Abstract
The effective radiative forcing due to aerosol-cloud interactions (ERFaci) is difficult to quantify, leading to large uncertainties in model projections of historical forcing and climate sensitivity. In this study, satellite observations and reanalysis data are used to examine the low-level cloud radiative responses to aerosols. While some studies assume that the activation rate of cloud droplet number concentration (Nd) in response to variations in sulfate mass concentration (SO42-) has a one-to-one relationship, we find this assumption to be incorrect. Our analysis estimates a global mean activation rate of 0.35 +/- 0.17 (90 % confidence) and demonstrates that explicitly accounting for the activation rate is crucial for accurate ERFaci estimation. This is corroborated through a "perfect-model" cross-validation using state-of-the-art climate models. Our results suggest a smaller and less uncertain value of the global ERFaci (-0.32 +/- 0.21 W m-2 for SO42-, 90 % confidence) than recent climate assessments (e.g., -0.93 +/- 0.7 W m-2, 90 % confidence), indicating that ERFaci may be less impactful than previously thought. Our results are also consistent with observationally constrained estimates of total cloud feedback and recent estimates that models with weaker ERFaci better match the observed hemispheric warming asymmetry over the historical period.