Why a plan to brighten clouds and cool the planet is so controversial
A city council meeting in Alameda, Calif. on Tuesday will take center stage in the global controversy over whether to try cool the planet by making clouds brighter.
Researchers at the University of Washington are studying a concept called “marine cloud brightening,” which aims to slow climate change by spraying clouds with sea salt. Salt particles help clouds form tiny, shiny water droplets, which reflect sunlight away from the earth before it can heat the planet.
In April, University of Washington scientists started testing a saltwater spraying machine on the deck of the USS Hornet, a retired aircraft carrier docked in Alameda. The city paused the experiment in May, citing health and environmental concerns — but outside consultants hired by the city later concluded the test doesn’t pose “a measurable health risk to the surrounding community.”
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The Alameda experiment isn’t meant to “alter clouds or any aspect of the local weather or climate,” according to Sarah Doherty, a University of Washington atmospheric scientist who runs the university’s marine cloud brightening program. The scientists are only testing whether their salt spray machine works and studying how salt particles move through the air.
“Frankly, it was about as innocuous an experiment as one can do,” said Gernot Wagner, a climate economist at Columbia Business School who wrote a book on planet-cooling technologies, “Geoengineering: the Gamble,” and is not involved in the study.
The episode highlights the stiff opposition scientists face when they research anything related to geoengineering, a broad category of techniques that aim to manipulate the climate. Some environmentalists argue that these ideas could have dangerous, unpredictable side effects — and are a distraction from cutting carbon emissions, the most surefire way to avoid climate change.
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“Geoengineering experiments, like the Marine Cloud Brightening project in the Bay Area, set a dangerous precedent and risk legitimizing a highly-speculative and harmful technology,” wrote Mary Church, who heads geoengineering advocacy for the Center for International Environmental Law (CIEL), an American and Swiss environmental nonprofit.
Environmental groups including the CIEL are calling on Alameda officials to end the University of Washington experiment. City council members will decide Tuesday whether the researchers can continue their study, which they hope to run for several more months.
What is marine cloud brightening?
Marine cloud brightening attempts to cool the planet by reflecting more sunlight back into space. Some scientists hope it could buy humanity more time to cut carbon emissions — or protect overheated ocean environments such as the Great Barrier Reef.
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The fluffy, white tops of certain clouds act like a natural sunscreen for the planet; the water droplets and ice crystals within reflect 30 to 60 percent of sunlight that hits them, according to NASA. Geoengineering researchers believe they can make clouds brighter — and increase their cooling effect — by increasing the number of droplets they contain.
Since 1990, researchers have theorized they could do this by spraying clouds with sea salt particles, which give the moisture in the air something to glom onto so they can form water droplets, or ice crystals. This already happens naturally when ocean winds blow sea foam high into the air, but scientists believe they can amp up the process to noticeably lower the temperature underneath a cloud.
But scientists don’t have machines that can reliably spray sea salt particles at the right size and in the right quantity to alter clouds, making it hard to try this in the real world. The experiment in Alameda is meant to test a new salt spray machine to see if it works outside of a lab — and to study some basic physics about how particles move through the air.
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Doherty stressed that the University of Washington researchers are not trying to brighten clouds in Alameda, but added that the experiment will help “study how clouds respond to particles … in the atmosphere and how this influences climate, including both the effects of pollution aerosols and the potential for brightening marine clouds to reduce climate warming.”
The shipping industry ran what amounted to an accidental test of the idea for decades, by emitting tons of sulfur dioxide into the atmosphere from ships’ smokestacks. The sulfur particles, like salt, helped form water droplets in clouds. When new rules forced the ships to stop emitting sulfur in 2020, ocean temperatures rose — largely because ocean clouds were no longer as bright, according to a study published last month in Communications Earth & Environment.
Australian researchers at Southern Cross University began a small experiment with marine cloud brightening near the Great Barrier Reef in 2020 but haven’t published conclusive results.
Why is marine cloud brightening controversial?
Some environmental groups oppose marine cloud brightening and other geoengineering techniques because they worry altering planetary systems will have unintended consequences and give polluters an excuse to keep pumping carbon into the atmosphere.
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More than 70 environmental nonprofits and activist groups wrote an open letter opposing this line of research last month. “Geoengineering our oceans is a dangerous distraction from the real solutions to the climate crisis and gives the fossil fuel industry a potential escape hatch while putting our oceans and coastal communities at serious risk,” they wrote.
Earlier this year, Harvard scientists gave up a decade-long quest to test a different geoengineering tactic that would involve releasing particles from a hot-air balloon high into the stratosphere to reflect sunlight. The researchers tried and failed to get approval to launch the balloon from Arizona, New Mexico and finally Sweden, whose government canceled the experiment under pressure from the Saami Council, which represents Indigenous groups in Finland, Russia, Norway and Sweden.
“There’s a fair number of people who think there shouldn’t be research [on geoengineering], and these early experiments have become a proxy battleground for this larger question about how to think about the development of these technologies,” said David Keith, who now directs the Climate Systems Engineering Initiative at the University of Chicago and used to be involved in the Harvard geoengineering experiment.
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Local fights over small experiments like the one in Alameda are likely to define the future of geoengineering research in the coming years, Keith said.
“This generation is not likely to be the one that makes decisions about actually deploying these technologies,” he said. “Those will only get made in 20 years by the next generation. Right now, our only real choice is: Do we research them or do we not?”
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