Shading the Earth
If we don’t cut fossil fuels fast enough, global warming may get out of hand. Some scientists say we need a plan B: a giant sunshade that would cool the whole planet.

Some call it hubris; others call it cool reason. But the idea that we might combat global warming by deliberately engineering a cooler climate—for instance, by constructing some kind of planetary sunshade—has lately migrated from the fringe to the scientific mainstream. We are already modify ing climate by accident, say proponents of geoengineering; why not do something intentional and intelligent to stop it? Hold on, say critics. Global warming shows we understand the Earth too little to engineer it without unintended and possibly disastrous consequences. Both sides worry that facts on the ground—rising seas melting ice, failing crops—may cut short the geoengineering debate. “If a country starts thinking it’s in their vital interests to do this, and they have the power, I find it hard to imagine them not doing it,” says Ken Caldeira, a climate expert at the Carnegie Institution.

Caldeira is talking about the easiest, cheapest form of geoengineering: building a sunshade in the stratosphere out of millions of tons of tiny reflective particles, such as sulfate. Planes, balloons, battleship guns pointed upward—there is no shortage of possible delivery vehicles. And there is little doubt you could cool Earth 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 continually, 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 cli mate 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 geoengineering, but there could be others—damage to the ozone layer, perhaps, or an increase in drought. If CO2 keeps rising, though, we may face greater emergencies. And what once seemed insane hubris just might become reality. —Robert Kunzig