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Climate change and energy

The hard lessons of Harvard’s failed geoengineering experiment

Some observers argue the end of SCoPEx should mark the end of such proposals. Others say any future experiments should proceed in markedly different ways.

April 4, 2024
the SCoPEx balloon diagram with a crimson "X" hovers in a blue background with black particles
Stephanie Arnett/MITTR | SCoPEx (device)

In late March of 2017, at a small summit in Washington, DC, two Harvard professors, David Keith and Frank Keutsch, laid out plans to conduct what would have been the first solar geoengineering experiment in the stratosphere.

Instead, it became the focal point of a fierce public debate over whether it’s okay to research such a controversial topic at all.

The basic concept behind solar geoengineering is that by spraying certain particles high above the planet, humans could reflect some amount of sunlight back into space as a means of counteracting climate change. 

The Harvard researchers hoped to launch a high-altitude balloon, tethered to a gondola equipped with propellers and sensors, from a site in Tucson, Arizona, as early as the following year. After initial equipment tests, the plan was to use the aircraft to spray a few kilograms of material about 20 kilometers (12.4 miles) above Earth and then fly back through the plume to measure how reflective the particles were, how readily they dispersed, and other variables. 

But the initial launch didn’t happen the following year, nor the next, the next, or the next—not in Tucson, nor at a subsequently announced site in Sweden. Complications with balloon vendors, the onset of the covid pandemic, and challenges in finalizing decisions between the team, its advisory committee, and other parties at Harvard kept delaying the project—and then fervent critiques from environmental groups, a Northern European Indigenous organization, and other opponents finally scuttled the team’s plans.

Critics, including some climate scientists, have argued that an intervention that could tweak the entire planet’s climate system is too dangerous to study in the real world, because it’s too dangerous to ever use. They fear that deploying such a powerful tool would inevitably cause unpredictable and dangerous side effects, and that the world’s countries could never work together to use it in a safe, equitable, and responsible way.

These opponents believe that even discussing and researching the possibility of such climate interventions eases pressures to rapidly cut greenhouse-gas emissions and increases the likelihood that a rogue actor or solitary nation will one day begin spraying materials into the stratosphere without any broader consensus. Unilateral use of the tool, with its potentially calamitous consequences for some regions, could set nations on a collision course toward violent conflicts.

Harvard’s single, small balloon experiment, known as the Stratospheric Controlled Perturbation Experiment, or SCoPEx, came to represent all of these fears—and, in the end, it was more than the researchers were prepared to take on. Last month, a decade after the project was first proposed in a research paper, Harvard officially announced the project’s termination, as first reported by MIT Technology Review.

“The experiment became this proxy for a kind of debate about whether solar geoengineering research should move forward,” Keith says. “And that’s, I think, the ultimate reason why Frank and I decided to pull the plug. There’s no way, given that weight that SCoPEx had come to hold, it made sense to move forward.”

I’ve been writing about solar geoengineering for more than a decade. I reported on the conference in 2017, and I continued to cover the team’s evolving plans over the following years. So the cancellation of the project left me puzzling over why it failed, and what that failure says about the latitude that researchers have to explore such a controversial subject.

In recent days, I asked a handful of people who were involved in the project or followed it closely for their insights and thoughts on what unfolded, what lessons can be drawn from the episode—and what it means for geoengineering research moving forward.

Few of the people I spoke with believe it spells the end of outdoor experiments in solar geoengineering, but some argue that it should—and others believe any future proposals should proceed in a very different way if researchers hope to avoid the same fate.

A short history of SCoPEx

Nature offered the inspiration for solar geoengineering: massive volcanic eruptions in the past have cooled global temperatures by emitting vast amounts of sulfur dioxide, which eventually form sulfuric acid aerosols that reflect away solar radiation. 

The 1991 eruption of Mount Pinatubo in the Philippines, for instance, blasted nearly 20 million tons of sulfur dioxide into the stratosphere, cooling global surface temperatures by around 0.5 °C for months.

But one concern about relying on the gas for geoengineering is that sulfuric acid also depletes the ozone layer, which shields life on Earth from harmful ultraviolet light. So some researchers, including Keith, have used computer models to explore whether we could reduce or even reverse that side effect by replacing sulfur dioxide with other substances, including diamond dust, alumina, or calcium carbonate

The SCoPEx researchers discussed the possibility of releasing several materials over a series of flights, including sulfuric acid, but they mainly emphasized calcium carbonate.

They hoped that the data from the launches could refine the accuracy of geoengineering simulations and improve our understanding of the technology’s potential benefits and risks.

“You have to go measure things in the real world, because nature surprises you,” Keith said at that conference in 2017.

He has continually stressed that the amount of material involved would represent a small fraction of the particulate pollution already emitted by planes, and that doing the same experiment for any other scientific purpose wouldn’t have raised an eyebrow.

But theirs became a lightning rod. In their effort to be upfront and transparent about their plans, Keith believes, they set off a self-reinforcing cycle of overheated press coverage and fierce attacks from critics, all of which inflated public concerns about what he contends was an ordinary experiment with negligible environmental impact. 

The team’s initial hopes for launching a balloon in Arizona in 2018 never came to fruition because the balloon vendor they were working with, World View, stopped launching payloads of the necessary weight, Keith says. (The company didn’t respond to an inquiry before press time.)

But the researchers continued to develop the equipment and aircraft in the labs at Harvard, and the university set up an oversight panel that began reviewing the team’s plans and developing guidelines for engaging with the public.

Eventually, the researchers shifted their focus to Sweden, where they began planning a launch to test the aircraft’s equipment, working with the Swedish Space Corporation. The balloon was set to lift off from the Esrange Space Center in Kiruna in the summer of 2021.

The aircraft would not have released any materials during that launch. But anti-geoengineering groups, environmental organizations, Swedish environmental activist Greta Thunberg, the Saami Council (which represents the Indigenous Saami peoples of Northern Europe), and the board of the Royal Swedish Academy of Sciences all criticized the plan, putting pressure on the aerospace company, the research team, and the advisors to halt the launch. 

Solar geoengineering "is a technology that entails risks of catastrophic consequences, including the impact of uncontrolled termination, and irreversible sociopolitical effects that could compromise the world’s necessary efforts to achieve zero-carbon societies,” the Saami Council wrote in a letter to the advisory committee. “There are therefore no acceptable reasons for allowing the SCoPEx project to be conducted either in Sweden or elsewhere.”

In response, the advisory committee recommended that the researchers delay their plans until they had conducted conversations about the project with the public and concerned parties. In late March of 2021, the team and the company agreed to stand down.

The project never regained traction from there.

Last spring, Keith moved to the University of Chicago, where he now leads the Climate Systems Engineering initiative, a multidisciplinary research effort dedicated to improving understanding of solar geoengineering, carbon removal, and other interventions that could counteract the effects of climate change.

A few months later, the research team informed the advisory committee that it had “suspended work” on the experiment. Then, last month, Keutsch officially confirmed he’s no longer pursuing the project.

“I felt that it was time to focus on other innovative research avenues in the incredibly important field of [solar radiation modification] that promise impactful results,” he said in an email.

Too dangerous to study

Plenty of observers are pleased with the outcome. 

Hundreds of researchers from a variety of disciplines have signed an open letter calling for an “International Non-Use Agreement on Solar Geoengineering,” stating that governments should commit to “ban outdoor experiments of solar geoengineering.”

Jennie Stephens, a professor of sustainability science and policy at Northeastern University, was one of the letter’s signatories. She argues that the SCoPEx experiment was particularly dangerous, because the funding, attention, and prestige of Harvard conferred legitimacy on planet-scale interventions that, to her mind, can never be safely governed or controlled.

She argues that even if the researchers have the best of intentions, solar geoengineering would ultimately be deployed by people or nations with money and power in ways that most benefit their interests, even if it meant disastrous consequences for other areas. Some research, for instance, suggests that solar geoengineering could significantly reduce rainfall in certain areas and might reduce the yields of some staple crops. While one block of nations might decide to use geoengineering to ease heat waves, what if that reduced the summer monsoons and the food supplies across parts of India or West Africa?

“There’s no way to even imagine deploying it on a global scale so that everybody would benefit,” she says. “Some people would be screwed, and some people may have reduced suffering. So it’s creating one more mechanism by which to interfere with the Earth systems and then privilege some and disadvantage others.”


But many believe it’s essential to learn more about the role that solar geoengineering could play in easing global warming, and whether the side effects could be moderated. There’s a simple reason: if it does work well, it could save many lives and ease suffering as climate change accelerates. 

For these observers, then, the question is: What lessons can be drawn to ensure that other experiments can go forward? And perhaps of equal importance: What lessons shouldn’t be drawn from SCoPEx?

Some researchers in the field fear that the broader takeaway from the termination of the project will be that the Harvard team chose to be too open about its intentions.

The “organized opposition to even the concept of outdoor experiments” makes it difficult for other research groups to pursue similar work and “may increase the probability of rogue actors,” says Michael Gerrard, faculty director of Columbia University’s Sabin Center for Climate Change Law, who served on the advisory committee. He notes that such activities are largely unregulated.

Immediately following the news that Harvard was no longer pursuing the project, several figures in the cleantech industry took to social media to say that people could, or should, release particles into the stratosphere on their own.

While the Harvard team’s public plans were going nowhere, several other individuals claimed to have simply started launching stratosphere-bound balloons without any announcements in advance. They include the CEO of Make Sunsets, a venture-backed geoengineering startup, as well as Andrew Lockley, an independent researcher in the UK. 

Meanwhile, earlier this week, a University of Washington-led research group conducted a small experiment in marine cloud brightening, another form of solar geoengineering, on a decommissioned aircraft carrier anchored off the coast of Alameda, California, according to the New York Times. The team “kept the details tightly held, concerned that critics would try to stop them,” the newspaper reported.

Keith himself is “strongly opposed” to doing anything “rogue,” in the sense of illegal, or to conducting any such research in this field outside of the normal scientific process. And he says that “not being open at all” isn’t the right strategy.  

But he is wrestling with how up-front researchers should be. The level of early notice and transparency they strived for “maybe really doesn’t work in a conflictual environment,” he observes. “So maybe we should have been significantly less open and had a few limited sets of checks.”

Sikina Jinnah, a professor of environmental studies at the University of California, Santa Cruz, who joined the project’s advisory committee after the Sweden decision, draws the opposite lesson about transparency and engagement. 

She says that the Harvard team never got to the point of engaging with the public about its plans in any formal way in Sweden, and she stresses that such conversations should begin much earlier in the process. (This was also one of the main conclusions in the committees’ final report on the experiment, which was released last month.)

“Early engagement, I think, is one of the big take-home lessons,” she says. “And not just sort of cursory ‘giving a public talk’ kind of engagement, but really moving to iterative engagement with communities about their concerns, about questions they may be interested in, and really starting to reframe that kind of engagement process as one that’s not detrimental to the research effort but can actually enhance research and enrich it in ways that are socially beneficial.”

Scientific merit

Other observers believe there was a more basic problem with SCoPEx.

“Most of the scientists in the field didn’t feel like it was a particularly essential experiment,” said Douglas MacMartin, an associate professor at Cornell University who focuses on solar geoengineering, in an email.

As a result, there wasn’t a rush to defend it, he added.

MacMartin explained that the project was more focused on studying alternative aerosols, mainly calcium carbonate, rather than addressing unknowns concerning the substance that most people think would be used: sulfur dioxide. 

That’s because scientists know much more about its overall effects and can model them more accurately, since volcanoes already add the gas to the stratosphere naturally. Climate models also suggest that the impact on ozone would be minimal “and thus not worrisome enough to justify turning to a less-well-understood material,” he said.

Alan Robock, a climate scientist at Rutgers who has highlighted the potential risks of geoengineering, echoed this concern. 

“I don’t think this project ever had a good science question,” he says. “I think it was more driven by wanting to build something, the engineering.”

MacMartin says the crucial starting questions for experiments in this field are what gaps in our understanding such research could fill and whether that information would help to inform decisions about geoengineering. And it’s the pursuit of those answers that should be communicated as the rationale to the public.

But, he says, too often the SCoPEx researchers articulated their case for the work along the lines of, “Hey, this is small—you should let us do it because we want to.”

In an email, Keutsch noted that one of the things they hoped to better understand through the experiment was how plumes of injected particles spread out and mix in the stratosphere. In addition, Keith noted that they did discuss releasing and studying sulfuric acid as well, though they tended to talk more about calcium carbonate.

Broader scientific program

Another concern about the project from early on was that it was a one-off, privately funded experiment, moving ahead outside of any broader, government-backed research program. (Funding came from grants that Harvard provided the researchers as new professors and through the university’s Solar Geoengineering Research Program, which has raised money from the Alfred P. Sloan Foundation, the Hewlett Foundation, the Pritzker Innovation Fund, and other groups and individuals.) For less touchy subjects, such an experiment might be funded and overseen through a federal scientific body like the US National Science Foundation. 

That meant the university had to set up an advisory board if the institution wanted standard scientific oversight—and it meant that that committee had to craft its own rules for how such experiments should proceed, even as the researchers were taking steps toward an initial launch to test out their hardware.

Given the sensitivity of the topic, some observers believe that outdoor solar geoengineering experiments should only proceed through broader, public research programs involving scientific bodies with established practices for evaluating scientific merit, ethics, and environmental impact. Ideally, such programs would include “society-wide engagement,” tapping a variety of experts to impartially inform significant portions of the public about such interventions, explore their areas of concern, and, crucially, use that input to inform the design of the research program, says Holly Buck, an environmental social scientist at the University at Buffalo and author of After Geoengineering.

“Unless government is convening a serious engagement process where they are going to incorporate what they hear into policy in this area, I would expect any sort of outdoor experiment to meet a similar kind of resistance,” she said in an email.

Several nations have set up small-scale research efforts in the field, including the US and China. But a comprehensive program of this sort would require far more funding than has been allocated to date. A 2021 National Academies report recommended that the US government establish a cross-agency research program in solar geoengineering, backed by $100 million to $200 million over a five-year period. 

Future experiments 

Keith himself owns up to several mistakes in the research effort, including failing to anticipate that opponents would raise concerns over the basic hardware test in Sweden. He also says the team was wrong to move ahead without having a public engagement plan in place. The public failure of SCoPEx, he believes, will probably make it more difficult for other experiments in the stratosphere to go forward.

“Which is really sad,” he says. “And I apologize, and it’s a failure.”

But he also says there is still room for other groups to pursue outdoor experiments, and he believes the odds are strong that someone will.

Indeed, there are numerous indicators of growing interest in researching this field and providing funding for it. As noted, the US government is developing a research program. The Environmental Defense Fund is considering supporting scientists in the area and recently held a meeting to discuss guardrails that should govern such work. And a number of major philanthropies that haven’t supported the field in the past are in advanced discussions to provide funding to research groups, sources tell MIT Technology Review.

Meanwhile, under Keith, the University of Chicago is working to hire 10 faculty researchers in this area.

He says he wouldn’t look to lead an outdoor experiment himself at this point, but he does hope that people working with him at the Climate Systems Engineering Initiative would, if it could offer insights into the scientific questions they’re exploring. 

“I absolutely want to see experiments happen from the University of Chicago,” he says.

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