Wildfires have been increasing across the western United States over the past four decades due to factors including declining forest stand health, increasing dry fuel loads, and multiple disturbances including drought and insect outbreaks. In the Sierra Nevada mountain range of California and Nevada, naturally-ignited wildfires are most often associated with hot and dry conditions that are linked to preceding periods (from months to years in length) of sustained low precipitation. Global climate change is expected to increase wildfires in this region by intensifying hot and dry periods, yet it is difficult to tease apart the potentially divergent effects of temperature versus precipitation change on wildfire occurrence. We used gridded temperature data from the Parameter-elevation Regressions on Independent Slopes Model (PRISM) and wildfire data from Monitoring Trends in Burn Severity (MTBS) to link air temperature extremes to increasing wildfire occurrence across the Sierra Nevada mountain range from 1984 to 2017. We were particularly interested in determining if increases to the magnitude of high and extreme high air temperatures led to commensurate increases in naturally-ignited wildfires, or if wildfire occurrence was more strongly influenced by other factors.
Results/Conclusions
Of the 364 naturally-ignited wildfires in the Sierra Nevadas from 1984-2017, the majority (352) occurred in summer and fall. Fire occurrence in fall increased from the 1980s to 2010s (1980s: 1.3 events per year, 1990s: 1.4, 2000s: 2.0, 2010s: 2.5) and corresponded to a sustained increase in fall air temperature magnitude. Conversely, sustained high temperatures in the early 2000s were associated with the highest occurrence of summer wildfire events (1980s: 9.3 events per year, 1990s: 6.4, 2000s: 10.7, 2010s: 7.9), after which summer fire occurrence declined. Wildfire increases therefore corresponded with high temperatures, yet the nature of temperature change may have divergent effects on seasonal wildfire occurrence. We found that 33% of wildfires occurred in locations that experienced an extreme high temperature event in the month of fire ignition, which exceeded the occurrence of extreme high temperature events in locations that did not experience an ignition event. This suggests that wildfire occurrence can be enhanced by near-term extreme high temperature pulses, yet the wildfire occurrences outside extreme temperature months also underscore the importance of additional factors. These preliminary results suggest a potential relationship between extreme high temperatures and wildfire ignition that may improve understanding of wildfire patterns.