Quantifying Cloud Brightening Based on Natural and Inadvertent Manmade Experiments in Marine Stratocumulus

Brightening marine stratocumulus in pristine ocean areas by seeding them with cloud condensation nuclei (CCN) is one of the main methods that are considered for solar radiation management, aimed at cooling the climate. General circulation models that have been employed for calculating the expected effects are lacking part of the needed physical processes, thus limiting our ability to quantify the aerosol cloud-mediated radiative forcing. Here we show the radiative effects of inadvertent experiments, such as ship tracks and air pollution that invades the ocean from densely populated land areas. Under optimal conditions, i.e., large anticyclones, the radiative forcing can reach -100 Wm-2, mostly due to increasing cloud cover and liquid water path. Observing the effects of natural variability of sea spray aerosols in the storm track of the Southern Oceans shows that, on average, the radiative forcing can reach -20 Wm-2. The enhanced wind-driven sea spray has this effect. This means that an artificial spray-making method that produces a wide distribution of particles is likely to have a substantial cooling effect, as long as most of the particles are created in the sub-micron size range.