Key Concepts

• Many scientists now support serious research into “geoengineering,” deliberate actions taken to slow or reverse global warming.
• Of the various geoengi­neering proposals, the ones that shade the earth from the sun could bring about the most immediate effects. But all of them have drawbacks and side effects that probably cannot be anticipated.
• Pumping sulfur dioxide into the stratosphere, as volcanoes do, is the most well established way to block the sun. Other proposals call for brightening clouds over the oceans by lofting sea salt into the atmosphere and building a sunscreen in space.

When David W. Keith, a physicist and energy expert at the University of Calgary in Alberta, gives lectures these days on geoengineering, he likes to point out how old the idea is. People have been talking about deliberately altering climate to counter global warming, he says, for as long as they have been worrying about global warming itself. As early as 1965, when Al Gore was a freshman in college, a panel of distinguished environmental scientists warned President Lyndon B. Johnson that carbon dioxide (CO₂) emissions from fossil fuels might cause “marked changes in climate” that “could be deleterious.” Yet the scientists did not so much as mention the possibility of reducing emissions. Instead they considered one idea: “spreading very small reflective particles” over about five million square miles of ocean, so as to bounce about 1 percent more sunlight back to space—“a wacky geoengineering solution,” Keith says, “that doesn’t even work.”

In the decades since, geoengineering ideas never died, but they did get pushed to the fringe—they were widely perceived by scientists and environmentalists alike as silly and even immoral attempts to avoid addressing the root of the problem of global warming. Three recent developments have brought them back into the mainstream.

   
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