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The stratosphere might be thought of as earth’s second balcony. It sits above the troposphere, which is where clouds billow, trade winds blow, and hurricanes rage, and beneath the mesosphere, which is where meteors go to vaporize. The height of the stratosphere varies according to the season and the location; very roughly speaking, at the equator, the bottom of the stratosphere sits about eleven miles above the surface of the earth, and at the poles it sits much lower—about six miles above the surface. From a geoengineering point of view, what’s key about the stratosphere is that it’s stable—much more stable than the troposphere—and also reasonably accessible. Commercial jets often fly in the lower stratosphere, to avoid turbulence, and spy planes fly toward the middle, to avoid surface-to-air missiles. Materials injected into the stratosphere in the tropics will tend to drift toward the poles and then, after a few years, drop back to earth.
Since the point of solar geoengineering is to reduce the amount of energy reaching the earth, any sort of reflective particle, in principle at least, would do. “The best possible material probably is diamond,” Keutsch told me. “Diamonds really will not absorb any energy. So this would minimize the change in stratospheric dynamics. And diamond itself is extremely unreactive. The idea that this is expensive—I don’t care about that. If we had to engineer this on a big scale because it solves a big problem, we would figure out a way to do it.” Shooting tiny diamonds into the stratosphere struck me as magical, like sprinkling the world with pixie dust.
“But one of the things to think about is that all the material comes back down,” Keutsch continued. “Does that mean that people are inhaling these little diamond particles? It’s very likely that the amount would be so small it wouldn’t be a problem. But, somehow, I really don’t like that idea.”
Another option is to go full-on volcano and spray sulfur dioxide. Here, too, there are downsides. Loading the stratosphere with sulfur dioxide would contribute to acid rain. More significantly, it could damage the ozone layer. Following the eruption of Mount Pinatubo, in the Philippines, in 1991, there was a brief downturn in global temperatures of about 1°F. In the tropics, ozone levels in the lower stratosphere fell by as much as a third.
“Perhaps this is not a good phrase, but it’s the devil that we know,” Keutsch said.
Of all the substances that might be deployed, Keutsch was most enthusiastic about calcium carbonate. In one form or another, calcium carbonate turns up everywhere—in coral reefs, in the pores of basalt, in the ooze at the bottom of the ocean. It’s the main component of limestone, which is one of the world’s most common sedimentary rocks.
“There are vast amounts of limestone dust blowing around in the troposphere, where we live,” Keutsch observed. “So that makes it attractive.
“It has near-ideal optical properties,” he went on. “It dissolves in acid. So I can say with certainty that it will not have the same ozone-depleting impact that sulfuric acid has.”
Mathematical modeling has confirmed the mineral’s advantages, Keutsch told me. But until someone actually throws calcium carbonate into the stratosphere, it’s hard to know how much to trust the models. “There’s no other way around it,” he said.
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The first government report on global warming—though the phenomenon was not yet called “global warming”—was delivered to President Lyndon Johnson in 1965. “Man is unwittingly conducting a vast geophysical experiment,” it asserted. The result of burning fossil fuels would, almost certainly, be “significant changes in the temperature,” which would, in turn, lead to other changes.
“The melting of the Antarctic ice cap would raise sea level by four hundred feet,” the report noted. Even if the process took a thousand years to play out, the oceans would “rise about four feet every ten years,” or “forty feet per century.”
Carbon emissions in the 1960s were growing fast—by about five percent a year. And yet the report made no mention of reversing or even just trying to slow this growth. Instead, it advised that “the possibilities of deliberately bringing about countervailing climatic changes…be thoroughly explored.” One such possibility was “spreading very small reflecting particles over large oceanic areas.
“Rough estimates indicate that enough particles to cover a square mile could be produced for perhaps one hundred dollars,” the report stated. “Thus a one percent change in reflectivity might be brought about for about five hundred million dollars a year”—roughly $4 billion a year in today’s money. Considering “the extraordinary economic and human importance of climate, costs of this magnitude do not seem excessive,” the report concluded.
None of the authors of the report is still alive, so it’s impossible to know why the committee jumped straight to a multimillion-dollar dump of reflective particles. Perhaps it was just the zeitgeist. In the 1960s, climate- and weather-control proposals were all the rage, both in the United States and the USSR. Project Stormfury, a collaboration between the U.S. Navy and the Weather Bureau, targeted hurricanes. These, it was believed, could be weakened by sending aircraft to seed the clouds around the eyewall with silver iodide. Operation Popeye, a secret weather-modification scheme run by the Air Force during the Vietnam War, was supposed to increase rainfall over the Ho Chi Minh Trail, once again by seeding clouds with silver iodide. An astonishing twenty-six hundred seeding sorties were flown by the 54th Weather Reconnaissance Squadron before Popeye was exposed in The Washington Post and shut down. (A related program—Operation Commando Lava—involved dumping a mix of chemicals on the trail in an effort to destabilize the soil.) Other climate-modification plans pursued at government expense aimed at reducing lighting strikes and suppressing hail.
The Soviets’ schemes were, depending on your perspective, even more farsighted or more off-the-wall. In a book titled Can Man Change the Climate? an engineer named Petr Borisov suggested melting the Arctic ice cap with a dam across the Bering Strait. Hundreds of cubic miles’ worth of cold water could then, somehow or other, be pumped from the Arctic Ocean into the Bering Sea, which would draw in warmer water from the North Atlantic and, according to Borisov’s calculations, produce milder winters not just in the polar regions but also in the mid-latitudes.
A rendering of the proposed dam across the Bering Strait
“What mankind needs is a war against cold, rather than a ‘cold war,’ ” Borisov declared.
Another Soviet scientist, Mikhail Gorodsky, recommended creating a washer-shaped band of potassium particles around the earth, something like the rings of Saturn. The band would be positioned to reflect sunlight in summer. Gorodsky believed this arrangement would result in much warmer winters in the far north and also lead to a thawing of the world’s permafrost, a development that he welcomed. Man Versus Climate, a survey of this and other Soviet proposals translated into English by a Moscow-based outfit called Peace Publishers, ended with the declaration:
New projects for transforming nature will be put forward every year. They will be more magnificent and more exciting, for human imagination, like human knowledge, knows no bounds.
In the 1970s, climate engineering fell out of favor. Once again, it’s hard to say exactly why. Public concern about the environment probably had something to do with it, as did the growing scientific consensus that cloud-seeding was a bust. Meanwhile, more and more reports were appearing, in both English and Russian, warning that humans were already modifying the climate, and on a massive scale.
In 1974, Mikhail Budyko, a prominent scientist at the Leningrad Geophysical Observatory, published a book titled Climatic Changes. Budyko laid out the dangers posed by rising CO2 levels but argued that their continued climb was inevitable: The only way to hold down emissions was to cut fossil-fuel use, and no nation was likely to do that.
Following this logic, Budyko arrived at the idea of “artific
ial volcanoes.” Sulfur dioxide might be injected into the stratosphere using planes or “rockets and different types of missiles.” Budyko wasn’t intent on improving on nature, in the fashion of Project Stormfury or damming the Bering Strait. Rather, he was thinking along more revanchist lines, as in the dictum from The Leopard: “If we want everything to remain as it is, everything must change.”
“In the near future, climate modification will become necessary in order to maintain current climatic conditions,” Budyko wrote.
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David Keith, a professor of applied physics at Harvard, has been described as “perhaps the foremost proponent of geoengineering,” a characterization that he bristles at. “I’m a proponent of reality,” he wrote in a letter to the editor of The New York Times in 2015. Keith founded the university’s Solar Geoengineering Research Program in 2017, and he regularly receives hate mail. Twice he’s gotten death threats worrisome enough to report to the police. His office is just down the hall from Keutsch’s, in a building known as the Link.
“Solar geoengineering is not a thing you can study in the abstract,” he told me when I went to speak to him a few days after I’d visited Keutsch. “It depends on human choices about how we use it. So whenever anybody makes a statement that solar geoengineering will imperil millions or save the world or whatever, you should always ask, ‘What solar geoengineering? Done what way?’ ”
Keith is tall and angular, with a Lincoln-esque beard. An avid mountaineer, he describes himself as a “tinkerer,” a “technophile,” and “an oddball environmentalist.” He grew up in Canada and for about a decade taught at the University of Calgary. While he was working there, he founded a company, Carbon Engineering, which competes with Climeworks on direct air capture. (Carbon Engineering has a pilot plant in British Columbia that I once visited; it has a spectacular view of Mount Garibaldi, a dormant volcano that rises to a height of nine thousand feet.) Nowadays, he splits his time between Cambridge and Canmore, a town in the Canadian Rockies.
Keith believes that the world will eventually cut its carbon emissions if not all the way down to zero, then close to it. He also believes carbon-removal technologies can eventually be scaled up to take care of the rest. But all this—quite possibly—will not be enough. During the period of “overshoot,” a great many people will suffer and changes that are, for all intents and purposes, irreversible may occur, like the demise of the Great Barrier Reef.
The best way forward, he argues, is to do everything: cut emissions, work on carbon removal, and look a lot more seriously at geoengineering. On the basis of computer modeling, he’s proposed that the safest option would be to put up enough aerosols to cut warming in half, rather than to counteract it entirely—what might be called “semi-engineering.”
“If you did not try to restore temperatures to pre-industrial levels, then the evidence from, really, all climate models is that most of the big climate hazards that people know about—extreme precipitation, extreme temperatures, changes in water availability, sea-level rise—are reduced,” he told me. This is true, he said, “basically everywhere, in the sense that there are no obvious regions that are made worse off. That result, I think, is really stunning.”
I asked Keith about what is sometimes called the “moral hazard” problem. If people think geoengineering is going to avert the worst effects of climate change, won’t that reduce their motivation to cut emissions? He agreed this was a worry. But he said the opposite was also possible: “opening up the range of options” could inspire greater action.
Solar geoengineering could potentially be used to “cut the top off” the risks of climate change.
“Moving away from the kind of monomania that says, ‘The only thing we can do is cut emissions,’ or the more narrow version, which says, ‘The only thing we can do is renewables,’ I think may actually secure broader political agreement to deal with the problem. People might be more willing to spend the big money to cut emissions as part of a project that, overall, wasn’t going to just limit the damage but was actually going to make the world better.”
I suggested humans didn’t have a very good track record when it came to the sort of intervention he was studying. Though importing poisonous amphibians hardly compares to blocking the sun, I cited the example of cane toads.
Keith suggested I was revealing my own biases: “To people who say most of our technological fixes go wrong, I say, ‘Okay, did agriculture go wrong?’ It’s certainly true that agriculture had all sorts of very unexpected outcomes.
“People think of all the bad examples of environmental modification,” he went on. “They forget all the ones that are more or less working. There’s a weed, tamarisk, originally from Egypt. It’s spread all around the desert Southwest and has been very destructive. After a bunch of trials, they imported some bug that eats the tamarisk, and apparently it’s kind of working.
“To be clear, I’m not saying that modifications mostly do work. I’m saying it’s a wide, undefined set.”
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Geoengineering is not something you can do with a mail-order kit in your kitchen. Still, as world-altering projects go, it looks to be surprisingly easy. The best method for delivering aerosols would probably be via airplane. The plane would need to be capable of reaching an altitude of around sixty thousand feet and of carrying a payload on the order of twenty tons. Researchers who looked into the configuration of such a craft, which they dubbed a Stratospheric Aerosol Injection Lofter, or SAIL, concluded that development costs would run to about $2.5 billion. This may sound like a lot of money, but it’s only about a tenth of what Airbus spent to develop its “superjumbo” A380, a plane it stopped producing after a dozen years. To deploy a fleet of SAILs would cost another $20 billion or so per decade. Again, this is nothing to sneeze at, but the world now spends more than three hundred times that amount every year on fossil-fuel subsidies.
“Dozens of countries would have both the expertise and the money to launch such a program,” the researchers—Wake Smith, a lecturer at Yale, and Gernot Wagner, a professor at NYU—observed.
Solar geoengineering would not just be cheap, relatively speaking; it would also be speedy. Pretty much as soon as the fleet of SAILs went into operation, cooling would begin. (A year and a half after Tambora erupted, the cucumbers in New England were frozen.) As Keutsch told me, it’s the only way to “do something fast” about climate change.
But if a fleet of SAILs looks like a quick, cut-rate solution, that’s primarily because it isn’t a solution. What the technology addresses are warming’s symptoms, not its cause. For this reason, geoengineering has been compared to treating a heroin habit with methadone, though perhaps a more apt comparison would be to treating a heroin habit with amphetamines. The end result is two addictions in place of one.
Since calcite or sulfate (or diamond) particles lofted into the stratosphere drop back down after a couple of years, they’d need constant replenishing. If the SAILs flew for a few decades and then, for whatever reason—a war, a pandemic, unhappiness with the results—they stopped, the effect would be like opening a globe-sized oven door. All the warming that had been masked would suddenly manifest itself in a rapid and dramatic temperature run-up, a phenomenon that’s become known as “termination shock.”
Meanwhile, to keep pace with warming, the SAILs would need to deliver bigger and bigger payloads. (In “artificial volcano” terms, this would be the equivalent of staging increasingly violent eruptions.) Smith and Wagner based their cost calculations on the kind of protocol that Keith has proposed, which would halve the rate of warming going forward. The two estimated that around a hundred thousand tons of sulfur would have to be dispersed in the program’s first year. By the tenth year, that figure would rise to more than a million tons. During that period, the number of flights would ramp up accordingly, from four thousand a year to mor
e than forty thousand. (Each flight, awkwardly enough, would generate many tons of carbon dioxide, causing more warming, entailing more flights.)
The more particles injected into the stratosphere, the greater the chance of weird side effects. Researchers who looked into using solar geoengineering to offset carbon dioxide levels of five hundred and sixty parts per million—levels that could easily be reached later this century—determined it would change the appearance of the sky. White would become the new blue. The effect, they noted, would cause “the sky over formerly pristine areas to look similar to the sky over urban areas.” Another, more felicitous result, they observed, would be glorious sunsets, “similar to those seen after large volcanic eruptions.”
Alan Robock is a climate scientist at Rutgers and one of the leaders of the Geoengineering Model Intercomparison Project, or GeoMIP. Robock maintains a list of concerns about geoengineering; the latest version has more than two dozen entries. Number 1 is the possibility that it could disrupt rainfall patterns, causing “drought in Africa and Asia.” Number 9 is “less solar electricity generation,” and number 17 is “whiter skies.” Number 24 is “conflicts between countries.” Number 28 is “do humans have the right to do this?”
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For several years, Keith and Keutsch have collaborated on a project known as the Stratospheric Controlled Perturbation Experiment, or SCoPEx (pronounced “scope-ex”). The experiment is supposed to take place somewhere treeless, like the American Southwest, at an altitude of twelve miles. It will feature a pound or two of reflective particles and a zero-pressure balloon attached to a gondola loaded with scientific instruments.
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