Optimal choice of proxy for cloud condensation nuclei reduces uncertainty in aerosol-cloud-climate forcing

Aerosol-cloud interactions (ACI) remain the largest uncertainty in anthropogenic climate forcings. Observation-based estimates of instantaneous radiative forcing from ACI (RF_aci; the Twomey effect) rely on the choice of aerosol quantities as proxies for cloud condensation nuclei (CCN) concentrations, which differ in their ability to represent cloud-base CCN and data accuracy. Using diverse observations and aerosol-climate models, we evaluate the utility of different proxies with two independent approaches. Both approaches reveal that surface CCN exhibits the smallest bias in predicting RF_aci (+5%), followed by aerosol index, surface sulfate and column CCN with similar biases of +25%, while aerosol optical depth and column sulfate show the largest biases (-60% and +92%). Constraining RF_aci with the optimal proxy reduces uncertainty from 66 to 43%, yielding a less negative RF_aci (-1.0 W m^-2) than the unconstrained case (-1.2 W m^-2). Our findings highlight the crucial role of proxy constraint in reconciling and improving RF_aci estimates.