New research published this month shows that the current drought in the Western United States is the worst seen in data going back to the year 800. Scientists developed estimates of precipitation during previous centuries using tree-ring reconstruction and found 2000–2021 was the driest 22-year period in the last 1,200 years.
Since the year 2000, southwestern North America (SWNA) has been unusually dry due to low precipitation totals and heat, punctuated most recently by exceptional drought in 2021. From 2000 to 2021, mean water-year (October– September) SWNA precipitation was 8.3 percent below the 1950–1999 average and temperature was 0.91 °C above average.
In summer of 2021, water levels at Lakes Mead and Powell, both on the Colorado River, reached their lowest levels on record, triggering unprecedented restrictions on Colorado River usage, in part because the 2-year naturalized flow out of Colorado River’s upper basin in water-years 2020–2021 was likely the lowest since at least 1906. Despite an active North American monsoon in 2021, the United States Drought Monitor classified more than 68 percent of the western United States as under extreme or exceptional drought for nearly all of July–October, 2021.
The researchers concluded that anthropogenic climate change accounts for 42 percent of the SWNA soil moisture anomaly in 2000–2021 and 19 percent in 2021.
Drought can have a very significant effect on wildland fire behavior. It affects vapor pressure deficit (VPD), soil moisture, relative humidity, and moisture in live and dead vegetation, or fuels. VPD is an absolute measure of the moisture deficit of the atmosphere and is more closely related to water stress on vegetation than relative humidity.
Soil moisture is a particularly important integrator of drought. Of all 22-year periods since 800, only two (1130–1151 and 1276–1297) contained more years with negative soil moisture anomalies than the 18 observed during 2000–2021.
The authors wrote that the 22-year long current drought is highly likely to continue through a 23rd year.
The research was conducted by A. Park Williams, Benjamin I. Cook, and Jason E. Smerdon.