Elevation-dependent intensification of fire danger in the western United States

Abstract

Studies have identified elevation-dependent warming trends, but investigations of such trends in fire danger are absent in the literature. Here, we demonstrate that while there have been widespread increases in fire danger across the mountainous western US from 1979 to 2020, trends were most acute at high-elevation regions above 3000 m. The greatest increase in the number of days conducive to large fires occurred at 2500–3000 m, adding 63 critical fire danger days between 1979 and 2020. This includes 22 critical fire danger days occurring outside the warm season (May–September). Furthermore, our findings indicate increased elevational synchronization of fire danger in western US mountains, which can facilitate increased geographic opportunities for ignitions and fire spread that further complicate fire management operations. We hypothesize that several physical mechanisms underpinned the observed trends, including elevationally disparate impacts of earlier snowmelt, intensified land-atmosphere feedbacks, irrigation, and aerosols, in addition to widespread warming/drying.

Topohydrological Influences on Wildfire Severity in the American Southwest

“…The general trend of the elevation models shows that as elevation increases, fire severity also increases. Similar to my models, the idea that higher elevations are experiencing more high-severity fire is becoming more common in recent west-wide studies of contemporary elevation-fire severity relationships (Alizadeh et al, 2023). This result is supported by many other studies, demonstrating the widespread importance of elevation as a driver of wildfire severity (Dillon et al, 2011;Holden et al, 2009;Pascolini-Campbell et al, 2022;Prichard & Kennedy, 2014).…”

“…Topography directly affects fire behavior spread, and can be broken down into aspect, slope, elevation, and topographic position (i.e. valley bottoms, mid-slopes, and ridge tops) (Alizadeh et al, 2023;Bradstock et al, 2010;Harris & Taylor, 2015).…”

“…In contemporary wildfire studies, higher elevations are experiencing significantly higher severity fire (Alizadeh et al, 2023). For example, north-facing slopes and valley bottoms are generally more mesic and therefore burn at lower fire severity, both historically and in recent wildfires (Barton & Poulos, 2018;Beaty & Taylor, 2001;Poulos et al, 2021;Taylor et al, 2021;Taylor & Skinner, 2003).…”

Land and Atmosphere Precursors to Fuel Loading, Wildfire Ignition and Post‐Fire Recovery

“…The occurrence of sudden positive anomalies in VPD, leading to increased atmospheric aridity and subsequent vegetation evaporation, is remarkable across all western climate regions approximately 2 to 3 months prior to the ignition of fires. The presence of consistently elevated VPD signifies a heightened susceptibility of ecosystems to ignition and the rapid spread of fires, resulting in larger and more intense wildfires (Alizadeh et al., 2023). Additionally, when the land surface experiences water limitation, interactions between the land and atmosphere can trigger elevated VPD levels (Zhou et al., 2019), creating a positive feedback loop that further exacerbates fuel drying.…”

Interdependencies Between Wildfire‐Induced Alterations in Soil Properties, Near‐Surface Processes, and Geohazards

“…In the western United States, the average annual area burned by large ( > 400 ha) forest fires has increased ten-fold over the past 50 years (Abatzoglou et al, 2021;Westerling, 2016). Fires are also occurring at higher elevations at an unprecedented rate in modern history (AghaKouchak, et al, 2018;Alizadeh et al, 2020Alizadeh et al, , 2023. With projected climatic changes, the increasing trend in burn severity and frequency is expected to continue (Abatzoglou et al, 2021;Jin et al, 2015) The devastating wildfires that occurred between 2019 and 2021, which destroyed thousands of structures and resulted in several hundred direct and indirect deaths, were a stark reminder of the destructive impact of climate change.…”