Marine Cloud Brightening Could Weaken Super El Niños, Study Says

Marine cloud brightening, a once-fringe solar geoengineering idea, could be aimed directly at a single El Niño event to blunt its worst effects, a team of Scripps Institution of Oceanography researchers reported on July 8 in the journal Science Advances. The team built a climate model that ran deliberate Pacific cloud brightening against the historically strong El Niños of 2015 and 1997, and found it could weaken their global footprint while boosting the cooling and drying effects normally associated with La Niña by more than 40%. Independent scientists who reviewed the work call the modeling credible while warning that deploying it would be a very different problem from running it on a supercomputer.

The timing is not a coincidence. A Super El Niño is forming in the tropical Pacific right now, and the same recipe of droughts, intense rains, floods, and Pacific cyclones has cost the global economy hundreds of billions of dollars in past strong events. The Scripps team’s argument is that regional, temporary brightening could be a fast-acting tool layered on top of flood control and disaster preparation. Most geoengineering research has instead chased decades-long planetary cooling programs.

A Super El Niño Is Already Forming in the Pacific

This year’s El Niño is on track to be among the strongest on record, and it is already reshaping weather patterns around the world. El Niño develops naturally in the tropical Pacific every few years, when weakened trade winds push heat from the ocean toward the coast of South America. That shift tilts the odds toward higher-than-average global temperatures, as well as droughts in some regions, intense rains and floods in others, and more cyclones across the Pacific. Piled on top of warming driven by burning fossil fuels, a strong El Niño can mean hundreds of billions of dollars in economic losses.

One reason researchers are calling the new modeling urgent is that the same compounding that makes an El Niño expensive also makes it dangerous. A strong El Niño on top of a warming climate stacks two weather risks, and the combined effect can multiply the damage. That is the case the new Science Advances study, led by Kate Ricke and Jessica Wan of Scripps, tries to address with a regional geoengineering fix.

• Australia’s 2019 to 2020 “Black Summer” bushfires produced almost 1 million metric tons of smoke into the stratosphere.
• More than 10,000 bushfires burned across Australia in that season.
• Australia’s bushfire smoke also helped trigger a rare triple-dip La Niña after 2020.
• Strong El Niños have cost the global economy hundreds of billions of dollars in past events.

How Cloud Brightening Could Cool the Pacific

Marine cloud brightening, or MCB, is one of a handful of solar geoengineering methods designed to reflect sunlight back into space. Where other techniques such as planes injecting aerosols into the stratosphere can only work globally, MCB has the potential to act as a regional cooling solution.

The technique itself is straightforward in concept. It entails spraying seawater into low-lying marine clouds, which makes the cloud droplets smaller and more numerous, enhancing the clouds’ reflectivity. Some pilot projects and randomized controlled trials have tested the technique’s efficacy, but only on very small scales. The Scripps team did not run a new field experiment; they used what nature handed them.

Geoengineering techniques have traditionally been framed as planetary countermeasures to the use of fossil fuels, an extremely controversial approach. The new study makes the case that some forms of geoengineering would be better aimed at regional events such as El Niño.

The pitch is not that MCB should be turned on across the Pacific tomorrow. Kate Ricke, a climate scientist with appointments at Scripps and UC San Diego’s School of Global Policy and Strategy, frames it as a tool to layer on top of existing flood control and disaster preparation. “The reason people do research on solar geoengineering is because we might end up in a world where we need it,” Ricke said.

The 2019 Australian Bushfires as a Natural Test

Researchers could not test MCB in the real world for fear of disastrous unintended consequences, so they turned to a natural experiment instead. Australia’s Black Summer bushfires in 2019 and 2020 incinerated tens of millions of acres and contributed to the deaths of hundreds of people. The fires also produced plumes of smoke filled with sun-reflecting particles, which mixed with clouds over the Pacific Ocean. That smoke was, in effect, an unplanned version of what deliberate cloud brightening would do.

The scale was unusual. More than 10,000 bushfires raged across the country, producing almost 1 million metric tons of smoke, one of the largest inputs of smoke into the stratosphere that humans have observed with satellite technology. The reflective particles in that smoke brightened the clouds, increasing the amount of solar radiation bouncing back to space, and previous research found this had helped trigger a rare triple-dip La Niña, the opposite phase of El Niño.

This event, Ricke said, enabled her and her coauthors to address a question they had been sitting on for years: whether regional interventions can help relieve the pressure events like El Niño put on the global climate system. The team created a model based on the bushfire smoke’s brightening effect and ran it against two historic El Niño events. The 2023 study that mapped the bushfire smoke was led by John Fasullo of the National Center for Atmospheric Research, and Fasullo is also a co-author of the new paper. “It was the key breakthrough to this becoming a viable research question,” Ricke said. “Without that validation opportunity, I don’t think our findings would be so credible.”

What the Model Showed

The Scripps team isolated the cloud brightening impacts of the Australian fires and used a seasonal forecasting model to simulate the effect of a similar brightening event happening before two historically strong El Niños that originated in 2015 and 1997. The simulation suggested that targeted marine cloud brightening would indeed attenuate each event’s effects. The earlier in an El Niño the technique is deployed, the more effective it would be, the study concluded.

If deployed over the central Pacific, marine cloud brightening could increase the cooling and drying effects associated with La Niña by more than 40%. The researchers also reported that the approach could significantly reduce the magnitude of those El Niño events and their global impact. The point is not to stop an El Niño, but to shave its worst edges. Economic analyses cited by the authors show that recent large El Niños have cost society trillions of dollars in damage.

Wan, the paper’s co-lead author, framed the regional approach as a way to break the long-deployment deadlock that has hung over solar geoengineering. “If we could target natural variability, we could get some of the benefits of geoengineering without having to employ it indefinitely,” she said.

Ricke made a similar argument. “One of the biggest social concerns around geoengineering is the fact that if we use it to reduce long-term climate risks, we have to deploy it continuously for an indefinite period of time,” she said. The El Niño case is different, she argued, because the intervention could be short and tied to a specific event, and the work is “just a proof of concept … the only thing we’ve shown is that it’s worth further study.”

The Scripps release is careful on what it is not claiming, with the researchers saying they are not aware of any proposals to test this on the El Niño brewing now. The team is also not arguing for actual deployment. They argue that a regional tool targeting super El Niños is at least scientifically plausible enough to be researched, a point laid out in the climate model showing cloud brightening weakens El Niños and in the marine cloud brightening El Niño paper in Science Advances.

It’s a different way of thinking about geoengineering. We need to understand a lot more, but if there is a way to use this in addition to the risk reduction tools to mitigate El Niños, why wouldn’t we consider it?

Why Some Scientists Urge Caution

Outside scientists are quick to separate the modeling from the implementation. “The thesis seems quite reasonable,” said Andrew Dessler, a professor of atmospheric science at Texas A&M University, of the Scripps study. “I think this is a really interesting paper, and I learned a few things reading it, but I certainly would not say that this is a great idea and we should implement it.” James Haywood, a professor of atmospheric science at the University of Exeter, said there remain “many, many unanswered questions and uncertainties as to the viability of marine cloud brightening” in terms of controlling its cooling impact. The technique can be steered only as well as the clouds it is targeting can be predicted. “You have to think very carefully about trade-offs,” Ricke acknowledged.

The main scientific worry is a swing too far in the other direction. La Niñas can bring extreme weather too, including increased rainfall and flooding in parts of Asia and Australia, and drier-than-normal conditions in parts of South America and the United States. Haywood raised the possibility of a mega La Niña “many, many times stronger than we’ve experienced before.”

Not every region loses out in an El Niño. Some are adapted to its impacts; California, for instance, relies on the heavy rain an El Niño typically brings to replenish water reservoirs, even if those same storms can be dangerous. Suppressing an El Niño could rob California of that precipitation. Ricke said the technology is “probably best to think about for now in terms of super El Niños, where most people, most places are losers and really extreme, damaging events are most possible.”

• Australia’s bushfire smoke could not be steered; deliberate MCB would have to be aimed and shut off, with no test of what happens if it goes wrong.
• El Niño is not uniformly bad; California and other regions depend on its wet winters, so suppressing an event could leave winners and losers in its wake.
• The modeling is partial; the team ran the climate response, not the regional economy, so deployment decisions would be made on incomplete information.

The Hardware and Politics Are Both Far From Ready

The hardware is not ready. “Almost two decades after research started, marine cloud brightening sprayers have spray rates that are at least a factor of a hundred too small for practical use,” said David Keith, a geophysical sciences professor at the University of Chicago, and the technique may be physically possible but “the technology simply doesn’t exist.”

Even if the sprayers were ready, the politics look worse. Dessler warned that actually executing something like this would be “a political nightmare,” with conflict or war a real risk if something went wrong in a worst-case scenario. Using it as a regional El Niño brake does not make the politics easier, just shorter, and the standard solar geoengineering critique, that any such tool would have to be sustained indefinitely and require unprecedented international cooperation, still applies to MCB when used as a planetary tool.

Haywood pointed to a separate ethical hurdle: who gets to decide if the world opts for this technique. A region that brightens clouds over the central Pacific is intervening in the climate of every country downstream, and the same cloud-brightening that protects California from floods could starve another country’s farms. The new study does not propose a governance model, but the broader NOAA research on the viability of marine cloud brightening outlines why the policy gap is one of the field’s largest open questions.

The Scripps team is clear that this is not yet a deployment proposal, with the researchers saying they are not aware of any proposals to test this on the El Niño brewing now. The study, funded by the National Science Foundation, NASA, the Department of Energy, and NOAA, is the kind of modeling exercise that can run for years before anyone has the tools to try it for real. For now, it sits where most of solar geoengineering sits, in a computer, with the political fight still ahead.

Frequently Asked Questions

What is marine cloud brightening?

Marine cloud brightening is a solar geoengineering technique that sprays seawater into low-lying marine clouds, making the cloud droplets smaller and more numerous so the clouds reflect more sunlight back into space. Unlike stratospheric aerosol injection, MCB can be aimed at a specific region of the ocean rather than the whole planet, which is why researchers are now asking whether it could be aimed at single weather events such as El Niño.

Could marine cloud brightening really weaken a strong El Niño?

The new Science Advances study modeled deliberate MCB over the central Pacific during the strong El Niños of 2015 and 1997 and reported it could increase the cooling and drying effects normally associated with La Niña by more than 40%. The findings come from a climate model, not a field test, and the study concluded that the earlier in an El Niño the technique is deployed, the more effective it would be.

How did the Australian bushfires help test the idea?

Australia’s 2019 to 2020 “Black Summer” bushfires sent plumes of sun-reflecting smoke into the Pacific stratosphere, an unplanned natural version of what MCB would do. Earlier research by John Fasullo and colleagues at the National Center for Atmospheric Research showed that the smoke from those fires had helped trigger a rare triple-dip La Niña after 2020, giving the Scripps team a real-world anchor to model what deliberate brightening might do when timed against an El Niño.

What are the biggest risks?

Scientists who reviewed the study raised three main concerns. The first is a miscalibrated cooling effect that could trigger a “mega La Niña” worse than the El Niño it was meant to blunt. The second is the regional winners-and-losers problem, since some areas, including California, depend on El Niño’s heavy winter rain. The third is the politics: running MCB over the central Pacific would intervene in the climate of every country downstream, with no governance model in place. Even the paper’s authors frame the work as “just a proof of concept” and stress they are not aware of any proposals to deploy it on the El Niño brewing now.

Is anyone planning to use this for the El Niño forming now?

The Scripps team said it is not aware of any proposals to test MCB on the El Niño brewing in the Pacific. The technology does not yet exist at the scale required; one outside expert estimated current sprayers are at least a factor of a hundred too small for practical use. The work was funded by the National Science Foundation, NASA, the Department of Energy, and NOAA, and the team has not set a timeline for when the technique could be ready for a real-world test.