Examining the causes of the record drought and heat in California and Nevada during Water Year 2021

Aerial view of Hoover Dam, NV

During Water Year 2021 (October 2020 – September 2021), California and Nevada were once again at the epicenter of drought and heat — with precipitation at its second lowest, and temperatures at their second highest since 1895. In a new study led by the Physical Sciences Laboratory, researchers investigate the factors related to simultaneous precipitation and temperature extremes in this region. The study examines whether the co-occurrence of human-caused climate change and La Niña – two principal climate drivers during Water Year 2021 – altered the risk of unprecedented compound dry and hot extremes. Their findings will be published in the Bulletin of the American Meteorological Society.

Water Year 2021 was the first time since 1895 that precipitation fell below -1.5 standardized departures and temperatures exceeded 1.5 standardized departures concurrently for the two-state average. By comparing past (1921-1950) and recent (1991-2020) climates, the study found that the risk of compound dry and hot extremes like those in Water Year 2021 significantly increased by at least six times. The increased risk was the result of statistically significant shifts to both lower precipitation and higher temperatures. Different factors contributed to different elements of the compound risk: La Niña caused the lower precipitation that increased dry extreme risks over the western United States and anthropogenic forcing alone increased regional temperatures.

This assessment investigates the factors related to simultaneous precipitation and temperature extremes during Water Year 2021 in California and Nevada that lead to heightened societal impacts such as fire risk and reduced water resources compared to individual extreme events. A better understanding of changes in hot and dry risk amid climate change can better prepare society to cope with these compound events in the future.

Andrew Hoell (PSL), Xiao-Wei Quan (PSL/CIRES), Martin Hoerling (PSL), Henry F. Diaz, Rong Fu, Cenlin He, Joel R. Lisonbee (NIDIS/CIRES), Justin S. Mankin, Richard Seager, Amanda Sheffield (NIDIS/CIRES), Isla R. Simpson, and Eugene R. Wahl (NCEI): Water Year 2021 Compound Precipitation and Temperature Extremes in California and Nevada. Bull. Amer. Meteor. Soc., https://doi.org/10.1175/BAMS-D-22-0112.1.