Cooperative Institute for Research in the Atmosphere

Wildfire-Driven Thunderstorms Cause a Volcano-Like Stratospheric Injection of Smoke

Intense heating by wildfires can generate a deep, smoke-infused thunderstorm, known as pyrocumulonimbus (pyroCb). This extreme weather phenomenon can release a large quantity of smoke particles into the upper troposphere and lower stratosphere. The meteorology driving pyroCb occurrence, combined with increasingly active fire seasons, indicates that pyroCb are a significant and endemic summertime feature in several temperate regions worldwide. A single fire season in western North America can include more than 25 intense single or multi-updraft pyroCb events. Since 2000, several stratospheric aerosol layers, first thought to be of volcanic origin, have been reclassified as originating from pyroCb activity. To date, however, the impact of pyroCb on climate has never been systematically explored, and remains almost completely unquantified. Here we quantify the mass of smoke aerosol injected into the lower stratosphere from five near-simultaneous intense pyroCb observed in western North America on 12 August 2017. We find that the stratospheric aerosol mass injected by this extreme event was comparable to a moderate volcanic eruption, and at least an order of magnitude larger than previous benchmarks for extreme pyroCb activity. The resulting high-altitude smoke layer encircled the Northern Hemisphere over several months. Our results demonstrate that extreme pyroCb activity significantly influences the lower-stratosphere in a manner similar to infrequent volcanic intrusions. We anticipate that this study will establish a foundation for understanding the effects of pyroCb smoke on lower-stratospheric chemistry and dynamic circulation.