Figure: Schematic summarising sources of anthropogenic aerosol and their direct interaction with radiation and their impact on clouds which in turn alter the balance of incoming and outgoing radiation. Processes associated with a high degree of uncertainty with respect to their overall radiative effect are labelled with a question mark.
This comprehensive review was lead by Nicolas Bellouin (University of Reading) and Johannes Quaas (University of Leipzig) and summarises the current state of knowledge of how aerosol emitted by human activity have altered Earth's radiative balance until now. Aerosol distributions and their chemical composition are far more heterogeneous than greenhouse gases, which increases the uncertainty of their radiative impact substantially. Bellouin et al. (2020) quantifies the radiative perturbation generated by anthropogenic aerosol, called the radiative effect, based on published studies of the past 40 years.
During the review process, all researchers with a diverse background in aerosol science contributed best estimates of the radiative effect of each process during which anthropogenic aerosol alter the radiative balance directly, or by changing the reflectivity or absorption of clouds. These individual contributions were summed up to provide a total estimate of the effective radiative forcing of anthropogenic aerosol. The total process-based bottom-up estimate was further constrained by the observed temperature change and change in radiative fluxes over the past century.
Combining a wide variety of observation-based, theory-based and modelling-based estimates, this review estimates the effective aerosol radiative forcing to lie between -1.6 to -0.6 W/m2 with a 68% confidence interval. This estimate is more negative than the previous community-based estimate published in the 5th assessment report of the Intergovernmental Panel on Climate Change (IPCC).
Thus the research of the past years indicates that anthropogenic aerosol have (with increased certainty) exerted a net cooling effect since the pre-industrial era.
The remaining substantial uncertainty about this forcing estimate is due to the high degree of uncertainty of the radiative effect generated by cloud amount changes and cloud cover changes in low clouds due to anthropogenic aerosol emissions, as well as numerous highly uncertain processes within mixed-phase and ice clouds around the world.