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.

Keith argues that such a project, usually known as solar geoengineering, is technologically feasible and — with a back-of-the-envelope cost of under $1 billion annually — ought to be fairly cheap from a cost-benefit perspective, considering the economic damages potentially forestalled: It might do good for a world unable to cut carbon-dioxide emissions enough to prevent further temperature increases later this century.

What surprised me, then, as Keith paced around his Harvard office one morning in early March, was his listing all the reasons humans might not want to hack the environment. “Actually, I’m writing a paper on this right now,” he said. Most of his thoughts were related to the possible dangers of trying to engineer our way out of a climate problem of nearly unimaginable scientific, political and moral complexity. Solar geoengineering might lead to what some economists call “lock-in,” referring to the momentum that a new technology, even one with serious flaws, can assume after it gains a foothold in the market. The qwerty keyboard is one commonly cited example; the internal combustion engine is another. Once we start putting sulfate particles in the atmosphere, he mused, would we really be able to stop?

Another concern, he said, is “just the ethics about messing with nature.” Tall, wiry and kinetic, with thinning hair and a thick beard that gives him the look of the backcountry skier he is, Keith proudly showed me the framed badge that his father, a biologist, wore when he attended the landmark United Nations Conference on the Human Environment in Stockholm in 1972. Now 53, Keith has taken more wilderness trips — hiking, rock climbing, canoeing — than he can properly recall, and for their recent honeymoon, he and his wife were dropped off by helicopter 60 miles from the nearest road in northern British Columbia. “It was quite rainy,” he told me, “and that ended up making it even better.” So the prospect of intentionally changing the climate, he confessed, is not just unpleasant — “it initially struck me as nuts.”