PNNL

Abstract

Uncertainty in anthropogenic forcing driven by aerosol-cloud interactions (aci) limits our ability to infer the sensitivity of the Earth system to forcing from historical records. The driver of aci is the change in cloud droplet number concentration (Nd) due to changes in aerosol that can serve as cloud condensation nuclei (CCN). Here, we combine a perturbed parameter ensemble (PPE) run in a global Earth system model with observations of CCN and Nd at surface sites in the Azores, the Southern Great Plains, and Ascension Island to provide a constraint on the anthropogenic contribution to present-day Nd. These observational lines of evidence constrain the preindustrial to present-day change in Nd to be between 16 and 31 cm-3. This is consistent with the upper end of some previous estimates but has a higher minimum perturbation, pointing to a stronger historical aerosol cooling.