Research ArticleCLIMATOLOGY

Relative impacts of mitigation, temperature, and precipitation on 21st-century megadrought risk in the American Southwest

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Science Advances  05 Oct 2016:
Vol. 2, no. 10, e1600873
DOI: 10.1126/sciadv.1600873
  • Fig. 1 Megadrought risk estimates for the American Southwest shown with model-projected changes in mean hydroclimate.

    (A to C) Megadrought risk estimates for the American Southwest (shading) shown with model-projected changes in mean hydroclimate under the RCP 8.5 (high emissions) scenario for (A) annual precipitation and JJA soil moisture (PDSI, 30-cm soil moisture, and 2-m soil moisture) in the CESM LENS, (B) annual precipitation from all CMIP5 models, and (C) JJA soil moisture indicators derived from a 17-model subset of CMIP5 for which all variables needed to compute these quantities were available (7). In all panels, the interquartile range of the ensemble is shown (the full range is shown in the Supplementary Materials). Model-based variables are normalized to unit variance over a historical reference period (1951–2000) and compared with midcentury changes (2051–2080). The shading shows the 2D PDF of megadrought risk for combinations of changes in the mean (Δμ) and variability (δσ) of a normalized drought indicator time series [z′(t)].

  • Fig. 2 Megadrought risk expressed as a function of both mean precipitation and temperature for the American Southwest compared with projected changes in temperature and precipitation.

    (A) Megadrought risk expressed as a function of both mean precipitation and temperature for the American Southwest (shading) compared with projected changes in temperature and precipitation (symbols) for two scenarios: RCP 2.6 (low emissions, blue triangles) and RCP 8.5 (high emissions, red circles). CMIP5 estimates of change are expressed as the difference between the historical reference period (1951–2000) and the midcentury average (2051–2080). The megadrought risk surface (shading) is the average of all 2D PDFs calculated at each grid point in the Southwest for each combination of temperature and precipitation change. JJA PDSI is used as the reference normalized drought indicator time series [z′(t)]. The vertical dashed line marks no change in precipitation. (B) Marginal distribution of precipitation change in CMIP5 models, binned at 5% intervals from −30 to +30% of historical climatology. (C) Marginal distribution of temperature changes, binned at 0.5°C intervals from zero to six.

  • Fig. 3 Megadrought risk estimates for fixed mean precipitation changes, shown as a function of mean annual temperature and compared with CMIP5 projections of mean warming from 2051 to 2100 compared to 1951 to 2000.

    Contours show risks for constant levels of mean precipitation change (ΔP), derived from the 2D PDF in Fig. 2. The dashed lines denote the median warming (again comparing 2051–2100 to 1951–2000) from RCP 2.6 (1.9°C) and RCP 8.5 (4.5°C) and their corresponding risks assuming no change in precipitation (ΔP = 0%).

  • Fig. 4 Maps of megadrought risk for the American Southwest under different levels of warming, and the required increase in precipitation to compensate for that warming.

    (A to C) Maps of megadrought risk for the entire American Southwest domain at constant (historical) precipitation climatology (ΔP = 0%) and various levels of warming. These estimates are based on the Monte Carlo procedure of observational and reanalysis data, not on CMIP5 (see Materials and Methods). (D to F) Increases in precipitation (blue shading) needed to maintain megadrought risks below 50% for different levels of regional warming. Contours map the projected changes in precipitation derived from the multimodel CMIP5 mean and are shown for reference at each level of temperature change.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/10/e1600873/DC1

    Analytical PDF of megadrought

    fig. S1. Joint (2D) PDF of Southwest megadrought risk for a normalized drought indicator time series [z′(t)] with various changes in the mean (Δμ) and changes in the variance (δσ).

    fig. S2. Full range of changes in mean (Δμ) and variability (δσ) simulated by a CMIP5 model subset.

    fig. S3. Megadrought PDF for various combinations of seasonal changes.

    fig. S4. Reduction of variance in smoothed time series (Xw) as a function of smoothing window length (w).

    fig. S5. Two-dimensional PDF of prolonged drought risk computed from the analytical expression for megadrought probability.

    fig. S6. Megadrought 2D PDF for changes in mean and variance but for different autocorrelation characteristics of the underlying data.

  • Supplementary Materials

    This PDF file includes:

    • Analytical PDF of megadrought
    • fig. S1. Joint (2D) PDF of Southwest megadrought risk for a normalized drought indicator time series z′(t) with various changes in the mean (Δμ) and changes in the variance (δσ).
    • fig. S2. Full range of changes in mean (Δμ) and variability (δσ) simulated by a CMIP5 model subset.
    • fig. S3. Megadrought PDF for various combinations of seasonal changes.
    • fig. S4. Reduction of variance in smoothed time series (Xw) as a function of smoothing window length (w).
    • fig. S5. Two-dimensional PDF of prolonged drought risk computed from the analytical expression for megadrought probability.
    • fig. S6. Megadrought 2D PDF for changes in mean and variance but for different autocorrelation characteristics of the underlying data.

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