It sounds like something out of science fiction: A country suffering from heat, flooding or crop failures decides on its own to send out a fleet of aircraft to spray a fine, sun-blocking mist into the earth’s atmosphere, reducing temperatures and providing relief to parched populations. Other countries view it as a threat to their own citizens and ready a military response.

But members of the U.S. intelligence community and other national security officials were worried enough last year to plot how to avert a war triggered by this kind of climate engineering. In a role-playing exercise, they practiced managing the tensions that would be unleashed, according to people familiar with the exercise, a sign that they see it as a credible threat in need of a strategy.

The practice, known as solar geoengineering, is theoretically possible. And as the world’s most vulnerable populations suffer more sharply from rising temperatures, global decision-makers will likely come under heavy pressure to deploy the technology, scientists and policymakers say. Compared to other methods to combat the effects of climate change, it’s likely to be cheaper and faster.

Because the technique could weaken the sun’s power across the globe — not just above whichever country decided to deploy it — security officials are concerned about the potential to spark conflict, since a single capital could make decisions that shape the entire world’s fate.

“Parts of the U.S. government are rightfully focused on trying better to understand this,” said Sherri Goodman, a senior fellow at the Wilson Center, referencing last year’s geoengineering exercise. “If you don’t understand it, you can’t manage it.”

The science is evolving, said Goodman, a longtime expert on the intersection of climate change and security. But global discussions haven’t kept up, leaving a powerful technology largely unregulated internationally.

“It could be weaponized by a country to either try to improve the climate and reduce the temperatures in their own location or against an adversary,” Goodman said. “It could be threatened in a way that could cause fear or panic among populations.”

So far, the United States is in the lead on research, but China, India and others are also working on it. The White House is in the process of developing a five-year research plan and strategy for managing the technology, at the behest of Congress, which mandated it last year.

The lack of global coordination on geoengineering is resulting in increasing anxiety about the risks of disagreements. A climate change-focused National Intelligence Estimate from 2021 — the distillation of U.S. spy agencies’ assessments about the top risks facing the United States — warned that the absence of regulations could mean that “state or nonstate actors will independently develop or deploy the technology — possibly covertly — in a manner that risks conflict if other nations blame them for a weather disaster they believe was caused by geoengineering.”

“It’s a powder keg,” said Janos Pasztor, who was a top United Nations official focused on climate change before devoting himself to fostering a global discussion about the governance of geoengineering. “If there are already challenges at the geopolitical level in certain regions … you have potentially some really serious issues.”

Now some U.S. intelligence officials are focused on understanding the challenges. A favorite read among some of them is a 2020 novel, “The Ministry for the Future,” by Kim Stanley Robinson. It envisions a near future in which, among other developments, India decides unilaterally to deploy solar geoengineering in defiance of a global ban after a heat wave kills 20 million of its citizens. Former president Barack Obama named it one of his favorite books of that year.

Some policymakers say that as the discussion evolves, existing tensions could worsen between industrialized nations that are responsible for the bulk of historic carbon emissions and less industrialized countries that are often hit hardest by climate change.

Solar geoengineering “has potential social and ethical issues around public consent and has the potential to spawn international conflicts,” Ajay K. Sood, the principal scientific adviser to the Indian government, told a climate-focused conference in New Delhi this month, adding that research into the technology in India was “nascent.”

“This technology may end up concentrating power in rich countries or nonstate actors in the global north,” he said.

In the global south, he said, solar geoengineering “may appear appealing given the context of dire climate consequences faced by them.”

Even basic research such as outdoor small-scale tests could turn controversial in some areas, some people say, let alone full-scale deployment.

“In troubled neighborhoods like ours, the protocols of the trials will need to be coordinated and collectively monitored, which becomes an impossibility in the highly fractured and polarized environments like the one that exists between Pakistan and India,” said Malik Amin Aslam, a former Pakistani minister for climate change.

Scientists say the technology necessary to hack the atmosphere is not complicated. Airplanes would spray sulfur into the sky at high altitude. Done right, the material would condense, reflecting a portion of the sun’s rays and shielding the Earth. That would mimic the cooling effect of a large volcanic explosion, which happened after the 1991 eruption of Mount Pinatubo in the Philippines. That volcano heaved about 15 million tons of sulfur dioxide into the atmosphere and lowered the average global temperature by about 1 degree Fahrenheit over the next 15 months, according to NASA estimates.

“It’s increasingly clear that putting a bunch of aerosols in the stratosphere could decrease the global average temperature,” said Chris Field, the director of the Stanford Woods Institute for the Environment. He chaired a National Academies of Sciences, Engineering and Medicine committee that recommended in 2021 that the Biden administration fund a federal research program into the technology.

“The future really depends on getting an ambitious response to the climate crisis put in place. And we just need to be really open to recognizing that some kinds of approaches that are fraught with downsides might still deserve to be considered just because the alternatives are so serious,” Field said.

Some scientists say the risks could be significant. Changes to the atmosphere would shift weather patterns and create droughts. A less-intense sun could lower crop yields and lead to hunger. There are concerns temperatures could build outside the sulfur dioxide layer that, if the spraying is stopped, could unleash a catastrophic heat wave across the world. Some climate experts worry that societies might use the technique to stall on the emissions cuts that are needed to fix the root of the problem.

There are other kinds of geoengineering techniques that would be more localized. Seawater could be sprayed into clouds, brightening them so that they shield more of the sun’s rays. Tiny glass beads could protect Arctic ice from melting. Iron dust in the oceans could feed phytoplankton that would then pull carbon dioxide out of the air. Those techniques might have risks — but they would be less likely to cross borders.

What could make the sulfur technique so much more effective is exactly what makes it diplomatically complicated: It doesn’t stay above the country that does the spraying. Instead, it spreads across the globe. Beijing’s decisions could have climate consequences in the United States — and vice versa.

“I can see this going wrong someday,” said Elizabeth Chalecki, a professor of political science at University of Nebraska Omaha who has studied the intersection of climate and security issues. “You can’t even govern based on carbon emissions. How are you going to govern a technology that could purposefully alter planetary living conditions?”

In the absence of global consensus, some private groups are taking matters into their own hands. One company, Making Sunsets, declared late last year that it had released some balloons filled with a compound that could reflect sunlight back into the atmosphere. The release was done in Mexico, prompting a backlash and the Mexican government to ban experimentation last month.

Most geoengineering activity so far remains inside computer models or laboratories.

China has had a state-backed solar geoengineering program at Beijing Normal University for much of the past decade, but it does not appear to have done open-air testing. The country has a long history of smaller-scale efforts to modify the climate, such as when it spent $30 million shooting salt and minerals to clear skies before the 2008 Beijing Olympics, or when it seeds clouds to make it rain ahead of other politically sensitive events.

In the United States, research has also been expanding. “There’s a wealth of scientific evidence in the last decade that these technologies really could significantly reduce the human environmental costs of climate change,” said David Keith, a scientist at Harvard who is a prominent researcher in the field.

His colleagues have struggled to address sensitivities about outdoor testing. A Harvard research project that sought to conduct a small-scale open-air solar geoengineering experiment in Sweden was called off in 2021 after the country’s indigenous Saami Council opposed it. The researchers are still considering other locations.

Around the world, a growing number of researchers are exploring what solar engineering might mean for their regions, even if their home countries are unlikely to deploy the technology.

Inés Camilloni, a climate scientist at the University of Buenos Aires, said the technology still might be able to address some short-term challenges. She has conducted research about the consequences of solar geoengineering on the Plata River basin that stretches across a densely populated region in South America. Deploying the technology, she found using lab modeling, would likely lead to higher river levels, decreasing drought but increasing flooding.

Researchers in South Asia say that it is easy to imagine disagreements across borders about solar geoengineering — especially given that cooperation on some issues is already tenuous at best. China, India, Pakistan and Bangladesh share river systems, but they don’t always give each other the rainfall and river level data that would be needed to predict and prepare for floods.

Since solar geoengineering could change weather patterns, that could make predicting rainfall even more challenging, said Mohammed Abu Syed, a senior fellow at the Bangladesh Center for Advanced Studies, who is doing research about how geoengineering could change rain patterns in his country.

There is little trust across borders, he said. One country’s deploying a geoengineering program “would influence the whole hydrological regime of South Asia,” he said, but Bangladesh’s broad floodplains might bear the brunt of new, unpredictable weather.

If solar geoengineering turns out to have negative consequences, he said, “We would be the one who would most suffer from this.”

A previous version of this article identified Sherri Goodman as the vice chair of the State Department’s International Security Advisory Board. Goodman was speaking in her personal capacity as a longtime expert on the intersection of climate change and security, not as an outside adviser to the State Department.