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Solar geoengineers would combat climate change with a haze of Sun-blocking aerosols.

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Geoengineers inch closer to Sun-dimming balloon test

For years, the controversial idea of solar geoengineering—lofting long-lived reflective particles into the upper atmosphere to block sunlight and diminish global warming—has been theoretical. It’s starting to get real: Today, after much technical and regulatory wrangling, Harvard University scientists are proposing a June 2021 test flight of a research balloon designed to drop small amounts of chalky dust and observe its effects. 

This first flight would not inject the particles; it would only be a dry run of the steerable balloon and instruments needed to study chemical reactions in the stratosphere, the calm, cold layer more than 10 kilometers up. Even so, the project, called the Stratospheric Controlled Perturbation Experiment (SCoPEx), must first win the approval of an independent advisory board, a decision that could come in February 2021.

The need to study the real-world effects of releasing reflective particles is pressing, says David Keith, a Harvard energy and climate scientist and one of SCoPEx’s lead scientists. Solar geoengineering is no substitute for cutting greenhouse gas emissions, he says, but it could ameliorate the worst damage of global warming, such as the extreme heat waves and storms that claim many lives today. “There is a real potential, maybe a significant potential, to reduce the risks of climate change this century—by a lot.”

Ideas for geoengineering come in many flavors. There are the so-called negative emissions technologies—sucking carbon dioxide out of the air using rocks or trees or machines—that would reduce Earth’s ability to trap heat. Solar geoengineering would reduce the heat Earth receives in the first place. One idea, based on the tracks of ocean ships, is to seed reflective clouds; another is inspired by volcanoes, which can spew sulfate aerosols into the stratosphere and appreciably cool the planet.

But research in solar geoengineering has long been taboo, says Faye McNeill, an atmospheric chemist at Columbia University who is unaffiliated with SCoPEx. “We didn’t want it to appear that we were encouraging it.” One fear is that solar geoengineering could be done unilaterally by groups or nations, with unknown effects on plant growth and rainfall patterns. Another worry is that it would encourage a sort of addiction, adding more and more particles to block warming while not addressing the root problem of mounting emissions. But now, with so much warming already locked in, “the urgency of the climate problem has escalated,” McNeill says.

SCoPEx is not only a science experiment, but also an important test of the governance of geoengineering, says Peter Frumhoff, chief climate scientist at the Union of Concerned Scientists. “We need to learn about the advisory process as much as the experiment itself.” A new wrinkle for SCoPEx is that the flight will be in Sweden, not the southwestern United States, as previously envisioned. The team will now use balloons launched by the Swedish Space Corporation, flying out of Kiruna. “That raises a number of questions around what the role of public consent and informed discussions in Sweden will look like,” Frumhoff says, adding that the advisory board is dominated by U.S. experts. 

For all of the precedents SCoPEx will set, the proposed experiment is quite modest. It will cost several million dollars and has been funded by private donors, including Microsoft co-founder Bill Gates. After much investigation, the team settled on using calcium carbonate—chalk, essentially—as an ideal light-blocking particle. Unlike sulfates, which can lead to ozone loss, calcium carbonate is not particularly reactive. But because it does not exist naturally in the stratosphere, models for its behavior are uncertain, Keith says. “Models rest on previous data. And where that previous data is scanty, it’s important to do a lot of experiments,” both in the lab and field, he says.

When the team is ready for its first research flight, which will depend on the performance of the test flight, the SCoPEx balloon would release up to 2 kilograms of calcium carbonate into the stratosphere and double back to observe the resulting plume. Keith’s previous calculations suggested the particles might help replenish the ozone layer by reacting with ozone-destroying molecules. But now lab experiments from the Harvard team, published today in Communications Earth & Environment, have found the compound to be relatively inert to that chemistry—still a step up from ozone-depleting sulfates, however. 

This lab work, however, only scratches the barest surface of how calcium carbonate will behave in the stratosphere, says Daniel Cziczo, an atmospheric chemist at Purdue University who is skeptical of SCoPEx. “This is the most basic start on the most basic material they’ve proposed,” he says. Even if it doesn’t deplete ozone, calcium carbonate will react with other gases and particles in the stratosphere, changing its composition— and potentially seed clouds in the lower atmosphere that might cool or warm the planet, he says. Much more about the downstream reactions of the altered calcium carbonate should be studied in the lab without any atmospheric release, he adds.

The bar for intentionally releasing particles into the atmosphere needs to be high, even if it is a pittance compared with the aerosols spewed by a single airplane flight, says Alan Robock, a climate scientist and geoengineering modeler at Rutgers University, New Brunswick. “The only reason to do that is if we have scientific questions that can’t be answered indoors.” Decades ago, lab work was enough to figure out the complex chemistry that was depleting the ozone hole, Cziczo says. “Nobody doing ozone depletion work felt they had to go into the stratosphere and cause chemical reactions.” Is SCoPEx, he asks, so different?