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Photographer: David Harlow/US Geological Survey/The LIFE Picture Collection/Getty Images Photographer: David Harlow/US Geological Survey/The LIFE Picture Collection/Getty Images

There’s a cheap, quick, dirty, and controversial way to combat global warming that isn’t on the agenda of the United Nations climate summit in Paris, which runs from Nov. 30 to Dec. 11. It involves replicating the planet-cooling effect of a volcanic eruption. When Mount Pinatubo in the Philippines blew in 1991, its emissions briefly reversed most of the global warming that had occurred since the start of the Industrial Revolution. The idea is to mimic Pinatubo by using a fleet of modified business jets to inject fine droplets of sulfuric acid into the stratosphere, where they would combine with water vapor to form fine sulfate particles that reflect sunlight away from the earth.

Scientists estimate that a few grams of sulfate would be enough to counteract the warming effect of a ton of carbon dioxide. The cost of this planetary protection? Perhaps 0.01 percent of the annual world gross domestic product. In other words, almost nothing. The cost of stopping the entire planet from warming would be not much more per decade than the $6 billion the Italian government is spending to protect one city, Venice, from rising sea levels. That’s the calculation of a leading figure in the debate over so-called geoengineering, David Keith, a professor of applied physics at Harvard and of public policy at Harvard’s Kennedy School of Government.

Featured in Bloomberg Businessweek, Nov. 30, 2015. Subscribe now. Illustration: Justin Metz; Source: Getty Images (2)

Naturally, there’s a catch. Several, in fact. The sun shield would merely mask the rising concentration of greenhouse gases, like perfuming a skunk. It adds one pollutant to counteract another. It could reverse progress toward closing the hole in the ozone layer by ripping apart ozone molecules. Sulfate particles falling from the sky could cause air pollution deaths. It would leave fertile coral reefs exposed to deadly bleaching because it wouldn’t do anything about ocean acidification. It could even become a cause for war if one country decided it was harmed by another’s climate meddling. Even Keith allows that it’s a “brutally ugly technical fix.”

Critics’ biggest worry: It would be perceived as a get-out-of-jail-free card for the planet. If pausing global warming is as easy as sending a fleet of modified Gulfstream G650s into the stratosphere with payloads of sulfuric acid, the weak pressure to cut back on emissions of greenhouse gases might get even weaker. That’s why you won’t find it mentioned on the agenda of the Paris summit, a major event that’s expected to draw 45,000 people from more than 190 countries.

This leaves humanity in a strange place. An effective but flawed technique for stopping global warming is shunted aside while negotiators try to fix the problem the right way, through cutting emissions. In Paris, China and India will point fingers at the wealthier nations, which will point right back. Meanwhile, temperatures keep going up. As the Parisian philosopher Voltaire might have reminded the UN: The perfect is the enemy of the good.

Every few years, geoengineering, the umbrella term for ideas such as volcano replication, gets rediscovered—and promptly beaten back down. Edward Teller, the father of the hydrogen bomb and an inspiration for Stanley Kubrick’s Dr. Strangelove, was an early enthusiast of what backers ambitiously call solar radiation management. That pedigree hasn’t exactly won over environmentalists. Steven Levitt and Stephen Dubner were castigated for hyping it in their 2009 best-seller, SuperFreakonomics. Advocates have been accused of playing God, committing chemical terrorism, weaponizing weather, even risking the collapse of civilization. Climate scientist Raymond Pierrehumbert of the University of Chicago calls the acid-spraying idea “barking mad.”

Keith has actually mapped out how it could be done. And unlike those who consider it a break-glass-in-an-emergency option, he suggests going ahead with it soon as a complement to—not a substitute for—emissions reductions and other mainstream measures. He says his plan could be rolled out gradually, as soon as 2020 if authorities approved tests now (which they won’t) and if the tests were successful (which he thinks they would be).

Keith isn’t committed to the sulfuric acid gambit. Sprinkling tiny particles of diamond or aluminum oxide into the stratosphere could also work, and would be only slightly more expensive and probably avoid health effects and damage to the ozone layer, according to an article he and other Harvard scientists published in the journal Atmospheric Chemistry and Physics last month. The trade-off is that less is known about that approach. Sulfuric acid is “the devil we know,” Keith said in a Nov. 22 interview.

The idea is to mimic the effects of a volcanic eruption by spraying sulfuric acid into the stratosphere

The fight over geoengineering is as much a clash of worldviews as it is an argument over the intricacies of solar radiation management. The Right considers global warming a nonproblem, whereas the Left condemns quick fixes. Keith, a self-described “oddball environmentalist,” is trying to navigate between the two sides. The thin, bearded 52-year-old is a lifelong tinkerer. He taught himself oxyacetylene welding to rebuild the rusted frame of his first car, as he recalls in his 2013 book, A Case for Climate Engineering. He’s taken long solo ski trips in the high Arctic, he writes, and has a freezer full of last year’s mule deer.

Rutgers University climatologist Alan Robock is highly skeptical of what he sees as tinkering on a planetary scale. Robock, 66, is a onetime Peace Corps volunteer whose website features pictures of himself braving the cold in Antarctica and posing with Fidel Castro in Cuba. The late Edward Lorenz, a pioneer of chaos theory, was Robock’s adviser on his Ph.D. thesis in meteorology at MIT. Robock’s list of 26 downsides to geoengineering ranges from the vital (“whose hand is on the thermostat?”) to eye of the beholder (“affect stargazing”).

Strangely, Keith and Robock wrote a paper together last year with other authors and agree on much of the basic science. Where they disagree is on how to weigh costs and benefits. Without singling out Robock, Keith says many scientists are exaggerating the risks because they don’t trust the world’s governments to handle such a powerful instrument. “I share their concern, but I believe in democracy,” Keith says. “I don’t believe that secret scientific societies should make the decisions.”

Several major environmental groups have given at least guarded support to research on geoengineering, including the Environmental Defense Fund and the Natural Resources Defense Council. The small Arctic Methane Emergency Group has gone as far as to support immediate deployment to “refreeze the Arctic.” Actual funding has been scarce, though. One of the few sources of money is Bill Gates, who has personally given a few million dollars for work by Keith and Kenneth Caldeira, an atmospheric scientist at the Carnegie Institution for Science.

The U.S. government has no organized research program, although that could change as global warming gets worse. Early this year the National Research Council issued a report saying that even though attempts to enhance earth’s albedo, or reflectivity, “pose considerable risks and should not be deployed at this time,” it advocated research “that furthers basic understanding of the climate system and its human dimensions.”

The planning needs to cover ideas for how to ensure that a rogue nation or even a company doesn’t take matters into its own hands. In his new book, The Planet Remade, Economist editor Oliver Morton imagines just such a scenario in which a cooling operation is launched in secret by a conspiracy of small nations.

If India suffers famine because a heat wave is killing its crops, the only option for rapid temperature relief would be some type of sun shield, Caldeira says. Even a smallish country could undertake the fix unilaterally. “We can hope they won’t be tempted, but if they are, it’s important to have the research done now so someone won’t accidentally destroy the ozone layer or shift rainfall patterns,” he says.

“At this point, no international organization has a direct mandate to address the full spectrum of possible geoengineering activities,” the Congressional Research Service said in a 2013 report. A 2013 article in Issues in Science & Technology advocated “an independent, broadly based advisory group” on research whose purview would include ethical concerns.

For now the climate talkers in Paris have other priorities. “To the first order of approximation, the world has done nothing for climate change, nothing. And it’s very hard to see very much happening going forward,” says Nathan Myhrvold, co-founder and chief executive officer of Intellectual Ventures, the technology research and patent company where Lowell Wood, another geoengineering theorist, is an inventor-in-residence. “So what’s Plan B? Now, maybe Plan A will get its act together tomorrow, but we think, hey, we should think about Plan B.”

Caldeira also wants a good Plan B. (His last name is Portuguese for “boiler,” not that that matters.) Says Caldeira: “Criticism is of the form of, ‘Oh, it’s not perfect.’ That’s not the question that policymakers will be faced with. It will be, ‘Will it help? ’ ” In Hack the Planet, his 2010 book, science writer Eli Kintisch called geoengineering “a bad idea whose time has come.”

—With Ashlee Vance