Record-setting fires in the western United States over the last decade caused severe air pollution, loss of human life, and property damage. Enhanced drought and increased biomass in a warmer climate may fuel larger and more frequent wildfires in the coming decades. Applying an empirical statistical model to fires projected by Earth System Models including climate–ecosystem–socioeconomic interactions, we show that fine particulate pollution over the US Pacific Northwest could double to triple during late summer to fall by the late 21st century under intermediate- and low-mitigation scenarios. The historic fires and resulting pollution extremes of 2017–2020 could occur every 3 to 5 y under 21st-century climate change, posing challenges for air quality management and threatening public health. 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 ∼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 μg/m3. 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.

Abstract Using observations and model simulations (ESM4.1) during 1988–2018, we show large year-to-year variability in western U.S. PM2.5 pollution caused by regional and distant fires. Widespread wildfires, combined with stagnation, caused summer PM2.5 pollution in 2017 and 2018 to exceed 2 standard deviations over long-term averages. ESM4.1 with a fire emission inventory constrained by satellite-derived fire radiative energy and aerosol optical depth captures the observed surface PM2.5 means and extremes above the 35 μg/m3 U.S. air quality standard. However, aerosol emissions from the widely used Global Fire Emissions Database (GFED) must be increased by 5 times for ESM4.1 to match observations. On days when observed PM2.5 reached 35–175 μg/m3, wildfire emissions can explain 90% of total PM2.5, with smoke transported from Canada contributing 25–50% in northern states, according to model sensitivity simulations. Fire emission uncertainties pose challenges to accurately assessing the impacts of fire smoke on air quality, radiation, and climate.