Decomposing the Effective Radiative Forcing of anthropogenic aerosols based on CMIP6 Earth System Models

Abstract. Anthropogenic aerosols play a major role for the Earth-Atmosphere system by influencing the Earth’s radiative budget and climate. The effect of the perturbation induced by changes in anthropogenic aerosols on the Earth's energy balance is quantified in terms of the effective radiative forcing (ERF) which is the recommended metric for perturbations affecting the Earth’s top-of-atmosphere energy budget since it is a better way to link this perturbation to subsequent global mean surface temperature change. In this work, the present-day ERF of anthropogenic aerosols is quantified using simulations from Earth system models (ESMs) participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6). The ERFs of individual aerosol species, such as sulphates, organic carbon (OC), and black carbon (BC) are calculated along with the ERF due to all anthropogenic aerosols and the transient ERF over the historical period (1850–2014). Additionally, ERF is analyzed into three components: (a) ERF_ARI, representing aerosol-radiation interactions, (b) ERF_ACI, accounting for aerosol-cloud interactions, and (c) ERF_ALB, which is mainly due to the contribution of surface albedo changes caused by anthropogenic aerosols. Here, the total anthropogenic aerosol ERF (calculated using the piClim-aer experiment) is estimated to be -1.11 ± 0.26 W m^-2, mostly due to the large contribution of ERF_ACI (-1.14 ± 0.33 W m^-2), compared to ERF_ARI (-0.02 ± 0.20 W m^-2) and ERF_ALB (0.05 ± 0.07 W m^-2). The total ERF caused by sulphates (piClim-SO₂) is estimated at -1.11 ± 0.31 W m^-2, the OC ERF (piClim-OC) is -0.35 ± 0.21 W m^-2, whereas the ERF exerted by BC (piClim-BC) is 0.19 ± 0.18 W m^-2. On top of that, our analysis reveals that ERF_ACI clearly prevails over the largest part of the Earth except for the BC experiment where ERF_ARI prevails over land. By the end of the historical period (1995–2014), the global mean total aerosol ERF is estimated at -1.28 ± 0.37 W m^-2 (calculated using the histSST experiment). We find that sulphates dominate both present-day and transient ERF spatial patterns at the top of the atmosphere, exerting a strongly negative ERF especially over industrialized regions of the Northern Hemisphere, such as North America, Europe, East and South Asia. Since the mid-1980s ERF has become less negative over Eastern North America and Western and Central Europe, while over East and South Asia there is a steady increase in ERF magnitude towards more negative values until 2014.