The Next Climate Conversation Is About Marine Clouds

Researchers are trying to find out: could we cool the planet using sea salt?

(Photo by Planet One Images/Universal Images Group via Getty Images.)

The climate debate is in a strange place. We’re told we face an epochal, civilization-ending calamity within our lifetimes. But when scientists bring up unconventional new ways of managing that risk, we’re told we mustn’t even talk about them. 

Why? Because, alas, some of their most promising ideas got slapped with the label “geoengineering”: a term so scary it seems to shut down people’s prefrontal cortex altogether.

The result is a weirdly misshapen public debate, where a strong taboo weighs over a whole branch of atmospheric science. Our climate debate refuses to acknowledge something top researchers now strongly suspect: that we could reverse global warming quickly and affordably using nothing scarier than sea salt. 

The idea is called Marine Cloud Brightening (MCB). It would work by making clouds reflect more of the sun’s energy back out into space. As we speak, researchers are working on the basic science needed to figure out if it can really work at scale. 

I’ve been talking to some of them. 

They’re not wild-eyed idealists or crazed Bond villains in the making. They’re cautious, evidence-seeking scientists working to push the frontiers of our knowledge in a deeply hostile information environment.

Every one of them sees Marine Cloud Brightening as an “in case of emergency, break glass” option, one that it would be much better never to have to use. Every one of them is keenly aware that the technique may have consequences nobody intended. They always preface their ideas with provisos about the imperative to curb carbon emissions first. They’re allergic to overpromising and terrified of having their work misrepresented by a sensationalist press prone to scaremongering with their research. 

And yet they’re passionate. Because in a world awash with charlatans peddling nonsense plans to save the world, they know they’re working on an idea that actually might.

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The science behind Marine Cloud Brightening isn’t new. Back in the 1970s, an Irish government meteorologist named Sean Twomey showed that some clouds are much better at shielding the earth from solar radiation than others. Clouds that form around fewer, bigger aerosol particles don’t cool the earth beneath them very much, but clouds made up of more, smaller droplets are much brighter, and therefore much better at cooling whatever is beneath them. 

That insight, known as the Twomey Effect, has been the object of intense research for 50 years. As far back as 1990, British physicist John Latham took it one logical step further: might it be possible to alter existing clouds to make them more reflective? What if you sprayed large numbers of very small particles of, say, sea salt into them? Wouldn’t that increase the number of droplets and, therefore, turn them into better cooling shields? Done at a big enough scale, wouldn’t that cool the whole planet?

Well, yes. In theory, yes. 

But the devil is in the detail.

Professor James Haywood of the University of Exeter in the UK told me we have some quite strong hints that MCB is likely to work. We know marine clouds react readily to human intervention because, without meaning to, we ran a multi-decade experiment that proved it. Exhaust fumes from dirty container ships and tankers crisscrossing the world’s oceans measurably cooled the Earth by helping bright clouds form in the ships’ wake. Research on these “ship tracks” suggests the effect might be replicated in a much more environmentally sound way, using benign, non-toxic sea salt as an aerosol instead of gnarly exhaust fumes.

Dr. Michael Diamond of Florida State University echoed the fascination with ship tracks, but stressed that the idea might not work at all, and that the “how” isn’t straightforward. He’s mindful of Dr. Haywood’s modeling work at Exeter which shows that, done too fast, Marine Cloud Brightening could create a sort of supercharged La Niña event, which could wreak havoc with the hydrology of the Amazon. Nobody wants that, so everyone agrees much more detailed modeling work is needed to decide which clouds to brighten—and when—to avoid undesirable effects. 

The engineering challenges are serious, too. Designing a nozzle able to spray sea salt particles small enough to take advantage of the Twomey Effect, for instance, is far from straightforward. Designing the right nozzle for MCB was the object of a decade-long research project by some eminent retired Xerox print-nozzle engineers. This isn’t some minor detail: spray the wrong sized salt particle into clouds and you can end up making them less reflective, not more. 

Then, take the seemingly simple question of how a plume of nanometer scale salt particles might disperse through the atmosphere. Scientists aren’t exactly sure how that happens. Out of every 100 salt particles you spray on a warm, sunny day, how many will remain in the air 50 meters away from the spray site? How about 200 meters away? Of what precise sizes? In what concentrations? How do those proportions change as weather conditions change? 

To make an MCB plan credible, scientists need detailed answers to those questions. And there are no short cuts. To find out what happens when you spray tiny salt particles into the air, you have to head outdoors and do an experiment.

That’s precisely what a team led by Professor Sarah Doherty of the University of Washington set out to do. Working off the deck of the U.S.S. Hornet, a decommissioned aircraft carrier-turned floating museum in Alameda, California, they sprayed small amounts of salt into the air on the windward end of the carrier deck, then measured what happened to them with detectors set up on the leeward side of the deck. It’s painstaking work.

Dr. Doherty is keen to stress that her experiment was not about brightening the clouds over the Bay Area. “We’re not at that stage yet,” she says. For now, they need to answer a more basic question about how tiny salt particles behave once you waft them up into the air. 

Her team’s goal was to provide a reality check that can be used to improve computer models that simulate the diffusion of nanometer scale salt particles through the air. Those models need to be just right, because they’ll be the basic building blocks for the bigger, more general models that will allow scientists to predict what MCB might mean if implemented at scale.

“It’s really just the first baby step,” is how Dr. Doherty put it. It will give researchers a much more solid foundation for modeling a full-fledged MCB proposal. 

And yet the taboo on these techniques is now so entrenched that even baby-step research faces knee-jerk opposition. Earlier this year, in a paroxysm of anti-science NIMBYism, Alameda City Council shut down Dr. Doherty’s research on the U.S.S. Hornet. The decision will set her research program back months, if not years. But then, nothing about getting this research done is going to be easy.

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The knowledge gaps aren’t just at the molecular level. They are as daunting as you scale up, first to the level of a cloud, then to the level of a weather system, and finally to the whole planet. 

But in principle—and with enough resources—each of the questions are answerable. The Apollo program and the Large Hadron Collider show that when governments and universities really put their minds to it, and politicians fund them at an appropriate level, scientists and engineers can solve big, intractable problems like these. 

So what might a Large Hadron Collider-style program for Marine Cloud Brightening look like?

Earlier this year, some of America’s most prominent atmospheric scientists outlined the answer in a fascinating paper in Science Advances. They proposed a far-reaching collaboration between aerosol-cloud interaction scientists, climate model builders, remote-sensing experts and sea salt-mist engineers. After rattling off the areas of uncertainty, the authors called for a research effort to include a permanent experimental site, preferably on a low-lying island, to study the impact of spraying truly tiny salt particles into the lower atmosphere at scale.

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All COPs are Bastards

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Dr. Graham Feingold of the National Oceanic and Atmospheric Administration’s Chemical Sciences Laboratory was the lead author on that study. In an interview from his lab in Boulder, Colorado, Dr. Feingold told me that Congress mandated NOAA to look into whether MCB could really be used to manage the risks from climate change. His job as a scientist is to inform policy-makers about what scientists would need to do to find the answer.

Dr. Feingold and Dr. Doherty think of themselves as scientists, not Marine Cloud Brightening advocates. Their goal is to gain a much finer grained understanding of the way clouds and aerosols interact. They know, of course, that such knowledge will have implications for MCB—but my sense is that they’d be doing the work even if they knew it wouldn’t. 

So when I asked Dr. Feingold how long it might take to figure out if MCB could work at scale, he chose his words carefully: “Ten years is a length of time when we can expect to make significant progress.”

Which is just as well. In ten years solutions that seem “out there” in 2024 may no longer look so unthinkable. Because CO₂ does its thing slowly: even if all carbon emissions stopped tomorrow—which they won’t—the climate will continue to deteriorate for decades to come.

The answers today’s MCB researchers are seeking may seem niche. But before too long, we’re going to need those answers. If we mean what we say when we call climate change a global emergency, we can’t afford to pre-judge our options for fighting it.

Quico Toro is a contributing editor at Persuasion and writes the Substack One Percent Brighter.

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Postscript from the editors:

Lately, we’ve published a number of pieces about climate change—from Simon Glynn’s exploration of the hidden cross-party support for climate action, to Yascha’s essay on “effective environmentalism,” to today’s reported piece by Quico Toro—a frequent champion of nuclear energy in our pages—on one of the most intriguing new research veins in climate science. We hope these pieces shed some much-needed light at a time when performance antics and bad science all too often hijack the urgent discussions we need to have about the environment.

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