Large contribution from anthropogenic warming to an emerging North American megadrought

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Science  17 Apr 2020:
Vol. 368, Issue 6488, pp. 314-318
DOI: 10.1126/science.aaz9600
  • Fig. 1 Summer soil-moisture reconstruction for SWNA.

    (A) Cross-validated reconstruction skill (R2) using tree-ring records that extend to 800 and 1700 CE (green contour: R2 ≥ 0.5; gray: reconstruction does not extend to 800 CE; yellow box: SWNA). (B) Time-resolved cross-validated R2 of the SWNA regional reconstruction. The inset shows observations versus cross-validated reconstructions during the 1901–1983 calibration interval using tree-ring records extending back to 800 and 1700 CE. (C) Time series of reconstructed (red) and observed (blue) 19-year running-mean standardized SWNA soil moisture (gray: 95% reconstruction confidence interval; blue horizontal line: 2000–2018 mean; pink and green shading: the five drought and pluvial periods with the most-negative and most-positive 19-year soil-moisture anomalies, respectively). (D) Maps of the local rank of the most negative 19-year anomaly to occur during each of the five drought events highlighted in (C). In the maps, the aqua color indicates no negative 19-year anomaly, and numbers in the top left corners indicate the rank of the most negative regionally averaged 19-year anomaly during each drought event.

  • Fig. 2 Effects of anthropogenic climate trends.

    (A to G) Time series plots showing the 19-year running-mean observed anomalies (black lines) in SWNA (yellow box in maps) for mean annual temperature (T) (A), mean annual vapor pressure deficit (VPD) (B), annual reference evapotranspiration (ETo) (C), annual precipitation (P) (D), and soil moisture [(E) to (G)]. Solid and dotted colored lines represent 19-year running-mean CMIP5 multimodel mean and IQ trends, respectively (gray lines: trends from 31 models). CMIP5 trends are evaluated for P, T, and relative humidity (RH). In (E) to (G), CMIP5 trends show contributions to observed soil-moisture anomalies since 1901. The maps show CMIP5 multimodel mean contributions to 2000–2018 anomalies (dots: >75% model agreement on sign; gray: masked out because reconstruction does not extend to 800 CE). (H) Percent contribution of CMIP5 (bars) multimodel mean climate trends to the 2000–2018 SWNA soil-moisture anomaly (whiskers: model IQs). Anomalies are relative to 1921–2000 in (A) to (D) and 800–2018 CE in (E) to (G).

  • Fig. 3 Development of the most severe 19-year droughts since 800 CE.

    Time series of cumulative SWNA summer soil-moisture anomalies for the 20 prolonged droughts with the most-negative 19-year soil-moisture anomalies. The drought periods analyzed here begin 18 years before the most-negative 19-year anomaly. The dark blue line shows 2000–2018 cumulative anomalies after removing CMIP5 multimodel mean climate trends. The shaded regions represent 95% confidence intervals for the four reconstructed megadroughts shown with the light colored lines.

  • Fig. 4 Trends in summer soil moisture simulated directly from coupled models.

    (Left) CMIP5 19-year running-mean 0- to 200-cm summer soil-moisture anomalies for historical (1850–2005) and 21st-century (2006–2018) scenarios (standardization relative to 1850–2018; gray represents 26 models; brown represents multimodel mean, IQs). (Right) Multimodel mean anomalies in 2000–2018 (dots represent >75% model agreement on sign; yellow box indicates SWNA).

Supplementary Materials

  • Large contribution from anthropogenic warming to an emerging North American megadrought

    A. Park Williams, Edward R. Cook, Jason E. Smerdon, Benjamin I. Cook, John T. Abatzoglou, Kasey Bolles, Seung H. Baek, Andrew M. Badger, Ben Livneh

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Supplementary Text 
    • Figs. S1 to S21
    • Tables S1 and S2 
    • References 
    Correction (29 October 2020): Methods sections 3.1 and 3.3 have been updated to reflect a revision to the number of ring-width index chronologies used in this study. All supplemental figures that draw on tree-ring data have also been updated (Figs. S2 to S4, S12, and S15 to S21).
    The original version is accessible here.

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