Air pollution and associated human mortality: the role of air pollutant emissions, climate change and methane concentration increases from the preindustrial period to present

Abstract. Increases in surface ozone (O₃) and fine particulate matter (≤2.5 μm aerodynamic diameter, PM_2.5) are associated with excess premature human mortalities. We estimate changes in surface O₃ and PM_2.5 from pre-industrial (1860) to present (2000) and the global present-day (2000) premature human mortalities associated with these changes. We extend previous work to differentiate the contribution of changes in three factors: emissions of short-lived air pollutants, climate change, and increased methane (CH₄) concentrations, to air pollution levels and associated premature mortalities. We use a coupled chemistry-climate model in conjunction with global population distributions in 2000 to estimate exposure attributable to concentration changes since 1860 from each factor. Attributable mortalities are estimated using health impact functions of long-term relative risk estimates for O₃ and PM_2.5 from the epidemiology literature. We find global mean surface PM_2.5 and health-relevant O₃ (defined as the maximum 6-month mean of 1-h daily maximum O₃ in a year) have increased by 8 ± 0.16 μg m⁻³ and 30 ± 0.16 ppbv (results reported as annual average ±standard deviation of 10-yr model simulations), respectively, over this industrial period as a result of combined changes in emissions of air pollutants (EMIS), climate (CLIM) and CH₄ concentrations (TCH4). EMIS, CLIM and TCH₄ cause global population-weighted average PM_2.5 (O₃) to change by +7.5 ± 0.19 μg m⁻³ (+25 ± 0.30 ppbv), +0.4 ± 0.17 μg m⁻³ (+0.5 ± 0.28 ppbv), and 0.04 ± 0.24 μg m⁻³ (+4.3 ± 0.33 ppbv), respectively. Total global changes in PM_2.5 are associated with 1.5 (95% confidence interval, CI, 1.2–1.8) million cardiopulmonary mortalities and 95 (95% CI, 44–144) thousand lung cancer mortalities annually and changes in O₃ are associated with 375 (95% CI, 129–592) thousand respiratory mortalities annually. Most air pollution mortality is driven by changes in emissions of short-lived air pollutants and their precursors (95% and 85% of mortalities from PM_2.5 and O₃ respectively). However, changing climate and increasing CH₄ concentrations also contribute to premature mortality associated with air pollution globally (by up to 5% and 15%, respectively). In some regions, the contribution of climate change and increased CH₄ together are responsible for more than 20% of the respiratory mortality associated with O₃ exposure. We find the interaction between climate change and atmospheric chemistry has influenced atmospheric composition and human mortality associated with industrial air pollution. Our study highlights the benefits to air quality and human health of CH₄ mitigation as a component of future air pollution control policy.