Cloudy with a chance of cooling the planet
Could reflecting more sunlight help tackle global warming? A cadre of retired physicists and engineers have decided to try.
IN THE BEGINNING, the story goes, the Earth was formless and empty, and darkness shrouded the surface of the deep. Several millennia later, an independent scientist named James Lovelock started to ponder this primordial arrangement: He imagined a fictional planet called Daisyworld, carpeted with black and white daisies. The pale flowers reflected the sun, while the dusky ones absorbed light and heat. As the sun’s rays grew more powerful, the balance of blossoms shifted, stabilizing the globe’s temperature.
A similar process, Lovelock speculated, might have maintained Earth’s temperature over geological time. He believed the planet’s systems could self-regulate, and in the 1970s, he proposed what came to be known as the Gaia hypothesis. In 1987, shortly after Congressional hearings on climate change—during which a Republican senator from Rhode Island concluded that “the scientific evidence … is telling us we have a problem”—Lovelock was no longer satisfied with metaphors. Along with a team of atmospheric scientists, he formed a less hypothetical model in which the amount of algae in the ocean influenced cloud development, not unlike his daisy effect.
Soon, a cloud physicist took things one step further. In 1990, John Latham, a professor at the University of Manchester Institute of Science and Technology, suggested that low-lying clouds over the ocean could intentionally be made more reflective to compensate for greenhouse gas emissions making Earth hotter. In 2002, outlining a process later dubbed marine cloud brightening, Latham proposed that ships could spray sea salt into the atmosphere, where the additional particles would help water vapor condense. That, then, would increase the number of droplets in the clouds—and, like white daisies, send more sunlight back into space.
In the 30 years since Latham first proposed cloud brightening, the warming effect of carbon in the atmosphere has increased by more than 40 percent. As temperatures rise, global changes in events like droughts, wildfires, and extreme rainfall are happening faster than scientists initially predicted. With the climate crisis accelerating, the question has become more pressing: Can tinkering with clouds really cool the planet?
ON THE MANICURED grounds of the Palo Alto Research Center, a roof’s low silhouette is abruptly interrupted by a tall white event tent. Even for the Silicon Valley–based research and development company, the experiment inside is a moonshot. A giant fan spins to life as engineers test specialized instruments designed to measure tiny aerosols. Then someone opens a valve on a compressed air pump, and with a loud whir, a nozzle begins to spray seawater. The tent fills with gauzy mist, shot through with California sunshine. “It’s like being inside a cloud,” says Sarah Doherty, an atmospheric physicist and senior research scientist at the University of Washington. The water droplets dry quickly, but the salt crystals they contained are left hanging until the air tastes like tears.
Influenced by Lovelock and Latham’s ideas about maintaining a stable planetary temperature, a small group of retired physicists and engineers from some of the world’s top tech companies gather here every week with the goal of developing a system that can spray seawater from ships to brighten marine clouds. Affectionately nicknamed the Old Salts, many are in their 70s, some well into their 80s. “We’re officially getting old,” says Armand Neukermans, a mechanical engineer and applied physicist, but “climate change isn’t going to just go away.”
Neukermans helped create the first generation of inkjet printers at the Hewlett-Packard Company, where he met Lovelock when the visiting scientist was a consultant for the tech giant. Lovelock would later open Neukermans’ eyes to Latham’s vision of cloud brightening. Over the course of his career, Neukermans spent a lot of time trying to design nozzles that could spray toner as uniformly sized droplets. Today, he’s applying those lessons to generate consistently tiny salt crystals, between 30 and 100 nanometers in diameter, orders of magnitude smaller than the holes in a printhead. Atmospheric scientists have learned that clouds form around particles called cloud condensation nuclei, which help water vapors condense—and reflect sunlight. Sizing the salt particles just so turns them into extra nuclei: Too small, and the particles won’t work; too big, and they not only take more energy to spray aloft, but they can have the opposite effect and trigger rainfall, causing cloud loss.
The clouds best suited to tinkering are low-lying ones called stratocumulus, which naturally occur over colder oceans—like the Pacific off the coasts of California and Chile. Because stratocumulus cover about one-fifth of the Earth’s oceans, brightening them has the potential to collectively make a big impact. A 2021 model from a University of Washington scientist suggests that making them 16 percent more reflective could roughly offset a doubling of carbon dioxide, buying the world more time to reduce emissions.
In fact, things like dust, pollution, and salt from ocean spray already have this effect. Exhaust from shipping vessels, for example, helps water vapor condense. This has made clouds along major shipping channels more reflective. But as smokestacks and shipping fuel get cleaner, this silver lining is starting to evaporate.
The overall picture is much dimmer. Using satellite observations, a September 2022 study published in Atmospheric Chemistry and Physics found that aerosols’ cooling effects, including their interactions with clouds, has decreased by 30 percent since 2000. That essentially amplifies the amount of warming from recent carbon dioxide emissions by as much as half, writes Johannes Quaas, a lead author and a climate scientist at Leipzig University.
Cloud research also has potential far beyond climate engineering. These commonplace wonders have a huge impact on global temperature, but are still one of the least understood elements of the atmosphere. Brightening gives investigators a novel way to conduct controlled experiments by intentionally introducing particles and studying their effects. The results could improve the accuracy of predictions and guide progress on potential mediations. Federal funding for this and other interventions, Doherty says, will also advance basic science.
As alarm grows over rising temperatures, the Old Salts aren’t the only people working on ways to reflect more sunlight and slow climate change. The top circles of academia and government are increasingly discussing these kinds of tactics, often called solar radiation management or geoengineering. Ideas include removing cirrus clouds, which absorb long-wave radiation and warm the planet, and—inspired by the cooling effect of volcanic eruptions—injecting sulfates or other compounds like calcium carbonate into the stratosphere to reflect light.
Some researchers are leery of these schemes, fearing they could destabilize ecosystem processes, intensify problems like ocean acidification, or create all-new problems. But the Old Salts view the effort as an insurance policy. In the absence of real progress in reducing emissions, many scientists now see this course as one that must be explored.
Increasingly, they warn of irreversible tipping points—like the looming collapse of the Thwaites glacier—that make it urgent to find new ways to control the crisis. Temperatures have increased so much that a September 2022 paper in the journal Science predicts that critical planetary functions may be disrupted even if emissions are curtailed. “I hope we never have to use marine cloud brightening,” Doherty says, “but I just don’t see us on a pathway to avoiding dangerous levels of climate warming.”
NEUKERMANS’ WORK on clouds started as an ad hoc effort. In the late 2000s, he began pulling together bright minds interested in addressing climate change. Sudhanshu Jain, a retired electrical engineer who designed computer chips for 25 years before winning a Santa Clara city council seat in 2020, met Neukermans at a fundraiser and wanted to see what he was up to. “And I just never left,” he recalls. “That’s the kind of guy he is.”
As the group of high-powered retirees grew, garnering its briny nickname, member Bob Ormond, who worked at a company that makes automated water treatment systems, offered the friends a borrowed room in his company’s offices in nearby Sunnyvale. Around 2009, Bill Gates gave them $300,000 in seed money. Once they burned through that, Neukermans recalls, “Everybody worked for free, and I paid the expenses.”
As longtime Silicon Valley entrepreneurs, the Old Salts were aided by their connections. At one point, they needed sheets of synthetic diamond film to make spray nozzles. “I traded the sheets for a couple of boxes of biscotti,” Neukermans says.
For expertise in atmospheric science, they turned to collaborators like Doherty and others at the University of Washington. The new additions helped them understand that not all clouds respond equally to additives: Some are already at their maximum brightness, or heavily influenced by other processes. Stratocumulus clouds tend to have lower droplet concentrations, so adding salt particles that act as nuclei can increase how reflective they are. If less sunlight hits the heat-absorbing dark water beneath them, that maximizes the cooling effect.
How, precisely, this research will transfer from the lab to an intervention on the ocean is still in the early stages of planning. But the idea is that a Goldilocks-calibrated nozzle, capable of spraying just the right size of salt crystal, would continuously run from a boat positioned under a marine cloud. “To do this at scale, you’d need thousands of ships over a large area,” Doherty points out. “But as long as that system keeps working, you’d have a steady supply of seawater.”
Initially, the Old Salts ran into many hurdles with their tech: Early attempts clogged nozzles, and the saltwater corroded equipment. But as grim new weather vocabulary like heat dome entered the lexicon, interest in their work surged. In 2015, the idea of climate interventions reached sufficient prominence that the National Academies of Sciences, Engineering, and Medicine published a report examining possible strategies. Lynn Russell, a distinguished professor at the Scripps Institution of Oceanography at the University of California, San Diego, who has spent her career studying aerosols and clouds and served on the report committee, says her impression was that the discussions focused on whether the research should even be pursued.
By 2019, the Old Salts had moved out of their borrowed offices and into the Palo Alto Research Center, where they finally had access to things like a machine shop, as well as the expertise PARC offered. And “Armand wanted younger people to get involved,” says Sean Garner, a general manager at Xerox, which owns PARC. In its new home, the team tried a fresh approach: It designed an effervescent spray nozzle that pumps air and saltwater through a single channel. This forces the liquid to the sides and compresses the air so that it leaves the nozzle at the speed of sound. Released into a much lower-pressure environment, the air explodes the surrounding ring of water, forming those elusive tiny droplets.
Jéssica Medrado, a young mechanical engineer at PARC who is collaborating with the Old Salts, says the system has finally proved capable of consistently generating droplets of the right size, and they are now trying to improve its efficiency—reducing the energy needed. Medrado is enthusiastic about the project; she’s originally from Brazil, where the dangers of climate change have become increasingly clear. Near her parents’ house, a woman recently drowned in a flash flood after being pulled into a street drain. “People shouldn’t be dying like that,” she says with horror. Such stories keep her motivated, even when the team hits roadblocks.
In 2021, the National Academies produced a second geoengineering report. This time, there was no hesitation: They recommended the US embark on brightening research and laid out a plan for designing and governing the programs. Chris Field, the director of the Stanford Woods Institute for the Environment, who worked on the report, says the planet’s trajectory has made him “much more open to looking at a broader range of options.” UCSD’s Russell agrees the need to research these kinds of strategies has only grown since the 2015 review. Even if the technology can be developed, though, she’s concerned about the risks. “The smart idea is reducing carbon emissions,” Russell says. “Marine cloud brightening, or any kind of climate engineering, is a dumb idea—but it may not be as dumb as doing nothing.”
EVENTUALLY, the team at PARC will need to try its equipment outdoors. Though it’s found a nearby industrial location suitable for a trial, it’s run into resistance to testing in the real world.
“One of the things people misunderstand is how far we are from being able to field any of these systems,” Xerox’s Garner says of climate interventions. “The role of science here is to reduce uncertainty and give policymakers tools so they can decide to use them down the road.”
To date, only one cloud brightening project has made it outside. In 2020, researchers from Australia’s Southern Cross University took a first step as part of emergency efforts to mitigate the effect of rising ocean temperatures on the Great Barrier Reef. They tested only whether the salt particles—sprayed by boat-mounted prototype nozzles that Neukermans advised on—could be detected in the atmosphere. The trial succeeded, and next the researchers hope to scale up and cool the ocean over the bleaching corals. “I’m glad they’re working on the reef with urgency,” Garner says, but he notes that it’s not an area known for stratocumulus clouds, meaning its brightening potential may be low.
In 2021, a Harvard-supported team hoped to conduct an outdoor experiment to advance its own solar geoengineering research, which, unlike the Old Salts’ efforts, centers on introducing particles like sulfates into the stratosphere. The researchers planned to deploy a balloon-powered gondola and equipment in northern Sweden as a first step toward testing brightening aerosols. But the Indigenous Saami council wrote a letter criticizing the project, stating the plans “constitute a real moral hazard,” adding that the technology “entails risks of catastrophic consequences.” An advisory committee subsequently ruled the trial should be delayed.
Each climate intervention strategy carries its own benefits and risks. The science behind stratospheric aerosols injections—aptly demonstrated by large volcanic eruptions—is fairly well understood. But Russell says they can persist for a year or more, and some of the compounds proposed, like sulfates, can have toxic effects. Some scientists also warn that if, say, a government were to suddenly halt an intervention, the resulting “termination shock” could cause temperatures to rebound sharply, accelerating warming.
Marine cloud brightening research has run into criticism as well. Though salt dissipates from the atmosphere within a week or two, the Old Salts acknowledge that the method may have unintended consequences. Adding brine might harm local ecosystems, and, if done broadly enough to have a cooling effect, it could also redistribute precipitation—perhaps dramatically. “We’re not going rogue,” Jain says about their efforts. “We’re actually saying, ‘Let’s put in place governance and good, peer-reviewed science before you have a bunch of rogue efforts.’”
If marine brightening and other programs move forward, it will be important for scientists to engage community members in upfront conversation about the risks. “If in advance you know that decreasing global average temperature makes drought more likely in the Philippines, what do you do?” Stanford’s Field asks. As a contributor to Intergovernmental Panel on Climate Change reports since 2007, he understands the drive to look at drastic interventions, given the planet’s trajectory. But he believes it’s critical to include junctures when researchers or governments can decide to stop further efforts. “Say after the fact, you learn that an intervention could result in a partial failure of the Indian monsoon—you may need an exit ramp,” he explains.
Field argues that initial interventions, at least, would be technically straightforward to stop. “People view solar geoengineering as a fork in the road that, once you take, you’re totally committed to,” he says. “That might be true at high levels, but if we get to a point where we want to deploy this stuff, it’s relatively easy to turn on and off.” The Old Salts argue it’s important to conduct this research in order to determine safe parameters ahead of any possible deployment.
The National Academies now recommend the US develop a program to navigate all these concerns, complete with a code of conduct, permitting systems for experiments, and an expert committee to help make difficult decisions on an international level—essentially providing guardrails. To do so will require funding: “A reasonable initial investment for this program is in the range of $100-200 million total over 5 years,” the National Academies concluded in its 2021 report, though the authors note that this should remain a small fraction of a budget for national climate research.
Currently, funding for climate intervention research is piecemeal. SilverLining, a nonprofit dedicated to climate research, for example, has given $3 million in grants to investigate mitigations. Recent federal spending bills included $13 million for the National Oceanic and Atmospheric Administration to study the stratosphere. Some of that has gone toward establishing baseline information, including about the ozone layer, the region that absorbs most of the sun’s ultraviolet radiation. The agency is deploying high-altitude balloons to measure the aerosol particles that reflect light in the stratosphere. Such data is bound to advance our knowledge of how clouds actually work.
That’s critical information for climate change projections. Models don’t generally agree on which clouds will grow or shrink as temperatures increase—a primary reason current warming estimates vary so dramatically. “It’s one of the biggest uncertainties related to climate,” Doherty says. Since satellites started tracking clouds, the types targeted by marine brightening have been a constant fixture in the skies, “but as the planet gets warmer, there’s no guarantee.”
Meanwhile, back in California, the team hopes to expand its own testing. It would like to try out its nozzle system under less controlled outdoor conditions within the next year. Neukermans is still going into the lab, although recent medical issues have sometimes made it painful for him to be on his feet for long. “He’s really suffering, but he shows up,” Jain says. The pandemic has highlighted everyone’s mortality. “We’re in a hurry. We don’t have time.”
One recent afternoon, Neukermans put down his calculations to think about his ten grandchildren, and the world he is leaving them. “There is nothing we do that doesn’t have consequences,” he says with a sigh. “We have to try everything—we don’t know exactly what’s going to work.”
This story originally appeared in the High Issue of Popular Science. Read more PopSci+ stories.