DEATH BY A THOUSAND PAPER CUTS: CLIMATE CHANGE AND DROUGHT IN THE WESTERN UNITED STATES

The western United States (WUS) (defined as the contiguous US west of 100^oW) has in recent decades been impacted by what has been termed the most severe megadrought in the last 1200 years [1]. Through an extreme gradient boosting machine learning model, this study uses NOAA’s divisional weather data (1896-2022) to examine “effective precipitation”, or total precipitation minus “loss” due to increased temperature’s impact on evapotranspiration (E-T) during this interval.

The current WUS megadrought (2000-2022) is the driest 23-year period on record with regard to both Palmer Drought Indices and effective precipitation. However, two 23-year periods (1918-1940 and 1943-1965) received less precipitation than the current megadrought. Enhanced E-T has roughly doubled the strength of the current drought. Drought has been exacerbated in the warmest climates (Southern Plains and Southwest), rapidly warming climates (N. Plains, Rocky Mountains, and Northwest), and in areas with low interannual precipitation variability where ecosystems are not adapted to long-term droughts (Rocky Mountains).

Temperature-induced E-T effects occur “invisibly”, not through any one particular event, heat wave, or year, but at a near constant, increasing rate over time that tracks Earth’s total warming. Precipitation deficits dominate short-term (1-5 year) drought signals, while temperature-induced E-T dominates long-term signals. Increased temperatures will further force the aridification of the WUS, as additional precipitation -- not strongly predicted by climate models -- will be necessary to counteract the drying effects of increased temperatures. Future work will examine additional causes of precipitation-water availability inefficiencies such as increased runoff due to climate change induced strengthening storms, increased water usage, possible changes to precipitation seasonality, a rising snow line, and earlier, faster snow melt.

[1] Williams et al. 2022, Nature Climate Change:232-4