(A) Drought severity time series of 12-month moving cumulative P′−PET′, P′, and −PET′ during 1948–2022 averaged over the WUS (with cosine latitude weighting); the thin lines represent 12-month cumulative values, while the thick lines are their 20-year moving average; the yellow-shaded area represents drought periods identified when average P′−PET′ falls below its 30th percentile value for the 1948–1999 climatological period (marked by the gray dashed horizontal line); the vertical dotted line separates 1948–1999 (P1) and 2000–2022 (P2). We multiply PET′ by −1 for direct comparison with P′. (B) Time series of drought coverage and contributions from P′ and −PET′; thin lines represent total areas within the WUS (11 contiguous US states, 3.12 × 106 km2 in total) that are in drought condition (local P′−PET′ below the 30th percentile value for any grid point; black) and those where PET′ (red line) or P′ (blue line) alone was strong enough to cause drought (Materials and Methods); thick lines are their 20-year moving average. (C) Map of averaged PET′ contribution to drought severity, i.e., −PET′/(P′−PET′), during drought periods in P1; the thick black line marks the boundary of the WUS region. (D) Same as (C), but for drought periods in P2. (E) Change of PET′ contribution from P1 to P2, i.e., the difference between (D) and (C); gray dotted areas indicate insignificant change (P ≥ 0.05; P values are adjusted using the false discovery rate (FDR) criterion of αFDR < 0.05).
Click the link to access the article on the Science Advances website (Yizhou Zhuang, Rong Fu, Joel Lisonbee, Amanda M. Sheffield, Britt A. Parker, and Genoveva Deheza). Here’s the abstract:
Historically, meteorological drought in the western United States (WUS) has been driven primarily by precipitation deficits. However, our observational analysis shows 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, mostly attributable to anthropogenic warming according to analyses of both observations and climate model simulations, is the main cause of the increased drought severity and coverage. 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 like the 2020–2022 event will transition from a one-in-more-than-a-thousand-year event in the pre-2022 period to a 1-in-60-year event by the mid-21st century and to a 1-in-6-year event by the late-21st century.
