How solar geoengineering may change our skies

Add aerosols to the stratosphere, and sunlight is reflected into space, cooling Earth’s surface. Volcanoes have done it, so why can’t humans? That’s the basic idea behind stratospheric aerosol injection (SAI), a controversial, and currently theoretical, geoengineering strategy. Some scientists believe that SAI could help mitigate the worst effects of global warming. Others say unintended consequences from SAI and the lack of strict international governance to ensure the technology is not abused make it too risky to consider.

Modelers have projected SAI’s impact on global and local climates, but there have been few studies on the visual effect of adding excess aerosols into the stratosphere. Now researchers have simulated what the sky may look like after injecting a blanket of various aerosols into the upper atmosphere (Environ. Res. Lett. 2025, DOI: 10.1088/1748-9326/ada2ae).

Ansar Lemon, an engineering PhD student at Harvard University, says the team modeled how the brightness and color of the sky changed with the injection of sulfate, calcium carbonate, and diamond aerosols. Then, using previous human perception research, the team calculated how noticeable humans would find the changes.

Although sky whitening is cited by some as a concerning visual effect, Lemon says the team’s results suggest that changes to twilight, to sunsets, and to the solar aureole—the bright-white glow around the sun—will be far more noticeable. The sky colors at twilight and sunset may become more vibrant, and the solar aureole in the day may become larger. But Lemon says the team calculated that only half of observers would perceive a change in sky whiteness, and only with the help of visual aids.

“It seems like the concerns over sky whitening are a little overblown,” he says. It would likely take a keen observer to notice a change, especially in places with high air pollution in the troposphere.

The new work offers a precise measurement of how SAI deployments may change the sky’s appearance, climate scientist Alan Robock of Rutgers University–New Brunswick says. “It shows that if you put a lot of stuff in the stratosphere, people will notice,” he says. This is particularly true for those living in areas with low air pollution.

The researchers’ findings could help people make educated decisions about deploying SAI campaigns, Robock says. Changing the way the sky looks “might rub some people the wrong way,” he adds.

In addition to this paper, Lemon and his coauthors published an online version of their simulation. There, curiosos can compare for themselves how the sky might change under different SAI scenarios and decide if the difference is perceptible.