Extreme fire spread events and their drivers in the western US

Abstract

Wildfire activity in recent years not only have large total area burned but also large, single-day fire spread events that pose challenges to ecological systems and human communities. Our objective was to better understand the relationships between extreme single-day fire spread events, annual area burned, and fire-season climate, and predict changes under future warming. We employ a satellite-derived dataset of daily fire spread events in the western USA and gridded climate data over this region to assess relationships between extreme single-day fire spread events, annual area burned, and fire-season maximum temperature, climate moisture deficit, and vapor pressure deficit over a time period of 2002-2020. We then develop models to predict fire activity under a 2°C warming scenario. Extreme single-day fire spread events >1100 ha (the top 16%) accounted for 70% of the cumulative area burned over the period of analysis. Annual area burned was correlated with number and mean size of spread events, and those largest of these large fire spread events. In 2020, wildfires burned over 4 million ha in the US and we identified 441 extreme events in 2020 alone that together burned 2.2 million ha across our study area. In contrast, the average extreme events between 2002 and 2019 was 168 per year that burned 0.5 million ha. Fire season climate variables correlate strongly with the annual number of extreme events and area burned. Our models predict that the annual number of extreme fire spread events more than doubles under a 2°C warming scenario, with an attendant doubling in area burned. Exceptional fire seasons like 2020 will likely be more common, and wildfire activity under future extremes will likely exceed anything witnessed yet. Safeguarding human communities and supporting resilient ecosystems may require new lines of scientific inquiry, novel land management approaches, and accelerated climate mitigation efforts.