A transition to temperature dominated droughts over the Western United States in the 21st Century

Historically, drought in the western United States (WUS) has been driven primarily by precipitation deficits, including during the period of 1948-1999. However, we have observed that since around 2000, rising surface temperature and the resulting high evaporative demand have contributed more to drought severity (62%) and coverage (66%) over the WUS than precipitation deficit. This increase in evaporative demand during droughts, which is mostly attributable to anthropogenic warming according to both observational flow analogue analysis and climate models’ simulations, is the main cause of the increased drought severity and coverage during 1948-2022. The unprecedented 2020-2022 WUS drought exemplifies this shift in drought drivers, with high evaporative demand accounting for 61% of its severity, compared to 39% from precipitation deficit. Climate model simulations corroborate this shift and project that under the fossil-fueled development scenario (SSP5-8.5), droughts similar to the 2020-2022 event will become 1-in-63-year occurrences by the mid-21^st century and alarmingly 1-in-6-year occurrences by the late-21^st century.