Decreasing fire season precipitation increased recent western US forest wildfire activity

Abstract

Western United States wildfire increases have been generally attributed to warming temperatures, either through effects on winter snowpack or summer evaporation. However, near-surface air temperature and evaporative demand are strongly influenced by moisture availability and these interactions and their role in regulating fire activity have never been fully explored. Here we show that previously unnoted declines in summer precipitation from 1979 to 2016 across 31-45% of the forested areas in the western United States are strongly associated with burned area variations. The number of wetting rain days (WRD; days with precipitation ≥2.54 mm) during the fire season partially regulated the temperature and subsequent vapor pressure deficit (VPD) previously implicated as a primary driver of annual wildfire area burned. We use path analysis to decompose the relative influence of declining snowpack, rising temperatures, and declining precipitation on observed fire activity increases. After accounting for interactions, the net effect of WRD anomalies on wildfire area burned was more than 2.5 times greater than the net effect of VPD, and both the WRD and VPD effects were substantially greater than the influence of winter snowpack. These results suggest that precipitation during the fire season exerts the strongest control on burned area either directly through its wetting effects or indirectly through feedbacks to VPD. If these trends persist, decreases in summer precipitation and the associated summertime aridity increases would lead to more burned area across the western United States with far-reaching ecological and socioeconomic impacts.

Observed Impacts of Anthropogenic Climate Change on Wildfire in California

“…This was observed previously across broader portions of the western United States (e.g., and may be partly representative of the importance of fine dead fuels to fire spread, which can quickly equilibrate with atmospheric moisture content (Matthews, 2014). However, correlative analyses with a single variable may artificially confound or inflate its importance due to covariance with other variables or factors (Holden et al, 2018;Williams, Seager, Macalady, et al, 2015). For example, VPD is negatively related to precipitation (cloud shade and soil moisture negatively force VPD), so the effect of one variable is entrained in the correlation between burned area and the other variable.…”

“…The FFWI is a proxy for fire potential and spread that is based on wind speed, humidity, and temperature with no memory of antecedent conditions (Fosberg, 1978) and has been linked to significant wind-driven fires in southern California (e.g., Barbero et al, 2014;Moritz et al, 2010). We also evaluate wet-day frequency in October-November, defined as days when precipitation ≥2.54 mm (Holden et al, 2018). For FFWI, we commence our analyses in 1958 due to an unrealistic positive trend in 1948-1957 NCEP-NCAR 10-m wind speed that is likely an artifact of the widespread expansion of rawinsonde measurements during this period.…”

“…To represent the timing of onset of the winter precipitation season, we evaluate the number of days needed to reach 10% of the fall (October-December) long-term mean precipitation total. We also evaluate wet-day frequency in October-November, defined as days when precipitation ≥2.54 mm (Holden et al, 2018).…”

“…In the western United States, annual area burned increased substantially in recent decades due to increased frequency and size of large wildfires Balch et al, 2018;Dennison et al, 2014;Westerling, 2016). It is well established that this observed increase in wildfire activity was promoted in many areas by reduced fuel moisture due to warming-induced increases in evaporative demand, reduced snowpack, and reduced warm-season precipitation frequency Holden et al, 2018;Kitzberger et al, 2017;Westerling, 2016). These recent climate trends are broadly consistent with those expected from anthropogenic climate change , but anthropogenic climate effects on wildfire can vary greatly across space and time due to confounding factors such as natural climate variations, land and fire management practices, ignitions from humans, spatial diversity in vegetation type, and the complex ways in which these processes interact .…”

Coupled ecohydrology and plant hydraulics modeling predicts ponderosa pine seedling mortality and lower treeline in the US Northern Rocky Mountains

“…Weather inputs required to run Ech2o-SPAC include minimum and maximum temperature and relative humidity, shortwave and longwave radiation, precipitation and wind speed. Daily gridded temperature, relative humidity and solar radiation inputs for simulations at each stake row point were extracted from 250 m grids (Holden et al, 2018). Daily precipitation data were extracted from 4 km PRISM data and resampled to 250 m resolution by bilinear interpolation (Daly et al, 2008).…”