PNNL

Abstract

Cloud reflectivity changes due to anthropogenic aerosol emissions are still a source of significant uncertainty in future climate predictions. Satellite observations are subject to confounding effects influencing both aerosols and cloud properties. Inferring aerosol effects from observations therefore relies on ‘natural experiments’ of localized emissions, so that polluted clouds can be compared to unpolluted ones nearby. Under certain circumstances, shipping emissions form ship tracks, long lines of polluted clouds that are visible in satellite images. However, only a fraction of the clouds polluted by shipping show these features. Here, we apply a new method to investigate all clouds polluted by shipping, regardless of whether they exhibit visible ship tracks. We show that formerly invisible emissions change cloud properties considerably. Cloud droplet number increases are roughly half as strong as in visible tracks. The response in liquid water path in non-precipitating clouds is positive in the invisible tracks, rather than negative. Together, this implies a higher reflectivity, even though it is not visible to the naked eye. We also for the first time directly detect shipping-induced cloud property changes in the trade cumulus regions of the Atlantic. These regions show a comparable droplet number increase and a strong positive liquid water response. Our results indicate that previous studies of ship tracks were strongly biased by selecting only visible tracks. The strong liquid water path response we find translates to a larger aerosol cooling effect on the climate, masking greenhouse gas induced warming.