In 2020-2021, drought deepened across the U.S. Southwest against the backdrop of two decades of accumulated drought damages that exceeded $131.4 billion and caused alarm about potential water delivery shortages in the Colorado River basin. During June-September 2020, precipitation averaged over the Four Corners states of Arizona, New Mexico, Colorado, and Utah was the lowest since at least 1895. New research to be published in the Bulletin of the American Meteorological Society (BAMS), examines whether human-caused climate change influenced an unprecedented failure of 2020 summer monsoon rains. For the study, NOAA and CIRES researchers from the Physical Sciences Laboratory and their colleagues focused on the Four Corners states during June-September 2020 and used observations, historical climate models, atmospheric models, and event-attribution experiments to conduct their analysis. Their findings appear in the BAMS special report, Explaining Extreme Events of 2020 from a Climate Perspective.

Record low precipitation in June-September 2020 over the Four Corner states capped off a three-year stretch of below-average rainfall; however, the findings show no significant trend since 1895. In addition, no statistically significant change in the frequency of low precipitation was noted from past to recent climates. Given the brevity of observations, the researchers used multiple models and large ensembles, controlled in various ways for historical climate drivers, to test the effect of climate change on low precipitation occurrences.

Four of five models indicated significant decreases in average June-September precipitation from past to recent climates over the Four Corners states. Significant increases in the risk of extreme low seasonal precipitation in recent climate were found in these four models. Though four models indicated that record low June-September 2020 precipitation in the Four Corners states was made more likely due to climate change, our confidence in this result is low because such a change has not been observed since 1895 and the models do not perfectly reproduce precipitation statistics in the region.

The findings indicate that record low June-September 2020 precipitation in the Four Corners states was made more likely due to climate change. However, it also highlights the uncertainty in climate change attribution studies, and signals a need for greater investments in observations and models that may help to project and predict the behavior and frequency of future extreme weather and climate events like droughts and floods.

Hoell, Andrew (PSL), Xiao-Wei Quan (PSL/CIRES), Martin Hoerling (PSL), Rong Fu, Justin Mankin, Isla Simpson, Richard Seager, Cenlin He, Joel Lisonbee (NIDIS/CIRES), Ben Livneh, Amanda Sheffield (NIDIS/CIRES) (2022): Record Low North American Monsoon Rainfall in 2020 Reignites Drought over the American Southwest. Bull. Amer. Meteor. Soc. https://doi.org/10.1175/BAMS-D-21-0129.1.

Posted: February 15, 2022