For the past few years, the Harvard professor David Keith has been sketching this vision: Ten Gulfstream jets, outfitted with special engines that allow them to fly safely around the stratosphere at an altitude of 70,000 feet, take off from a runway near the Equator. Their cargo includes thousands of pounds of a chemical compound — liquid sulfur, let’s suppose — that can be sprayed as a gas from the aircraft. It is not a one-time event; the flights take place throughout the year, dispersing a load that amounts to 25,000 tons. If things go right, the gas converts to an aerosol of particles that remain aloft and scatter sunlight for two years. The payoff? A slowing of the earth’s warming — for as long as the Gulfstream flights continue.