Researchers find the current Western drought is worst in 1,200 years

It is intensified by climate change

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low water level drought Lake Powell, Glen Canyon National Recreation Area
The public launch ramp at Antelope Point in late March, 2021 at Lake Powell, Glen Canyon National Recreation Area. NPS photo.

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.

Precipitation, temperature, and vapor pressure anomaly, 2000 to 2021
Observed climate anomalies. Anomalies in water-year (WY: October–September) (a) precipitation total, (b) temperature, and (c) vapour-pressure deficit (VPD). Maps on left show the average WY anomaly during 2000–2021. Yellow box: Southwestern North America (SWNA) study region. Anomalies are relative to 1950–1999. Time series on right show regionally averaged WY anomalies in SWNA (black) annually and as (red) 22-year running means visualized on the final year in each 22-year period. Geographic boundaries in maps were accessed through Matlab 2020a. From the paper.

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.

Soil moisture, 800 to 2021
Extended drought events. Summer soil moisture anomalies, expressed as standard deviations from the 800–2021 mean (σ), during the longest 8 extended drought events during the 800–2021 study period. The pink background bounds the years of each extended drought event. The horizontal dotted black line represents the 800–2021 mean. For the first 7 droughts shown, soil moisture anomalies come from our tree-ring reconstruction. For the final drought (2000–2021), anomalies come from our observation-based record. From the paper.

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.

Percent of US with extreme or exceptional drought, 2000 to 2022
Extreme and exceptional drought in the western United States (US). Weekly percentage of western continental United States (west of 103°W) classified by the United States Drought Monitor (USDM) as under extreme or exceptional drought from January 1, 2000 to December 28, 2021. Calculations were made form weekly shapefiles of USDM drought classifications, available at as of January 9, 2022. The USDM is developed by the National Drought Mitigation Center (NDMC), the U.S. Department of Agriculture (USDA) and the National Oceanic and Atmospheric Administration (NOAA). From the paper.

The research was conducted by A. Park Williams, Benjamin I. Cook, and Jason E. Smerdon.

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Author: Bill Gabbert

After working full time in wildland fire for 33 years, he continues to learn, and strives to be a Student of Fire.