Australian Wildfires cause the largest stratospheric warming since Pinatubo.

Global mean lower stratosphere temperatures rose abruptly in January 2020 reaching values not experienced since the early 1990s. Anomalous lower stratospheric temperatures were recorded for 4 months at highly statistically significant levels (p-values of 0.0004 to 0.02). While the warming event of 1991-1993 has been definitively attributed to absorption of sunlight by stratospheric sulfate from the eruption of Pinatubo, no candidate volcanic eruption for explaining the 2020 stratospheric heating exists. Here, we use a combination of satellite and surface-based remote sensing observations to derive a time-series of stratospheric biomass burning aerosol optical depths originating from the intense 2019/20 S.E. Australian wildfires and apply these to a state-of-the-art climate model. We show beyond doubt that the biomass burning aerosols emitted by the S.E. Australian wildfires are the cause of this lower stratospheric warming, with implications for stratospheric dynamics and stratospheric ozone should this type of event become more frequent in the future.