Ship-track-based assessments overestimate the cooling effect of anthropogenic aerosol

TL;DR

This study reveals that ship-track experiments overestimate the cooling effect of anthropogenic aerosols on stratocumulus clouds by up to 200%, highlighting the need for revised estimates in climate models.

Contribution

The paper introduces a novel analysis comparing ship-track data with satellite observations, showing that previous estimates of aerosol cooling effects are significantly overstated.

Findings

Ship-track studies overestimate aerosol cooling by up to 200%.

Satellite data contradict the generalization of ship-track effects.

Offsetting warming effects are significant in aerosol-cloud interactions.

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. 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.