Volcanic eruptions, like the 2006 Augustine Volcano eruption in Alaska, release plumes of aerosols into the atmosphere that can affect global temperatures, proof of concept for advocates of solar radiation management. (Photo by Smith Collection/GADO/Getty Images)

Stratospheric aerosol injection — the type of SRM described above—is one of a suite of technologies aimed at large-scale manipulation of the climate, known as geoengineering. There are two broad categories: solar geoengineering to block or reflect sunlight (like SRM), and “negative emissions technologies” to suck greenhouse gases out of the air. Negative emissions technologies have been factored into climate modeling for years and have a long-established (if controversial) role in IPCC mitigation strategies; solar geoengineering has been more fringe.

But in the past few years, the governments of the United Kingdom and United States have each sponsored research into solar geoengineering, outlets from The New Yorker to the New York Times have commissioned lengthy articles on it, and an editor at The Economist wrote a whole book on the subject, The Planet Remade. Just after the release of the new IPCC report, Council on Foreign Relations President Richard N. Haass opined that world governments should “accelerate R&D on geoengineering,” and Rep. Ro Khanna (D-Calif.) called to “incentivize oil companies to help.”

SRM has even found a research base in the Ivy League. Thanks to Harvard’s Solar Geoengineering Research Program, founded in 2017, the first steps toward implementation may soon float above the United States through the program’s Stratospheric Controlled Perturbation Experiment.

Known as Scopex, the experiment is not an SRM test, per se—to truly test the concept would require a global scale. But it is an attempt to go beyond computer modeling to understand the chemistry and microphysics of how particular aerosols interact with the stratosphere. A propeller-powered balloon will disseminate the aerosols; a small gondola, attached, will carry sensors. Researchers hope to launch in 2019.

Opposition to SRM has increased as well. As IPCC researchers met in South Korea to finalize their report, 110 civil society organizations across five continents, from First Nations peoples to think tanks to environmental NGOs, issued an anti-geoengineering manifesto: Hands Off Mother Earth, or HOME.

It reads, in part: “Geoengineering perpetuates the false belief that today’s unjust, ecologically and socially devastating industrial model of production and consumption cannot be changed and that we therefore need techno-fixes to tame its effects.” Signatories fear the development of SRM will be used as an excuse to continue the carbon status quo, calling it a “dangerous distraction” from real solutions and a “further entrenchment of fossil fuel economies.” Fossil fuel corporations have yet to throw their weight behind SRM specifically, but Shell’s chief climate change advisor, David Hone, praises the “sulphur solution” (referring to SRM) as the simplest in his book, Putting the Genie Back.

Harvard scientist David Keith, a contributor to the Scopex project, has emerged as something of a public face of SRM. He disagrees that geoengineering protects fossil fuel companies, envisioning it instead as a complement to decarbonization. “If solar geoengineering was much better understood, I don’t think it would make all the environmental forces give up and let Exxon win,” he says. “There’s not an alternative to cutting fossil fuels. We have to get them out of the energy system.”

Keith isn’t pushing for SRM deployment—just research— and says it’s important to understand all of the risks before moving forward. He worries about the Trump administration or other fossil fuel-friendly governments grasping onto SRM as a perceived quick fix. Still, he thinks we should be able to simultaneously research SRM and take action against fossil fuels.

The HOME manifesto, however, calls for a ban on open-air field experiments like Scopex. “[Scopex] makes no sense if you are not going to pursue deployment later,” says Silvia Ribeiro, one of the manifesto’s authors and the Latin America director of ETC Group, which addresses the social and environmental impacts of new technologies. “All powerful technologies have started with small experiments.”

Oxford physicist Raymond Pierrehumbert, a lead author on the IPCC’s third assessment report in 2001, sees Scopex as a crossing of the Rubicon. “Proceeding to field experimentation,” he writes, “crosses a thin red line beyond which lies the slippery slope down to ever-larger field trials and, ultimately, deployment.”

Skeptics also note that SRM only addresses some of the effects of global warming—those stemming directly from temperature. It does nothing to reduce the amount of carbon dioxide in the air, which means ocean acidification would continue apace, causing irreversible damage to coral reefs and other ocean life that will reverberate up the food chain. The continued accumulation of greenhouse gases would also heighten the impacts of potential termination shock.

Aerosol injections could increase exposure to UV radiation, as well, by wearing away at the ozone layer, most seriously at the North and South Poles, but also around the middle latitudes where most people live. Cooler temperatures do mean greater crop yields, which seems to be a mark in SRM’s favor. But a paper released this summer in Nature found that sunlight scattered by SRM would impede photosynthesis, negating any positive effects.

Study author Jonathan Proctor put it this way: “If we think of geoengineering as experimental surgery, our findings suggest that the side effects of treatment are as bad as the original disease.”

Even in the best case, the global effects of SRM will be extremely difficult to predict, says Janos Pasztor, director of the Carnegie Council for Geoengineering Governance Initiative. He is working on a regulatory framework around geoengineering research and deployment.

“The most important impact is hopefully positive, which is the reduction of global temperatures—otherwise you wouldn’t do it,” Pasztor says. But the most worrying effects may be what Pasztor calls the “unknown unknowns,” impacts we simply don’t know how to predict. “The global atmosphere is unbelievably complex,” he says. “We have advanced computer modeling with supercomputers, but we still don’t really know how to model it.” He believes better models and better data should be prerequisites for any deployment. “We only have one atmosphere, and we cannot mess it up.”

A small group of researchers in the Global North, dubbed the “Geoclique” by science writer Eli Kintisch, makes the case that it’s too soon to write off SRM, which accounts for a small percentage of climate research overall. While foundations and wealthy individuals have spent a few billion dollars on these efforts (most notably Bill Gates), clique members operate on modest budgets.

Another danger is that this so-called quick fix could be deployed unilaterally, side effects on everyone else be damned. It would cost just a few billion dollars a year to mount fleets of jets equipped with spray cannons, well within the means of a small country, a large corporation or a Bill Gates.

Could a group of low-lying countries, devastated by storms and rising seas, reach a tipping point and pull the trigger? Could deployment result from a geopolitical power play between the United States and China? Critics believe that the more SRM is legitimized through experiments and positive mainstream attention, the more likely it is that these speculative scenarios become real.