A little before 9 a.m. on Tuesday, an engineer named Matthew Gallelli crouched on the deck of a decommissioned aircraft carrier in San Francisco Bay, pulled on a pair of ear protectors, and flipped a switch.
A few seconds later, a device resembling a snow maker began to rumble, then produced a great and deafening hiss. A fine mist of tiny aerosol particles shot from its mouth, traveling hundreds of feet through the air.
It was the first outdoor test in the United States of technology designed to brighten clouds and bounce some of the sun’s rays back into space, a way of temporarily cooling a planet that is now dangerously overheating. The scientists wanted to see whether the machine that took years to create could consistently spray the right size salt aerosols through the open air, outside of a lab.
If it works, the next stage would be to aim at the heavens and try to change the composition of clouds above the Earth’s oceans.
As humans continue to burn fossil fuels and pump increasing amounts of carbon dioxide into the atmosphere, the goal of holding global warming to a relatively safe level, 1.5 degrees Celsius compared with preindustrial times, is slipping away. That has pushed the idea of deliberately intervening in climate systems closer to reality.
Universities, foundations, private investors and the federal government have started to fund a variety of efforts, from sucking carbon dioxide out of the atmosphere to adding iron to the ocean in an effort to store carbon dioxide on the sea floor.
“Every year that we have new records of climate change, and record temperatures, heat waves, it’s driving the field to look at more alternatives,” said Robert Wood, the lead scientist for the team from the University of Washington that is running the marine cloud brightening project. “Even ones that may have once been relatively extreme.”
Brightening clouds is one of several ideas to push solar energy back into space — sometimes called solar radiation modification, solar geoengineering, or climate intervention. Compared with other options, such as injecting aerosols into the stratosphere, marine cloud brightening would be localized and use relatively benign sea salt aerosols as opposed to other chemicals.
And yet, the idea of interfering with nature is so contentious, organizers of Tuesday’s test kept the details tightly held, concerned that critics would try to stop them. Although the Biden administration is funding research into different climate interventions, including marine cloud brightening, the White House distanced itself from the California study, sending a statement to The New York Times that read: “The U.S. government is not involved in the Solar Radiation Modification (SRM) experiment taking place in Alameda, CA, or anywhere else.”
David Santillo, a senior scientist at Greenpeace International, is deeply skeptical of proposals to modify solar radiation. If marine cloud brightening were used at a scale that could cool the planet, the consequences would be hard to predict, or even to measure, he said.
“You could well be changing climactic patterns, not just over the sea, but over land as well,” he said. “This is a scary vision of the future that we should try and avoid at all costs.”
Karen Orenstein, director of the Climate and Energy Justice Program at Friends of the Earth U.S., a nonprofit environmental group, called solar radiation modification “an extraordinarily dangerous distraction.” She said the best way to address climate change would be to quickly pivot away from burning fossil fuels.
On that last point, the cloud researchers themselves agree.
“I hope, and I think all my colleagues hope, that we never use these things, that we never have to,” said Sarah Doherty, an atmospheric scientist at the University of Washington and the manager of its marine cloud brightening program.
She said there were potential side effects that still needed to be studied, including changing ocean circulation patterns and temperatures, which might hurt fisheries. Cloud brightening could also alter precipitation patterns, reducing rainfall in one place while increasing it elsewhere.
But it’s vital to find out whether and how such technologies could work, Dr. Doherty said, in case society needs them. And no one can say when the world might reach that point.
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In 1990, a British physicist named John Latham published a letter in the journal Nature, under the heading “Control of Global Warming?,” in which he introduced the idea that injecting tiny particles into clouds could offset rising temperatures.
Dr. Latham later attributed his idea to a hike with his son in Wales, where they paused to look at clouds over the Irish Sea.
“He asked why clouds were shiny at the top but dark at the bottom,” Dr. Latham told the BBC in 2007. “I explained how they were mirrors for incoming sunlight.”
Dr. Latham had a proposal that may have seemed bizarre: create a fleet of 1,000 unmanned, sail-powered vessels to traverse the world’s oceans and continuously spray tiny droplets of seawater into the air to deflect solar heat away from Earth.
The idea is built on a scientific concept called the Twomey effect: Large numbers of small droplets reflect more sunlight than small numbers of large droplets. Injecting vast quantities of minuscule aerosols, in turn forming many small droplets, could change the composition of clouds.
“If we can increase the reflectivity by about 3 percent, the cooling will balance the global warming caused by increased C02 in the atmosphere,” Dr. Latham, who died in 2021, told the BBC. “Our scheme offers the possibility that we could buy time.”
A version of marine cloud brightening already happens every day, according to Dr. Doherty.
As ships travel the seas, particles from their exhaust can brighten clouds, creating “ship tracks,” behind them. In fact, until recently, the cloud brightening associated with ship tracks offset about 5 percent of climate warming from greenhouse gases, Dr. Doherty said.
Ironically, as better technology and environmental regulations have reduced the pollution emitted by ships, that inadvertent cloud brightening is fading, as well as the cooling that goes along with it.
A deliberate program of marine cloud brightening could be done with sea salts, rather than pollution, Dr. Doherty said.
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Brightening clouds is no easy task. Success requires getting the size of the aerosols just right: Particles that are too small would have no effect, said Jessica Medrado, a research scientist working on the project. Too big and they could backfire, making clouds less reflective than before. The ideal size are submicron particles about 1/700th the thickness of a human hair, she said.
Next, you need to be able to expel a lot of those correctly sized aerosols into the air: A quadrillion particles, give or take, every second. “You cannot find any off-the-shelf solution,” Dr. Medrado said.
The answer to that problem came from some of the most prominent figures in America’s technology industry.
In 2006, the Microsoft founder, Bill Gates, got a briefing from David Keith, one of the leading researchers in solar geoengineering, which is the idea of trying to reflect more of the sun’s rays. Mr. Gates began funding Dr. Keith and Ken Caldeira, another climate scientist and a former software developer, to further their research.
The pair considered the idea of marine cloud brightening but wondered if it was feasible.
So they turned to Armand Neukermans, a Silicon Valley engineer with a doctorate in applied physics from Stanford and 74 patents. One of his early jobs was at Xerox, where he devised a system to produce and spray ink particles for copiers. Dr. Caldeira asked if he could develop a nozzle that would spray not ink, but sea salt aerosols.
Intrigued, Dr. Neukermans, who is now 83, lured some of his old colleagues out of retirement and began research in a borrowed lab in 2009, with $300,000 from Mr. Gates. They called themselves the Old Salts.
The team worked on the problem for years, eventually landing on a solution: By pushing air at extremely high pressure through a series of nozzles, they could create enough force to smash salt crystals into exceedingly small particles of just the right size.
Their work moved to a larger laboratory at the Palo Alto Research Center, a former Xerox research facility now owned by SRI International, a independent nonprofit research institute. Dr. Medrado became the lead engineer for the project two years ago. By the end of last year, the sprayer had been assembled and was waiting in a warehouse near San Francisco.
The machine was ready. The team needed somewhere to test it.
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As the researchers were perfecting the sprayer, a profound transformation was happening outside their laboratory.
Since Dr. Latham first proposed the idea of marine cloud brightening, the concentration of heat-trapping gases in the atmosphere has increased by about 20 percent. Last year was the hottest in recorded history and the World Meteorological Organization projects that 2024 will be another record year. Global ocean temperatures have been at record highs for the past year.
As the effects of climate change continue to grow, so has interest in some sort of backup plan. In 2020, Congress directed the National Oceanic and Atmospheric Administration to study solar radiation modification. In 2021, the National Academies of Sciences, Engineering and Medicine published a report saying the United States should “cautiously pursue” research into the idea. Last month, scientists from NOAA and other federal agencies proposed a road map for researching marine cloud brightening.
Interest is growing overseas, as well. In February, an Australian team of researchers at Southern Cross University, which was advised by Dr. Neukermans, conducted a monthlong experiment off the country’s northeast coast, spraying aerosols from a ship and measuring the response of clouds.
Daniel P. Harrison, the lead researcher, called the tests “the smallest of baby steps aimed at confirming and refining the underpinning theory in the real world.” He said it was too early to discuss any findings.
Private funding is also growing. Kelly Wanser is a former technology executive who helped establish the marine cloud brightening project at the University of Washington. In 2018 she created SilverLining, a nonprofit organization to advance research into what she calls “near-term climate interventions” like cloud brightening.
Ms. Wanser’s group is contributing part of the funding for the research at the University of Washington and SRI, which is budgeted at about $10 million over three years, she said. That includes the study aboard the Hornet, which is expected to cost about $1 million a year.
Finding money for that work has gotten easier as record heat has “really shifted attitudes” among funders, Ms. Wanser said. Donors include the Quadrature Climate Foundation, the Pritzker Innovation Fund and the Cohler Charitable Fund, established by the former Facebook executive Matt Cohler, according to Ms. Wanser.
Last year, Ms. Wanser spoke with a member of the board that runs the Hornet, which now operates as a museum affiliated with the Smithsonian. Would they host a first-of-its-kind study?
The museum agreed. The test was a go.
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The flight deck of the Hornet rises 50 feet above the shore of Alameda, a small town on the east side of San Francisco Bay. On Tuesday, it held a series of finely calibrated sensors, perched atop a row of scissor lifts reaching into the air.
Underneath a United States flag at the far end of the flight deck was the sprayer: Shiny blue, roughly the shape and size of a spotlight, with a ring of tiny steel nozzles around its three-foot-wide mouth. The researchers call it CARI, for Cloud Aerosol Research Instrument.
On one side of the sprayer was a box the size of a shipping container that housed a pair of compressors, which fed highly pressurized air to the sprayer through a thick, black hose. On the other side was a tank of water. A series of switches, turned in careful sequence, fed the water and air into the device, which then shot a fine mist toward the sensors.
The goal was to determine whether the aerosols leaving the sprayer, which had been carefully manipulated to reach a specific size, remained that size as they rushed through the air in different wind and humidity conditions. It will take months to analyze the results. But the answers could determine whether marine cloud brightening would work, and how, according to Dr. Wood.
Ms. Wanser said she hoped the testing, which could continue for months or longer, will demystify the concept of climate intervention technologies. Toward that aim, the equipment will remain on the Hornet and be on display during hours when the ship is open to the public. Even if the equipment is not ultimately used to cool the planet, the data it generates can add to the understanding of how pollution and other aerosols interact with clouds, the researchers said.
Dr. Wood estimated that scientists could need another decade of tests before they were in a position to potentially use marine cloud brightening at the scale required to cool the Earth.
Ms. Wanser is already looking ahead to the next phase of that research. “The next step is go out to the ocean,” she said, “aim up the spray a little higher, and touch clouds.”
