The Role of Humidity in Past and Future Fire Weather Trends in the Contiguous United States

Grant Buster, Tianqi Zhang, Jiafu Mao, Weichen Liu, Stefan Rahimi

There has been evidence of increasing fire activity in the United States since approximately the 1980s, now estimated to cause tens to hundreds of billions of dollars of damage per year. Weather is one of the key drivers of fire activity, with hot, dry, and windy weather commonly associated with high-risk conditions. We are still, however, developing an understanding of how these weather variables have contributed to changing fire weather regimes over the last several decades and whether these changes will continue into the future. In this work, we investigate the effect of declining humidity in arid regions of the United States on fire weather trends. We find that these drying trends have been the predominant contributor to historical increasing trends in fire weather in the Western United States based on a common fire weather index, and that this signal is largely missing from downscaled weather projections based on Earth system models. Accordingly, a near-future (2020–2039) fire weather scenario derived from downscaled Earth System Model data produces little to no change in the frequency of extreme fire weather days across the Western United States. However, under a continued drying scenario, many regions in the West experience 20–70% increases in the annual occurrence of extreme fire weather days.