Co-benefits of mitigating global greenhouse gas emissions for future air quality and human health

Actions to reduce greenhouse gas (GHG) emissions often reduce co-emitted air pollutants, bringing co-benefits for air quality and human health. Past studies typically evaluated near-term and local co-benefits, neglecting the long-range transport of air pollutants, long-term demographic changes, and the influence of climate change on air quality. Here we simulate the co-benefits of global GHG reductions on air quality and human health using a global atmospheric model and consistent future scenarios, via two mechanisms: reducing co-emitted air pollutants, and slowing climate change and its effect on air quality.We use new relationships between chronic mortality and exposure to fine particulatematter and ozone, global modelling methods and new future scenarios. Relative to a reference scenario, global GHG mitigation avoids 0.5±0.2, 1.3±0.5 and 2.2±0.8 million premature deaths in 2030, 2050 and 2100. Global average marginal co-benefits of avoided mortality are US$50–380 per tonne of CO2, which exceed previous estimates, exceed marginal abatement costs in 2030 and 2050, and are within the low range of costs in 2100. East Asian co-benefits are 10–70 times the marginal cost in 2030. Air quality and health co-benefits, especially as they are mainly local and near-term, provide strong additional motivation for transitioning to a low-carbon future. Past studies have estimated that the human health co-benefits of GHG mitigation, by reducing co-emitted air pollutants, can be substantial, and whenmonetized, range acrossmany studies from a small fraction of GHG mitigation costs to exceeding them. Here we estimate the co-benefits of global GHG reductions for air quality and human health using a global atmospheric model and future scenarios. We account for the influence of international air pollutant transport on health, the effect of methane on global ozone, increases in population and susceptibility to air pollution, and economic growth that increases valuation. In addition to direct co-benefits of reduced co-emitted air pollutants (mainly local and immediate), we account for a second co-benefits mechanism, not previously quantified, in which slowing climate change decreases its effects on air quality (global and long-term). Climate change has been shown to increase ozone in the US and Europe (although the magnitude and patterns differ among studies), for example, through increased photochemical reaction rates and biogenic emissions, and meteorological changes, but decrease ozone in remote areas. Fine particulate matter (PM2.5) may also increase in polluted regions, but the net effect of several influences of climate change is less clear.