Environmental Science and Engineering Seminar

Droughts and wildfires have long been integral components of the natural climate and ecosystems of the Western United States. However, both hazards have intensified rapidly in recent decades. The region has experienced drought that is comparable to the worst mega-drought of the past millennium and a rapid increase in burned area since the early 21st century. While these trends are broadly attributable to anthropogenic warming and the associated rise in atmospheric moisture demand, the relative contributions of natural climate variability versus human-induced climate change to the observed intensification of droughts and wildfires have remained unclear previously.
To address this question, we applied an ensemble-analogue approach to disentangle the influence of anomalous atmospheric circulation, which is mainly driven by natural climate variability, on drought severity and fire weather intensity, from the influence of changing thermodynamic conditions primarily driven by surface warming. Our results show that warming-induced increases in moisture demand account for at least 75% of the rise in fire weather intensity, as measured by vapor pressure deficit (VPD), and nearly 90% of the increase in drought severity and spatial extent across the Western United States since 2000. In fact, elevated evaporative demand has now surpassed precipitation deficits as the dominant contributor to drought severity and extent.
These findings highlight that anthropogenic forcing is the primary driver behind the intensification of fire weather and the ongoing mega-drought over the Western United States in the 21st century.