Assessing cloud sensitivity to shipping aerosol across large emissions ranges

Aerosol-cloud interactions remain a large source of uncertainty in anthropogenic climate forcing. One of the reasons for this uncertainty is the confounding role of meteorology, influencing both aerosols and cloud properties. To untangle these variables, ship tracks, the clouds polluted by shipping emissions, have been widely studied. Recently, the use of shipping emissions locations and amounts, combined with reanalysis winds, has allowed us to study polluted clouds by following ship emissions to the locations they are advected to by the time of a satellite measurement of clouds. This is possible even when no visible tracks appear in satellite images. Here, we additionally use emission amounts data and investigate their effect on key cloud characteristics like droplet numbers and liquid water. This per-ship emissions data is valuable as it allows us to investigate cloud property changes stratified by region or meteorology. Between the ships with the lowest and highest emissions, droplet number anomalies increase by an order of magnitude from 0.25% to 2.5%, but the effect saturates at high emissions. We furthermore present evidence that increases of liquid water are insensitive to the amount of aerosol increases. Crossing data with a set of machine-learning detected ship tracks, we show that emissions amount has a similarly saturating effect on the formation of visible tracks as on droplet number, increasing roughly linearly for a large range of emissions before saturating (and even declining) at high emissions. The saturation of cloud responses at relatively high emissions could indicate that clouds react strongly to reductions in aerosol emissions.