Why Solar Geoengineering Could Be a Vital Part of the Climate Crisis Solution

For years, Harvard climate scientist David Keith has been striving to have his research taken seriously. A pioneer in geoengineering, his work aims to address climate change with technological interventions. Past ideas have included dispersing iron in oceans to boost plankton growth for carbon absorption or directly capturing carbon from the air.

Keith started a company dedicated to creating technology that removes carbon from the atmosphere, but his primary focus is solar geoengineering—reflecting sunlight away from Earth to cut down on the heat trapped by greenhouse gases. Though the method remains unproven, models suggest it could succeed. Plus, real-world evidence from major volcanic eruptions supports the concept.

Soon, Keith and his team aim to conduct one of the first trials of their idea: a high-altitude balloon that will release small, reflective particles into the stratosphere, the uppermost layer of Earth's atmosphere. While the exact location and timing of the experiment are yet to be finalized, it represents an initial step in testing whether artificial particles in the stratosphere could help cool the planet in a way similar to natural volcanic eruptions.

However, the notion of using technological interventions to combat climate change remains controversial. The very discussion — let alone the research — of geoengineering has long been viewed with skepticism, as many fear it might undermine other efforts to combat climate change, particularly the essential work of reducing carbon emissions. As a result, geoengineering has been pushed to the fringes of climate research. But Keith suggests that public attitudes are shifting. He contends that while geoengineering alone cannot resolve the climate crisis, it could potentially reduce the damage if executed carefully alongside emission-reduction strategies.

In 2000, Keith published a comprehensive review of geoengineering research in the Annual Review of Energy and the Environment, highlighting that key climate assessments had largely ignored the topic until that point. Earlier this year, he gave a talk in Seattle about the state of the field at the annual meeting of the American Association for the Advancement of Science. Knowable Magazine sat down with Keith to discuss how the scientific, technological, and geopolitical landscape has evolved over the years.

Q&A with Climate Scientist David Keith

This conversation has been edited for length and clarity.

Two decades ago, you referred to geoengineering as 'deeply controversial.' How has the perception of that controversy shifted since then?

At that time, geoengineering was something only a small circle of climate experts were aware of, and they mostly agreed it was best not to talk about it. That was the extent of it. Today, the topic is much more openly discussed. I believe the taboo has diminished significantly. While it remains a controversial subject, there has clearly been a shift. More and more people within climate science, public policy circles, and environmental organizations agree that it’s a conversation worth having, even if many believe it should never be implemented. There's even growing support for research on it. The atmosphere around it feels different now.

Why was there a stigma against discussing geoengineering, and do you think it was justified?

The hesitation was well-meaning; people were concerned that discussing geoengineering could undermine efforts to reduce emissions. However, I don’t believe that fear of moral hazard is a valid reason to halt research. There were critics who argued that distributing the AIDS triple-drug cocktail in Africa might lead to misuse and resistance. Similarly, some opposed the introduction of airbags, fearing people would drive more recklessly. There’s a long history of people opposing risk-reducing technologies because of the potential for risk compensation — the idea that people might take on more risk when they feel safer. I think that argument is ethically flawed.

For me, the primary concern is that certain entities — like large fossil-fuel corporations with a vested interest in blocking emissions cuts — could leverage geoengineering as an excuse to avoid reducing emissions. This concern is probably the main reason some large civil-society organizations seek to suppress or limit discussions about geoengineering so that it doesn’t enter the broader climate debate. While I understand the concern, I believe the correct approach is to confront it openly, rather than avoiding the topic. I don’t want a world where important decisions are made behind closed doors by a select few.

Has there been an increase in geoengineering research over the last two decades?

Yes, significantly, particularly in recent years. When I published my paper for the Annual Reviews in 2000, there was almost no structured research in the field. A few researchers were occasionally looking into it, but they were dedicating only about 1 percent of their time to it.

Today, research programs on geoengineering can be found in many parts of the world. China has a notable initiative, Australia has a well-funded program that outpaces any efforts in the U.S., and several European countries are also conducting research in the field.

What has been the most unexpected discovery over the last 20 years regarding how solar geoengineering might function?

The biggest revelation has been the recent findings, including two studies I participated in, showing that the effects of a global solar geoengineering effort wouldn’t be as unevenly distributed geographically as originally feared. The key issue for real-world policy is determining who stands to lose the most.

In one paper published last year in Nature Climate Change, we utilized an extremely high-resolution computer model to compare two scenarios across the entire land surface: one with carbon dioxide levels at twice preindustrial levels and the other where solar geoengineering is used to cut the temperature rise in half. We then analyzed whether solar geoengineering would bring certain climate variables back to preindustrial levels, which we labeled as 'moderated,' or if it would push them further from preindustrial levels, termed 'exacerbated.' This comparison was made for each of the 33 geographic regions identified by the Intergovernmental Panel on Climate Change.

We focused on critical climate factors, such as extreme temperature changes, average temperature shifts, water availability changes, and extreme precipitation variations. The results were astonishing: in no region was any variable exacerbated. That outcome was quite surprising.

In another paper published in March in Environmental Research Letters, we applied the same analysis using a different model. We found that with solar geoengineering, every region saw moderated conditions, except for four dry regions that experienced increased precipitation. My assumption is that residents of these areas would likely prefer the wetter outcome, as people generally fear drought more than excess rain.

The model’s predictions might not fully match what happens in the real world, but if there's a key reason to seriously explore these technologies and test them through experiments, it’s findings like this one. It suggests that we might be able to reduce many of the major disruptions to the climate without severely impacting any specific region. That’s an impressive possibility.

How exactly will your proposed real-world experiment, known as the Stratospheric Controlled Perturbation Experiment (SCoPEx), work?

SCoPEx is an experiment involving a stratospheric balloon designed to release aerosols into the stratosphere and observe their behavior in the first few hours and over the first kilometer after being released in a plume. A high-altitude balloon will carry a gondola containing scientific instruments to an altitude of 20 kilometers. It will discharge a small quantity of substances such as ice, calcium carbonate (powdered limestone), or sulfuric acid droplets, known as sulfates. The gondola will feature propellers originally designed for airboats to help it travel through the released plume, gathering data along the way.

The amount of material released will be around 1 kilogram, a quantity too small to have any notable health or environmental effects once dispersed. The objective is not to alter the climate or attempt to reflect sunlight. Instead, the aim is to refine our understanding of how aerosols form in the stratosphere, particularly in plumes, which is crucial for understanding solar geoengineering. We hope to launch the experiment soon, but the timing and location will depend on the availability of the balloon and recommendations from an advisory committee.

We are aware of the health dangers posed by sulfuric acid pollution in the lower atmosphere. Could there be similar health risks from introducing sulfate aerosols into the stratosphere?

Anything we release into the stratosphere will eventually come back down to the Earth’s surface, which is one of the risks that needs to be considered. A large-scale solar geoengineering effort might involve injecting around 1.5 million tons of sulfur and sulfuric acid into the stratosphere every year. This could be carried out by a fleet of aircraft — approximately 100 planes that would have to continuously fly payloads up to around 20 kilometers (12 miles) in altitude. It may sound outlandish, but we know that sulfuric acid pollution in the lower atmosphere kills many people each year, so introducing it into the stratosphere undeniably poses risks. However, it's crucial to grasp the actual scale of 1.5 million tons annually.

The eruption of Mount Pinatubo in the Philippines in 1991 released roughly 8 million tons of sulfur into the stratosphere in just one year. This cooling effect impacted global climate and influenced various systems. Currently, global sulfur emissions total about 50 million tons annually into the lower atmosphere, which causes millions of deaths each year from fine particulate pollution. So, the relative risk posed by solar geoengineering seems relatively small, and it must be considered alongside the risks of not pursuing solar geoengineering.

How fast could a full-scale solar geoengineering program be implemented?

It could happen quite swiftly, but the scenarios in which it does so quickly are the problematic ones — essentially where a single country moves ahead without consulting others. The ideal situation would be for countries not to rush into it, but rather to develop clear strategies, establish checks and balances, and ensure proper governance.

If there is a significant increase in research over the next five to ten years — which is possible given the shift in attitudes — it's conceivable that a coalition of nations might begin to gradually implement a plan with concrete and visible actions that can be scrutinized by the scientific community by the end of this decade. While I don't expect it to unfold that quickly, I do think it's within the realm of possibility.

How does geoengineering complement other efforts to address climate change, such as reducing fossil fuel emissions and removing carbon from the atmosphere?

The most critical action we must take regarding climate change is decarbonizing our economy, which severs the connection between economic activity and carbon emissions. No matter what I say about solar geoengineering, the need to reduce emissions remains unchanged. Without that, we're in serious trouble.

Carbon removal, which entails capturing and storing carbon that has already been emitted, could disrupt the relationship between emissions and atmospheric carbon dioxide levels. Large-scale carbon removal becomes particularly relevant as emissions approach zero and we face the more challenging sectors of the economy to mitigate. Finally, solar geoengineering may offer a partial and imperfect solution to loosen, but not eliminate, the connection between atmospheric carbon dioxide levels and climate changes such as sea level rise, extreme weather events, and temperature shifts.

If you examine the overall greenhouse gas curve in the atmosphere, emissions cuts are like flattening the curve. Carbon removal helps you bring it down further. Meanwhile, solar geoengineering works by cutting off the peak of the curve, which helps mitigate the risks associated with the existing carbon dioxide in the air.

Some people argue that solar geoengineering should be seen as a last-resort option, used only in emergencies. Others suggest we should use it to rapidly revert to preindustrial climate conditions. I propose that we employ solar geoengineering to gradually taper the peak of the curve, starting and ending it slowly.

Do you feel hopeful about the potential for solar geoengineering to become a reality and significantly impact the climate crisis?

Right now, I'm not feeling particularly optimistic, as it seems we're far from an international environment that can foster rational policy. This issue is not exclusive to the U.S.; it affects many European nations with rising populist movements, Brazil, as well as more authoritarian countries like India and China. The world is becoming increasingly nationalistic, making it difficult to see a coordinated global effort in the near future. Nevertheless, I remain hopeful that these dynamics will change eventually.

This article was originally published in Knowable Magazine and is being republished here as part of Covering Climate Now, a global collaboration of journalists working to strengthen the climate narrative.