Sun Screen: Stationed between the Earth and sun at a point where the gravitational forces nearly cancel each other out, a 600,000-square-mile space "mirror" scatters sunlight with a mesh woven of fine metal wires. Photo by John MacNeill

David Keith never expected to get a summons from the White House. But in September 2001, officials with the President's Climate Change Technology Program invited him and more than two dozen other scientists to participate in a roundtable discussion called "Response Options to Rapid or Severe Climate Change." While administration officials were insisting in public that there was no firm proof that the planet was warming, they were quietly exploring potential ways to turn down the heat.

Most of the world's industrialized nations had already vowed to combat global warming by reining in their emissions of carbon dioxide, the chief "greenhouse gas" blamed for trapping heat in Earth's atmosphere. But in March 2001 President George W. Bush had withdrawn U.S. support for the Kyoto Protocol, the international treaty mandating limits on CO₂ emissions, and asked his administration to begin studying other options.

Keith, a physicist and economist in the chemical and petroleum engineering department at the University of Calgary, had for more than a decade been investigating strategies to curtail global warming. He and the other scientists at the meeting-including physicists from Lawrence Livermore National Laboratory who had spent a chunk of their careers designing nuclear weapons-had come up with some ideas for "geoengineering" Earth´s climate. What they proposed was tinkering on a global scale. “We already are inadvertently changing the climate, so why not advertently try to counterbalance it?” asks retired Lawrence Livermore physicist Michael MacCracken, a former senior scientist at the U.S. Global Change Research Program who helped organize the meeting.

“If they had broadcast that meeting live to people in Europe, there would have been riots,” Keith says. “Here were the bomb guys from Livermore talking about stuff that strikes most greens as being completely wrong and off-the-wall.” But today, a growing number of physicists, oceanographers and climatologists around the world are seriously considering technologies for the deliberate manipulation of Earth´s climate. Some advocate planetary air-conditioning devices such as orbiting space mirrors that deflect sunlight away from Earth, or ships that intensify cloud cover to block the sun´s rays. Others are suggesting that we capture carbon dioxide-from the air, from cars and power plants-and stash it underground or react it with chemicals that turn it to stone.