PNNL

Abstract

Fire-emitted aerosols play an important role in influencing Earth’s climate, directly by scattering and absorbing radiation and indirectly by influencing cloud microphysical properties. The quantification of fire-aerosol interaction, however, is highly uncertain and subject to the accuracy of surface emissions and vertical distribution of smoke plumes. Here we optimized fire-associated aerosol emissions in the Energy Exascale Earth System Model (E3SM) using the Global Fire Emissions Database version 4s (GFED4s) and AERONET aerosol optical depth (AOD) observations during 1997-2016. We distributed fire emissions from the surface through the upper troposphere using smoke plume heights derived from the Multi-angle Imaging SpectroRadiometer (MISR) satellite observations. With optimized global fire emissions, we estimated that global fires emit 45.5 Tg y-1 of primary particulate organic matter, 10.6 Tg y-1 of secondary organic aerosol, and 3.9 Tg y-1 of black carbon. We performed two climate simulations with and without optimized fire emissions to assess contemporary fire aerosol effects on near-surface meteorology and gross primary production (GPP). We find that fire aerosols statistically significantly increase global AOD by 14%, which contributes to a small positive direct radiative effect at the top of the atmosphere (0.25 ± 0.09 W m-2) and a reduction in net shortwave radiation at the surface (-1.48 ± 0.34 W m-2). Together, fire induced direct and indirect aerosol effects cause annual mean global land surface air temperature to decrease by 0.17 ± 0.15°C, relative humidity to increase by 0.4 ± 0.3%, and diffuse light fraction to increase by 0.5 ± 0.3%, with spatiotemporal differences substantially regulated by regional fire emissions. At the global scale fires cause GPP to decline by 2.8 Pg C y-1 as a result of offsetting large positive drivers (decreasing temperature, and increasing humidity and diffuse light) nearly cancelling out large negative drivers (decreasing surface shortwave radiation and soil moisture). Our analysis highlights the importance of wildfire in driving surface climate and plant CO2 sequestration.