A Transition from Precipitation- to Temperature-Dominated Drought in the Western United States

While precipitation deficits have long been the primary driver of drought, our observational analysis shows that since the year 2000, rising surface temperature and the resulting high evaporative demand have contributed more to drought severity (62%) and coverage (66%) across the western US (WUS). This increase in evaporative demand, largely driven by human-caused climate change, is the main cause of the observed increase in drought severity and coverage. The unprecedented 2020–2022 WUS drought, which led to widespread water shortages and wildfires, exemplifies this shift in drought drivers, with high evaporative demand accounting for 61% of its severity. Climate model simulations corroborate this shift and project that, under the fossil-fueled development scenario (SSP5-8.5), droughts like the 2020–2022 event will transition from being a very rare event (<0.1%) in the pre-2022 period to a 1-in-60-year event by mid-century (2040-2060) and to a 1-in-6-year event by the late 21st century (2080-2100). These projections highlight the urgent need for adaptation measures to mitigate the growing risk of severe drought in the WUS under a changing climate.