Atmospheric drying is a primary driver of past and future fire weather trends across the Western United States

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 historical trends in fire weather and whether these trends will continue. In this work, we investigate the impact of declining humidity in arid regions of the United States. We find that these drying trends have been the predominant contributor to historical increases 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.