Two hundred fifty years of aerosols and climate: the end of the age of aerosols

Carbonaceous and sulfur aerosols have a substantial global and regional influence on climate, resulting in a net cooling to date, in addition to their impact on health and ecosystems. The magnitude of this influence has changed substantially over the past and is expected to continue to change into the future. An integrated picture of the changing climatic influence of black carbon, organic carbon and sulfate over the period 1850 through 2100, focusing on uncertainty, is presented using updated historical inventories and a coordinated set of emission projections. We describe, in detail, the aerosol emissions from the Representative Concentration Pathway (RCP) 4.5 scenario and its associated reference scenario. While aerosols have had a substantial impact on climate over the past century, we show that, by the end of the 21st century aerosols will likely be only a minor contributor to radiative forcing due to increases in greenhouse gas forcing and a net global decrease in pollutant emissions. This outcome is even more certain under a successful implementation of a policy to limit greenhouse gas emissions as low-carbon energy technologies that do not emit appreciable aerosol or SO2 are deployed.