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Large Contribution from Anthropogenic Warming to an Emerging North American Megadrought A

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Large Contribution from Anthropogenic Warming to an Emerging North American Megadrought A RESEARCH DROUGHT used to develop previous continental drought atlases (16). Reconstruction skill is evaluated Large contribution from anthropogenic warming as the squared Pearson’s correlation (R2) be- tween observations during the 1901–1983 to an emerging North American megadrought calibration period and out-of-sample recon- struction values that were calculated by using A. Park Williams1*, Edward R. Cook1, Jason E. Smerdon1, Benjamin I. Cook1,2, John T. Abatzoglou3,4, leave-10-out cross-validation (30). Reconstruc- Kasey Bolles1, Seung H. Baek1,5, Andrew M. Badger6,7,8, Ben Livneh6,9 tion skill is highly significant (P <0.01)across much of SWNA (Fig. 1A). The cross-validated 2 Severe and persistent 21st-century drought in southwestern North America (SWNA) motivates R for the SWNA regionally averaged recon- comparisons to medieval megadroughts and questions about the role of anthropogenic climate change. struction is 0.86 back to 1700 CE (Fig. 1B). Skill We use hydrological modeling and new 1200-year tree-ring reconstructions of summer soil moisture reduces back in time owing to loss of tree-ring to demonstrate that the 2000–2018 SWNA drought was the second driest 19-year period since chronologies but remains above 0.73, even 800 CE, exceeded only by a late-1500s megadrought. The megadrought-like trajectory of 2000–2018 when using the subset of tree-ring chronolo- soil moisture was driven by natural variability superimposed on drying due to anthropogenic gies extending back to 800 CE (Fig. 1B). warming. Anthropogenic trends in temperature, relative humidity, and precipitation estimated from We evaluated 19-year running means of 31 climate models account for 47% (model interquartiles of 35 to 105%) of the 2000–2018 drought reconstructed and observed soil-moisture severity, pushing an otherwise moderate drought onto a trajectory comparable to the worst SWNA anomalies for explicit contextualization of the megadroughts since 800 CE. dry 2000–2018 period. Running-mean values are assigned to the final year in each 19-year window. During 800–2018 CE, there were 40 Downloaded from outhwestern North America (SWNA; west- conditions have been clearly promoted by nat- prolonged drought events with more than one ern United States and northern Mexico: ural Pacific Ocean variability (18–20), certain negative SWNA 19-yearrunning-meansoil- 30°N to 45°N, 105°W to 125°W) has been elements are also consistent with projected moisture anomaly. We rank the severity of anomalously dry and warm in the 21st drying due to anthropogenic radiative forcing each prolonged drought event based on the century relative to the 20th century (1–3). (21–23). Cold-season precipitation deficits across event’s most negative 19-year soil-moisture S http://science.sciencemag.org/ The 21st-century drought severity has been re- the southwestern United States and north- anomaly. Definitions of megadrought vary, flected in reduced snowpack (4), reduced river ern Mexico are consistent with modeled pole- but in North America, they generally refer to flow and lake levels (5), declines in ground- ward expansion of the subtropics, albeit with multidecade drought events that contained water availability (6, 7), shifts in agricultural large uncertainties in models and observa- periods of very high severity and were longer activities (8), forest drought stress (9), increased tions (24, 25). Observed warming since the lasting than any event observed in the 19th wildfire activity (10), and reduced vegetation early 1900s is more uniformly consistent with or 20th centuries (12). Here we identify the carbon uptake (11). model simulations of anthropogenic trends, strongest SWNA megadroughts in the recon- Paleoclimatic proxies indicate that SWNA decreasing SWNA runoff and warm-season struction as the prolonged drought events experienced many severe swings in hydrocli- soil moisture by reducing snowpack and in- that contained at least one 19-year anomaly mate before the observed period. In particular, creasing evaporative demand (26–28). Mod- that was 0.25 standard deviations (s)moreneg- tree-ring records reveal several megadrought els project that 21st-century SWNA summer ative than any observed in the 20th century. on April 16, 2020 events during the Medieval era and subse- droughts will intensify owing to declining The regionally averaged SWNA reconstruction quent centuries (~850–1600 CE) that dwarfed spring precipitation in the southern portion of (Fig. 1C) reveals four megadroughts that sat- all droughts in the following 400 years in in- the region and continued warming-induced isfy this criterion in the late 800s, mid-1100s, tensity and duration (12). These megadroughts reductions of summer runoff and soil mois- 1200s, and late 1500s. were likely associated with cool eastern trop- ture (22–24, 29). The 21st-century prolonged drought event ical Pacific sea surface temperatures, which pro- Here, we use 1586 tree-ring chronologies (still ongoing as of 2020 given our definition) mote an atmospheric wave train that blocks to reconstruct 0- to 200-cm summer (June to registered its first negative SWNA 19-year Pacific storms from reaching SWNA (13–15). August) soil moisture and snow water equiva- anomaly in 1996–2014, and its most negative Any attribution of recent drought to anthro- lent (hereinafter termed “soil moisture” collec- anomaly (2000–2018) was −0.74 s; the late- pogenic climate change must consider this re- tively) anomalies on a 0.5° latitude-longitude grid 1500s megadrought was the only reconstructed gion’scapacityforlargeinternal hydroclimatic back to 800 CE across western North America event with a more negative 19-year soil-moisture variability (16, 17). Although 21st-century drought [(30); Fig. 1]. Soil-moisture anomalies are anomaly than that in 2000–2018 (Fig. 1C). standardized relative to the entire 800–2018 The most severe SWNA 19-year soil moisture CE period, and the magnitude of negative anomaly during the late-1500s megadrought anomalies indicates drought severity. The soil- was −0.80 s in 1575–1593. The 2000–2018 1Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA. 2NASA Goddard moisture record targeted in the reconstruction drought severity was nevertheless within the Institute of Space Studies, New York, NY 10025, USA. covers 1901–2018 and is referred to as Noah- uncertainty ranges of several other 19-year 3Department of Geography, University of Idaho, Moscow, calibrated soil moisture (31). Because true ob- drought severities, and the late-1500s event 4 ID 83844, USA. Management of Complex Systems servations of soil moisture do not exist, this contained six 19-year anomalies more nega- Department, UC Merced, Merced, CA 95343, USA. 5Department of Earth and Environmental Sciences, soil-moisture record is modeled based on ob- tive than that in 2000–2018. Within SWNA, Columbia University, New York, NY 10027, USA. served climate. Monthly precipitation, temper- local drought rankings during the 21st-century 6 Cooperative Institute for Research in Environmental ature, humidity, wind speed, and radiation event were generally not as high as the rank- Sciences, University of Colorado Boulder, Boulder, CO 80302, USA. 7Universities Space Research Association, data are used to force a bucket-type water- ing of the regionally averaged drought (Fig. Columbia, MD 21046, USA. 8NASA Goddard Space Flight balance model with intermonth persistence 1D). Only 37% of SWNA experienced a local 9 Center, Greenbelt, MD, USA 20771, USA. Civil, tuned to emulate the Community Noah land- 19-year drought severity that ranked among Environmental, and Architectural Engineering, University of Colorado Boulder, Boulder, CO 80309, USA. surface model (32) (fig. S1). The reconstruction the top five since 800 CE, a smaller aerial *Corresponding author. Email: [email protected] method is the same method that has been extent of high-ranking drought than occurred Williams et al., Science 368, 314–318 (2020) 17 April 2020 1 of 5 RESEARCH | REPORT during any of the four most severe reconstructed prolonged SWNA droughts in at least 1200 years soil-moisture anomalies and those recalculated megadroughts (Fig. 1D). Notably, the four mega- (fig. S2, B and C). after removing model-estimated anthropogenic droughts in Fig. 1 were longer than the 21st- To address the contribution of anthropo- trends from the observed climate records [e.g., century drought thus far, giving grid cells more genic climate change, we evaluated 1901–2018 (3, 10)]. The positive effect of the slight multi- chances to register high-ranking 19-year drought trends in precipitation, temperature, and rela- model mean precipitation increase in northern severities. Conversely, the 21st-century drought tive humidity simulated with 31 climate mod- SWNA (Fig. 2E) is overwhelmed by the spa- is the only event in which all SWNA grid cells els in the fifth phase of the Coupled Model tially ubiquitous drying effect of increasing registered at least one below-average 19-year Intercomparison Project (CMIP5). During vapor-pressure deficit simulated by all models soil-moisture anomaly. 2000–2018, the multimodel mean anthropo- (Fig. 2F). Notably, the high intermodel spread The above results are consistent across alter- genic warming in SWNA was 1.2°C [model in anthropogenic precipitation trends causes nate reconstructions with longer calibration interquartiles (IQs): 1.0° to 1.5°C], with all high spread among estimated soil-moisture periods that extend beyond 1983 (using fewer models simulating warming (Fig. 2A).
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