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Stardust is tackling the wrong problem with the wrong structure

Posted on by Ben Peltz

By David Keith | April 30, 2026

In late 2025, Stardust, a for-profit startup, announced it had raised $60 million in venture capital funding to develop a proprietary system for solar geoengineering. The company’s mission is to commercialize sunlight reflection methods (SRM) by deploying specially engineered, “chemically inert” (i.e., safe) particles into the stratosphere via custom aircraft. This is a system that Stardust claims could be operational by the start of the next decade.

I want to see a technically detailed plan for the early deployment of sunlight reflection using sulfur in the stratosphere. Such a plan should extend from materials handling and dispersal from aircraft, through monitoring hardware, data assimilation, and the feedback controls needed to ensure that deployment achieves a goal such as ramping up cooling at a rate of 0.1 C per decade—roughly halving current warming—while maintaining hemispheric balance.

I want this plan to be detailed and public. Such a plan would inform decisions about governance and deployment. A critical review of SRM requires such a plan.

The SRM research community is doing an inadequate job of producing such a plan. So, there is an unmet need.

Stardust has a strong technical team with systems engineering expertise that could address this need. Yet I don’t expect Stardust to meet this need, first because as an opaque, for-profit company, it has the wrong structure; and second, it’s focused on the wrong problem.

Daniele Visioni and I argued that Stardust has the wrong structure in our 2025 op-ed in MIT Technology Review; proprietary technology and profit motives are directly at odds with the transparency needed to legitimize SRM research. As we said, “trust will be the most important single ingredient in making these decisions. And trust is the one product for-profit innovation does not naturally manufacture.” That is why I want to see a public not-for-profit doing this work instead.

Here I argue that Stardust is focused on the wrong problem. Stardust seems focused on finding an alternative aerosol that is better than sulfates. Yet the direct risks of sulfur—as opposed to other SRM aerosols—are quantitatively small and well understood, whereas the solutions Stardust advances are all-but-certain to introduce new unknown unknowns, which might entail potentially large, unanticipated risks.

As I see it, the main attraction of Stardust rests on two assumptions1:

  1. Sulfur is a risky way to do SRM, so there’s a big benefit in finding particles that are less dangerous than sulfur.
  2. If an SAI aerosol was made from something non-toxic it would be safer than aerosol made from sulfate.

Let’s start with assumption #1. Suppose one could completely remove the direct health impacts from the sulfate aerosols injected into the stratosphere? Suppose there were a magic aerosol that causes zero environmental or health impacts as it makes its way down to the ground and into lungs and ecosystems. How much would this change the risk of SRM?

Not much.

While SRM will impact human health though changes in air quality, sulfuric acid’s direct contribution to that harm is surprisingly small.

Seb Eastham’s 2018 paper showed that the direct impacts of sulfate SRM on air quality are small compared to the indirect effects of the SRM’s cooling and other climate change on air quality.

The amount of aerosols in the air we breathe is most commonly measured as PM2.5, the total mass in micro-grams of all aerosol particles smaller than 2.5 µm in each cubic meter of air.

The epidemiological evidence that aerosol particles are unhealthy is very strong. Very roughly, this data shows that if you are exposed to an extra ~30 µg/m³ of PM2.5 your life expectancy decreases by about a year.

We expect that aerosols added to the stratosphere will make their way to the surface atmosphere and add to PM2.5 causing harm. Seb Eastham aimed to quantify this impact of SRM. To his surprise, he found that while stratospheric aerosols descending to the surface did contribute to PM2.5, that contribution was small compared to the increase in PM2.5 caused by cooling. Why did cooling make more PM2.5? Mostly because it increased the amount of nitric acid in aerosols, nitric acid that mostly comes from industrial emission of NOx. (For more details see section 3.2 of Seb’s paper.)

Now, a new study from Daniele Visioni’s group using more modern methods has confirmed that result.

Assumption #1 is therefore false. It would be great to eliminate the direct air quality impacts of sulfate SAI aerosol, but these impacts are quantitatively small compared to the indirect effects of SRM on air quality.

Context matters: air quality impacts do not seem to be seen as a leading concern about SRM. (See my list of concerns here). Moreover, the combined impacts of sulfate SRM on human health counting aerosols (PM2.5), ground level ozone, and increased UV caused by damage to stratospheric ozone, are at least 10X smaller than the health benefits of SRM in the form of reduced deaths from heat. See Tony Harding’s paper in PNAS, and our commentary.

Onto assumption #2. If an SAI aerosol was made from something organic and “non-toxic,” it would be safer than an aerosol made from sulfate. This statement seems so obviously true as to be inarguable. Drink a glass of sulfuric acid and you die; eat a tablespoon of flour and you’re fine. Of course, an SAI aerosol made from organic, edible food-safe material would be safer than an aerosol made of sulfuric acid, right?

Maybe not.

Materials that are safe to eat or drink can sometimes be dangerous if inhaled. Wheat flour is safe to eat (unless one is intolerant). Yet inhaling half micron wheat-flour aerosol particles is dangerous.

The health hazards of aerosol particles cannot be deduced from the biocompatibility or toxicity of the materials out of which they are made.

While there are well established protocols for testing the safety of materials ingested in food or drink, there are no similar protocols for testing aerosol safety, particularly not chronic exposure to low concentrations of aerosols.

In many environments, sulfate is a good fraction of total mass of PM2.5, so regulations have pragmatically focused on sulfates.

But that does not mean that it’s the sulfuric acid in the aerosols that is causing harm. Despite decades of studying the health effects of PM2.5, scientists are still uncertain about what specific components of particulate matter are causing the harm.

Evidence suggests sulfate itself may not be the root cause. Mice exposed to pure sulfate aerosols in the lab suffer comparatively low health impacts. Here’s a recent paper.

A leading hypothesis for why PM2.5 aerosols are dangerous is the so-called “hitchhiker hypothesis,” which is the idea that these aerosols pick up tiny amounts of organics or metals and transport them into the lungs. Once there, the hitchhiking organics or metals cause the harm, not sulfate itself2.

So, it’s plausible that some organic, food-safe aerosol could be more dangerous than sulfate if the main driver of toxicity is the hitchhiking organics or metals and not sulfate itself.

Moreover, a Stardust-engineered aerosol injected into the stratosphere will undergo complex chemical reactions as it descends though the atmosphere and reaches someone’s lungs. So, a test of toxicity of the pure stardust aerosol could not accurately predict the impact in the real world just as tests of the toxicity of pure sulfate aerosol do not correctly predict the health impacts of PM2.5 aerosols.

Assumption #2 is therefore unknown. An SAI aerosol made from something non-toxic might be safer or less safe than an aerosol made from sulfate.

What we can say for sure is that a novel aerosol would have more uncertain health impacts than sulfate. There would be more basis for concern about unknown unknowns.

Sulfur aerosols are not safe. We know they cause mortality and morbidity, but we can estimate the size of their impacts with some confidence because of many thousands of scientific studies over the last century. This means there is little basis for concern about unknown unknowns with sulfate.

 

Summary

Stardust has the wrong structure because trust is the one product for-profit innovation does not naturally manufacture.

Stardust is focused on the wrong problem because (a) the direct health impacts of SAI sulfate can be assessed with some confidence using the immense scientific literature on their health impacts; and (b) assessments using this literature find that the direct health impacts of SAI sulfate are a small contributor to the overall risks of SAI; and finally, (c) the health and environmental risks of a novel particle would be more uncertain than the risks of sulfates.

I am no naïve booster of sulfates—I published some of the early papers on non-sulfate aerosols3. In addition to reducing health impacts, designer aerosols offer the chance to reduce three important side effects of sulfate SRM: heating of the lower stratosphere, damage to the ozone layer, and alteration of the visual appearance of the sky. It’s worth researching better methods of SRM.

But these three impacts only grow large when a large amount of sulfur is used, as would be needed to cool Earth more than about 0.5 or 1 degree Celsius. If SRM is gradually ramped up (in my view the only sensible way to use it) it will take a long time—perhaps half a century—until SRM is cooling Earth enough for these side effects to be important. Thus, it will be a long time before there is a sound reason to use designer aerosols.

In the long run, some new aerosols will be demonstrated that are better than sulfate. But it is very hard to argue the humanity should start with anything other than sulfate, because of the depth of our knowledge about its hazards and the inherent uncertainty in any designer particle.

I am convinced there is an urgent need for end-to-end systems engineering for SRM. While I disagree with what Stardust is doing, my conversation with the Stardust founders left me with a strong sense that they have good intentions. I believe their work is motivated by a shared desire to do this systems engineering for SRM. I wonder if they over-focused on the ‘ideal particle’ concept because it’s easier to sell novel tech to investors than it would be to raise funds developing a sulfate aerosol system.

In any case, the rise of Stardust has convinced me of the need for a public non-profit entity that can do this systems engineering for SRM using sulfate aerosols.

 

Acknowledgements: I thank Ben Peltz and Dakota Gruener for editorial suggestions.

 

Notes

1 On April 2, 2026 Stardust released a “A proposal for the safety and controllability requirements that SRM systems should meet” on arXiv.

2 See, https://hsph.harvard.edu/news/metals-and-sulfate-in-air-pollution-mixture-may-contribute-most-to-asthma-hospitalizations/, or https://documents1.worldbank.org/curated/en/810141630705865331/pdf/Are-All-Air-Pollution-Particles-Equal-How-Constituents-and-Sources-of-Fine-Air-Pollution-Particles-PM-2-5-Affect-Health.pdf

3 https://davidkeith.earth/publication/photophoretic-levitation-of-engineered-aerosols-for-geoengineering/, https://davidkeith.earth/publication/solar-geoengineering-using-solid-aerosol-in-the-stratosphere/, https://davidkeith.earth/publication/stratospheric-solar-geoengineering-without-ozone-loss/.

 

MIT Technology Review: Why the For-Profit Race into Solar Geoengineering is Bad for Science and Public Trust

Posted on by Ben Peltz

By David Keith and Daniele Visioni

Last week, an American-Israeli company that claims it’s developed proprietary technology to cool the planet announced it had raised $60 million, by far the largest known venture capital round to date for a solar geoengineering startup.

The company, Stardust, says the funding will enable it to develop a system that could be deployed by the start of the next decade, according to Heatmap, which broke the story.

As scientists who have worked on the science of solar geoengineering for decades, we have grown increasingly concerned about the emerging efforts to start and fund private companies to build and deploy technologies that could alter the climate of the planet. We also strongly dispute some of the technical claims that certain companies have made about their offerings.

Given the potential power of such tools, the public concerns about them, and the importance of using them responsibly, we argue that they should be studied, evaluated, and developed mainly through publicly coordinated and transparently funded science and engineering efforts. In addition, any decisions about whether or how they should be used should be made through multilateral government discussions, informed by the best available research on the promise and risks of such interventions—not the profit motives of companies or their investors.

The basic idea behind solar geoengineering, or what we now prefer to call sunlight reflection methods (SRM), is that humans might reduce climate change by making the Earth a bit more reflective, partially counteracting the warming caused by the accumulation of greenhouse gases.

There is strong evidence, based on years of climate modeling and analyses by researchers worldwide, that SRM—while not perfect—could significantly and rapidly reduce climate changes and avoid important climate risks. In particular, it could ease the impacts in hot countries that are struggling to adapt.

The goals of doing research into SRM can be diverse: identifying risks as well as finding better methods. But research won’t be useful unless it’s trusted, and trust depends on transparency. That means researchers must be eager to examine pros and cons, committed to following the evidence where it leads, and driven by a sense that research should serve public interests, not be locked up as intellectual property.

In recent years, a handful of for-profit startup companies have emerged that are striving to develop SRM technologies or already trying to market SRM services. That includes Make Sunsets, which sells “cooling credits” for releasing sulfur dioxide in the stratosphere. A new company, Sunscreen, which hasn’t yet been announced, intends to use aerosols in the lower atmosphere to achieve cooling over small areas, purportedly to help farmers or cities deal with extreme heat.

Our strong impression is that people in these companies are driven by the same concerns about climate change that move us in our research. We agree that more research, and more innovation, is needed. However, we do not think startups—which by definition must eventually make money to stay in business—can play a productive role in advancing research on SRM.

Many people already distrust the idea of engineering the atmosphere—at whichever scale—to address climate change, fearing negative side effects, inequitable impacts on different parts of the world, or the prospect that a world expecting such solutions will feel less pressure to address the root causes of climate change.

Adding business interests, profit motives, and rich investors into this situation just creates more cause for concern, complicating the ability of responsible scientists and engineers to carry out the work needed to advance our understanding.

The only way these startups will make money is if someone pays for their services, so there’s a reasonable fear that financial pressures could drive companies to lobby governments or other parties to use such tools. A decision that should be based on objective analysis of risks and benefits would instead be strongly influenced by financial interests and political connections.

The need to raise money or bring in revenue often drives companies to hype the potential or safety of their tools. Indeed, that’s what private companies need to do to attract investors, but it’s not how you build public trust—particularly when the science doesn’t support the claims.

Notably, Stardust says on its website that it has developed novel particles that can be injected into the atmosphere to reflect away more sunlight, asserting that they’re “chemically inert in the stratosphere, and safe for humans and ecosystems.” According to the company, “The particles naturally return to Earth’s surface over time and recycle safely back into the biosphere.”

But it’s nonsense for the company to claim they can make particles that are inert in the stratosphere. Even diamonds, which are extraordinarily nonreactive, would alter stratospheric chemistry. First of all, much of that chemistry depends on highly reactive radicals that react with any solid surface, and second, any particle may become coated by background sulfuric acid in the stratosphere. That could accelerate the loss of the protective ozone layer by spreading that existing sulfuric acid over a larger surface area.

(Stardust didn’t provide a response to an inquiry about the concerns raised in this piece.)

In materials presented to potential investors, which we’ve obtained a copy of, Stardust further claims its particles “improve” on sulfuric acid, which is the most studied material for SRM. But the point of using sulfate for such studies was never that it was perfect, but that its broader climatic and environmental impacts are well understood. That’s because sulfate is widespread on Earth, and there’s an immense body of scientific knowledge about the fate and risks of sulfur that reaches the stratosphere through volcanic eruptions or other means.

If there’s one great lesson of 20th-century environmental science, it’s how crucial it is to understand the ultimate fate of any new material introduced into the environment.

Chlorofluorocarbons and the pesticide DDT both offered safety advantages over competing technologies, but they both broke down into products that accumulated in the environment in unexpected places, causing enormous and unanticipated harms.

The environmental and climate impacts of sulfate aerosols have been studied in many thousands of scientific papers over a century, and this deep well of knowledge greatly reduces the chance of unknown unknowns.

Grandiose claims notwithstanding—and especially considering that Stardust hasn’t disclosed anything about its particles or research process—it would be very difficult to make a pragmatic, risk-informed decision to start SRM efforts with these particles instead of sulfate.

We don’t want to claim that every single answer lies in academia. We’d be fools to not be excited by profit-driven innovation in solar power, EVs, batteries, or other sustainable technologies. But the math for sunlight reflection is just different. Why?

Because the role of private industry was essential in improving the efficiency, driving down the costs, and increasing the market share of renewables and other forms of cleantech. When cost matters and we can easily evaluate the benefits of the product, then competitive, for-profit capitalism can work wonders.

But SRM is already technically feasible and inexpensive, with deployment costs that are negligible compared with the climate damage it averts.

The essential questions of whether or how to use it come down to far thornier societal issues: How can we best balance the risks and benefits? How can we ensure that it’s used in an equitable way? How do we make legitimate decisions about SRM on a planet with such sharp political divisions?

Trust will be the most important single ingredient in making these decisions. And trust is the one product for-profit innovation does not naturally manufacture.

Ultimately, we’re just two researchers. We can’t make investors in these startups do anything differently. Our request is that they think carefully, and beyond the logic of short-term profit. If they believe geoengineering is worth exploring, could it be that their support will make it harder, not easier, to do that?

David Keith is the professor of geophysical sciences at the University of Chicago and founding faculty director of the school’s Climate Systems Engineering Initiative. Daniele Visioni is an assistant professor of earth and atmospheric sciences at Cornell University and head of data for Reflective, a nonprofit that develops tools and provides funding to support solar geoengineering research.

Original post in MIT Technology Review

Preserving Earth’s Reflectivity: Supplementary Comments on “A Responsible Way to Cool the Planet”

Posted on by Ben Peltz

By David Keith | September 21, 2025

Supplementary comments on “A responsible way to cool the planet” with Zeke Hausfather, an opinion essay in the NYT published 21 September 2025.

My guess is that a fair-minded understanding of the risks and benefits of deploying a small amount (example below) of sunlight reflection would yield surprisingly broad support for limited deployment.

Yet, any talk of limited deployment comes with one giant caveat: the fear that once started, it would be politically hard to stop. If there’s a big problem, it’s easy to stop. But if it starts and goes roughly to plan, then there will be a strong temptation to keep doing more, while weakening efforts to cut emissions or remove carbon.

How to set up political structures that avoid over deployment? There are clever proposals for voting rules that tie the right to vote in decisions about SRM to successful emissions cuts, but there are no easy answers.

Zeke and I propose a simple rule: limit use of SRM to maintaining Earth’s reflectivity against the decrease in reflectivity that will continue as pollution is cleaned up. This rule has two useful consequences. First, it ties use of SRM to clean up of pollution, and since this pollution is mostly from burning fossil fuels, this is roughly equivalent to tying use of SRM to cuts in fossil fuels; and second, it provides a non-arbitrary fixed upper limit on how much SRM can be deployed.

One can imagine this rule is simple enough to be useful in international negotiation over sunlight reflection.

Any such limit can obviously be overridden. It’s impossible to bind the hands of the future, and it would be unwise to do so, even if it were possible.

 

Notes:

Here’s what a small amount could look like: injecting sulfur into the stratosphere using high-flying aircraft. Starting in the early 2030s and gradually increasing over time, the injection could, after about 50 years, reflect enough sunlight to cool Earth by 0.5 C. That would lower projected 2080 temperatures from about 2.5 C above pre-industrial levels without SRM to around 2.0 C, while balancing the cooling across both hemispheres. This a bit larger than current cooling now caused by pollution, which is more like 0.4 C, but it’s similar magnitude. The human and environmental benefits of reducing temperatures by half a degree are enormous as demonstrated by the IPCC 1.5 report.

A strong temptation to keep doing more. This is geoengineering’s moral hazard, perhaps better called “mitigation deterrence”. It can arise from political exploitation or collective addition, as I describe here: https://davidkeith.earth/publication/toward-constructive-disagreement-about-geoengineering-a-shared-taxonomy-of-concerns-may-help/ . It can arise from political exploitation or collective addition as I describe here: https://davidkeith.earth/publication/toward-constructive-disagreement-about-geoengineering-a-shared-taxonomy-of-concerns-may-help/

Here are my previous NYT op-eds:

The New York Times: What’s the Least Bad Way to Cool the Planet?

The New York Times: Blocking the Sky to Save the Earth

The New York Times: A Responsible Way to Cool the Planet

Posted on by Ben Peltz

By Zeke Hausfather and David Keith

Since the Industrial Revolution, burning coal and oil has filled the air with sulfur, shortening the lives of billions of people. In response, countries passed stringent air pollution laws requiring coal plants to scrub out sulfur and ships to switch to cleaner fuels. Global sulfur emissions have fallen some 40 percent since 2006. China alone has slashed them by about 70 percent.

We should celebrate cleaner air, but we also have to reckon with an unintended consequence. It turns out that by reflecting sunlight back into space, tiny sulfur particles protected Earth from about a third of the warming caused by human emissions of carbon dioxide. Now more of the underlying greenhouse gas warming is showing through, accelerating climate change. As The Economist recently put it, “If India chokes less, it will fry more.”

For some of us in the world of climate science, this raises a thorny question: Should we explore replacing the inadvertent cooling effects of sulfur with a cleaner, deliberate version?

Geoengineering the climate in this way is not a new idea; it was in the first U.S. high-level climate report that reached President Lyndon Johnson’s desk in 1965. While several options have been proposed recently, the most plausible way to make Earth more reflective is to use a small fleet of high-altitude aircraft to increase the amount of sulfuric acid droplets in the upper atmosphere. We know this can work; when volcanic eruptions put large amounts of sulfur in the upper atmosphere, such as Pinatubo in 1991, Earth was noticeably cooler for a few years.

We should take such an idea seriously because the costs of losing accidental sulfur cooling were made painfully evident this year when heat waves pushed temperatures above 120 degrees Fahrenheit in the Middle East and North Africa. If sunlight reflection could save lives and protect the environment, it is at least worth discussing.

Because sulfur is much more effective at cooling the planet when put into the upper atmosphere compared with what’s released into the lower atmosphere when we burn fossil fuels, we’d have to add far less of it. And for the same amount of cooling produced from burning fossil fuels, sulfur in the upper atmosphere would cause at least 100 times smaller health impacts.

Sunlight reflection is no panacea. Putting sulfur in the upper atmosphere will damage the ozone layer, allowing more ultraviolet radiation through. Even if it reduces deaths from heat and extreme weather, large-scale deployment could exacerbate climate change in some locations, perhaps by shifting rainfall patterns.

There is also a more fundamental limitation to sunlight reflection. It is effectively a Band-Aid that treats the symptoms of climate change but not the underlying disease of greenhouse gases. And unlike a skinned knee, the Earth does not heal from climate change on any time scale that matters for human societies. Warming from carbon dioxide is astonishingly persistent; much of what we emit today will warm the planet for many thousands of years to come.

Even if the world drives emissions down to zero, the planet wouldn’t cool down for millenniums. The only durable way to return to cooler temperatures is to remove the excess carbon we have already added, and removing enough to reverse even 0.1 degree Celsius of warming would cost tens of trillions of dollars.

This means that the real risk of geoengineering is not some Hollywood-style catastrophe, but complacency. A cheap way to delay the effects of warming risks undermining the need to rapidly reduce emissions, and going down that path would risk locking our children into a dependency where even stopping the process becomes too expensive to contemplate.

Given all this, we are not advocating deploying geoengineering today. But if policymakers decide that it is needed, a more modest approach would be to run a small, carefully scaled program that slightly increases the upper atmosphere’s reflectivity to compensate for the loss of cooling as sulfur pollution is eliminated.

The goal would not be to dial the Earth to some preferred temperature, nor to offset all greenhouse warming. It would be to keep the total cooling from sulfur roughly constant for a period of time, reducing near-term climate risk while decarbonization efforts continue.

As world leaders gather in New York for the U.N. General Assembly and Climate Week, any discussions of sunlight reflection should have a clear, enforceable commitment to never cool the Earth more than today’s current sulfur emissions do. And it should come with a clear off-ramp: As the world reaches net-zero emissions and scales up carbon removal technologies later this century, the program should end.

Pacing matters as much as limits. If society ever chooses to test this approach, it should start small and move slowly. Tying it to reductions in air pollution allows a slow ramp-up, resulting in increments that are imperceptible to most of us but visible to satellites and sensors. This should be coupled with regular checkpoints to assess side effects on regional rainfall, the atmosphere and ozone. The intent is to buy a modest, temporary buffer, not to start a new arm of climate control.

If we stay focused on the cure of reducing emissions and consider bounded and temporary sunlight reflection, we could preserve cleaner air, avoid a near-term temperature surge and not betray the generations to come, who will live with the consequences of our choices today.

Original post in The New York Times. 

Beyond Offsets: How to Build a Carbon Removal Industry

Posted on by Ben Peltz

Carbon offsets aren’t enough. To truly tackle climate change, we need a global industry dedicated to pulling carbon out of the air and at massive scale. Join hosts John and Sonia inside the innovation race to scale carbon removal technologies, featuring insights from leading voices in the field.

They speak with Dr. David Keith, a pioneering climate scientist and founder of Carbon Engineering, who unpacks the technological, policy, and economic hurdles to direct air capture and other approaches. You’ll also hear from two recent XPRIZE Carbon Removal winners, Mike Kelland of Planetary Technologies and Jim Mann of UNDO about how their startups are using ocean alkalinity and enhanced rock weathering to permanently sequester CO₂, while also delivering benefits to farmers and marine ecosystems.

Together, they explore whether the world can build a scalable, measurable, and credible carbon removal industry – one capable of drawing down billions of tons of CO₂ annually.

Original post on RBC

Comparing the benefits and risks of solar geoengineering

Posted on by vekstrom

By: David Keith and Anthony Harding (Georgia Tech)

Climate change has risks—and those risks are only increasing. Many of these might be reduced by solar geoengineering. Solar geoengineering also has risks, and it requires rigorous, transparent research before it is deployed. One major question though is how does the reduction in risks from solar geoengineering compare to the additional risks its use entails? Or equivalently, how big are the benefits of solar geoengineering compared to its harms?

Our paper, Impact of solar geoengineering on temperature-attributable mortality, is a first effort to provide a quantitative risk-risk comparison for any solar geoengineering method. It answers the call of an emerging consensus that if we’re going to avoid the worst effects of climate change, we need to evaluate every potential solution—including solar geoengineering. Importantly, highly credible scientific organizations like the National Academies of Sciences, Engineering, and Medicine (NASEM), and a growing chorus of experts, are among those who recognize solar engineering deserves exploration.

In our new paper, we quantify and compare what we consider to be a few of the largest physical risks for sulfate aerosol injection—mortality risk from temperature, air pollution, and ozone loss.

Injecting sulfate aerosol into the stratosphere will cool the planet, reducing mortality from heat, one of the leading risks of climate change. Sulfate aerosol air pollution is a leading cause of environmental mortality worldwide, so it is one of the most obvious risks of sulfate aerosol geoengineering. Sulfate aerosols in the stratosphere can also damage the ozone layer, causing an increase in mortality from skin cancers.

Comparing these three risks, we find that the reduction in mortality from cooling—a benefit—is roughly ten times larger than the increase in mortality from air pollution and ozone loss—a harm. Like any statement about solar geoengineering this result depends on the scenario we evaluated along with a host of other assumptions.

Our view is that quantitative analysis of the expected benefits and costs of a possible policy action is crucial input to sensible debate about public policy. This perspective is particularly relevant to solar geoengineering given its uncertainties, risks, and distributional effects. Good benefit-cost analysis should consider structural uncertainties and consider the distribution of those impacts across different affected groups. Benefit-cost analysis should not (and does not) mechanically determine policy outcomes, but good policy analysis and debate should take benefit-cost analysis seriously. Note that our view about the importance of benefit-cost analysis is reflected by the fact that we have both taught this topic in public policy schools.

In the remainder of this essay, we offer some notes and then a set of answers to questions we imagine readers might ask.

Some notes:

  • Our paper is a collaboration between us and Princeton collaborators Gabe Vecchi and Wenchang Yang.
  • Our paper relies on a state-of-the-art method for estimating the impact of warming on local mortality led by my UChicago colleague Michael Greenstone. The estimate of added mortality due to the addition air pollution and ozone loss comes from Seb Eastham’s paper.
  • Our paper links to two prior papers. Tony led a paper on the impact of solar geoengineering on income inequality that used related econometric methods. David was part of prior collaboration with Gabe Vecchi which produced an important estimate of solar geoengineering’s potential to reduce regional climate hazards.
  • Many groups have called for risk-risk analysis of solar geoengineering including the National Academy, NASEM 2021, The Carnegie Climate Governance Initiative, C2G and the call-for-balance letter with Peter Singer, James Hansen, and Bjorn Stevens, as signatories, see paper.
  • The air pollution mortality estimate by Eastham combines the direct impacts of injected aerosol that makes it to the surface with climate-mediate changes in the amount of surface ozone and particulate air pollution produced from given industrial emissions. Air pollution mortality due to particular matter increases as the climate cools, a larger increase in mortality than direct impact of the sulfate injected into the stratosphere (see Figure 2 of Eastham). But this effect depends on air pollution emissions which will likely be lower late in the century than is assumed in the Eastham paper. It may be better to compare our estimated change in temperature-attributable mortality to Eastham’s estimate of direct impacts of the descending injection mass—yielding a benefit-harm ratio of about 40:1.
  • Our work examines only three risk pathways: temperature-attributable mortality, air pollution, and the impact of increased ultraviolet due to reduced ozone. It is just one step toward a broad quantitative risk–risk assessment of solar geoengineering. While not comprehensive, these are important risk pathways: temperature-attributable mortality may account for more than half of the monetized harms of climate change, and air pollution and ozone loss are among the most salient impacts of stratospheric sulfate geoengineering.

How will the positive and negative impacts of solar geoengineering be distributed geographically? Research consistently suggests that those who are expected to be most harmed by a warming world are those in poorer and hotter regions of the world. Broadly, we find the converse for solar geoengineering. Cooling by solar geoengineering reduces temperature-attributable mortality in hotter regions while it increases mortality in cooler regions (Figure 1 of paper). Global warming does the converse. This, combined with the fact that mortality impacts are greater when people are poorer, means that the benefits of solar geoengineering are concentrated in hotter and poorer regions.

Did we get our result by choosing an unrealistically positive scenario for deploying solar geoengineering? On the one hand, any statement about SRM is necessarily scenario dependent. Here’s how Parson and Keith put it:

SRM presents two fundamental policy-relevant scientific questions. How effectively could it reduce climate risks? And what additional harms or risks, of what severity, would it introduce? Answers to these questions about SRM’s effects rely partly on knowledge derived from scientific research, but they also depend on assumptions about how SRM is used, under what background conditions of greenhouse gas emissions and climate change. The three principal dimensions of choice in how SRM is used are how much global-average cooling or radiative forcing is pursued, how changes in radiative forcing are distributed around the world, and what SRM method is used.

If, for example, one deployed a massive amount of SRM in only one hemisphere the results would be bad.

On the other hand, our paper expresses many of its results as ratios either of risk-to-risk or of SRM’s effect to the effect if the same climate change were caused by removal of CO2, and these ratios are strongly dependent on the amount of SRM. We expect, for example, that the 13:1 risk-risk ratios would be very roughly the same if one was cooling the world only 0.1 C or as much as 2 C, and roughly independent of how quickly emissions were cut or carbon was removed. But our scenario does assume a roughly hemispherically balanced uniform SAI deployment.

How sure are you about your results? The abstract says there is only a 60% chance that benefits would outweigh the harms. The 60% figure is almost completely driven by the uncertainty in the effect of temperature-related mortality. Forget SRM and just think about CO2-driven climate change: though we don’t calculate it exactly, there is a large probability that the benefits of reduced deaths in cooler regions would be larger than the harms of increased deaths in hot regions—so our 60% figure really comes from the fact that the model we use doesn’t have high confidence that CO2-driven climate change is bad for average mortality. Things look very different if you look at a hot or cold region alone.

What next? Do you believe this type of climate intervention should be deployed? And who should have the authority to make this kind of decision? Our role as scientists is to expand the base of knowledge. Governments must decide if, when and how to put theory into practice. We hope the main impact of this paper is to spur our colleagues to provide more and better quantitative comparisons between risks and benefits.

Comment on Buying Time by David Gelles, New York Times, 1st August 2024

Posted on by Robin Dolezal

By David Keith | August 5, 2024

Good journalism builds stories out of facts. And good stories need characters.

The NY Times recognized the growing significance of debates about solar geoengineering and commissioned a story that humanized the topic by building it around a character, me.

David Gelles’ article is fair, but as the following examples illustrate, the imperative of storytelling may leave readers with an exaggerated impression of both my importance and the differences between my views and the views of other interviewees he quotes.

“My” solution.  Gelles’ article refers to solar geoengineering as “his [my] solution”. This is doubly wrong. I was a child when the idea of injecting sulfur into the stratosphere to slow global warming was first proposed. These ideas are not mine but are the work of many people over more than half a century. Moreover, solar geoengineering is not a solution, at best it’s a band-aid; a supplement to emissions cuts but not a substitute for them.

Moral hazard. The article says, “Opponents worry it would distract from the urgent work of transitioning away from fossil fuels”, adding that opponents cite “moral hazard” as one of the main risks of solar geoengineering. This is accurate, but in crediting this concern to ‘opponents’, Gelles suggests an “all-in” vs “all-out” dichotomy, which in turn suggest that people like me who are willing to contemplate deployment are blind to the risks, while people Gelles labels as opponents are blind to the benefits. Concern about moral hazard is, in fact, widespread within the solar geoengineering research community. I fear that fossil fuel interests or fossil-rich nations will exploit solar geoengineering by exaggerating its effectiveness and minimizing its risk to weaken controls on carbon emissions. I also believe I was the first to call this concern a moral hazard almost a quarter century ago[i].

Frank Keutsch. The article says “Dr. Keutsch is less sanguine than Dr. Keith when considering [SRM’s] potential risks” and quotes Frank as saying, “I compare stratospheric solar geoengineering with opiates,” and “They only treat the symptom and not the actual cause. You can get addicted to it if you don’t actually address the cause. In addition, like any painkiller, you’re going to have side effects. And then there are withdrawal symptoms, and that’s termination shock.”

I fully agree with Frank’s comments. I love his opiate analogy and use it occasionally. Many of us use similar analogies, including myself when I compared geoengineering to chemotherapy in 2010 congressional testimony. It’s possible that Frank is more concerned than me about geoengineering’s risks, but we talk often, and my overall impression is that while we see some things differently, we share a similar overall level of concern.

David Suzuki. The article quotes David Suzuki saying, “The whole notion of spraying sulfur compounds to reflect sunlight is arrogant and simplistic”, and “The fundamental problem is that we think we’re so smart that we don’t have to pay attention to nature’s boundaries.” A reader might assume that David Suzuki opposed research on solar geoengineering—the central question at issue today—yet in a conversation I had with David Suzuki in March 2022 as part of a documentary, Suzuki’s response to a question about research on solar geoengineering was “absolutely we need more information just to show us how ignorant we are.”

Shuchi Talati, the founder of a nonprofit organization called the Alliance for Just Deliberation on Solar Geoengineering, called the technology “a double-edged sword.” Adding that “…it can also exacerbate suffering if used in a bad way.”  Yes. Solar geoengineering could cause immense harm through deliberate misuse. To dramatize its terrible possibilities, I computed that solar geoengineering could theoretically be used to cool the earth over a century, freezing the oceans to the equator and, a quote that Michal Spector picked up for a 2012 New Yorker article.

Gelles’ follows Shuchi’s quote with a statement implying that I disagree, saying that I “countered that the risks posed by solar geoengineering are well understood, not as severe as portrayed by critics and dwarfed by the potential benefits”. I believe this statement is correct for the physical risks of deployments under specific conditions (e.g., hemispherically balanced and offsetting less than half the CO2-driven warming). Yet I emphatically agree with Shuchi’s view that solar geoengineering could most certainly “exacerbate suffering if used in a bad way.” Caveats matter.

Summary. I am excited to see solar geoengineering getting above-the-fold coverage in the NY Times. Readers should bear in mind that story-driven reporting tends to exaggerate differences by portraying people as stylized representatives of sharply distinct points in a landscape of opinion.

No single person’s judgment should count for much. The news that should, in my view, matter to readers, is that there is rapidly growing agreement that research on this topic makes sense, as does active debate about how these technologies might be used and governed. There are, of course, strong voices against research from climate experts such as Ray Pierrehumbert, but evidence from surveys and formal consensus documents suggest that these opinions are held by a small minority[ii]. Additionally, to the extent we can gauge public opinion, there is a surprising level of support for research which is strongest in the developing world[iii].

If you want to learn more about solar geoengineering I suggest the One Atmosphere report from the UN Environment Program. For my views in my own words, have a look at my 2021 essay in the NY Times or my short book. My website has all my articles. If you want ones most relevant to general readers use this search. If you want my take on the links between nature and climate engineering, see my µ-autobiography. Or, see my Facts & values: my thoughts on talking about solar geoengineering.

Minor errors

  • “Risk is negligible compared to the benefits”. If I said “negligible” then I misspoke, but I doubt I said negligible as I have been speaking about this comparison a lot and have a standard way to describe it. The comparison between the reduction in mortality from heat and the increase in mortality from air pollution is from a paper by Tony Harding. An early version is public as an RFF whitepaper; a full version is under peer review. I have been speaking about this quite a bit and I typically provide a quantitative comparison of benefits to harms and include caveats as we do in the paper. Since the deaths from air pollution would be due to deliberate introduction of sulfur, I don’t think it’s ethical to dismiss them as negligible.
  • SCoPEx history. Firstly, SCoPEx was led by Frank Keutsch in the period the article refers to https://www.keutschgroup.com/scopex. I collaborated closely with Frank, but an account that leaves his name out overstates my role. Second, no test was planned over Arizona in 2018. Frank and I were developing hardware and working with balloon launch providers, but we were a long way from being able to schedule a flight. Third, the article says, “When details of that plan became public, a group of Indigenous people objected and issued a manifesto against geoengineering.” The manifesto was authored by ETC, founded in Montreal as part of an anti-geoengineering campaign. ETC did get signatures from some indigenous organizations but the article inaccurately characterizes the group that issued the manifesto.

[i] See page 276 of https://davidkeith.earth/publication/geoengineering-the-climate-history-and-prospect-2/

[ii] Dannenberg, A., Zitzelsberger, S. Climate experts’ views on geoengineering depend on their beliefs about climate change impacts. Nat. Clim. Chang. 9, 769–775 (2019); Dai, Z., Burns, E.T., Irvine, P.J. et al. Elicitation of US and Chinese expert judgments show consistent views on solar geoengineering. Humanit Soc Sci Commun 8, 18 (2021).

[iii] Sugiyama, M., Asayama, S., & Kosugi, T. (2020). The North–South Divide on Public Perceptions of Stratospheric Aerosol Geoengineering?: A Survey in Six Asia-Pacific Countries. Environmental Communication, 14(5), 641–656; Low, S., Fritz, L., Baum, C.M. et al. Public perceptions on solar geoengineering from focus groups in 22 countries. Commun Earth Environ 5, 352 (2024).

 

 

 

 

 

 

 

 

 

 

A radical solution to address climate change, with David Keith

Posted on by molliefreeman

Climate change can feel like an impossible crisis these days. Every week there is some new report about the irreversible damage we’re doing to our planet and the havoc it will bring to people’s lives. We all know cutting emissions is the solution, yet governments and companies seem no closer to meeting the goals that scientists say we must hit. It can feel hopeless.

There is one possible controversial solution to climate change many in the mainstream haven’t discussed. It’s so controversial, in fact, that some experts say we shouldn’t even be discussing it. But University of Chicago Professor David Keith says we need to talk about it. It’s called solar geoengineering—the process in which you reflect a small fraction of sunlight back into space using aerosols. As the founding director of the Climate Systems Engineering Initiative at UChicago, Keith is leading a team that will research solar geoengineering and other novel solutions to climate change.

Original post on Big Brains Podcast

The Economist: David Keith on why carbon removal won’t save big oil but may help the climate

Posted on by molliefreeman

Occidental, an American oil major, recently agreed to buy Carbon Engineering, a Canadian carbon-removal company, for $1.6bn. The deal underlines big oil’s growing interest in carbon-capture technologies, which suck carbon dioxide from the air. What does it mean for the climate?

Suppose a trucker dumped a load of manure on your front lawn and then demanded a fee to haul it away. Big oil made the fuel that is cooking our planet, so the idea that it might profit from cleaning it up strikes many people as obscene.

Critics argue that big oil is using carbon removal as a tool to protect its core business. As Occidental’s chief executive, Vicki Hollub, sees it, carbon removal means “we don’t need to ever stop oil.” Defenders argue that big oil can help meet social demands for decarbonisation by pivoting to carbon neutrality while bringing technical expertise to new low-carbon markets.

Greenwash or swords-to-ploughshares? My guess—informed by my experience as a climate-focused academic and as the founder of Carbon Engineering (on whose board I still sit)—is that the oil majors will be unsuccessful at both. Greenwashing will not protect them; nor will they smoothly pivot from being oil suppliers to carbon removers. Yet big oil’s carbon-removal play may nevertheless yield substantial climate benefits, in part because it is unlikely to play out as well as the companies hope.

Big oil will trumpet its green achievements, both real and imaginary. This will dampen public disapproval and help recruit talent, but it is hard to see how it reduces the threat to the core business, which is driven by accelerating climate policies and the decreasing cost of electric vehicles.

A world with large-scale carbon removal is a world with carbon prices high enough and decarbonisation policies strong enough to drive oil demand down sharply. Permanent carbon removal is likely to cost over $150 per tonne of carbon dioxide for at least a decade or two. That is equivalent to a penalty of almost $70 per barrel of oil. Though it may provide a green aura, an oil company’s carbon-removal business, however successful, will not protect its legacy oil business from strong carbon prices and policies. Neither greenwashing nor green reality changes the fundamentals.

The feasibility of a swords-to-ploughshares pivot rests on the premise that expertise transfers from oil and gas to carbon removal—or even beyond to solar power and other clean technologies. Although engineering skills are transferable, the business pivot is less plausible. A management culture built to succeed at making risky bets on big hydrocarbon plays such as ultra-deep offshore oil is different from the management culture needed to succeed in clean energy or carbon removal.

When oil companies build thriving carbon-removal businesses, the interests of these business units will be misaligned with the legacy oil business. Legacy oil wants low carbon prices and high energy prices. Carbon removal wants the opposite. Big institutional investors such as pension funds prefer pure plays, so they will push to cleave carbon removal from legacy oil. History suggests that incumbents rarely survive fundamental shifts in the underlying business. ibm was an exception, but it is now dwarfed by Apple and Microsoft. The benefits of synergy are usually outweighed by the costs and conflicts of maintaining the legacy business.

So even when big oil succeeds in carbon removal the most likely outcome is freestanding cleantech companies alongside legacy oil rather than successfully integrated conglomerates. Environmentalists can thus welcome big oil’s move into carbon removal for the skills it brings with guarded optimism that the swords-to-ploughshares pivot will do little to protect the legacy oil business.

And the skills are desperately needed. Building billion-dollar battery factories, hydrogen infrastructure or plants to extract carbon from the air requires engineering and management skills that are concentrated in industries like oil and commodity chemicals. Occidental, for example, plans to build plants that can remove and store up to 30m tonnes of carbon per year at King Ranch in Texas. That is the equivalent of decarbonising 30m-60m transatlantic passenger flights per year. Although Occidental has never built a direct-air-capture plant, Carbon Engineering’s technology knits together existing industrial processes to achieve the new goal of carbon removal, and Occidental has experience with almost all the components required for direct air capture, including potassium hydroxide, a chemical used in the process, and CO2 sequestration. A startup cannot build plants with tens of millions of tonnes of capacity without the skills of a company that has built industrial plants at scale.

Big oil’s pivot to clean should be celebrated as a marker of the power of environmental advocacy, not a sign of its weakness. These investments did not happen simply because big oil woke up feeling woke. The driving force is policy. Today’s most important driver is Joe Biden’s clean-energy incentives. But these incentives did not just happen because the American president woke up green. They are the fruit of decades of environmental advocacy.

Greenwashing is a risk. Environmentalists are right to worry. Big oil will try to use carbon removal to defend the status quo. But there is a political upside. In a decarbonising world in which big oil only does oil and gas, its only future is extinction and it will fight progress with its back to the wall. If, however, the industry is also in the decarbonisation business, its interests—and the interests of the communities that depend on it—are split, with the low-carbon business units fighting for strong climate policy even as the legacy businesses oppose it. My hope is that this blurring of interests will lubricate the political bargains needed to accelerate climate progress.

Original post on The Economist