A controversial geoengineering plan aims to spray reflective particles into the stratosphere
The stratosphere is a dry and frigid place, where the air is thin and clouds are scarce. Up there, 10 to 50 kilometers above the Earth’s surface, ozone molecules absorb the sun’s ultraviolet light, protecting life far below. This second layer of the atmosphere is serene and mostly void of life. It’s also become the subject of one of today’s most contentious scientific proposals.
The proposal calls for what’s known as “solar geoengineering”: cooling the planet by deflecting sunlight that would otherwise strike the planet. Later this year, researchers hope to release a balloon that will ascend to 20 km, where their airborne platform will undergo various tests. Eventually, they’ll add equipment to spray tiny aerosol particles of calcium carbonate, the compound found in limestone, blackboard chalk, and Tums antacids. The particles will act like microscopic mirrors that should reflect sunlight back into space.
Little is known about how, or whether, solar geoengineering might work and how the particles would react and move in the stratosphere. Even less is understood about the potential risks to people and the environment—could the particles deplete the ozone layer, for example, or significantly alter the weather? But as Earth’s rising temperatures trigger a cascade of calamitous effects, and as humans pump more greenhouse gases into the air, a prominent group of scientists is urging the world to seriously consider the stratospheric option.
One of those scientists is Harvard University’s Frank Keutsch, who is leading the high-profile Stratospheric Controlled Perturbation Experiment (SCoPEx). He says the group hopes to launch the balloon in mid-2022. Keutsch believes solar geoengineering’s many unknowns are precisely why SCoPEx is worth doing. If global warming continues unabated, and if the world veers toward catastrophe, it would be better to have tools ready to avoid the most dire outcomes, he says.
“This question of whether climate change becomes so bad that we have to do something to dull its impacts and reduce human suffering is a very big one,” says Keutsch, an atmospheric chemist. “Research takes a long time. If we only start research when people say, ‘Oh, I think we need this,’ then it’s too late.”
The SCoPEx data could help improve computer models, which today rely mainly on assumptions and predictions, not observations. The quantity of calcium carbonate to be released—about 1 kilogram—won’t be enough to trigger any measurable cooling, and it roughly equals the particle pollution that a large commercial airliner releases every minute of flight, says David Keith, a physics and public-policy professor at Harvard who helps lead the project. There were nearly 40 million such flights in 2019. (Keith is also the founder of Carbon Engineering, a Canadian firm building a sprawling facility in West Texas that will pull carbon dioxide directly from the air.)
Solar geoengineering could be a “painkiller” for the planet—not a substitute for curbing emissions, but a balm that makes life on Earth more bearable in a sweltering future.
Yet even basic solar geoengineering research is proving to be controversial. Critics say that pursuing the technology is a dangerous distraction from the more essential task of rapidly reducing greenhouse gas emissions. For many, the mere idea of purposefully intervening in the climate system is problematic and worth shutting down before it gains traction. The first SCoPEx test was originally planned for early 2021 in northern Sweden, but backlash from environmental and indigenous groups prompted the team to cancel the launch.
Still, Keutsch says he’s optimistic that this year’s test will go forward, once the researchers find a new balloon partner and launch site. “The more we learn about the reality of the state of climate change, there’s a greater realization that this research is something we have to start sooner rather than later,” he says.
Solar geoengineering is not a substitute for curbing emissions
The reality is that the planet is warming at an alarming rate. Each of the last four decades has been the warmest on record, scientists say. Concentrations of greenhouse gases are at record levels, mostly because of the coal, oil, and natural gas that gets burned for electricity, heat, and transportation. Today, the average global temperature is about 1.09 °C hotter than in the late 19th century, according to the Intergovernmental Panel on Climate Change (IPCC), the United Nations–run scientific authority on global warming. “It is unequivocal that human influence has warmed the atmosphere, ocean, and land,” the IPCC stated in a comprehensive report released in August.
Scientists warn that global warming is likely to hit 1.5 °C within the next two decades, a level that will bring devastating and long-term effects such as catastrophic flooding, severe drought, deadly heat waves, and mass die-offs of coral reefs. Preventing further, more perilous levels of warming will require immediately shifting away from fossil fuels, scaling up renewable energy, and potentially even removing carbon dioxide from the atmosphere.
In this unnerving context, solar geoengineering could be a “painkiller” for the planet—not a substitute for curbing emissions or restoring carbon-trapping forests, but a balm that makes life on Earth more bearable in a sweltering future, Keutsch says.
Not everyone agrees. Painkillers can lead to harmful addictions, the U.S. climate scientist Michael E. Mann has said. He has likened solar geoengineering to “climate methadone,” because once the world starts injecting sun-dimming particles, it likely won’t be able to stop.
Field experiments aim to improve our understanding of solar geoengineering
Harvard scientists first proposed SCoPEx in a 2014 research paper. They called for “small-scale, in situ experimentation” that could help remove some of the uncertainties and “unknown unknowns” surrounding solar geoengineering, which in the paper they called solar-radiation management. Two years earlier, two of the paper’s coauthors—David Keith and James G. Anderson, a professor of atmospheric chemistry at Harvard—had floated the possibility of launching a balloon and injecting “micro” amounts of sulfate particles into the air.
That idea didn’t advance, but in 2017, Keith became faculty director of Harvard’s Solar Geoengineering Research Program. The program, of which SCoPEx is the centerpiece, has so far raised US $16.2 million from Microsoft cofounder Bill Gates, the William and Flora Hewlett Foundation, and other philanthropic organizations.
SCoPEx has two main goals: to observe how plumes of particles disperse in the stratosphere, and to explore which types of particles have the fewest side effects.
This year’s planned field experiment will be carried aloft by a zero-pressure balloon that stretches 27 meters in diameter, roughly the length of two school buses. The balloon isn’t particularly novel; every year, NASA conducts up to 15 stratospheric balloon flights from launch sites worldwide to collect data and test technologies for space missions.
What’s unique is SCoPEx’s gondola, an aluminum and carbon-fiber frame that holds an array of hardware. A Raspberry Pi 4–based flight computer will receive commands and log data. Two Globalstar satellite phones will enable communication between the gondola and ground equipment. Twin airboat propellers will allow scientists to reposition the gondola while it’s airborne.
During its first flight, SCoPEx will test how well the platform operates when exposed to temperatures down to −60 °C as well as direct sunlight. The gondola won’t carry any chemicals or particle-spraying tools. The initial test will last 4 to 6 hours at an altitude of nearly 20 km—more than twice the height of Mount Everest. Although researchers can simulate the stratosphere in a thermal vacuum chamber, it’s difficult to know how real-world conditions will affect equipment.
“Before I put a lot of sophisticated instrumentation on an airborne platform, I want to know that that airborne platform works,” Keutsch said over videoconference from his native Germany, where he was on sabbatical.
The first SCoPEx test was planned for an early 2021 launch from the Esrange Space Center in northern Sweden, but backlash prompted the team to cancel the launch.Swedish Space Corp.
Assuming the platform passes muster, the next test will be to spray and track particles in the stratosphere. A spraying device will release the calcium carbonate into a kilometer-long wake created by the propellers. The balloon will then move back and forth through the wake, while lidar tracks how far the particle plume travels. Another laser-based instrument, a lightweight Portable Optical Particle Spectrometer, will measure the size and number of particles. Other equipment will collect data on the moisture and ozone in the stratosphere.
“This is not a test of whether solar geoengineering works,” Keith says. “These are things we need to do if we’re going to improve the science of solar geoengineering.” SCoPEx will allow researchers to evaluate potential side effects, which could be significant, including possible ozone depletion, increased air pollution, and changes in weather patterns, with some regions likely to be more negatively affected than others.
The key question for society, he says, is whether the risks of solar geoengineering are worth taking, to avoid the extreme consequences of global warming. “We don’t face a risk-free decision,” Keith says. “The issue is about risk trade-offs.”
Protests forced the cancellation of SCoPEx’s first atmospheric test
SCoPEx’s first test flight was supposed to launch in June 2021 from Esrange Space Center, in Kiruna, a town in Sweden’s Lapland region. The site, managed by the Swedish Space Corp., is a large patch of dirt surrounded by endless trees above the Arctic Circle.
Åsa Larsson Blind says she first learned about SCoPEx in February 2021. Larsson Blind is vice president of the Saami Council, an organization that represents the Sámi indigenous peoples, whose traditional territory is in the north of Sweden, Finland, Norway, and Russia’s Kola peninsula. A U.S.-based indigenous group emailed the council, informing them of the planned launch. Shortly after, the Saami Council and environmental groups sent a letter to the SCoPEx Advisory Committee and Swedish officials protesting the SCoPEx test flight.
Larsson Blind, who comes from a family of reindeer herders, spoke to IEEE Spectrum by video call from her home in the northernmost part of Sweden. She says the council is active in climate-policy issues because it wants to secure the natural systems that “our traditional livelihoods and culture is based on.” Solar geoengineering is “so not in line with how we believe nature should be respected.” Even small-scale initiatives like SCoPEx, she says, are “a step towards legitimizing the development of the technology.”
The pushback prompted SCoPEx’s committee of external advisors to recommend postponing the launch until a process of “robust and inclusive public engagement” occurs in Sweden. On 31 March 2021, the SCoPEx team agreed to suspend its equipment test flight until the advisory committee recommends otherwise.
The stalled launch highlighted questions about solar-geoengineering research—how or whether it should occur, and who gets to decide. As it stands, a small number of researchers from wealthy Western institutions are contemplating an approach that, if deployed, could impact everyone on Earth. Just days before the 2021 postponement, the National Academies of Sciences, Engineering, and Medicine called for developing “international governance mechanisms” and global scientific partnerships to ensure solar-geoengineering research moves forward in a “socially responsible manner.” In its report, the academies also recommended that the U.S. government invest $100 million to $200 million over five years in a national solar-geoengineering research program; the funding would go toward scientific activities as well as oversight, peer review, and potentially permits for field experiments. But at present, there is no United Nations agency to build a global consensus or U.S. agency to provide oversight. The SCoPEx Advisory Committee, which is tasked with making sure the project is conducted thoughtfully and transparently, is an early effort to address this discrepancy.
Critics say that pursuing solar geoengineering is a dangerous distraction from the more essential task of rapidly reducing greenhouse gas emissions.
“That’s why SCoPEx has had some difficulty, because they’re trying to develop the governance structure while they’re trying to do the science. The cart’s a bit in front of the horse,” says David W. Fahey, who directs the Chemical Sciences Laboratory of the U.S. National Oceanic and Atmospheric Administration, in Boulder, Colo. Fahey says that he supports Harvard’s research initiative, and his laboratory loaned SCoPEx a particle spectrometer used in earlier stages of the project.
Establishing a governing body isn’t a guarantee that the interests of wealthy, powerful nations won’t overtake those of poorer, more vulnerable countries, says Jennie C. Stephens, director of Northeastern University’s School of Public Policy and Urban Affairs, in Boston. She points to existing international efforts, such as those to reduce greenhouse gas emissions or distribute COVID-19 vaccines, that have struggled to balance the needs and desires of disparate populations.
Solar geoengineering is “a very narrow way of looking at the climate crisis,” she says. “All it’s thinking about is reducing the global average temperature and the physical system, without thinking about the distribution...and how different people in the world will be impacted if we were to try to modify and manipulate the Earth’s climate system.”
Lab studies continue to advance the science behind SCoPEx
As Harvard’s balloon flight garners both support and scrutiny, parallel research continues in the lab. Earlier iterations of SCoPEx proposed using sulfate particles, which exist in the stratosphere and are known to cause cooling. In 1991, when Mount Pinatubo erupted in the Philippines, it created a haze of particles so dense that it temporarily cooled the planet by about 0.6 °C. But sulfate aerosols—the combination of particles and water—can destroy the planet-protecting ozone layer. They can also heat up the stratosphere, changing air circulation and weather patterns.
Models suggest that calcium carbonate might be more benign. Calcium carbonate is bountiful in the lower atmosphere, in the form of calcite dust, but it doesn’t exist in the stratosphere. In New York City, Han Huynh studied the substance as a Ph.D. candidate in V. Faye McNeill’s group at Columbia University.
For her experiments Huynh used a glass aerosol flow-tube reactor, coupled with a chemical-ionization mass spectrometer. She measured the reaction between calcite aerosols and hydrogen chloride, a stratospheric trace gas that can, through chain reactions, ultimately impact the stratospheric ozone level. The flow reactor was kept at around −66 °C using a layer of circulated coolant sealed in a vacuum layer and encased in foam. Researchers continuously monitored the number of calcite aerosols, their surface area, and other factors.
Huynh and McNeill recently studied how calcium carbonate could affect global ozone. Based on their results, “what we see is that the ozone impact uncertainty is really, really high,” Huynh says. “There’s no way to tell right now, based on our study, whether or not it will have a positive or negative impact.” That’s largely because not enough is known about how the aerosols react with relevant gases in the stratosphere. “You need to continue these lab studies a lot longer before [you can] say, ‘Okay, this is a good idea. We should go and test this outside.’”
At Harvard, scientists are conducting such studies using a tabletop device built by research assistant Zhen Dai. The apparatus simulates and measures chemical reactions between calcium carbonate and hydrogen chloride and chlorine nitrate; the goal is to begin quantifying how, and how quickly, the particles might react under stratospheric conditions
As research continues in the lab and, eventually, outdoors, Frank Keutsch says he’s working to expand SCoPEx’s team to include scientists from Latin America, Africa, and the Asia-Pacific region. “This global conversation is really important, because people’s views on these technological solutions vary drastically across different cultural backgrounds and different areas,” he says. “It should be a little bit more diverse than a few people from Harvard.”
This article appears in the January 2022 print issue as "Planet-Cooling Tests Could Start in 2022."
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