Exceptional heat and atmospheric dryness amplified losses of primary production during the 2020 U.S. Southwest hot drought

Authors: DANNENBERG, M.P. - University Of Iowa; YAN, D, - University Of Arizona; BARNES, M.L. - University Of Indiana; SMITH, W.K. - University Of Arizona; JOHNSTON, M - Iowa State University; Scott, Russell - Russ; Biederman, Joel; KNOWLES, J.F. - California State University; WANG, X. - University Of Arizona; DUMAN, T. - University Of New Mexico; LITVAK, M - University Of New Mexico; KIMBALL, J.S. - University Of Montana; WILLIAMS, A.P. - University Of California; ZHANG, Y. - Peking University

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/2022
Publication Date: 4/22/2022
Citation: Dannenberg, M., Yan, D., Barnes, M., Smith, W., Johnston, M., Scott, R.L., Biederman, J.A., Knowles, J., Wang, X., Duman, T., Litvak, M., Kimball, J., Williams, A., Zhang, Y. 2022. Exceptional heat and atmospheric dryness amplified losses of primary production during the 2020 U.S. Southwest hot drought. Global Change Biology. 28(16):4794-4806. https://doi.org/10.1111/gcb.16214.
DOI: https://doi.org/10.1111/gcb.16214

Interpretive Summary: Droughts are among our costliest and deadliest natural disasters. Human-caused warming enhances the severity of drought and its impacts on ecosystems both by quickening soil drying and through plant responses to warming-enhanced atmospheric dryness. During the summer and autumn of 2020, much of the U.S. Southwest experienced its hottest and driest conditions since the late 1800s, resulting in large reductions in plant photosynthesis across the region (enough to feed ~50 million cattle for a month across shrublands and grasslands alone). Importantly, much of this reduction in productivity was driven by exceptionally high heat and atmospheric dryness, both of which are largely the result of recent warming, suggesting amplified impacts of drought on Earth’s ecosystems in a hotter future world.

Technical Abstract: Earth’s ecosystems are increasingly threatened by “hot drought,” which occurs when anthropogenic warming intensifies the hydrological, physiological, and ecological effects of precipitation deficits by enhancing evaporative losses of soil moisture and increasing plant stress due to higher vapor pressure deficit (VPD). Drought-induced reductions in gross primary production (GPP) exert a major influence on the terrestrial carbon sink, but the extent to which hotter and atmospherically drier conditions will amplify the effects of naturally-occurring precipitation deficits on Earth’s 21st century carbon cycle remains largely unknown. During summer and autumn 2020, the U.S. Southwest experienced one of the most intense “hot droughts” on record, with record-low precipitation and record-high air temperature and VPD across the region. Here, we evaluate the effects of this hot drought on GPP and further decompose those negative GPP anomalies into their constituent meteorological and hydrological drivers. We found a 140 Tg C (>25%) reduction in GPP below the 2015-2019 mean, by far the lowest regional GPP over the Soil Moisture Active Passive satellite record. Roughly half of the estimated GPP loss was attributable to low soil moisture (likely a combination of record-low precipitation and warming-enhanced evaporative depletion), but record-breaking air temperature and high VPD further amplified the reduction of GPP, together contributing roughly 40-45% of the GPP anomaly. Both temperature and VPD are very likely to continue increasing over the next century, likely leading to more frequent and intense hot drought events and substantially enhancing drought-induced GPP reductions.