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.”