In the western United States (US), the annual area of forest fire increased by 600-800% during 1984-2016. This increase is largely the result of a significant increase in fuel aridity over the past few decades. Fuel aridity is a multi-metric proxy for environmental dryness and annual values of this metric averaged across western US forested areas account for 75% of the variability in the logarithm annual western US forest fire area during 1984-2016. This indicates a non-linear relationship, where each incremental increase in fuel aridity causes more burned area than the last. Over the past century, fuel aridity across western US forests has been positively forced by a multi-model mean anthropogenic warming of approximately 1.5°C. To investigate the influence of anthropogenic warming on fuel aridity and forest fire, we subtracted the modeled anthropogenic trends in temperature and humidity from the observed records of these variables and calculated “non-anthropogenic” fuel aridity records, representing a hypothetical world without anthropogenic warming trends.
Anthropogenic warming has significantly increased fuel aridity and burned area in the western US. Considering the past decade (2007-2016), mean fuel aridity across western US forests was 0.63 standard deviations higher than non-anthropogenic fuel aridity. This translates to 87% more forest area experiencing high fuel aridity in a given year and an average of 37% more extreme fire-danger days per year. Based on the strong empirical relationship between fuel aridity and forest fire area, we use fuel aridity to estimate burned area in the observed versus non-anthropogenic world to estimate the burned area attributable to anthropogenic warming. Of the observed 9.9 million ha that actually burned during 1984-2016, we attribute 4.5 million ha to anthropogenic warming trends [95% confidence: 2.9-6.8 million ha]. Importantly, the non-linear response of burned area to fuel aridity dictates that the impact of anthropogenic warming on forest fire area is particularly strong when combined with natural climate variations that promote dry conditions in the western US (e.g., cold east Pacific sea surface temperatures), as was the case for much that past 20 years. Possibilities for the next 20 years will be discussed.