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Smoke particles can be injected by pyrocumulonimbus (pyroCb) in the upper troposphere and lower stratosphere, but their effects on the radiative budget of the planet remain elusive. Here, by focusing on the record-setting Pacific Northwest pyroCb event of August 2017, we show with satellite-based estimates of pyroCb emissions and injection heights in a chemical transport model (GEOS-Chem) that pyroCb smoke particles can result in radiative forcing of similar to 0.02 W/m(2) at the top of the atmosphere averaged globally in the 2 months following the event and up to 0.9 K/day heating in the Arctic upper troposphere and lower stratosphere. The modeled aerosol distributions agree with observations from satellites (Earth Polychromatic Imaging Camera [EPIC], Cloud-Aerosol Transport System [CATS], and Cloud-Aerosol Lidar with Orthogonal Polarization [CALTON), showing the hemispheric transport of pyroCb smoke aerosols with a lifetime of 5 months. Hence, warming by pyroCb aerosols can have similar temporal duration but opposite sign to the well-documented cooling of volcanic aerosols and be significant for climate prediction.

Plain Language Summary Extreme fire events can produce towering smoke plumes, which can result in the injection of smoke aerosols into the lower stratosphere (similar to 10 km above the surface in the midlatitudes). These stratospheric aerosols are significant because they stay in the atmosphere longer than those closer to the surface. In this study, we modeled the effects emanating from one of the largest of these fire events that happened in British Columbia, Canada, and Washington, USA, on 12 August 2017. We found that the smoke particles from this fire event had a lifetime of around 5 months and resulted in a net positive radiative forcing with warming focused in the stratosphere because smoke particle contain soot, an efficient absorber of solar radiation. This net positive radiative forcing contrasts with the cooling effects of analogous volcanic eruptions that are long thought to be dominant sources of stratospheric aerosols. Accounting for these smoke aerosols from large forest fire events in studies of atmospheric composition and climate may be more significant in the future as more large fire events are expected in a warmer climate.