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Brief CommuniCation
https://doi.org/10.1038/s41558-022-01290-z
1Department of Geography, University of California, Los Angeles, Los Angeles, CA, USA. 2Lamont-Doherty Earth Observatory of Columbia University,
Palisades, NY, USA. 3NASA Goddard Institute for Space Studies, New York, NY, USA. ✉e-mail: williams@geog.ucla.edu
A previous reconstruction back to 800
ce
indicated that the
2000–2018 soil moisture deficit in southwestern North
America was exceeded during one megadrought in the
late-1500s. Here, we show that after exceptional drought
severity in 2021, ~19% of which is attributable to anthropo-
genic climate trends, 2000–2021 was the driest 22-yr period
since at least 800. This drought will very likely persist through
2022, matching the duration of the late-1500s megadrought.
Since the year 2000, southwestern North America (SWNA,
30–45° N, 105–125° W) has been unusually dry due to low precip-
itation totals and heat, punctuated most recently by exceptional
drought in 20211. From 2000 to 2021, mean water-year (October–
September) SWNA precipitation was 8.3% below the 1950–1999
average and temperature was 0.91 °C above average (Extended
Data Fig. 1). No other 22-yr period since at least 1901 was as dry
or as hot. While there have been single-year breaks in these anom-
alous conditions, aridity has dominated the 2000s, as evidenced
by declines in two of North America’s largest reservoirs, Lakes
Mead and Powell, both on the Colorado River. In summer 2021,
these reservoirs reached their lowest levels on record, trigger-
ing unprecedented restrictions on Colorado River usage2, in part
because the 2-yr naturalized flow out of Colorado River’s upper
basin in water-years 2020–2021 was likely the lowest since at least
1906 (Extended Data Fig. 2). Aridity was especially extreme and
widespread from summer 2020 through summer 2021 (Extended
Data Fig. 3). Despite an active North American monsoon in 2021,
the United States Drought Monitor (USDM3) classified >68% of
the western United States as under extreme or exceptional drought
for nearly all of July–October 2021 (Extended Data Fig. 4),
a record-high proportion of drought extent in the USDM’s 22-yr
history.
Soil moisture is a particularly important integrator of drought.
Soil moisture impacts runoff ratios and therefore streamflow, agri-
cultural productivity and irrigation demand, ecosystem productiv-
ity and health, wildfire activity and land–atmosphere feedbacks such
as heatwave intensity. Summer soil moisture is particularly crucial,
as summer is when water demand from ecosystems, humans and
the atmosphere is generally highest, and also the season of focus
in most tree-ring reconsructions of drought severity4. According
to a bucket-type water-balance model forced by monthly climate
data5, SWNA 0–200 cm soil moisture in summer (June–August)
was below average in 18 of the 22 years from 2000–2021 (Extended
Data Fig. 5). This turn-of-the-twenty-first-century drought was last
investigated by Williams et al.5 through 2018, who speculated that
the extended drought event may have been terminating in 2019
due to abundant precipitation that year. Dry conditions returned in
2020 and intensified substantially in 2021, however, indicating that
the turn-of-the-twenty-first-century drought is not over.
To understand the longer-term context of the turn-of-
the-twenty-first-century drought in SWNA, Williams et al.5
extended the SWNA summer soil moisture record back to 800
using a tree-ring reconstruction and compared the observed
drought of 2000–2018 to the infamous megadroughts that occurred
repeatedly from 800–16006. These megadroughts were multidecade
droughts that exceeded any subsequent event through the 1900s in
terms of duration and severity7. Williams et al.5 found that the 19-yr
soil moisture anomaly from 2000 to 2018 was probably the second
driest in at least 1,200 years, exceeded only by a 19-yr interval dur-
ing the last of the megadroughts, in the late 1500s. The authors
refrained from classifying 2000–2018 as an official megadrought
because the reconstructed megadroughts lasted longer than 19 yr.
Here, we update the Williams et al.5 analysis through 2021 to
compare the now 22-yr-long turn-of-the-twenty-first-century
drought to the reconstructed megadroughts. Our updated SWNA
regionally averaged soil moisture reconstruction is highly skil-
ful (cross-validated R2 = 0.74 – 0.85) and nearly identical to that of
Williams et al.5 (Extended Data Fig. 6). We find that 2000–2021
ranks as the driest 22-yr period since at least 800 , with a 22-yr
mean soil moisture anomaly of 0.87 s.d. (σ) below the 800–2021
mean (Fig. 1a). The second-driest 22-yr period was 1571–1592,
with a reconstructed soil moisture anomaly of –0.83 σ, although
the reconstruction’s 95% uncertainty range overlaps with the 2000–
2021 anomaly.
The reconstructed megadroughts and the current event were not
exclusively dry across time or space but 2000–2021 was particu-
larly dry in both regards. Of all 22-yr 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
(Extended Data Fig. 7a). Subregionally, 2000–2021 drought rank-
ings were generally less severe relative to past megadroughts but
2000–2021 still ranked among the five driest 22-yr periods locally
across 61% of SWNA (Fig. 1b). This represents the largest SWNA
area to experience a top- five 22-yr drought-severity ranking in at
least 1,200 years (Extended Data Fig. 7b).
Exceptionally dry soil in 2021 was critical for the current drought
to escalate and overtake the 1500s megadrought as the period with
the highest 22-yr mean severity (Fig. 2a). The 2021 soil moisture
anomaly (–2.58 σ) was nearly as dry as that of 2002 (–2.59 σ), the
driest year in the 1901–2021 observational record and notable for
its severe impacts on forest ecosystems and wildfire8,9. The fact
Rapid intensification of the emerging
southwestern North American megadrought in
2020–2021
A. Park Williams 1,2 ✉ , Benjamin I. Cook2,3 and Jason E. Smerdon 2
NATURE CLIMATE CHANGE | VOL 12 | MARCH 2022 | 232–234 | www.nature.com/natureclimatechange
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