The Big Idea
Published: July 15, 2009
Shading Earth
Art: Bryan Christie Design. Not to scale
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 modifying 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.”

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