  The effect of anthropogenic aerosol on the reflectivity of stratocumulus
cloud decks through changes in cloud amount is a major uncertainty in climate
projections. The focus of this study is the frequently occurring
non-precipitating stratocumulus. In this regime, cloud amount can decrease
through aerosol-enhanced cloud-top mixing. The climatological relevance of this
effect is debated because ship exhaust does not appear to generate significant
change in the amount of these clouds. Through a novel analysis of detailed
numerical simulations in comparison to satellite data, we show that results
from ship-track studies cannot be generalized to estimate the climatological
forcing of anthropogenic aerosol. We specifically find that the
ship-track-derived sensitivity of the radiative effect of non-precipitating
stratocumulus to aerosol overestimates their cooling effect by up to 200%. This
offsetting warming effect needs to be taken into account if we are to constrain
the aerosol-cloud radiative forcing of stratocumulus.
