronomer and telescope designer at the University of Arizona. Angel has proposed launching trillions of two-foot-wide, thinner-than- Kleenex disks of silicon nitride---each disk an autonomous robot weighing less than a gram--- into space between Earth and the sun, where they could deflect sunlight. By Angel's own reckoning, the scheme would take decades and cost trillions of dollars. With that much time and money, we could wean ourselves from fossil fuels and actually solve the climate problem---by far the better outcome, as Angel and most proponents of geoengineering would agree. Unfortunately, though the recession has temporarily slowed the rise in carbon dioxide emissions, we've made no real progress toward that goal. Some say we're running out of time. If we put up a sunshade without restraining emissions and the sunshade later fails, the climate accident would become a train wreck: The global warming we'd been masking would come rushing at us all at once. That might be the worst unintended consequence of geoengineer- ing, but there could be others---damage to the ozone layer, perhaps, or an increase in drought. If CO keeps rising, though, we may face greater emergencies. And what once seemed insane hubris just might become reality. ---Robert Kunzig Mount Pinatubo's cloud of ash and pumice was deadly, but the whole Earth was cooled by its sulfur. O Good or bad idea? Vote online at ngm.com/bigidea.