An idea once dismissed as science fiction — cooling Earth by scattering sunlight-reflecting particles into the upper atmosphere — is now being taken seriously by researchers. This proposed technique, called stratospheric aerosol injection (SAI), aims to offset global warming by mimicking the cooling effect of volcanic eruptions. Hundreds of scientific models have explored how it might work. Yet researchers at Columbia University caution that those models overlook how complex, uncertain, and potentially risky such an effort would be.
“Even when simulations of SAI in climate models are sophisticated, they’re necessarily going to be idealized. Researchers model the perfect particles that are the perfect size. And in the simulation, they put exactly how much of them they want, where they want them. But when you start to consider where we actually are, compared to that idealized situation, it reveals a lot of the uncertainty in those predictions,” says V. Faye McNeill, an atmospheric chemist and aerosol scientist at Columbia’s Climate School and Columbia Engineering.
“There are a range of things that might happen if you try to do this — and we’re arguing that the range of possible outcomes is a lot wider than anybody has appreciated until now.”
Exploring the Limits of Solar Geoengineering
In a study published in Scientific Reports, McNeill and her team examined the physical, political, and economic barriers that make SAI far more complicated in reality than in theory. They reviewed existing studies to understand how the results of SAI would depend on the details of how and where it is deployed. Key factors include the altitude and latitude of particle release, the time of year, and the total amount of material injected into the atmosphere.
Among these variables, latitude appears to have the greatest influence. SAI efforts concentrated near the poles, for example, could disrupt tropical monsoons, while releases near the equator might alter jet streams and interfere with global air circulation.
“It isn’t just a matter of getting five teragrams of sulfur into the atmosphere. It matters where and when you do it,” says McNeill. These variabilities suggest that, if SAI takes place, it should be done in a centralized, coordinated fashion. Given geopolitical realities, however, the researchers say that is unlikely.
Lessons From Volcanoes
Most climate models studying SAI assume the use of sulfate aerosols, similar to the compounds produced by volcanic eruptions. When Mount Pinatubo erupted in 1991, for instance, global temperatures dropped by nearly one degree Celsius for several years. That event is often cited as evidence that SAI could temporarily cool the planet.
But volcanic activity also highlights the risks. Pinatubo’s eruption disrupted the Indian monsoon system, reduced rainfall across South Asia, and contributed to ozone depletion. Similar side effects could result from artificial sulfate releases, including acid rain and soil contamination. These concerns have pushed scientists to investigate other, potentially safer materials.
Searching for Better Materials
Proposed alternatives include minerals such as calcium carbonate, alpha alumina, rutile and anatase titania, cubic zirconia, and even diamond. While much attention has been paid to how well these materials might scatter sunlight, other essential questions — such as their availability and practicality — have been less explored.
“Scientists have discussed the use of aerosol candidates with little consideration of how practical limitations might limit your ability to actually inject massive amounts of them yearly,” says Miranda Hack, an aerosol scientist at Columbia University and the new paper’s lead author. “A lot of the materials that have been proposed are not particularly abundant.”
Diamond, for instance, would perform well optically but is far too scarce and expensive to use. Cubic zirconia and rutile titania could meet demand in theory, but economic modeling by the Columbia team suggests production costs would skyrocket with increased demand. Only calcium carbonate and alpha alumina are abundant enough to be feasible at scale, yet both face serious technical problems during dispersion.
Small Particles, Big Problems
For SAI to work, particles must remain extremely small — less than one micron in size. However, the mineral alternatives tend to clump together into larger aggregates. These larger clusters scatter sunlight less effectively and behave unpredictably in the atmosphere.
“Instead of having these perfect optical properties, you have something much worse. In comparison to sulfate, I don’t think we would necessarily see the types of climate benefits that have been discussed,” says Hack.
A Strategy Full of Uncertainty
According to the researchers, the many unknowns surrounding SAI — from deployment logistics to material performance — make the technique even more uncertain than previously believed. These challenges should be clearly acknowledged when policymakers and scientists discuss the future of solar geoengineering.
“It’s all about risk trade-offs when you look at solar geoengineering,” says Gernot Wagner, a climate economist at the Columbia Business School and a close collaborator with the Climate School. Given the messy realities of SAI, he says, “it isn’t going to happen the way that 99 percent of these papers model.”
The study also lists Daniel Steingart, co-director of the Columbia Electrochemical Energy Center, as a coauthor. Together, the team emphasizes that while SAI may seem like an attractive quick fix for global warming, the path to actually cooling the planet could be far more perilous and unpredictable than it appears.