Future inhibition of ecosystem productivity by increasing wildfire pollution over boreal North America

Abstract. Biomass burning is an important source of tropospheric ozone (O₃) and aerosols. These air pollutants can affect vegetation photosynthesis through stomatal uptake (for O₃) and light scattering and absorption (for aerosols). Wildfire area burned is projected to increase significantly in boreal North America by the mid-century, while little is known about the impacts of enhanced emissions on the terrestrial carbon budget. Here, combining site-level and satellite observations and a carbon–chemistry–climate model, we estimate the impacts of fire emitted O₃ and aerosols on net primary productivity (NPP) over boreal North America. Fire emissions are calculated based on an ensemble projection from 13 climate models. In the present day, wildfire enhances surface O₃ by 2 ppbv (7 %) and aerosol optical depth (AOD) at 550 nm by 0.03 (26 %) in the summer. By mid-century, area burned is predicted to increase by 66 % in boreal North America, contributing more O₃ (13 %) and aerosols (37 %). Fire O₃ causes negligible impacts on NPP because ambient O₃ concentration (with fire contributions) is below the damage threshold of 40 ppbv for 90 % summer days. Fire aerosols reduce surface solar radiation but enhance atmospheric absorption, resulting in enhanced air stability and intensified regional drought. The domain of this drying is confined to the north in the present day but extends southward by 2050 due to increased fire emissions. Consequently, wildfire aerosols enhance NPP by 72 Tg C yr⁻¹ in the present day but decrease NPP by 118 Tg C yr⁻¹ in the future, mainly because of the soil moisture perturbations. Our results suggest that future wildfire may accelerate boreal carbon loss, not only through direct emissions increasing from 68 Tg C yr⁻¹ at present day to 130 Tg C yr⁻¹ by mid-century but also through the biophysical impacts of fire aerosols.