14 Mar 2022
14 Mar 2022

Radiative impacts of the Australian bushfires 2019–2020 – Part 1: Large-scale radiative forcing

Pasquale Sellitto1,2, Redha Belhadji1, Corinna Kloss3, and Bernard Legras4 Pasquale Sellitto et al.
  • 1Univ. Paris Est Créteil and Université de Paris, CNRS, Laboratoire Interuniversitaire des Systèmes Atmosphériques, Institut Pierre Simon Laplace, Créteil, France
  • 2Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, Catania, Italy
  • 3Laboratoire de Physique de l’Environnement et de l’Espace, Orléans, France
  • 4Laboratoire de Météorologie Dynamique, UMR CNRS 8539, École Normale Supérieure, PSL Research University, École Polytechnique, Sorbonne Universités, École des Ponts PARISTECH, Institut Pierre Simon Laplace, Paris, France

Abstract. As a consequence of extreme heat and drought, record-breaking wildfires developed and ravaged south-eastern Australia during the fire season 2019–2020. The fire strength reached its paroxysmal phase at the turn of the year 2019–2020. During this phase, pyro-Cb developed and injected biomass burning aerosols and gases into the upper-troposphere–lower-stratosphere (UTLS). The UTLS aerosol layer was massively perturbed by these fires, with aerosol extinction increased by a factor 3 in the visible spectral range in the Southern Hemisphere, with respect to a background atmosphere, and stratospheric aerosol optical depth reaching values as large as 0.015 in February 2020. Using the best available description of this event by observations, we estimate the radiative forcing (RF) of such perturbations of the Southern-Hemispheric aerosol layer. We use offline radiative transfer modelling driven by observed information of the aerosol extinction perturbation and its spectral variability obtained from limb satellite measurements. Based on hypotheses on the absorptivity and the angular scattering properties of the aerosol layer, the regional (at three latitude bands in the Southern Hemisphere) clear-sky TOA (top-of-atmosphere) RF is found varying from small positive values to relatively large negative values (up to -2.0 W/m2), and the regional clear-sky surface RF is found to be consistently negative and reaching large values (up to -4.5 W/m2). We argue that clear-sky positive values are unlikely for this event, if the aging/mixing of the biomass burning plume is mirrored by the evolution of its optical properties. Our best estimate for the area-weighted global-equivalent clear-sky RF is -0.35 ± 0.21 (TOA RF) and -0.94 ± 0.26 W/m2 (surface RF), thus the strongest documented for a fire event and of comparable magnitude with the strongest volcanic eruptions of the post-Pinatubo era. The surplus of RF at the surface, with respect to TOA, is due to absorption within the plume that has contributed to the generation of ascending smoke vortices in the stratosphere. Highly reflective underlying surfaces, like clouds, can nevertheless swap negative to positive TOA RF, with global average RF as high as +1.0 W/m2 assuming highly absorbing particles.

Pasquale Sellitto et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-42', Anonymous Referee #1, 07 Apr 2022
  • RC2: 'Comment on egusphere-2022-42', Anonymous Referee #2, 27 Apr 2022

Pasquale Sellitto et al.

Pasquale Sellitto et al.


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Short summary
As a consequence of extreme heat and drought, record-breaking wildfires ravaged south-eastern Australia during the fire season 2019–2020. Fires injected a smoke plume very high up to the stratosphere which dispersed quite quickly to the whole Southern Hemisphere and interacted with solar radiation, reflecting and absorbing part of it – thus producing impacts on the climate system. Here we estimate this impact on radiation and we study how it depends on the properties and ageing of the plume.