A potential emergent constraint on cloud liquid water path adjustments to aerosol–cloud interactions

Abstract. Emergent constraints are relationships between an observable in the present-day climate (such as cloud state variables) and an unobservable response (such as adjustments to radiative forcing) of the climate system to a perturbation. Here we present a candidate emergent constraint arising from the relationship across members of a perturbed parameter ensemble (PPE) between the observable present-day cloud droplet number–liquid water path (𝑁_𝑑–𝓛) correlation and the unobservable liquid water path adjustment to anthropogenic aerosol–cloud forcing (RA_𝓛). Emergent constraint candidates require scrutiny to distinguish them from, for example, spurious correlations. The candidate presented here meets several criteria delineated by Klein and Hall (2015): high correlation coefficient, plausible underlying physical mechanism, and emergence from a PPE that perturbs the physical parameters relevant to both the observable and the climate response. Constraining the observable 𝑁_𝑑–𝓛 regression slope to present-day satellite estimates yields a constrained estimate for the ratio of present-day to preindustrial cloud liquid water path 𝓛_PD/𝓛_PI = 0.976 ± 0.009 (PPE regression slope uncertainty only) with a regression coefficient of 0.92. The constrained 𝓛_PD/𝓛_PI implies a robustly positive RA_𝓛; this disagrees in sign with all other general circulation model (GCM) estimates, but agrees with non-GCM lines of evidence. However, the constrained estimate requires extrapolating the emergent-constraint relationship past the minimum 𝓛_PD/𝓛_PI produced by any of the PPE members.