Experimental physicist David Keith has a plan: dump hundreds of thousands of tons of atomized sulfuric acid into the upper atmosphere; watch the acid particles reflect additional sunlight; wait for global temperature to drop. Many of Keith’s peers think this geoengineering scheme is crazy, least of which are the possible unknown and unmeasured side-effects, but this hasn’t stopped the healthy debate. One thing is becoming increasingly clear — humans need to take collective action.

[div class=attrib]From Technology Review:[end-div]

Here is the plan. Customize several Gulfstream business jets with military engines and with equipment to produce and disperse fine droplets of sulfuric acid. Fly the jets up around 20 kilometers—significantly higher than the cruising altitude for a commercial jetliner but still well within their range. At that altitude in the tropics, the aircraft are in the lower stratosphere. The planes spray the sulfuric acid, carefully controlling the rate of its release. The sulfur combines with water vapor to form sulfate aerosols, fine particles less than a micrometer in diameter. These get swept upward by natural wind patterns and are dispersed over the globe, including the poles. Once spread across the stratosphere, the aerosols will reflect about 1 percent of the sunlight hitting Earth back into space. Increasing what scientists call the planet’s albedo, or reflective power, will partially offset the warming effects caused by rising levels of greenhouse gases.

The author of this so-called geoengineering scheme, David Keith, doesn’t want to implement it anytime soon, if ever. Much more research is needed to determine whether injecting sulfur into the stratosphere would have dangerous consequences such as disrupting precipitation patterns or further eating away the ozone layer that protects us from damaging ultraviolet radiation. Even thornier, in some ways, are the ethical and governance issues that surround geoengineering—questions about who should be allowed to do what and when. Still, Keith, a professor of applied physics at Harvard University and a leading expert on energy technology, has done enough analysis to suspect it could be a cheap and easy way to head off some of the worst effects of climate change.

According to Keith’s calculations, if operations were begun in 2020, it would take 25,000 metric tons of sulfuric acid to cut global warming in half after one year. Once under way, the injection of sulfuric acid would proceed continuously. By 2040, 11 or so jets delivering roughly 250,000 metric tons of it each year, at an annual cost of $700 million, would be required to compensate for the increased warming caused by rising levels of carbon dioxide. By 2070, he estimates, the program would need to be injecting a bit more than a million tons per year using a fleet of a hundred aircraft.

One of the startling things about Keith’s proposal is just how little sulfur would be required. A few grams of it in the stratosphere will offset the warming caused by a ton of carbon dioxide, according to his estimate. And even the amount that would be needed by 2070 is dwarfed by the roughly 50 million metric tons of sulfur emitted by the burning of fossil fuels every year. Most of that pollution stays in the lower atmosphere, and the sulfur molecules are washed out in a matter of days. In contrast, sulfate particles remain in the stratosphere for a few years, making them more effective at reflecting sunlight.

The idea of using sulfate aerosols to offset climate warming is not new. Crude versions of the concept have been around at least since a Russian climate scientist named Mikhail Budkyo proposed the idea in the mid-1970s, and more refined descriptions of how it might work have been discussed for decades. These days the idea of using sulfur particles to counteract warming—often known as solar radiation management, or SRM—is the subject of hundreds of papers in academic journals by scientists who use computer models to try to predict its consequences.

But Keith, who has published on geoengineering since the early 1990s, has emerged as a leading figure in the field because of his aggressive public advocacy for more research on the technology—and his willingness to talk unflinchingly about how it might work. Add to that his impeccable academic credentials—last year Harvard lured him away from the University of Calgary with a joint appointment in the school of engineering and the Kennedy School of Government—and Keith is one of the world’s most influential voices on solar geoengineering. He is one of the few who have done detailed engineering studies and logistical calculations on just how SRM might be carried out. And if he and his collaborator James ­Anderson, a prominent atmospheric chemist at Harvard, gain public funding, they plan to conduct some of the first field experiments to assess the risks of the technique.

Leaning forward from the edge of his chair in a small, sparse Harvard office on an unusually warm day this winter, he explains his urgency. Whether or not greenhouse-gas emissions are cut sharply—and there is little evidence that such reductions are coming—”there is a realistic chance that [solar geoengineering] technologies could actually reduce climate risk significantly, and we would be negligent if we didn’t look at that,” he says. “I’m not saying it will work, and I’m not saying we should do it.” But “it would be reckless not to begin serious research on it,” he adds. “The sooner we find out whether it works or not, the better.”

The overriding reason why Keith and other scientists are exploring solar geoengineering is simple and well documented, though often overlooked: the warming caused by atmospheric carbon dioxide buildup is for all practical purposes irreversible, because the climate change is directly related to the total cumulative emissions. Even if we halt carbon dioxide emissions entirely, the elevated concentrations of the gas in the atmosphere will persist for decades. And according to recent studies, the warming itself will continue largely unabated for at least 1,000 years. If we find in, say, 2030 or 2040 that climate change has become intolerable, cutting emissions alone won’t solve the problem.

“That’s the key insight,” says Keith. While he strongly supports cutting carbon dioxide emissions as rapidly as possible, he says that if the climate “dice” roll against us, that won’t be enough: “The only thing that we think might actually help [reverse the warming] in our lifetime is in fact geoengineering.”

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