Stratospheric aerosol injection is not being used at scale by any government or organization. No country is deploying SAI to cool the planet, and no large-scale program is underway. The only known actor actively releasing material into the atmosphere is a small startup called Make Sunsets, which has launched weather balloons containing sulfur dioxide from sites in the United States. The amounts released are tiny compared to what would be needed to affect global temperatures.
What Make Sunsets Has Done
Make Sunsets, a private company, has conducted over 124 balloon launches carrying small amounts of sulfur dioxide gas. The company sells “cooling credits” based on these releases, framing its activity as a commercial geoengineering service. Mexico banned the company’s operations after early launches were conducted there without government approval. In 2025, the U.S. Environmental Protection Agency issued a formal demand for information from the company, pressing for answers about its unregulated releases of sulfur dioxide into the air.
The EPA has confirmed that as of July 2025, Make Sunsets is the only private actor it is aware of that has actively deployed stratospheric aerosol injection in the United States. The material released to date is extremely small relative to natural events like volcanic eruptions, which can inject millions of tons of sulfur into the stratosphere in a single event.
No Government Is Deploying SAI
The U.S. government has stated clearly that it is not engaged in any form of outdoor solar geoengineering testing or large-scale deployment. The EPA has also confirmed that solar geoengineering is not occurring through commercial aircraft and is not associated with aviation contrails.
The federal government does fund limited research into solar radiation modification. Congress has directed NOAA to fund research as part of its Earth’s Radiation Budget Program, and NOAA and NASA are cooperating on missions to sample the lower stratospheric aerosol layer using high-altitude research aircraft. In 2023, the White House Office of Science and Technology Policy released a congressionally mandated research plan outlining how the U.S. could study SAI’s potential risks and benefits. That plan emphasized modeling, observation, and societal analysis, not preparation for deployment.
Australia is supporting research and limited deployment of a related but distinct technique called marine cloud brightening, which aims to shade and cool ocean water over the Great Barrier Reef. This is not stratospheric aerosol injection.
The Most Prominent Experiment Was Cancelled
Harvard University’s Stratospheric Controlled Perturbation Experiment, known as SCoPEx, was the highest-profile scientific effort to test SAI technology in the real atmosphere. The project would have released a small amount of material from a high-altitude balloon to study how particles behave in the stratosphere. It never got that far. The research team suspended work in August 2023, and the lead scientist, Frank Keutsch, later announced he was no longer pursuing the experiment. The balloon platform developed for the project is expected to be repurposed for unrelated atmospheric research.
International Rules Remain Incomplete
There is no international framework governing solar geoengineering research, experiments, or deployment. The Convention on Biological Diversity adopted a precautionary position in 2010, stating that no geoengineering activities that may affect biodiversity should take place until there is an adequate scientific basis to justify them, appropriate risk assessment, and effective regulatory mechanisms. A narrow exception exists for small-scale scientific studies conducted in controlled settings. Countries including the UK, France, Canada, and Bolivia have submitted formal positions in response, but the resolution is non-binding and lacks enforcement power.
This governance gap is a central concern among researchers and policymakers. Without agreed-upon rules, any nation or private company could theoretically begin injecting aerosols unilaterally, which is part of why Make Sunsets’ activities drew such sharp regulatory attention.
How SAI Would Work in Theory
The concept behind SAI is straightforward: inject reflective particles into the stratosphere (roughly 12 to 30 miles above the surface) so they bounce a small fraction of incoming sunlight back into space before it warms the Earth. The idea comes from observing volcanic eruptions. When Mount Pinatubo erupted in 1991, it sent massive quantities of sulfur into the stratosphere and global temperatures dropped by about half a degree Celsius for the following year.
Most proposals focus on injecting sulfur dioxide, which reacts in the stratosphere to form tiny droplets of sulfuric acid and water. These droplets scatter sunlight effectively. To offset the warming from a doubling of pre-industrial carbon dioxide levels, estimates suggest a sustained injection of roughly 1 to 10 million metric tons of sulfur per year would be needed.
A Harvard analysis estimated that a global SAI program using purpose-built high-altitude aircraft would cost an average of $2 to $2.5 billion per year over its first 15 years, with roughly $2 billion in upfront development costs for new airframes and $350 million to modify existing engines. That price tag is remarkably low compared to the economic damages of unchecked warming, which is part of what makes SAI both appealing and worrying to policymakers: it could be cheap enough for a single nation to deploy on its own.
Known Risks and Trade-Offs
Sulfur-based SAI would not simply reverse warming. Modeling studies show it would slow the global water cycle, reducing overall precipitation even as it lowers temperatures. Regional effects could be significant. One study found that SAI designed to stabilize global temperatures could reduce rainfall over the Indian subcontinent by roughly 11.6%, creating drought-like conditions, while increasing rainfall over southern India and parts of China. The Asian summer monsoon, which hundreds of millions of people depend on for agriculture, would weaken due to changes in upper-atmosphere wind patterns and pressure systems.
Sulfur dioxide injection also damages the ozone layer. Studies estimate that injecting enough sulfate to meaningfully cool the planet would reduce the ozone column by 1 to 13%, depending on the scenario. It would also heat the lower tropical stratosphere, increasing water vapor concentrations and triggering additional ozone destruction.
Researchers have explored alternatives. Calcite, essentially powdered limestone, could potentially cool the planet while simultaneously repairing the ozone layer rather than damaging it. Calcite particles neutralize the acids that drive ozone-destroying chemical reactions. One analysis found that 2.1 million metric tons per year of calcite particles could produce meaningful cooling with a simultaneous 3.8% increase in ozone. Calcite would also warm the lower stratosphere about ten times less than sulfate aerosols producing the same cooling effect. This remains theoretical, however, with no real-world atmospheric testing completed.
The Termination Problem
One of the most discussed risks of SAI is what happens if it stops suddenly. Aerosol particles only remain in the stratosphere for one to two years before settling out, so the cooling effect requires continuous replenishment. If a global SAI program were halted abruptly, whether due to war, economic collapse, or political disagreement, the masked warming would return rapidly. This “termination shock” could expose ecosystems and human systems to decades of suppressed warming compressed into just a few years, a rate of change far faster than what would have occurred without intervention. This concern is a major reason why the U.S. research plan emphasizes studying sudden termination scenarios through climate models.