Abstract
Previous studies have identified a recent increase in wildfire activity in the western United States (WUS). However, the extent to which this trend is due to weather pattern changes dominated by natural variability versus anthropogenic warming has been unclear. Using an ensemble constructed flow analogue approach, we have employed observations to estimate vapor pressure deficit (VPD), the leading meteorological variable that controls wildfires, associated with different atmospheric circulation patterns. Our results show that for the period 1979 to 2020, variation in the atmospheric circulation explains, on average, only 32% of the observed VPD trend of 0.48 ± 0.25 hPa/decade (95% CI) over the WUS during the warm season (May to September). The remaining 68% of the upward VPD trend is likely due to anthropogenic warming. The ensemble simulations of climate models participating in the sixth phase of the Coupled Model Intercomparison Project suggest that anthropogenic forcing explains an even larger fraction of the observed VPD trend (88%) for the same period and region. These models and observational estimates likely provide a lower and an upper bound on the true impact of anthropogenic warming on the VPD trend over the WUS. During August 2020, when the August Complex "Gigafire" occurred in the WUS, anthropogenic warming likely explains 50% of the unprecedented high VPD anomalies.
Highlights
Western United States j fire weather j attribution j atmospheric circulation j anthropogenic warming mortality from bark beetles [19], earlier and reduced springtime snowmelt [7], reduced summer precipitation [20], cloud shading [21], vegetation cover [22], fog frequency [23], live fuel moisture content [23,24,25], and increase in fire-prone wind patterns [26, 27]
Our observation-based estimate suggests ∼onethird of the vapor pressure deficit (VPD) trend is attributable to natural variability of atmospheric circulation, whereas ∼two-thirds is explained by anthropogenic warming
Our results show that the daily analogue VPD0 explains a large fraction of the total variance of the observed VPD0 averaged over the western United States (WUS) for all warm season days during 1979 to 2020 (R2 = 77%), indicating that the analogue method successfully captures the influence of synoptic variations in circulation patterns on VPD
Summary
Western United States j fire weather j attribution j atmospheric circulation j anthropogenic warming mortality from bark beetles [19], earlier and reduced springtime snowmelt [7], reduced summer precipitation [20], cloud shading [21], vegetation cover [22], fog frequency [23], live fuel moisture content [23,24,25], and increase in fire-prone wind patterns [26, 27]. According to the National Interagency Fire Center (NIFC) report, the area burned by wildfire during the 2020 warm season reached 8.8 million acres [13, 14], more than five times the average during 1984 to 2000 This rapid increase in burned area has been observed across most of the WUS except in Wyoming. The number of days per year with high VPD (defined as days with VPD larger than the 90th percentile value of VPD in the climatological period of 1979 to 2010) increased by 94% during 2001 to 2018 relative to 1984 to 2000 (Fig. 1 C and D) Many factors and their complex interactions can contribute to increased fire activity. The western United States (WUS) has experienced a rapid increase of fire weather (as indicated by vapor pressure deficit, VPD) in recent decades, especially in the warm season.
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