Effective radiative forcing from emissions of reactive gases and aerosols – a multi-model comparison

This paper quantifies the pre-industrial (1850) to present-day (2014)effective radiative forcing (ERF) of anthropogenic emissions of NOX,volatile organic compounds (VOCs; including CO), SO2, NH3, black carbon, organic carbon, andconcentrations of methane, N2O and ozone-depleting halocarbons, usingCMIP6 models. Concentration and emission changes of reactive species cancause multiple changes in the composition of radiatively active species:tropospheric ozone, stratospheric ozone, stratospheric water vapour,secondary inorganic and organic aerosol, and methane. Where possible we breakdown the ERFs from each emitted species into the contributions from thecomposition changes. The ERFs are calculated for each of the models thatparticipated in the AerChemMIP experiments as part of the CMIP6 project,where the relevant model output was available. The 1850 to 2014 multi-model mean ERFs (± standard deviations) are−1.03 ± 0.37 W m−2 for SO2 emissions, −0.25 ± 0.09 W m−2 for organic carbon (OC), 0.15 ± 0.17 W m−2 for blackcarbon (BC) and −0.07 ± 0.01 W m−2 for NH3. For thecombined aerosols (in the piClim-aer experiment) it is −1.01 ± 0.25 W m−2. The multi-model means for the reactive well-mixed greenhousegases (including any effects on ozone and aerosol chemistry) are 0.67 ± 0.17 W m−2 for methane (CH4), 0.26 ± 0.07 W m−2for nitrous oxide (N2O) and 0.12 ± 0.2 W m−2 forozone-depleting halocarbons (HC). Emissions of the ozone precursors nitrogenoxides (NOx), volatile organic compounds and both together(O3) lead to ERFs of 0.14 ± 0.13, 0.09 ± 0.14 and 0.20 ± 0.07 W m−2 respectively. The differences inERFs calculated for the different models reflect differences in thecomplexity of their aerosol and chemistry schemes, especially in the case ofmethane where tropospheric chemistry captures increased forcing from ozoneproduction.