Making tracks
The magnitude of the effect of anthropogenic aerosols on the formation of clouds is an important unknown about how humans are affecting climate. Studies of stratocumulus cloud tracks that are formed by ship exhaust have been used to estimate the radiative impact of this process, but Glassmeier et al. now show that this approach overestimates the cooling effect of aerosol addition by up to 200%. These findings underscore the need to quantify stratocumulus cloud responses to anthropogenic aerosols to understand the climate system.
Science, this issue p. 485
Abstract
The effect of anthropogenic aerosol on the reflectivity of stratocumulus cloud decks through changes in cloud amount is a major uncertainty in climate projections. In frequently occurring nonprecipitating stratocumulus, cloud amount can decrease through aerosol-enhanced cloud-top mixing. The climatological relevance of this effect is debated because ship exhaust only marginally reduces stratocumulus amount. By comparing detailed numerical simulations with satellite analyses, we show that ship-track studies cannot be generalized to estimate the climatological forcing of anthropogenic aerosol. The ship track–derived sensitivity of the radiative effect of nonprecipitating stratocumulus to aerosol overestimates their cooling effect by up to 200%. The offsetting warming effect of decreasing stratocumulus amount needs to be taken into account if we are to constrain the cloud-mediated radiative forcing of anthropogenic aerosol.
