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Since the 1970s, human-caused increases in temperature and vapor pressure deficit have enhanced fuel aridity across western continental US forests, accounting for approximately over half of the observed increases in fuel aridity during this period. These anthropogenic increases in fuel aridity approximately doubled the western US forest fire area beyond that expected from natural climate variability alone during 1984–2015. The growing ACC [anthropogenic climate change] influence on fuel aridity is projected to increasingly promote wildfire potential across western US forests in the coming decades and pose threats to ecosystems, the carbon budget, human health, and fire suppression budgets (Flannigan et al 2013, Millar & Stephenson 2015) that will collectively encourage the development of fire-resilient landscapes (Smith et al 2016). Although fuel limitations are likely to eventually arise due to increased fire activity (Westerling et al 2011), this process has not yet substantially disrupted the relationship between western US forest fire area and aridity. We expect anthropogenic climate change and associated increases in fuel aridity to impose an increasingly dominant and detectable effect on western US forest fire area in the coming decades while fuels remain abundant.
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Effects
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Forests and Ecosystems
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Fire
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Wildfire Trends
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West
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The duration of the fire-weather season increased significantly across western US forests (+41%, 26 d for the all-metric mean) during 1979–2015, similar to prior results (10) (Fig. 4A and Table S2). Our analysis shows that ACC [anthropogenic climate change] accounts for ∼54% of the increase in fire-weather season length in the all-metric mean (15–79% for individual metrics). An increase of 17.0 d per year of high fire potential was observed for 1979–2015 in the all-metric mean (11.7–28.4 d increase for individual metrics), over twice the rate of increase calculated from metrics that excluded the ACC signal (Fig. 4B and Table S2). This translates to an average of an additional 9 d (7.8–12.0 d) per year of high fire potential during 2000–2015 due to ACC.
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Effects
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Forests and Ecosystems
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Fire
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Wildfire Trends
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West
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Fuel aridity averaged across western US forested areas showed a significant increase over the past three decades, with a linear trend of +1.2 σ (95% confidence: 0.42–2.0 σ) in the all-metric mean for 1979–2015 (Fig. 3A, Top and Table S1). The all-metric mean ACC [anthropogenic climate change] contribution since 1901 was +0.10 σ by 1979 and +0.71 σ by 2015. The annual area of forested lands with high fuel aridity (>1 σ) increased significantly during 1948–2015, most notably since 1979 (Fig. 3A, Bottom). The observed mean annual areal extent of forested land with high aridity during 2000–2015 was 75% larger for the all-metric mean (+27% to +143% range across metrics) than was the case where the ACC signal was excluded.
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Effects
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Forests and Ecosystems
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Fire
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Interactions With Other Factors
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National
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v3.2
