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Climate Change

Will the world ever be ready for solar geoengineering?

Many object to shielding the planet from the sun’s rays by injecting particles into the atmosphere, but a small band of researchers think the approach is worth studying before it’s too late

by Tien Nguyen
March 26, 2018 | A version of this story appeared in Volume 96, Issue 13
Image of the Sun with the Earth in the distance on the dark background of space.

Credit: Will Ludwig/C&EN/Shutterstock


The first time Frank Keutsch heard about solar geoengineering, he thought the idea was terrifying. To the Harvard University atmospheric chemist, schemes such as spraying millions of tons of sulfate particles into the sky to reflect the sun’s rays and cool the planet seemed perilous. Not only might the strategies disrupt the atmosphere in unexpected ways, but they might also dramatically alter the weather and harm the lives of Earth’s inhabitants.

In brief

Once considered taboo, discussions about solar geo­engineering are growing louder.Many worry that the idea of solar geoengineering—mainly strategies to cool the planet by shooting sun-reflecting particles into the atmosphere—will distract from efforts to reduce carbon emissions and thus the underlying cause of climate change. Meanwhile, others argue that we should start thinking about how these strategies might be governed, and a few say it’s time to study solar geoengineering in earnest to understand its hazards. As the debate rages on, one group is pushing forward with outdoor experiments possibly later this year. Risky as these techniques may be, society may one day have to wrestle with whether to implement them.

“It’s a very contentious topic, and for good reason,” Keutsch says. Sure, the unknowns of opening what amounts to a chemical sunshade over our heads are worrisome. But even more troubling, Keutsch says, is the “moral hazard” of solar geoengineering: the idea that instead of dealing with the cause behind climate change directly, by cutting back on the use of fossil fuels, humans would fall back on solar geoengineering to merely stave off its symptoms. The term “moral hazard,” borrowed from economists, describes the temptation for people to make riskier decisions when they feel protected from the consequences.

Scientists have discussed solar geoengineering in hushed tones for years, but fears like Keutsch’s meant that experiments had been taboo. That started changing in 2006, when Paul J. Crutzen, who had shared the Nobel Prize in Chemistry more than a decade prior for his work on ozone depletion, penned a controversial essay calling for stratospheric geoengineering research (Clim. Change 2006, DOI: 10.1007/s10584-006-9101-y). While he hoped for a world in which we reduced carbon emissions to the point that these risky measures were never needed, he wrote, “Currently, this looks like a pious wish.”

In the intervening decade, a relatively small number of research groups took up Crutzen’s charge, mostly conducting theoretical studies. Now some say they’re just about ready to run experiments in the real world, possibly by later this year.

But most scientists still find this idea deeply troubling. Among them is Daniel Cziczo, an atmospheric scientist at Massachusetts Institute of Technology. To Cziczo, the notion of injecting sulfates into the air as a response to climate change is a nonstarter because it would destroy ozone. It also doesn’t address ocean acidification, one of the most harmful side effects of climate change.

“Scientists are throwing out proposals which are sometimes absolutely crazy,” Cziczo says. It’s “totally illogical,” he says, to instruct people to reduce carbon emissions while pushing forward an option that lets them ignore your advice.

Still, government officials and supporters of geoengineering research continue to evaluate their options. Two main classes compose geoengineering: solar geoengineering—also known as albedo modification—which focuses on reflecting sunlight before it hits Earth, and direct air capture, a suite of techniques to suck carbon dioxide from the ambient air. In 2015, the U.S. National Academies of Sciences, Engineering & Medicine assessed proposals for both types of approaches in a pair of reports and concluded that there wasn’t enough information to recommend any of these geoengineering technologies for large-scale deployment.

Some in Congress are now calling for the National Academies to reassess their studies, especially of solar geoengineering.

At the end of last year, after the first congressional hearing on geoengineering, focused mainly on solar technologies, in the U.S. House of Representatives, Democrats Jerry McNerney of California and Eddie Bernice Johnson of Texas proposed H.R. 4586 to “provide for the National Academies to study and report on a research agenda to advance the understanding of albedo modification strategies, and for other purposes.” Direct air capture, the other branch of geoengineering, has also caught federal interest. This month, Congress introduced a tax incentive designed to spur spending on carbon capture, both from point sources like power plants and straight out of the surrounding air.

A graphic showing the proposed SCoPEx balloon apparatus releasing particles into the air, lifting out of the plume, and then analyzing it.
Credit: SCoPEx team
Researchers at Harvard propose, later this year, to launch the first real-life solar geoengineering experiment: a balloon flown 20 km above the ground that will release a plume of particles, such as ice and CaCO3, then study their physical and chemical properties.

Meanwhile, an ocean away, representatives from many of the world’s nations were gathering in Bonn to discuss the next steps in emissions reductions to meet the goals of the 2015 Paris Agreement, which aims to limit the rise in global temperatures to below 2 °C by 2100. The U.S. wasn’t present at this meeting, having left the agreement a few months earlier.

Amid all these scientific and policy discussions, a team of researchers at Harvard is proposing to launch the first stratospheric outdoor solar geoengineering experiment near Tucson later this year. The group plans to loft 20 km into the air a balloon carrying instruments that would release a plume of particles 1 km long and 100 meters wide, then turn around and study the particles’ basic physical properties in the upper atmosphere.

Leading this stratospheric test, despite his initial trepidation, is Keutsch.

“I actually still think it’s a terrifying concept,” he says. “But at the same time, if you look at predictions of climate change, I think they are also very frightening.”

Keutsch, like the other scientists who study solar geoengineering, is emphatic that reducing carbon emissions should be society’s first priority. But in the meantime, he says, better understanding the risks involved with solar geoengineering through experiments could be useful. “Knowledge is better than ignorance.”

If the fledgling field moves forward—and some hope to ensure that it doesn’t—solar geoengineering researchers will have no shortage of questions to answer: What types of particles should be released into the sky? How many particles and where? What happens when they fall to Earth? And perhaps most pressing: Who gets to decide if and when humankind presses “go”?

Predicting a hazy future

Solar geoengineering, though it sounds like science fiction, wasn’t conceived from thin air. Scientists took their inspiration from a natural event: the earth-shaking eruption of Mount Pinatubo in 1991 in the Philippines. The second-largest volcanic outburst of the 20th century, it spewed ash and 17 megatons of sulfur dioxide 35 km skyward. For several years afterward, the SO2 stayed in the atmosphere, reacting with atmospheric chemicals to form sulfate aerosol particles that reflected sunlight and caused global temperatures to drop by 0.5 °C. Once those particles eventually drifted down to Earth, temperatures rebounded.

Credit: UCAR
Advanced models show that injecting sulfate particles into the atmosphere could cool Earth as greenhouse gas emissions continue to cause global temperatures to rise, but these strategies come with risky side effects, such as ozone depletion, acid rain, and changes in weather patterns.

Taking their lead from the tropical blast, some scientists developed climate models that simulated what would happen if sulfates were sprayed into the atmosphere at the equator, where Mount Pinatubo is located. Others went even simpler, taking sulfates out of the equation and simulating what would happen if we could turn down the sun’s brightness with an imaginary dimmer switch. Both types of models revealed that global temperatures would drop; however, cooling would occur unevenly, with more of it happening in the tropics and less in Arctic areas.

Last fall, several of these solar geo­engineering modeling researchers who had teamed up in a collaboration spanning four institutions debuted one of the most advanced solar geoengineering models. The model accounts for complex atmospheric chemistry, atmospheric dynamics, and sulfate aerosol formation and, for the first time, allows scientists to design, instead of just predict, specific climate outcomes (J. Geophys. Res.: Atmos. 2017, DOI: 10.1002/2017jd026888).

According to the model, which assumes that humans aren’t going to succeed at cutting back on their emissions, if solar geoengineering began in 2020, global temperatures could be stabilized at that year’s level for the remainder of the century. The strategy would involve spraying increasing amounts of sulfur dioxide at four locations 15° and 30° north and south of the equator. By 2090, according to the team’s calculations, we would need to annually inject an amount of SO2 equivalent to up to half the total volume that burning fossil fuels releases globally each year.


I actually still think it’s a terrifying concept.
Frank Keutsch, chemist leading proposed outdoor solar geoengineering experiments, Harvard University

“It’s not something we see as a plan B,” emphasizes Simone Tilmes, one of the coauthors of the study and a project scientist at the U.S. National Center for Atmospheric Research. We still need to cut carbon emissions, Tilmes says, but this approach might keep temperatures low while we do so. The world may have only five to 10 more years to enact a plan to fight climate change, she says, before we’re unable to meet the Paris Agreement goals. “To me, it’s not even buying time; we already lost the time that we may have to make up for,” she says.

But for each degree of cooling we gain from sending up sulfate aerosols, the team sees a possible assortment of dangerous side effects.

While sulfate particles reflect and scatter light, they can also absorb solar radiation, heating the lower stratosphere and changing the transport of atmospheric chemicals, leading to unpredictable effects on weather patterns and precipitation, the researchers say. Large sulfate particles, which would form as SO2 is continually pumped into the air, aren’t as good as small, high-surface-area particles at reflecting light and can fall out of the sky faster, potentially as acid rain, they add.


Arguably, the most serious side effect is that sulfates could lead to the destruction of ozone. Ozone loss occurs when halogen molecules, such as hydrochloric acid and chlorine nitrate, transform into halogen radicals, which destroy ozone. Sulfate particles speed up this process by providing a surface for radical formation.

Solar geoengineering’s side effects could be numerous, from the moment an atmospheric treatment is deployed to the moment it’s abruptly cut off. Because solar geoengineering addresses only the symptoms and not the cause of climate change—greenhouse gases—stopping treatment could lead to devastating consequences, says ecologist Christopher Trisos, a postdoctoral fellow at the National Socio-Environmental Synthesis Center.

Global temperatures would rocket right back to previous levels so quickly that many species might struggle to survive, he says.

Trisos coauthored a study that evaluated the speed, called climate velocity, at which a species would have to move to get away from negative climate effects if solar geoengineering were abruptly cut off (Nat. Ecol. Evol. 2018, DOI: 10.1038/s41559-017-0431-0).

Species would have to move, on average, two to six times as fast as what would be required by climate change without solar geoengineering to escape harmful climate effects on their habitat, according to the model. Howler monkeys, for example, are able to migrate into a new habitat at about 1 km/year. If temperatures changed drastically after solar geoengineering termination, they would need to move at 10 km/year.

Given these risks, ecologists have issued one of the few directives on geoengineering. In 2010, the Convention of Biological Diversity, an institute of the United Nations with more than 190 parties—excluding the U.S.—issued what amounts to a moratorium on any large-scale climate intervention activities, including solar geoengineering or carbon capture, until there is enough scientific evidence to justify such strategies.

The decision does, however, allow small-scale research experiments, like Harvard’s balloon experiment, to march forward in an attempt to gather that evidence.

Pushing ahead

While modeling studies have been exploring the effects of sulfates in the stratosphere, researchers have also been investigating the possible consequences of another solar geoengineering approach that steers clear of the upper atmosphere.

This strategy aims a little lower—at clouds. Called marine cloud brightening, the approach was inspired by container ships, whose exhaust creates clouds often hundreds of kilometers long called “ship tracks” as they crisscross the ocean. These ship tracks reflect sunlight, but they may also disturb precipitation in unpredictable ways.

How solar geoengineering could cool the planet
In theory, strategies to reflect sunlight, by brightening clouds with sea spray or injecting SO2 into the stratosphere to generate sulfate particles, could shield the planet from the sun’s warming rays.

Credit: C&EN

Instead of using exhaust, researchers argue that sea spray, which is safer, can also do the trick. Researchers involved with the Marine Cloud Brightening Project, a collaboration among atmospheric scientists, engineers, and social scientists, have developed a special, high-pressure nozzle that ejects nanometer-sized saltwater aerosol particles into the sky. To generate a single ship track, says Robert Wood, one collaborator at the University of Washington, 500 such nozzles would need to be grouped together and installed on each specially purposed ship, a resource that requires more funding. The team has been waiting for funding for quite some time, Wood says, estimating that it might take 10 or 20 more years. “I’m not holding my breath,” he says.

Wood acknowledges the fears many have about solar geoengineering’s moral hazard but doesn’t think that developing a technology means it will necessarily get used. “I’m not doing this to prove it works,” he says. Instead, he’s trying to prove that it doesn’t work, which he says can also reveal useful information.

Other labs, like those of Keutsch and his collaborator, vocal geoengineering proponent David Keith, who’s also at Harvard, have investigated chemical alternatives to sulfate aerosols. “It’s a great chemistry problem,” Keutsch says.

Using models and testing simulated conditions in the lab, Keutsch and his colleagues hope to find particles capable of trapping less heat in the atmosphere and of reflecting more light skyward. Among the chemical candidates are diamond, titanium dioxide, and calcium carbonate. Because it’s inert, diamond has the best properties, but its price points to impracticality. TiO2 may prove problematic, researchers say, because the compound is a known photocatalyst and could thus cause unwanted side reactions.

That leaves the cheap and readily available CaCO3. According to the team’s models, CaCO3 may heat the lower stratosphere one-tenth the amount that sulfate aerosols do, and it may counter ozone loss by neutralizing acids emitted by humans (Proc. Natl. Acad. Sci. USA 2016, DOI: 10.1073/pnas.1615572113).

V. Faye McNeill, an atmospheric chemist who studies the impact of aerosol particles on Earth’s climate at Columbia University, cautions that it remains to be seen how CaCO3 will interact with stratospheric ozone in reality. McNeill contrasts these calcite particles with sulfate aerosols, which have been thoroughly studied because they’re found naturally in the atmosphere and because they were released in abundance after Mount Pinatubo’s eruption. “So far what I have seen in the literature are reasonable, educated guesses for the stratospheric heterogeneous chemistry of CaCO3,” she says, “but this needs to be backed up with lab data.”

The Harvard team is currently studying particles of the material in the lab, using flow reactors to expose the CaCO3 to various atmospheric gases. Another idea is to coat the CaCO3 particles with compounds that would make their surfaces less inviting to the formation of ozone-depleting radicals.

The team plans to test extremely small amounts of CaCO3 in its outdoor experiment, slated to take place over Tucson, after first releasing ice particles, a lower-risk material.

“The experiment itself won’t affect anybody, but it has implications for everybody,” Keutsch says.

The public has expressed concern, and sometimes much stronger emotions, about these types of experiments. Chemtrail conspiracy theorists, for instance, who posit that the government has been poisoning the public with chemicals spread by commercial planes, have reacted rather vociferously to the idea of spraying aerosols into the atmosphere.

Over the years, Keith says he’s received hundreds of emails from people about solar geoengineering, ranging from polite curiosity to threatening. Others in the solar geoengineering field have also received hate mail and even death threats, including Douglas MacMartin of Cornell University, who testified at last year’s congressional hearing on geoengineering. MacMartin reported that he’d seen an uptick in such missives last year. “There’s some risk that somebody might get hurt,” Keith says.

Public opinion is partly what derailed an attempt to conduct an outdoor solar geoengineering experiment in the U.K. in 2012. Called Stratospheric Particle Injection for Climate Engineering, or SPICE, the project proposed pumping water up through a 1-km-long hose to a tethered balloon, which would spray the water into the atmosphere.

Controversy could definitely stop solar geoengineering in its tracks, says Tim Kruger, manager of the Oxford Geoengineering Programme and a member of the board that decided whether SPICE would be funded. He says amid the public concern, the experiment ultimately fell apart after it was revealed that another member of the funding board had filed a patent on similar technology with one of the SPICE researchers. Given the clear conflict of interest and simultaneous public backlash, the project was canceled, Kruger says.

The Harvard team has no plans to patent any of the technology from the experiment, Keutsch says. But he still worries about public opinion. “The thing I don’t want to happen is I say, ‘Oh we’re ready to do the experiment,’ and then there’s such a negative backlash that it makes future solar geoengineering research impossible for a long time.”

Who will decide?

Peter Frumhoff, director of science and policy at the Union of Concerned Scientists, coauthored an opinion piece last year with Northeastern University’s Jennie Stephens detailing ways that solar geoengineering researchers can move forward responsibly by getting informed consent from the public.

They have a few main points of advice: Scientists should set up an independent advisory board, which Keutsch’s team is in the process of doing, whose recommendations should be taken seriously. Researchers should also be transparent about funding sources and restrict funding to entities that are committed to deep cuts in greenhouse gas emissions. Scientists should bring a diverse set of civil society stakeholders into the conversation, especially those who are most at risk, such as developing countries, Frumhoff says.

The article concludes that “solar geoengineering field research should not take place unless and until greater societal legitimacy has been established.”

Various parties have offered similar guidance for responsible geoengineering research. In 2009, Kruger and a few other academics devised the “Oxford Principles,” and in 2015, legal scholars crafted a voluntary code of conduct for geoengineering research that has recently been updated by an international initiative called the Geoengineering Research Governance Project.

“There are bits and pieces of governance, but the totality does not add up to what’s needed,” says Janos Pasztor, former UN assistant secretary-general for climate change under Secretary-General Ban Ki-moon.

Pasztor is the executive director of the Carnegie Climate Geoengineering Governance Initiative (C2G2), a small group that encourages discussions about and development of governance frameworks for solar geoengineering but doesn’t take a position on deployment of these strategies.

Since last year, Pasztor and his team have been discussing solar geoengineering with communities from around the world, including in countries like Kenya and South Africa, which so far haven’t been a part of the conversation. While many people he speaks with are positive when it comes to learning about solar geoengineering, Pasztor says, “It’s a hard step to go from positive to proactive.”

The team hopes to facilitate the presentation of a resolution at the UN Environment Assembly in Nairobi, Kenya, in March 2019. That resolution would call for governments to agree not to deploy solar geoengineering techniques until scientists better understand the risks and society agrees on solar geoengineering’s governance. Although aspects of the resolution could change during negotiations, Pasztor says, the team hopes the action will head off any unilateral decisions by rogue nations to deploy solar geoengineering. This poses a risk, given the techniques’ relatively cheap price tag, recently estimated at about $50 billion for the initial hardware and then $12.5 billion each year afterward (Earth’s Future 2018, DOI: 10.1002/2017ef000735).

“The governance of this is very, very difficult,” Pasztor says. “It’s not too early to start talking about these issues, even if these technologies will not be relevant for a couple of decades, because that’s how long it might take to build the governance framework.”

Yet those who oppose geoengineering don’t think geoengineering governance is the main issue. Linda Schneider, senior program officer at the Heinrich Böll Foundation, a think tank affiliated with Germany’s Green Party, finds much of the discourse around geoengineering problematic.

“Most of the researchers in the field start from the assumption that we might need geoengineering,” she says, which she thinks leapfrogs the basic question of whether geoengineering should be done. Schneider says she was one of the few nonscientists and critical voices among attendees at the 2017 Climate Engineering Conference in Berlin in October, which drew mainly physical and social scientists.

Schneider and the foundation say it’s critical to stay focused on emissions reduction. At the conference, she led a session presenting a pathway to limit global temperature rise to 1.5 °C—a stricter version of the Paris Agreement goal. The plan involves radical emissions reduction and no geoengineering. The session essentially devolved into a “big fight,” she says.

As Schneider advocated for climate responses without geoengineering, proponents criticized scenarios relying only on drastic carbon reduction as politically unrealistic. “I think what made them upset is that they were forced to confront their own political values underlying their own research,” Schneider says. Scientists want their experiments to exist outside the real, political context, she argues.

Her group is opposed to any outdoor solar geoengineering research, including the Harvard balloon experiment. “If they do go forward, it would definitely be something we challenge publicly and openly,” she says.

Schneider says even if the Harvard group assembled an independent advisory board, she wonders whether members of a handpicked board would be truly independent. The Union of Concerned Scientists’ Frumhoff agrees that it’s a legitimate concern but also wonders who else would assemble a board.

Frumhoff says the discussion is incredibly difficult, but that doesn’t mean we should shy away from it. “The scientific community is wrestling with all manner of technologies that we didn’t take seriously a few years ago,” he says. “Now is the moment to have this conversation because it’s coming at us.”

For now, solar geoengineering is likely to continue being a contentious conversation. MIT’s Cziczo says he’s willing to be wrong about solar geoengineering but that “no one has shown me that it’s a good idea.”

Cziczo and Keutsch are colleagues in the same field and are actually good friends. They’ve even discussed their opposing viewpoints over beers. “I’m not sure that either of us will fully convince the other,” Cziczo says.

He does, however, think it’s important to teach the science of geoengineering and dedicates a few lectures to it in a special course for MIT undergraduates based on the National Academies’ geoengineering reports. He notes that these students will be the ones dealing with the impact of the climate decisions his generation makes.

After the lesson, he likes to ask the class if they would choose a low-cost solar geoengineering option or if they’d pay more to reduce emissions to respond to climate change. He says it may be the way he teaches the topic, but “the students always choose to address the underlying problem.”


CORRECTION: This story was updated on March 27, 2018, to clarify that the Harvard balloon experiment would be the first solar geoengineering experiment to take place in the stratosphere, not the first one ever conducted.


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