Modeling Climate and Health Impacts of Air Pollution

We applied an atmospheric chemistry–general circulation model to calculate the impacts of air pollution on climate and public health (SI Appendix, SI Methods). The model comprehensively accounts for emissions, multiphase chemistry, and other processes that control atmospheric composition. Model results include concentrations of ozone (O₃) and particulate matter, including PM_2.5 (particulates with a diameter <2.5 µm), being the main cause of morbidity and mortality (2 (link), 9 (link)). The results for PM_2.5 and O₃ served as input to the health impact calculations, based on the Global Burden of Disease methodology (2 (link)). We applied a Global Exposure Mortality Model (GEMM) for PM_2.5 that is based on an unmatched large number of cohort studies in many countries, and accounts for additional causes of death than considered previously (10 (link)). The GEMM calculations were complemented with those for O₃, accounting for about 3% of the total excess mortality rate. The atmospheric chemistry model was initially run for 20 y (excluding 5-y spin-up) with prescribed ocean temperatures to analyze health impacts and climate forcings, following IPCC recommendations (11 ), including changes in cloud reflectivity through the effects of aerosols on cloud condensation nuclei (CCN) (12 ). Uniquely, we included the increase in CCN activity of aeolian (wind-blown) dust particles is due to interaction with air pollution (chemical “aging”), which generally increases their ability to take up water. SI Appendix, Fig. S1 shows a comparison between modeled and satellite observed aerosol optical depth, SI Appendix, Fig. S2 for rainfall, SI Appendix, Fig. S3 for PM_2.5 and dust aerosol optical depth, and SI Appendix, Figs. S4 and S5 present the calculated aerosol radiative forcing of climate, which match the IPCC ensemble model estimates (11 ). Subsequently, the same model was rerun for 30-y periods (excluding 5-y spin-up) with an interactive ocean to compute equilibrium climate responses. We accounted for air pollution and greenhouse gases in idealized scenario calculations to characterize the public health and climate impacts of a hypothetical phaseout from fossil-fuel-related and other anthropogenic emissions, a distinction that could be essential for policy-making.