Gavin D. Madakumbura, A. Park Williams, Karen C. Short, Max A. Moritz, Bowen Wang, Caroline S. Juang
First published: 30 April 2026 https://doi.org/10.1029/2025EF007077

Abstract
Burned area and wildfire damages in the western United States (WUS) have increased dramatically in recent decades. Wildfire frequency, however, has shown trends that vary by data set, region, and fire size. Using a comprehensive fire occurrence data set screened for reporting artifacts from 1992 to 2020, we show a significant downward trend in annual fire frequency in WUS, with a significant contribution from decreasing human ignitions. Human-caused fires significantly declined in California and Arizona, but increased in Wyoming. The relationship between human-caused fires and population suggests a pyric transition, where fire frequency correlates positively to population in areas with very low population such as Wyoming and negatively where population is denser, such as in California and Arizona. Understanding the human influence on fire frequency trends beyond anthropogenic warming has strong implications for future projections of wildfire activity.

Plain Language Summary
Burned area and damages from wildfires in the western United States have increased in recent decades. Contrary to the common belief that the frequency of wildfires has also increased, trends in the number of fires have been less clear than trends in area burned, and they vary by data set, region, and fire size. Using a comprehensive fire occurrence data set, we find a significant decline in the annual number of wildfires in the western United States over the past three decades. Decreases in human-caused ignitions are most evident in California and Arizona, while human-caused ignitions increased in Wyoming. When examining how the number of human-caused fires relates to population patterns, we find that the relationship varies. In areas with very low population, fires increase as population increases. However, after the population reaches a certain level, further population growth is associated with fewer fires. Considering the influences of human populations and demographics on wildfire could improve both global and western United States estimates of future wildfire activity compared to estimates based on climate change alone.

1 Introduction
Wildfires in the western United States (WUS) have changed substantially in recent decades. Increases in burned area and fire-related damages have been documented (Abatzoglou & Williams, 2016; Burke et al., 2021; Higuera et al., 2023), as well as changes in fire size distributions and frequency in the 2000s (Iglesias et al., 2022). Anthropogenic warming has been suggested as a primary driver behind the increasing burned areas (Abatzoglou & Williams, 2016; Westerling, 2016) across all or much of the forested WUS. Fire frequency, or the number of ignitions, however, can be strongly influenced by anthropogenic factors beyond climate, as the number of fires depends not only on ignition potential related to climate variability and change (Abatzoglou et al., 2016), but also on human demographics, which influence both ignitions and fire suppression efforts (Bowman et al., 2011; Riley et al., 2019). Therefore, fire frequency can exhibit a wide range of spatial and temporal patterns beyond the common expectation of a ubiquitous increasing trend driven by anthropogenic warming or increased human activity (Doerr & Santín, 2016).

Observed wildfire frequency trends in the WUS have been shown to vary depending on the data set used (e.g., whether limited to “large” fires), the time period assessed, and the region of focus. Long-term perspectives from sedimentary charcoal records, fire scar data, and historical observations indicate that until the 18th century, temporal variation in regional mean fire activity across the WUS was primarily driven by climate (Margolis et al., 2025; Marlon et al., 2012). Increased ignitions during Anglo-European contact and territorial expansion were associated with land clearing, logging, agriculture, and railroad operations (Marlon et al., 2012). After the 1800s, however, a downward trend emerged, likely associated with land-use changes and an increased focus on protective measures like fire prevention and suppression (Marlon et al., 2012; Parks et al., 2025).

…