Stratospheric Smoke With Unprecedentedly High Backscatter Observed by Lidars Above Southern France

Extreme pyroconvection events triggered by wildfires in northwest Canada and United States during August 2017 resulted in vast injection of combustion products into the stratosphere. The plumes of stratospheric smoke were observed by lidars at Observatoire de Haute‐Provence (OHP) for many weeks that followed the fires as distinct aerosol layers with backscatter reaching unprecedentedly high values for a nonvolcanic aerosol layer. We use spaceborne CALIOP lidar to track the spatiotemporal evolution of the smoke plumes before their detection at OHP. A remarkable agreement between ground‐ and spaced‐based lidars sampling the same smoke plume on a particular date allowed us to extrapolate the OHP observations to a regional scale, where CALIOP reported extreme aerosol optical depth values as high as 0.21. On a monthly time scale, the lidar observations indicate that boreal summer 2017 forest fires had a hemisphere‐scale impact on stratospheric aerosol load, similar to that of moderate volcanic eruptions. Plain Language Summary Stratospheric aerosol plays a large role in global climate through negative radiative forcing. Volcanic eruptions are considered the major source of stratospheric aerosol. In the absence of strong eruptions, the permanent stratospheric aerosol layer is commonly attributed to sulphuric gases emitted at the surface and lofted into the stratosphere by deep convection. Recent studies have put in evidence that biomass burning is an important contributor to stratospheric aerosol budget. During Summer 2017, severe forest wildfires raged in North America, resulting in pyrocumulonumbus firestorms injecting large amounts of smoke and combustion products into the stratosphere. The smoke has been dispersed throughout a large part of northern hemisphere in a few weeks. The observations using ground‐based and space‐borne laser radars (lidars) indicate that the smoke layer had an unprecedentedly high optical depth for a non‐volcanic aerosol layer. On a monthly time scale, the boreal summer 2017 forest fires had a hemisphere‐wide impact on stratospheric aerosol load, similar to that of moderate volcanic eruptions. This study emphasizes the significance of biomass burning as a source of stratospheric aerosol and provides an opportunity for re‐evaluating the potential of wildfires to pollute the stratosphere. Key Points North American wildfires during summer 2017 and intense pyroconvection pollute the stratosphere with smoke Stratospheric smoke plumes dete