ArticlePublisher preview available

Rapid intensification of the emerging southwestern North American megadrought in 2020–2021

To read the full-text of this research, you can request a copy directly from the authors.

Abstract and Figures

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 anthropogenic 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. Southwestern North America has been experiencing lower than average precipitation and higher temperatures since 2000. This emerging megadrought, spanning 2000–2021, has been the driest 22-year period since the year 800 and 19% of the drought severity in 2021 can be attributed to climate change.
This content is subject to copyright. Terms and conditions apply.
Brief CommuniCation
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:
A previous reconstruction back to 800 
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 USDMs 22-yr
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 reconstructions 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
A. Park Williams 1,2 ✉ , Benjamin I. Cook2,3 and Jason E. Smerdon 2
NATURE CLIMATE CHANGE | VOL 12 | MARCH 2022 | 232–234 |
Content courtesy of Springer Nature, terms of use apply. Rights reserved
... smoothed annual flows on the Colorado River are now lower than at any other time in the last 12 centuries. This is consistent with recent findings by Williams et al. (2022), who report that 2000-2021 is the driest 22-year period across the southwest United States in the same time frame. ...
... Much of the research addressing the severity of the ongoing drought in the southwest United States relative to that of past droughts has relied upon a set of moisture-limited tree-ring chronologies that has been utilized to develop gridded reconstructions of drought-e.g., North American Drought Atlas (NADA; Cook & Krusic, 2004; E. R. Cook & Krusic, 2004), Living Blended Drought Atlas (LBDA; Cook et al., 2010), seasonal precipitation (Stahle et al., 2020), and soil moisture (Williams et al., 2020(Williams et al., , 2022. The numerous spatial analyses that have utilized this tree-ring network (e.g., Coats et al., 2015;Cook et al., 2015;Ho et al., 2016) have been limited to the time period when the network has coverage for large parts of the United States. ...
... The current period of drought has been found to be anomalous in the context of the instrumental record (Williams et al., 2022) and, as noted above, in the context of 1,200 years of reconstructed Colorado River streamflow reported in Meko et al. (2007). The new reconstruction now allows us to evaluate this drought over an even longer period of time. ...
Full-text available
Plain Language Summary The Colorado River drought we currently are experiencing is severe in the context of the 116‐year gage record (1906–2021), but how severe is it in a long‐term context? Existing tree‐ring based reconstructions of Colorado River streamflow have suggested that the 22‐year period 2000–2021 could be the worst drought in the southwestern United States in 1,200 years. The purpose of this study is to extend the Colorado River reconstruction back 2,000 years and to evaluate the current drought in a long‐term context. We find that an even more extreme drought occurred and persisted over much of the second century. Data are sparse this far back in time, but evidence from both tree‐ring data and paleoclimate data from lakes, bogs, and caves supports the existence and severity of this drought in the context of the last two millennia. Additional work is needed to learn more about this drought and its causes, but we now know that drought more persistent than even the well‐documented medieval period droughts occurred in the past, expanding our understanding of the range of natural climate variability.
... The existence of the recovery programs suggests consistent compliance of current and future federal water projects with the Endangered Species Act (Benson 2010). Compliance is achieved through the above-mentioned mitigation and management actions, but achieving their stated goal of recovery is becoming more and more unlikely as water availability continues to decline (Williams et al., 2022). Further, the programs generally perform management actions in designated critical habitats, which for most listed species includes very little habitat in smaller tributaries widely used by many fishes (Bottcher et al., 2013;Webber et al., 2013;Laub et al., 2018), and so protections for tributary habitats is limited. ...
... Frontiers in Environmental Science | May 2022 | Volume 10 | Article 870488 reduction from the 20th to 21st century and the predictions for the future are no better (Udall and Overpeck 2017;Williams et al., 2022). The second priority is rehabilitation of the riparian plant community across the riverscape by removing established nonnative vegetation, as described above. ...
Full-text available
Desert riverscape rehabilitation practitioners must contend with compounding effects of increasing human water demand, persistent drought, non-native species establishment, and climate change, which further stress desert riverine ecosystems such as rivers in the Colorado River basin, USA. Herein, we provide our perspective on the importance of natural flows, large floods in particular, for successful conservation and rehabilitation of riverscapes. We present ideas developed from our experience with rehabilitation projects across multiple desert tributary rivers with varying levels of habitat degradation and water abstraction. We propose spatially extensive measures such as protection of in-stream flows, tailoring rehabilitation efforts to available annual water availability, and working with nature using low-tech process-based techniques to more completely address the mechanisms of habitat degradation, such as flow reduction and vegetation-induced channel narrowing. Traditionally, rehabilitation efforts in the Colorado River basin take place at relatively small spatial extents, at convenient locations and, largely focus on reducing non-native plant and fish species. We suggest that we need to think more broadly and creatively, and that conservation or recovery of natural flow regimes is crucial to long-term success of almost all management efforts for both in-stream and riparian communities.
... Climate pressures have been steadily increasing and the past decade (2010-2019) has been the hottest recorded to date [19]. Additionally a recent study found that the southwestern US "megadrought" is the greatest drought in 1200 years [20]. As of Spring 2022, Washington state was already reporting severe and extreme droughts in the eastern part of the state, and these droughts are predicted to cause smaller wheat kernels [21,22]. ...
Full-text available
Through plant breeding and improved agronomy, the average wheat kernel size increased globally by about 40% from 1940 to 2000. Millers demand larger kernels because they contain more white flour (endosperm). Climate pressures are resulting in frequently reduced kernel size and routine rejection by the commodity system. If whole-wheat flour instead of white flour is the target, these smaller kernels have unrealized value. A total of 94% of Americans do not meet the recommended fiber intake, and inadequate fiber intake plays a role in the development of multiple chronic diseases. A total of 98% of the fiber in wheat is found in the bran. Bran content was measured in “big” (x¯ = 0.042 g/kernel) and “small” (x¯ = 0.023 g/kernel) kernels in nine varieties over locations and years. On average, small kernels contained 15.9% more bran than big kernels (n = 54, p < 0.001) and, thus, had higher mineral and fiber content. In the majority of cases, baking showed no difference in whole-wheat quality among flours within the same variety, regardless of kernel size, based on bread slice height and surface area. Wheat that was rejected by commercial mills as too small produced satisfactory bread. Favoring larger kernels and white flour production has unintended health consequences. Valuing smaller kernels and whole-wheat production provides an outlet for farmers dealing with increasing climate pressures and leads to an end-use product which can improve human health by increasing dietary fiber consumption.
... Convective activity associated with the summer monsoon is stronger at the more southerly SOR site. Furthermore, the SOR observations were conducted from June 1998 to November 2000, before the current southwest mega-drought (Williams et al., 2022). The CSU observations were conducted from March 1990 to 2010. ...
Full-text available
Utilizing 956 nights of Na lidar nocturnal mesopause region temperature profiles acquired at Fort Collins, CO (40.6°N, 105.1°W) over a 20‐year period (March 1990–2010), we deduce background nightly mean temperature T¯(z) $\bar{T}(z)$ and the square of the buoyancy frequency N²(z) at 2‐km resolution between 83 and 105 km. The temperature climatology reveals the two‐level mesopause structure with clarity and sharp mesopause transitions, resulting in 102 days of summer from Days 121 to 222 of the year. The same data set analyzed at 10‐min and 1‐km resolution gives the gravity wave (GW) temperature perturbations Ti'(z) and the wave variance Var(T′(z)) and GW potential energy Epm(z) between 85 and 100 km. Seasonal averages of GW Var(T′(z)) and Epm(z) between 90 and 100 km, show that Var(T′) for spring and autumn are comparable and lower than for summer and winter. Due mainly to the higher background stability, or larger N²(z) in summer, Epm(z) between 85 and 100 km is comparable in spring, summer, and autumn seasons, but ∼30%–45% smaller than the winter values at the same altitude. The uncertainties are about 4% for winter and about 5% for the other three seasons. The values for Epm are (156.0, 176.2, 145.6, and 186.2 J/kg) at 85 km for (spring, summer, autumn, and winter) respectively, (125.4, 120.2, 115.2, and 168.7 J/kg) at 93 km, and (207.5, 180.5, 213.1, and 278.6 J/kg) at 100 km. Going up in altitude, all profiles first decrease and then increase, suggesting that climatologically, GWs break below 85 km.
... Exceptional drought conditions across the southwest United States have persisted since 2000, making it the driest 19-year span since the late 1500s (Williams et al., 2022). Studies using previous CMIP models (CMIP3 and CMIP5) suggest a reorganization in the seasonal cycle of the NAM rainfall due to GHG forcing, with rainfall declines in the early monsoon season balanced by late rainfall increases (Cook & Seager, 2013; Seager & Vecchi, 2010). ...
Full-text available
Changes in the North American Monsoon (NAM), a circulation system that transports moisture into western Mexico and the southwest U.S., can have substantial impacts on water resources and agriculture. Here, we utilize future projections from Phase 6 and 5 of the Coupled Model Intercomparison Project (CMIP) to assess monthly changes in land precipitation and the leading mechanisms resulting from anthropogenic climate change. Historical CMIP6 simulations of seasonal precipitation demonstrate skill in reproducing NAM rainfall, but mimic precipitation biases observed in previous CMIP generations. Future climate projections from the SSP5‐8.5 pathway produce reductions in precipitation that persist throughout the monsoon season (June–August) but are balanced by precipitation increases during the late monsoon season (September–October) but are not shown in CMIP5 projections. Atmospheric moisture budget analysis reveals that early monsoon rainfall deficits are associated with a combination of greater evaporative demand, a negative dynamic response of vertical moisture advection, and anomalous subsidence. Increases in late monsoon season rainfall are attributed to a positive change in the dynamical term of vertical moisture advection and increases in upward motion. Although minimal changes in total land NAM rainfall are observed, seasonal shifts and the persistence of drier conditions can have significant ecological and societal consequences.
Full-text available
Tree die-off, driven by extreme drought and exacerbated by a warming climate, is occurring rapidly across every wooded continent - threatening carbon sinks and other ecosystem services provided by forests and woodlands. Forecasting the spatial patterns of tree die-off in response to drought is a priority for the management and conservation of forested ecosystems under projected future hotter and drier climates. Several thresholds derived from drought-metrics have been proposed to predict mortality of Pinus edulis, a model tree species in many studies of drought-induced tree die-off. To improve future capacity to forecast tree mortality, we used a severe drought as a natural experiment. We compared the ability of existing mortality thresholds derived from four drought metrics (the Forest Drought Severity Index, the Standardized Precipitation Evapotranspiration Index, and raw values of precipitation and vapor pressure deficit, calculated using 4km PRISM data) to predict areas of P. edulis die-off following an extreme drought in 2018 across the southwestern US. Using aerial detection surveys of tree mortality in combination with gridded climate data, we calculated the agreement between these four proposed thresholds and the presence and absence of regional-scale tree die-off using sensitivity, specificity, and the area under the curve (AUC). Overall, existing mortality thresholds tended to over predict the spatial extent of tree die-off across the landscape, yet some retain moderate skill in discriminating between areas that experienced and did not experience tree die-off. The simple precipitation threshold had the highest AUC score (71%) as well as fair sensitivity and specificity, but the Forest Drought Severity Index had the greatest sensitivity to die-off (85.9%). We highlight that empirically derived climate thresholds may be useful forecasting tools to identify vulnerable areas to drought induced die-off, allowing for targeted responses to future droughts and improved management of at-risk areas.
Full-text available
Environmental water markets have emerged as a tool for restoring flows in rivers across the world. Prior literature suggests that certain legal conditions are necessary for these markets to function. However, we find substantial market activity has occurred without these legal conditions through market and legal data collected in five core U.S. Colorado River basin states (Arizona, Colorado, New Mexico, Utah, and Wyoming) from 2014 to 2020. Ninety-five percent of the 446 water transactions sidestepped formal legal processes to transfer water rights to the environment. We also find that government regulatory and conservation programs, not private-sector investment, have driven most environmental water market activity. Government spending is the dominant funding source, with 90% of the $53 million spent coming from governments and 68% from the U.S. federal government alone. Finally, our analysis finds that current market activity would be insufficient to stave off future curtailment of critical water users under the Colorado River Compact and that $86–89 million annually in new investment is required to do so. In a basin experiencing a historic megadrought, our analysis suggests prioritizing such new investments over legal reform. Global implications are that such flow restoration is possible where legal regimes for environmental water markets do not already exist.
A partir del libro Los wixaritari. El espacio compartido y la comunidad, de Héctor Medina Miranda (2020), sobre la historia y antropología de los pueblos indígenas del Gran Nayar, una región que abarca el sur de la Sierra Madre Occidental de México, se abordan cinco temas generales de la antropología sociocultural: 1) la identidad, ubicación y organización política de los grupos lingüístico-culturales señalados en las fuentes etnohistóricas; 2) las teorías de territorialización, placemaking (la producción de lugares) y kiekari(«rancheridad» o la domesticación del espacio), que incluyen la noción de la tierra como propiedad en el estado-nación; 3) la organización de nuevos asentamientos indígenas sobre un área geográfica extensa poco estudiada hasta ahora; 4) el nexo entre ideología ritual y relaciones regionales de poder; y 5) la conexión entre la «invención de la tradición» y la producción de lugares como un acto performativo en el proceso histórico de continua reterritorialización.
Large, severe wildfires continue to burn in frequent-fire adapted forests but the mechanisms that contribute to them and their predictability are important questions. Using a combination of ground based and remotely sensed data we analyzed the behavior and patterns of the 2020 Creek Fire where drought and bark beetles had previously created substantial levels of tree mortality in the southern Sierra Nevada. We found that dead biomass and live tree densities were the most important variables predicting fire severity; high severity fire encompassed 41% of the area and the largest high severity patch (19,592 ha) comprised 13% of total area burned. Areas with the highest amounts of dead biomass and live tree densities were also positively related to high severity fire patch size indicating that larger, more homogenous conditions of this forest characteristic resulted in adverse, landscape-scale fire effects. The first two days of the Creek Fire were abnormally hot and dry but weather during the days of the greatest fire growth was largely within the normal range of variation for that time of year with one day with lower windspeeds. From September 5 to 8th the fire burned almost 50% of its entire area and fire intensity patterns inferred from remotely sensed brightness-temperature data were typical except on September 6th when heat increased towards the interior of the fire. Not only was the greatest heat concentrated away from the fire perimeter, but a significant amount of heat was still being generated within the fire perimeter from the previous day. This is a classic pattern for a mass fire and the high amount of dead biomass created from the drought and bark beetles along with high live tree densities were critical factors in developing mass fire behavior. Operational fire behavior models were not able to predict this behavior largely because they do not include post-frontal combustion and fire-atmosphere interactions. An important question regarding this mass fire is if the tree mortality event that preceded it could have been avoided or reduced or was it within the natural range of variation for these forests? We found that the mortality episode was outside of historical analogs and was exacerbated by past management decisions. The Creek Fire shows us how vulnerable of our current frequent-fire forest conditions are to suffering high tree mortality and offering fuel conditions capable of generating mass fires from which future forest recovery is questionable because of type conversion and probable reoccurring high severity fire.
Surface soil moisture (SSM) is of great importance in understanding global climate change and studies related to environmental and earth science. However, neither of current SSM products or algorithms can generate SSM with High spatial resolution, High spatio-temporal continuity (cloud-free and daily), and High accuracy simultaneously (i.e., 3H SSM data). Without 3H SSM data, fine-scale environmental and hydrological modeling cannot be easily achieved. To address this issue, we proposed a novel and integrated SSM downscaling framework inspired by deep learning-based point-surface fusion, which was designed to produce 1 km spatially seamless and temporally continuous SSM with high accuracy by fusing remotely sensed, model-based, and ground data. First, SSM auxiliary variables (e.g., land surface temperature, surface reflectance) were gap filled to ensure the spatial continuity. Meanwhile, the extended triple collocation method was adopted to select reliable in-situ stations to address the scale mismatch issue in SSM downscaling. Then, the deep belief model was utilized to downscale the original 9 km SMAP SSM and 0.1º. ERA5-Land SSM to 1 km. The downscaling framework was validated over three ISMN soil moisture networks covering diverse ground conditions in Southwestern US. Three validation strategies were adopted, including in-situ validation, time-series validation, and spatial distribution validation. Results showed that the average Pearson correlation coefficient (PCC), unbiased root mean squared error (ubRMSE), and mean absolute error (MAE) achieved 0.89, 0.034 m3m−3, and 0.032 m3m−3, respectively. The use of point-surface fusion greatly improved the downscaling accuracy, of which the PCC, ubRMSE, and MAE were improved by 3.73, 20.93, and 39.62% compared to surface-surface fusion method, respectively. Comparative analyses have also been carefully conducted to confirm the effectiveness of the framework, in terms of other downscaling algorithms, scale variations, and fusion methods. The proposed method is promising for fine-scale studies and applications in agricultural, hydrological, and environmental domains.
Full-text available
Abstract Over the last two decades, southwestern North America (SWNA) has been in the grip of one of the most severe droughts of the last 1,200 years, with one third to nearly one half of its severity attributable to climate change. We analyze how the risk of extreme soil moisture droughts in SWNA, analogous to the most severe 21‐year (≥ in magnitude to 2000–2020) and single‐year (≥ in magnitude to 2002) events of the last several decades, changes in projections from Phase 6 of the Coupled Model Intercomparison Project. By the end of the 21st century, SWNA experiences robust (R ≥ 0.80) soil moisture drying and substantial increases in extreme single‐year drought risk that scale strongly with warming, spanning an 8%–26% probability of occurrence across +2–4 K. Notably, our results show that 21‐year droughts analogous to 2000–2020 are up to 5 times more likely than extreme single‐year droughts under all levels of warming (≈50%). These high levels of 21‐year drought risk are largely invariant across scenarios because of large spring precipitation declines in half the models, shifting SWNA into a drier mean state. Despite projections of this sweeping and ostensibly inevitable increase in 21‐year drought risk, climate mitigation reduces their severity by reducing the magnitude of extreme single‐year droughts during these events. Our results emphasize both the importance of preparing SWNA for imminent increases in persistent drought events and constraining projected precipitation uncertainty to better resolve future long‐term drought risk.
Full-text available
Accurate quantification of global land evapotranspiration is necessary for understanding variability in the global water cycle, which is expected to intensify under climate change1–3. Current global evapotranspiration products are derived from a variety of sources, including models4,5, remote sensing6,7 and in situ observations8–10. However, existing approaches contain extensive uncertainties; for example, relating to model structure or the upscaling of observations to a global level11. As a result, variability and trends in global evapotranspiration remain unclear12. Here we show that global land evapotranspiration increased by 10 ± 2 per cent between 2003 and 2019, and that land precipitation is increasingly partitioned into evapotranspiration rather than runoff. Our results are based on an independent water-balance ensemble time series of global land evapotranspiration and the corresponding uncertainty distribution, using data from the Gravity Recovery and Climate Experiment (GRACE) and GRACE-Follow On (GRACE-FO) satellites13. Variability in global land evapotranspiration is positively correlated with El Niño–Southern Oscillation. The main driver of the trend, however, is increasing land temperature. Our findings provide an observational constraint on global land evapotranspiration, and are consistent with the hypothesis that global evapotranspiration should increase in a warming climate. Using a global mass-balance approach to calculate evapotranspiration, it is shown that global land evapotranspiration increased by 10% between 2003 and 2019, driven mainly by warming land temperatures.
Full-text available
California’s water resources rely heavily on cool‐season (November–March) precipitation in the Sierra Nevada. Interannual variability is highly volatile and seasonal forecasting has little to no skill, making water management particularly challenging. Over 1902–2020, Sierra Nevada cool‐season precipitation totals exhibited significant 2.2‐ and 13–15‐year cycles, accounting for approximately 40% of total variability and perhaps signifying potential as seasonal forecasting tools. However, the underlying climate dynamics are not well understood and it is unclear whether these cycles are stable over the long term. We use tree rings to reconstruct Sierra Nevada cool‐season precipitation back to 1400. The reconstruction is skillful, accounting for 55%–74% of observed variability and capturing the 20th‐century 2.2‐ and 13–15‐year cycles. Prior to 1900, the reconstruction indicates no other century‐long periods of significant spectral power in the 2.2‐ or 13–15‐year bands. The reconstruction does indicate significant cyclicity over other extended periods of several decades or longer, however, with dominant periodicities in the ranges of 2.1–2.7 and 3.5–8 years. The late 1700s through 1800s exhibited the highest‐amplitude cycles in the reconstruction, with periodicities of 2.4 and 5.7–7.4 years. The reconstruction should serve to caution against extrapolating the observed 2.2‐ and 13–15‐year cycles to guide future expectations. On the other hand, observations and the reconstruction suggest that interannual variability of Sierra Nevada cool‐season precipitation is not a purely white noise process and research should aim to diagnose the dynamical drivers of extended periods of cyclicity in this critical natural resource.
Full-text available
Snowpack in the Sierra Nevada Mountains accounts for around one-third of California’s water supply. Melting snow provides water into dry summer months characteristic of the region’s Mediterranean climate. As climate changes, understanding patterns of snowpack, snowmelt, and biological response is critical in this region of agricultural, recreational, and ecological value. Here we investigated the relationships between tree rings of montane conifer trees ( Tsuga mertensiana, Abies magnifica, Abies concolor, Calocedrus decurrens, Juniperus occidentalis, and Pinus ponderosa) and regional climate indices with the goal of reconstructing April 1 snow-water equivalent (SWE) in the North Fork American River watershed of the Sierra Nevada. Chronologies were positively correlated with April 1 SWE of the year prior to ring formation. Temporal trends in correlation between tree-ring chronologies and climate indices indicate strengthening tree growth response to climate over time. We developed a skillful, nested reconstruction for April 1 SWE, 1661–2013. Variability of the reconstruction is within the envelope of 20th and 21st-century variability; however, the 2015 record low snowpack is unprecedented in the tree-ring record, as in results from previous studies. Future research should focus on integrating modern snow sensor data into paleoclimate research and understanding mechanistic linkages between snow and tree growth response.
Full-text available
Severe and persistent 21st-century drought in southwestern North America (SWNA) motivates comparisons to medieval megadroughts and questions about the role of anthropogenic climate change. We use hydrological modeling and new 1200-year tree-ring reconstructions of summer soil moisture to demonstrate that the 2000–2018 SWNA drought was the second driest 19-year period since 800 CE, exceeded only by a late-1500s megadrought. The megadrought-like trajectory of 2000–2018 soil moisture was driven by natural variability superimposed on drying due to anthropogenic warming. Anthropogenic trends in temperature, relative humidity, and precipitation estimated from 31 climate models account for 47% (model interquartiles of 35 to 105%) of the 2000–2018 drought severity, pushing an otherwise moderate drought onto a trajectory comparable to the worst SWNA megadroughts since 800 CE.
Full-text available
There is a need for improved drought monitoring and assessment methods in the United States. Drought is the most costly natural disaster [Federal Emergency Management Agancy (FEMA 1995; Wilhite 2000)], but it is often neglected by developers of assessment and forecast products. Drought is more nebulous than other disasters and does not lend itself to traditional assessments or forecast methods. Its relatively slow onset and the complexity of its impacts are reasons for the new assessment methodology. Improvements in drought monitoring and forecasting techniques will allow for better preparation, lead to better management practices, and reduce the vulnerability of society to drought and its subsequent impacts. The Drought Monitor (additional information available online at http://drought.unl/edu/dm) was created with the goal of tracking and displaying the magnitude and spatial extent of drought and its impacts across the United States. The Drought Monitor is produced weekly and classifies drought severity into four major categories, with a fifth category threshold assigned to locations on a map are determined from a number of indicators, or tools, blended with subjective interpretation.
Full-text available
Megadroughts are comparable in severity to the worst droughts of the 20th century but are of much longer duration. A megadrought in the American Southwest would impose unprecedented stress on the limited water resources of the area, making it critical to evaluate future risks not only under different climate change mitigation scenarios but also for different aspects of regional hydroclimate. We find that changes in the mean hydroclimate state, rather than its variability, determine megadrought risk in the American Southwest. Estimates of megadrought probabilities based on precipitation alone tend to underestimate risk. Furthermore, business-as-usual emissions of greenhouse gases will drive regional warming and drying, regardless of large precipitation uncertainties. We find that regional temperature increases alone push megadrought risk above 70, 90, or 99% by the end of the century, even if precipitation increases moderately, does not change, or decreases, respectively. Although each possibility is supported by some climate model simulations, the latter is the most common outcome for the American Southwest in Coupled Model Intercomparison 5 generation models. An aggressive reduction in global greenhouse gas emissions cuts megadrought risks nearly in half.
Full-text available
By coordinating the design and distribution of global climate model simulations of the past, current, and future climate, the Coupled Model Intercomparison Project (CMIP) has become one of the foundational elements of climate science. However, the need to address an everexpanding range of scientific questions arising from more and more research communities has made it necessary to revise the organization of CMIP. After a long and wide community consultation, a new and more federated structure has been put in place.
The sensitivity of river discharge to climate-system warming is highly uncertain and governing processes are poorly understood, impeding climate-change adaptation. A prominent exemplar is the Colorado River, where meteorological drought and warming have been shrinking a water resource that supports more than USD 1 trillion per year of economic activity. Monte-Carlo simulation with a radiation-aware hydrologic model resolves the longstanding, wide disparity in sensitivity estimates and reveals the controlling physical processes. We estimate that annual-mean discharge has been decreasing by 9.3% per °C of warming due to increased evapotranspiration, mainly driven by snow loss and consequent decrease of reflection of solar radiation. Projected precipitation increases likely will not suffice to counter fully the robust, thermodynamically induced drying. Increasing risk of severe water shortages is expected.
Between 2000 and 2014, annual Colorado River flows averaged 19% below the 1906-1999 average, the worst 15-year drought on record. At least one-sixth to one-half (average at one-third) of this loss is due to unprecedented temperatures (0.9°C above the 1906-99 average), confirming model-based analysis that continued warming will likely further reduce flows. Whereas it is virtually certain that warming will continue with additional emissions of greenhouse gases to the atmosphere, there has been no observed trend towards greater precipitation in the Colorado Basin, nor are climate models in agreement that there should be a trend. Moreover, there is a significant risk of decadal and multidecadal drought in the coming century, indicating that any increase in mean precipitation will likely be offset during periods of prolonged drought. Recently published estimates of Colorado River flow sensitivity to temperature combined with a large number of recent climate model-based temperature projections indicate that continued business-as-usual warming will drive temperature-induced declines in river flow, conservatively -20% by mid-century and -35% by end–century, with support for losses exceeding -30% at mid-century and -55% at end-century. Precipitation increases may moderate these declines somewhat, but to date no such increases are evident and there is no model agreement on future precipitation changes. These results, combined with the increasing likelihood of prolonged drought in the river basin, suggest that future climate change impacts on the Colorado River flows will be much more serious than currently assumed, especially if substantial reductions in greenhouse gas emissions do not occur. This article is protected by copyright. All rights reserved.