Stratospheric Aerosol Injection Would Change Cloud Brightness

Intentional modification of the sunlight reflected back to space by Earth has received increasing attention as a potential tool to combat the current climate crisis. Two approaches have emerged as most viable: Stratospheric Aerosol Injection (SAI) and Marine Cloud Brightening. This study identifies a substantial but unrecognized source of cloud brightening caused by SAI, which we call “diffusion-brightening”; essentially, diffusion of the radiation field that would accompany SAI can result in sunlight entering clouds at steeper angles, which increases cloud albedo without actively injecting aerosols into clouds. We present idealized calculations that suggest the diffusion-brightening effect can lead to clouds reflecting about 10% more of the incoming sunlight depending on stratospheric aerosol, cloud, and Sun conditions. We show that the radiative effect of diffusion-brightening could exceed that of stratospheric aerosol reflection in many cloudy scenes, which has global relevance given that clouds cover around two-thirds of the planet.