Solar geoengineering moves into the spotlight as climate concerns grow

Earth’s average temperature is continuing to tick inexorably upward as the world’s nations stall at reducing their atmosphere-warming emissions. In the face of that grim future, strategies to try to turn down the planet’s thermostat are gaining traction. One strategy in particular — solar geoengineering, which aims to cool the planet by reflecting solar radiation back into space — may be having a moment in the sun.

Depending on whom you ask, it’s potentially highly dangerous, highly promising or highly uncertain. There aren’t any real guidelines. But, with the future of emissions restrictions also highly uncertain, some researchers say solar geoengineering needs to be on the table.

“It’s incredibly depressing that we’re here,” says Shuchi Talati, a climate technology governance expert who founded the nonprofit Alliance for Just Deliberation on Solar Geoengineering in Washington, D.C. “I don’t want to have this conversation. But this is where we are.”

So let’s have that conversation.

Here are the facts: Earth just had its hottest year on record — 2024 was the first year in recorded history in which the average planetary temperature rose above 1.5 degrees Celsius relative to preindustrial temperatures.

The heat had deadly consequences around the globe. And the planet’s temperature continues to tick upward, as humans’ emissions of greenhouse gases such as carbon dioxide continue to flood the atmosphere.

Another fact: Reducing those emissions, scientists agree, is the number one preferred strategy to hold Earth’s average temperature in check.

And one more: Nations may in principle agree on these points, but actual emissions reductions are just not happening fast enough to meet the goals outlined at the 2015 Paris climate summit: holding the planet’s average temperature to less than 2 degrees Celsius above preindustrial times.

“The goal posts keep moving because we keep missing the targets,” Talati says. 

A relatively quick geoengineering fix

In the face of this grim reality, global attention on — and funding for — climate intervention strategies is skyrocketing. Proposed climate interventions, or geoengineering, target different aspects of Earth’s climate system, hoping to find ways to mitigate the greenhouse effect. One strategy is to remove some of the climate-warming carbon dioxide from the atmosphere via large-scale tree-planting projects or ocean-based carbon-removal technologies.

Another is to intercept incoming radiation from the sun and send it back out into space. Generally known as solar geoengineering, the term is an umbrella for a variety of possible strategies to reflect sunlight, such as injecting reflective molecules into the stratosphere or brightening clouds over the ocean.

“I think that’s why solar radiation management has become a more prominent part of the discussion,” Talati says. “Is this a thing where maybe the benefits outweigh the harm? Is it a thing we have to start thinking about? Will it exacerbate injustices? Will it create new ones? I don’t know the answers to those questions.”

Weighing the pros and cons

One concern is the uncertainty over how solar geoengineering might shift temperature and precipitation changes around the globe, potentially exacerbating inequities in the impacts of climate change, particularly in the Global South. Another is how injecting aerosol particles into the stratosphere might affect the ozone layer or increase air pollution at the ground level when the particles sink.

On paper, though, the benefits of solar geoengineering might be great. In a 2019 study, a team of researchers used computer simulations to analyze how different parts of the globe would fare under an idealized solar geoengineering scenario, in which the technology halved the warming produced by humans’ increased carbon dioxide inputs. That amount of cooling, their models suggest, could offset most of the carbon dioxide–induced increase in tropical cyclone intensity. The study also found no clear evidence of heightened risks to poorer parts of the world, whether from temperature or extreme precipitation.

“Is [solar geoengineering] a thing where maybe the benefits outweigh the harm? Is it a thing we have to start thinking about? Will it exacerbate injustices? Will it create new ones? I don’t know the answers to those questions.”

Shuchi Talati
climate technology governance expert

Broadly speaking, the study found that “solar geoengineering is really hemispherically balanced and reduces hazards almost everywhere,” says David Keith, a climate scientist at the University of Chicago. “I would have thought that was an incredibly over-the-top statement 20 years ago, but I think it’s pretty much a summary of what the literature says.”

Keith also coauthored a recent study that compares the risk of death from heat on an Earth 2.5 degrees C warmer than preindustrial times against increased death risk from air pollution and ozone loss due to solar geoengineering. Their calculations suggested that for every 10 lives saved from 2050 to 2080 by cooling Earth’s temperatures, there would be only one extra death from air pollution and ozone loss, the team reported in December in Proceedings of the National Academy of Sciences. Moreover, the study suggests that the greatest benefits would be in the world’s hottest and poorest regions.

The study is intended to start putting some hard numbers on the big questions of social cost, says coauthor Anthony Harding, an environmental economist at Georgia Tech in Atlanta. “It’s important to have numbers rather than saying ‘this is good’ and ‘this is bad.’”

Talati agrees that there’s value in quantifying solar geoengineering’s benefits. “But I do think there can be a danger in painting solar radiation management, in a certain way, as too positive or too negative, when we haven’t done enough research to be able to say anything with that much certainty.” Parsing those benefits versus the risks is particularly challenging when it comes to conveying takeaways to policy makers, she says.

Keeping solar engineering in the lab — for now

There’s plenty of call to — cautiously — do more research on the topic. It shows up, for example, in recent reports by the United Nations’ Intergovernmental Panel on Climate Change and the U.S. National Academies of Sciences, Engineering and Medicine. But there’s also a great deal of wariness. In December, scientists with the European Union’s European Commission called for a moratorium across the continent on solar geoengineering research outside of the laboratory. Hundreds of scientists signed an open letter in January 2022 calling for a worldwide non-use agreement, including no public funding and no outdoor experiments.

To date, there haven’t really been any field experiments in solar geoengineering. In March 2024, Harvard University shuttered a solar geoengineering research program that would have included the world’s first outdoor stratospheric aerosol injection experiment. The Stratospheric Controlled Perturbation Experiment, or SCoPEx, had planned to launch a weather balloon into the stratosphere over northern Sweden in 2021. The team wanted to release a small amount of calcite into the atmosphere to get baseline data for how the particles might behave, collecting data on aerosol microphysics, turbulence and atmospheric chemistry. Just before launch, the Swedish government canceled the test flight due to resistance from environmental groups and the Indigenous Sámi Council. 

What happened with SCoPEx has cast a shadow over solar engineering in general, which frustrates project leader David Keith, then at Harvard. The experiment got a lot of undue attention, including from the press, he says, particularly since that balloon flight wasn’t going to introduce any reflective aerosols into the atmosphere. Articles made resistance appear to be widespread, but surveys of public perception on the technology itself tell a different story, Keith says. “I don’t believe there’s a robust sense that the public is pushing back.”

Efforts to understand public perception of solar geoengineering have found mixed results. In a 2021 survey of U.S. adults by the nonpartisan Pew Research Center, based in Washington, D.C., about 41 percent of the total respondents — and over 50 percent of Generation Z and millennials — said they think solar geoengineering would mitigate the effects of global climate change. But about 74 percent of the respondents also expressed at least some concern about the consequences of the technology.

Global surveys of public perception also offer a nuanced mixture of hope and concern. Two 2024 studies by the same team of researchers presented findings of public perceptions of solar geoengineering’s cost-benefit ratio from 44 focus groups in 22 countries across Europe, North America, South America, Africa and Asia. Many respondents conditionally favored the idea of small-scale field tests as part of a larger risk assessment, the team reported, though many others were opposed to any testing at all. Among respondents in nations in the Global South — which tend to be poorer and are more vulnerable to the impacts of climate change — there was overall more support for such limited field tests, but also a “richer range of concerns,” including over global inequities and possible geopolitical conflict.

The trouble is, it’s not clear how to conduct small, localized field tests that will both have no impact and provide information on long-term impacts around the globe — an impossibility that many respondents were well aware of, the team found. But one thing that respondents around the globe agreed on was the need for both government coordination and public engagement on how any such research should progress. 

Yet one reason why scientists have balked at moving solar geoengineering forward is that there isn’t a single, agreed-upon framework for best practices and oversight, Talati says. She served as cochair of an independent advisory committee that oversaw part of the SCoPEx project. The committee devised “an incredibly detailed governance structure” for the project.

But all that work was for just that one project — and the guidance had no teeth; the researchers didn’t even necessarily need to follow it. It’s not a sustainable way for the field to move forward, Talati says. “You need well-understood norms that researchers are going to abide by, and that the public knows are going to be abided by. And it’s also not fair to ask researchers to create a system of governance every single time from scratch.”

An uncertain path forward

The future of solar geoengineering research in the United States is murky. One thing seems clear, Talati says: There isn’t likely to be an increase in regulation at the federal level. But that doesn’t mean it will be smooth sailing, considering that there are many competing views within the administration.

“You have the [Elon] Musks and [Peter] Thiels, and they want deregulation, technology development. I can see that being a very easy space for the private sector to take advantage of,” Talati says. If the federal government does engage in funding any research, she says, it may shift from the National Oceanic and Atmospheric Administration to, for example, the Department of Defense — which, she adds, could potentially make the details of that research less transparent.

On the other hand, political stances on solar radiation management among the country’s primary political parties have also shifted significantly in recent years. “The classic thinking in, say, 2010 might be that Republicans would be more interested,” Talati says, “because it could be a cheaper way to address climate change that would allow the fossil fuel industry to continue business as usual.”

But in the first three months of 2025, over 16 states have introduced bills to ban solar geoengineering; a Tennessee ban on the technology passed into law in 2024. Many of the states considering such legislation are Republican-led, including Texas and Florida, Talati notes. And in some cases, the bills refer to a debunked conspiracy theory previously promoted by U.S. Health and Human Services secretary Robert F. Kennedy, Jr., that the government is already conducting weather modification via chemtrails, a misperception that planes’ condensation trails are actually injections of chemicals.

“It’s kind of astounding how many states we have now that have introduced this legislation, and I think that will change how different political parties engage with [solar geoengineering],” Talati says. “I don’t think anyone saw that coming.”