Research ArticleCLIMATOLOGY

Unprecedented 21st century drought risk in the American Southwest and Central Plains

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Science Advances  12 Feb 2015:
Vol. 1, no. 1, e1400082
DOI: 10.1126/sciadv.1400082
  • Fig. 1 Top: Multimodel mean summer (JJA) PDSI and standardized soil moisture (SM-30cm and SM-2m) over North America for 2050–2099 from 17 CMIP5 model projections using the RCP 8.5 emissions scenario.

    SM-30cm and SM-2m are standardized to the same mean and variance as the model PDSI over the calibration interval fromthe associated historical scenario (1931–1990). Dashed boxes represent the regions of interest: the Central Plains (105°W–92°W, 32°N–46°N) and the Southwest (125°W–105°W, 32°N–41°N). Bottom: Regional average time series of the summer season moisture balance metrics from the NADA and CMIP5models. The observational NADA PDSI series (brown) is smoothed using a 50-year loess spline to emphasize the low-frequency variability in the paleo-record. Model time series (PDSI, SM-30cm, and SM-2m) are the multimodel means averaged across the 17 CMIP5models, and the gray shaded area is the multimodel interquartile range for model PDSI.

  • Fig. 2 Interquartile range of PDSI and soil moisture from the NADA and CMIP5 GCMs, calculated over various time intervals for the Central Plains.

    The groups of three stacked bars at the top of each column are from the NADA PDSI: 1100–1300 (the time of the Medieval-era megadroughts, brown), 1501–1849 (the Little Ice Age, blue), and 1850–2005 (the historical period, green). Purple and red bars are for the modeled historical period (1850–2005) and late 21st century (2050–2099) period, respectively. Red dots indicate model 21st century drought projections that are significantly drier than the model simulated historical periods. Gray dots indicate model 21st century drought projections that are significantly drier than the Medieval-era megadrought period in the NADA.

  • Fig. 3 Same as Fig. 2, but for the Southwest.
  • Fig. 4 Kernel density functions of PDSI, SM-30cm, and SM-2m for the Central Plains and Southwest, calculated from the NADA and the GCMs. The NADA distribution (brown shading) is from 1100–1300 CE, the timing of the medieval megadroughts.

    Blue lines represent model distributions calculated from all years from all models pooled over the historical scenario (1850–2005 CE). Red lines are for all model years pooled from the RCP 8.5 scenario (2050–2099 CE).

  • Fig. 5 Risk (percent chance of occurrence) of decadal (11-year) andmultidecadal (35-year) drought, calculated from the multimodel ensemble for PDSI, SM-30cm, and SM-2m.

    Risk calculations are conducted for two separate model intervals: 1950–2000 (historical scenario) and 2050–2099 (RCP 8.5). Results for the Central Plains are in the top row, and those for the Southwest are in the bottom row.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/ content/full/1/1/e1400082/DC1

    Fig. S1. For the individual models, ensemble mean soil moisture balance (PDSI, SM-30cm, and SM-2m) for 2050–2099: ACCESS1.0, ACCESS1.3, BCC-CSM1.1, and CanESM2.

    Fig. S2. Same as fig. S1, but for CCSM4, CESM1-BGC, CESM-CAM5, and CNRM-CM5.

    Fig. S3. Same as fig. S1, but for GFDL-CM3, GFDL-ESM2G, GFDL-ESM2M, and GISS-E2-R.

    Fig. S4. Same as fig. S1, but for INMCM4.0,MIROC-ESM, MIROC-ESM-CHEM, NorESM1-M, and NorESM1-ME models.

    Fig. S5. Same as Fig. 1, but for the RCP 4.5 scenario.

    Fig. S6. Regional average moisture balance time series (historical + RCP 8.5) from the first ensemble member of each model over the Central Plains.

    Fig. S7. Same as fig. S6, but for the Southwest.

    Fig. S8. Pearson’s correlation coefficients for three time intervals from the models over the Central Plains: PDSI versus SM-30cm, PDSI versus SM-2m, and SM-30cm versus SM-2m.

    Fig. S9. Same as fig. S8, but for the Southwest.

    Fig. S10. Same as Fig. 2, but for the RCP 4.5 scenario.

    Fig. S11. Same as Fig. 3, but for the RCP 4.5 scenario.

    Fig. S12. Same as Fig. 4, but for the RCP 4.5 scenario.

    Fig. S13. Same as Fig. 5, but for the RCP 4.5 scenario.

    Table S1. Continuous model ensembles from the CMIP5 experiments (1850–2099, historical + RCP8.5 scenario) used in this analysis, including the modeling center or group that supplied the output, the number of ensemble members, and the approximate spatial resolution.

    Table S2. The number of soil layers integrated for our CMIP5 soil moisture metrics (SM-30cm and SM-2m), and the approximate depth of the bottom soil layer.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. For the individual models, ensemble mean soil moisture balance (PDSI, SM-30cm, and SM-2m) for 2050–2099: ACCESS1.0, ACCESS1.3, BCC-CSM1.1, and CanESM2.
    • Fig. S2. Same as fig. S1, but for CCSM4, CESM1-BGC, CESM-CAM5, and CNRM-CM5.
    • Fig. S3. Same as fig. S1, but for GFDL-CM3, GFDL-ESM2G, GFDL-ESM2M, and GISS-E2-R.
    • Fig. S4. Same as fig. S1, but for INMCM4.0,MIROC-ESM, MIROC-ESM-CHEM, NorESM1-M, and NorESM1-ME models.
    • Fig. S5. Same as Fig. 1, but for the RCP 4.5 scenario.
    • Fig. S6. Regional average moisture balance time series (historical + RCP 8.5) from the first ensemble member of each model over the Central Plains.
    • Fig. S7. Same as fig. S6, but for the Southwest.
    • Fig. S8. Pearson’s correlation coefficients for three time intervals from the models over the Central Plains: PDSI versus SM-30cm, PDSI versus SM-2m, and SM-30cm versus SM-2m.
    • Fig. S9. Same as fig. S8, but for the Southwest.
    • Fig. S10. Same as Fig. 2, but for the RCP 4.5 scenario.
    • Fig. S11. Same as Fig. 3, but for the RCP 4.5 scenario.
    • Fig. S12. Same as Fig.4, but for the RCP 4.5 scenario.
    • Fig. S13. Same as Fig. 5, but for the RCP 4.5 scenario.
    • Table S1. Continuous model ensembles from the CMIP5 experiments (1850–2099, historical + RCP8.5 scenario) used in this analysis, including the modeling center or group that supplied the output, the number of ensemble members, and the approximate spatial resolution.
    • Table S2. The number of soil layers integrated for our CMIP5 soil moisture metrics (SM-30cm and SM-2m), and the approximate depth of the bottom soil layer.

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