What is ‘cloud brightening’—and why did California just stop scientists from studying it?

When it comes to curbing the climate crisis, reducing our emissions won’t be enough on its own. Experts say we’ll likely need to take additional steps, like carbon removal or reflecting sunlight back into space.



One method of this so-called solar engineering is called cloud brightening, or marine cloud brightening, in which droplets of sea salt would be sprayed into clouds in order to reflect sunlight and cool the Earth. But this approach is controversial.



Recently, a study by the University of Washington and the Coastal Aerosol Research and Engagement Program on the USS Hornet got caught up in this controversy. Officials in Alameda, California, halted the research due to health concerns, referring to it as a “cloud brightening study.” (Local media picked up that language as well.)



But the study wasn’t actually an experiment on cloud brightening. Instead, it was research on how particles move through the atmosphere. That could eventually inform scientific models of cloud brightening, but it’s also important for understanding the effects of pollution.



This isn’t the first study related to cloud brightening that has faced pushback. But experts in the solar engineering field say concerns about the technology have no merit. Research that leads to more understanding of cloud brightening doesn’t mean the technology will be deployed, they add. But delaying such research could be harmful—especially because pollution is already causing cloud brightening. 



What is cloud brightening?



The idea of cloud brightening as a climate solution is to reflect the sun’s energy back into space. It’s one example of solar geoengineering —also called solar radiation management—along with actions like stratospheric aerosol injection (which mimics the eruption of a volcano by sending aerosols up into the stratosphere), space sunshades (in which mirrors are sent into space to reflect sunlight away from the planet), and others . 



Marine cloud brightening involves spraying saltwater into clouds above the ocean. The particles of salt then become “cloud condensation nuclei,” around which condensation forms. With more condensation in the air, clouds appear fuller and brighter. Brighter clouds then reflect more sunlight away from the Earth’s surface, offering a cooling effect.



One benefit of cloud brightening, researchers say, is its potential for specificity: Clouds could potentially be brightened just over coral reefs, Douglas MacMartin, a Cornell University researcher, told Carbon Brief , or over areas in the ocean that produce hurricanes, in order to reduce their strength. 



Cloud brightening is different from cloud seeding , which looks to stimulate clouds into producing more rain. “Cloud seeding is not a way to manage the climate,” says David Keith, a geophysical sciences professor at the University of Chicago. Instead, seeding clouds is a way to address drought. 



To use cloud brightening as a climate mitigation strategy would involve machinery that sends the salt particles up into clouds. (Prototypes that create small plumes have been used in some studies, such as the Alameda one, but there hasn’t yet been any large-scale, mass deployment of this equipment). But aerosol particles are already being released into the atmosphere through pollution. Think of the smog or haze that covers cities like Los Angeles, when particulate matter from car exhaust or wildfires fills the air. Those particles are already reflecting sunlight away from Earth and into space, essentially acting as cloud brightening. 



This reality poses a major concern for researchers. As we work to reduce pollution, we could inadvertently cause more global warming, because we’re removing the cloud brightening that pollution causes. A 2020 study found that China’s 10-year effort to improve its air quality and reduce pollution has actually caused more warming across the northern hemisphere. A 2024 study says specifically that recent heat waves in the Pacific may be a result of less pollution from factories in China. 



“Over the next 20 years, we’re expected to clean most of the [aerosol pollution] up, which means that we have this uncertainty about how much warming we’re going to get,” says Kelly Wanser, cofounder of and senior adviser to the University of Washington Marine Cloud Brightening Program. (Wanser is also the executive director of SilverLining, a nonprofit working with the university on this research.) Atmospheric experts say we need to study the effects of cloud brightening in order to better inform models that can tell us how much reducing pollution will warm the planet. 



Cloud brightening controversy 



Cloud brightening isn’t a new idea; Keith has been researching it since the 1980s. And over those years, he’s faced concerns and criticisms about his work. An experiment related to solar geoengineering he was involved with in 2021 was even halted after backlash from environmental organizations (even though that experiment wasn’t planning to release any particles).  



One of the most common concerns that critics raise is around “moral hazard—that even working on solar geoengineering will distract from the need to cut emissions,” Keith says, noting that it’s not a reason to avoid research. The same concern has been raised around carbon removal and carbon offsets. 



There have also been questions about the risks to either the environment or to people. Alameda city officials halted the study in California. “City staff are working with a team of biological and hazardous materials consultants to independently evaluate the health and environmental safety of this particular experiment,” officials said in a statement . “In particular, the City is evaluating the chemical compounds in the spray to determine if they are a hazard either inhaled in aerosol form by humans and animals, or landing on the ground or in the bay.” 



The University of Washington said in a statement that the plumes of saltwater spray emitted through the study were “well below established thresholds for environment or human health” impacts. Still, researchers say the pause isn’t unusual, and they were already in talks with officials to pause the study while the city does its review. “We respect the care that they take in looking after the environment of Alameda,” Wanser says. (Though there’s no timeline yet for the California study to resume, the city said it expects findings from its evaluation in June.)



One concern underlying the backlash has to do with the idea that we shouldn’t be manipulating the environment or altering the climate. But we already are: Pollution from cars, factories, planes, and more already adds particles into the air. The dramatic rise in greenhouse gas emissions has already affected the planet’s climate.



“The entire purpose of solar engineering is to reduce the amount of climate change,” Keith says. “It is certainly trying to change the climate, but it’s trying to change the climate in a way that reduces the human impact on the Earth, not increases it.” 



The case for research



Despite the controversy about solar engineering, and cloud brightening specifically, researchers say this work is still crucial. “There is increasingly strong evidence from climate models, from analogs, from basic physics for 25 years now that suggests that it really could reduce climate risk significantly,” Keith says. “And that the added risks might actually be quite small compared to the benefits.” 



Groups that oppose cloud brightening are within their rights not to support it, he adds. “But society has a right to want to know more. We want to know more about these technologies, including finding out maybe they don’t work. Or finding out that they work better.”



Studying how particles affect the atmosphere isn’t an endorsement of cloud brightening either. It’s not likely to be deployed for decades, if at all. But without conducting the research now, future generations won’t have the necessary information to decide whether to pursue it as a mitigation strategy.



And again, there’s already a cloud brightening phenomenon happening with pollution. Studies on particles in the atmosphere can help scientists learn just how much warming we might see when we improve air quality around the world. “This whole pathway of studying more closely how aerosols move in the atmosphere, especially aerosols relevant for the pollution aerosol problem—this is the biggest uncertainty in climate change,” Wanser says.



Solar geoengineering, then, could be a replacement for this cooling effect of pollution, once that pollution is reduced. “We’re doing this already. Could we study whether there’s a clean, safe way to try to do it? That’s really what this program is,” she adds. “It’s a direct analog, because pollution is flowing up into the low cloud layer. And the question is, well, could we use sea salt, which lives in nature, to flow up into the low cloud layer, to brighten clouds instead?”