Tripling of western US particulate pollution from wildfires in a warming climate

The air quality impact of increased wildfires in a warming climate has often been overlooked in current model projections, owing to the lack of interactive fire emissions of gases and particles responding to climate change in Earth System Model (ESM) projection simulations. Here, we combine multiensemble projections of wildfires in three ESMs from the Sixth Coupled Model Intercomparison Project (CMIP6) with an empirical statistical model to predict fine particulate (PM2.5) pollution in the late 21st century under a suite of Shared Socioeconomic Pathways (SSPs). Total CO2 emissions from fires over western North America during August through September are projected to increase from present-day values by 60 to 110% (model spread) under a strong-mitigation scenario (SSP1-2.6), 100 to 150% under a moderate-mitigation scenario (SSP2-4.5), and 130 to 260% under a low-mitigation scenario (SSP5-8.5) in 2080-2100. We find that enhanced wildfire activity under SSP2-4.5 and SSP5-8.5 could cause a twofold to threefold increase in PM2.5 pollution over the US Pacific Northwest during August through September. Even with strong mitigation under SSP1-2.6, PM2.5 in the western US would increase similar to 50% by midcentury. By 2080-2100, under SSP5-8.5, the 95th percentile of late-summer daily PM2.5 may frequently reach unhealthy levels of 55 to 150 mu g/m(3). In contrast, chemistry-climate models using prescribed fire emissions of particles not responding to climate change simulate only a 7% increase in PM2.5. The consequential pollution events caused by large fires during 2017-2020 might become a new norm by the late 21st century, with a return period of every 3 to 5 y under SSP5-8.5 and SSP2-4.5.