Bounding global aerosol radiative forcing of climate change

International audienceKey Points: 44 • An assessment of multiple lines of evidence supported by a conceptual model pro-45 vides ranges for aerosol radiative forcing of climate change; 46 • Aerosol effective radiative forcing is assessed to be between −1.60 and −0.65 W m −2 47 at the 16-84% confidence level; 48 • Although key uncertainties remain, new ways of using observations provide stronger 49 constraints for models. Abstract 51 Aerosols interact with radiation and clouds. Substantial progress made over the past 40 52 years in observing, understanding, and modeling these processes helped quantify the im-53 balance in the Earth's radiation budget caused by anthropogenic aerosols, called aerosol 54 radiative forcing, but uncertainties remain large. This review provides a new range of 55 aerosol radiative forcing over the industrial era based on multiple, traceable and arguable 56 lines of evidence, including modelling approaches, theoretical considerations, and obser-57 vations. Improved understanding of aerosol absorption and the causes of trends in sur-58 face radiative fluxes constrain the forcing from aerosol-radiation interactions. A robust 59 theoretical foundation and convincing evidence constrain the forcing caused by aerosol-60 driven increases in liquid cloud droplet number concentration. However, the influence 61 of anthropogenic aerosols on cloud liquid water content and cloud fraction is less clear, 62 and the influence on mixed-phase and ice clouds remains poorly constrained. Observed 63 changes in surface temperature and radiative fluxes provide additional constraints. These 64 multiple lines of evidence lead to a 68% confidence interval for the total aerosol effec-65 tive radiative forcing of −1.60 to −0.65 W m −2 , or −2.0 to −0.4 W m −2 with a 90% like-66 lihood. Those intervals are of similar width to the last Intergovernmental Panel on Cli-67 mate Change assessment but shifted towards more negative values. The uncertainty will 68 narrow in the future by continuing to critically combine multiple lines of evidence, es-69 pecially those addressing industrial-era changes in aerosol sources and aerosol effects on 70 liquid cloud amount and on ice clouds