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

Background/Question/Methods

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 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 use this natural experiment to evaluate the effects of hot drought on GPP and further decompose those negative GPP anomalies into their constituent meteorological and hydrological drivers.

Results/Conclusions

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 VPD amplified the reduction of GPP, together contributing roughly 40-45% of the GPP anomaly. Both air temperature and VPD are very likely to continue increasing over the next century, likely leading to more frequent and intense hot droughts and substantially enhancing drought-induced GPP reductions.