Global radiative forcing of stratospheric aerosols injected by the 2020 Australian extreme wildfire event

Abstract. During the last decades, extreme wildfires have injected large amounts of biomass-burning aerosols in the stratosphere. Partly composed of carbon, these aerosols absorb incoming solar radiation, inducing changes in the atmosphere's energy balance, and can self-loft to altitudes higher than 30 km, with an increased residence time of several months. In this study we estimate the radiative forcing of stratospheric aerosols from the Australian New Year Super Outbreak (ANYSO) in 2020. We first model individual self-lofting plumes from the Pacific Northwest Event (PNE) in Canada in 2017 and from ANYSO to constrain the aerosols' optical properties. We use observations to track them and model their heating rates. For the PNE plume a Single Scattering Albedo (SSA) of 0.95 is the best estimate to compute the heating rates, as well as a SSA of 0.90–0.95 for the ANYSO plume. Cloud cover and geometrical thickness of the plume have a crucial impact on these computations. We then compute the direct radiative forcing of Southern Hemisphere aerosols injected in the stratosphere by ANYSO in 2020. Cloud cover has a crucial impact on those forcings, especially at the top of the atmosphere (TOA) where it makes values ranging from negative (clear-sky) to positive (all-sky). The global TOA radiative forcing of stratospheric aerosols from ANYSO over 2020 was evaluated at 0.08 to 0.19 W.m⁻² in all-sky, and −0.17 to −0.12 W.m⁻² in clear-sky. The surface radiative forcing estimate is −0.04 to −0.06 W.m⁻² in all-sky and −0.29 to −0.24 W.m⁻² in clear-sky.