Uncertainty in aerosol–cloud radiative forcing is driven by clean conditions

Abstract. Atmospheric aerosols and their impact on cloud properties remain the largest uncertainty in human forcing of climate system. By increasing concentration droplets (Nd), reduce droplet size increase reflectivity clouds (a negative radiative forcing). Central to this is susceptibility number aerosol (β), diversity which explains much variation from aerosol–cloud interactions (RFaci) global models. This has made measuring β a key target for developing observational constraints forcing. While burden clean, pre-industrial atmosphere been demonstrated as forcing, here we show that behaviour under these clean conditions equal importance understanding spread estimates between models observations. means is, counterintuitively, driven by situations with little aerosol. Discarding produces close agreement different model response but does not provide strong constraint RFaci. makes constraining an important goal future studies.