Yizhou Zhuang, Rong Fu, Benjamin D. Santer, Robert E. Dickinson, and Alex Hall

Proceedings of the National Academy of Sciences

Published date November 9, 2021

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

Study key findings & significance

  • Higher temperatures raise the vapor pressure deficit (VPD), which is the difference between the maximum amount of moisture the air can hold and the amount of moisture currently in the air. As this value rises, the air becomes drier and absorbs more moisture from plants and the soil, increasing the risk of catching fire.
  • Observational data indicated that over two thirds (68%) of the increase in vapor pressure deficit between 1979 and 2020 is due to climate change, while the remaining 32% can be explained by natural variations
  • Climate models attribute ∼90% of the VPD trend to anthropogenic warming. 
  • Both estimates suggest that anthropogenic warming is the main cause for increasing fire weather and provide a likely range for the true anthropogenic contribution to the western US trend in VPD.

Author quotes

“We can expect more and more severe fire weather in the coming years. We really hope the public and the science community will realize what has happened and what will be coming in the future.”

Yizhou Zhuang, lead author of the study and postdoctoral student in atmospheric and oceanic sciences at UCLA

“The climate impact on fire weather is already happening. It’s not in the future; it’s now – it’s the recent past.”

Rong Fu, a professor of oceanic and atmospheric sciences and a co-author of the study


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.

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