Observational Constraints Suggest a Smaller Effective Radiative Forcing from Aerosol-Cloud Interactions

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 it is assumed that the activation rate of cloud droplet number concentration (N_d) in response to variations in sulfate aerosols (SO₄) or the aerosol index (AI) has a one-to-one relationship in the estimation of ERFaci, we find this assumption to be incorrect, and demonstrate 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, which compares our estimates with the “true” ERFaci. Our results suggest a smaller and less uncertain value of the global ERFaci than previous studies (-0.39 ± 0.29 W m^-2 for SO₄ and -0.24 ± 0.18 W m^-2 for AI, 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 “top-down” estimates that models with weaker ERFaci better match the observed hemispheric warming asymmetry over the historical period.