Spatial and temporal changes in wildfires and their attributes across the western United States

Abstract It is widely perceived that wildfire activity has increased across the western United States (WUS), with studies generally focusing on large wildfires. This study examines changes in the number of wildfire ignitions and burned area in the WUS using a comprehensive record of more than 750,000 wildfire incidents from 1992 to 2020 across biophysical gradients, seasons, and ignition causes. It further analyzes the environmental conditions associated with elevated fire activity. Our analysis comparing the periods 1992-2006 and 2007-2020 indicates that the annual number of recorded wildfires (>0.04 ha) in the WUS declined by 31% despite a 40% increase in burned area. This finding denotes that although wildfire prevention and mitigation strategies have decreased the overall number of fires, environmental conditions promoted larger fire sizes. Burned area increased in both forested and non-forested areas and across human- and naturally-ignited fires, with the greatest increase (84%) observed in lightning-caused forest fires. The seasonality of natural ignitions remained largely unchanged from 1992-2020, whereas the average day of year of ignition for human-caused wildfires shifted >12 days earlier (statistically significant), driven by an increasing number of springtime ignitions across the WUS. Ignitions increasingly occurred on days with abnormally drier-hotter weather compared to their climatology. Our analysis also showed that the median Energy Release Component (ERC) activation threshold associated with wildfire ignitions was ~42 (interquartile range, IQR: 34-50 across ecoregions). Activation threshold refers to the fire danger level beyond which the number of ignitions increases markedly. This threshold for fire size area was ~50 (IQR: 43-60). Findings of this study provide critical information for the development of wildfire prevention and mitigation strategies.