Quantifying contributions of natural variability and anthropogenic forcings on increased fire weather risk over the western United States

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.

Automated summary

Recent studies have shown that the increase in wildfire activity in the western United States in recent years is likely attributed to both natural weather pattern changes and anthropogenic warming, with approximately 68% of the observed trend in vapor pressure deficit (VPD) being due to human influence. Climate models indicate that anthropogenic forcing explains an even larger fraction (88%) of the VPD trend, providing a lower and upper bound on the true impact of anthropogenic warming on VPD trends in the region. In August 2020, during the occurrence of the August Complex Gigafire, it is estimated that anthropogenic warming explained 50% of the exceptionally high VPD anomalies.