Research ArticleCLIMATOLOGY

Unprecedented climate events: Historical changes, aspirational targets, and national commitments

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Science Advances  14 Feb 2018:
Vol. 4, no. 2, eaao3354
DOI: 10.1126/sciadv.aao3354
  • Fig. 1 The global change in probability of exceeding the historically unprecedented event at three levels of forcing.

    Global-scale cumulative distribution functions (CDFs) are calculated from all bootstrapped return interval ratios at all observationally available grid points for each level of anthropogenic forcing (see Materials and Methods). The horizontal axis is the change in probability calculated as the return interval ratio between the natural and anthropogenic forcing. For example, a ratio of 5 means that, in the anthropogenic forcing, the probability of exceeding the most extreme historically observed value is five times the probability in the world without human influence. The vertical axis is the cumulative fraction of all ratios calculated at all available grid points that are less than or equal to a given ratio. Insets show 1 minus the value on the vertical axis, which gives the fraction of ratios that are greater than a given ratio. For example, if a given CDF curve intersects 5 on the horizontal axis and 0.75 on the vertical axis, then 75% of all calculated return interval ratios are less than or equal to 5, and the inset will show that 25% of all calculated ratios are greater than 5. The dark gray vertical line in each panel shows where the return interval ratio between the natural and anthropogenic forcing is equal to 1, meaning that the probability of exceeding the most extreme historically observed value is equivalent in the natural and anthropogenic forcing. The three levels of anthropogenic forcing are the 1986–2005 period of the Historical simulations (~1500 GT CO2 emitted and ~1°C of global warming above the pre-industrial), the 2016–2035 period of the RCP8.5 simulations (~2500 GT CO2 and ~1° to 2°C), and the 2036–2055 period of the RCP8.5 simulations (~3500 GT CO2 and ~2° to 3°C).

  • Fig. 2 The change in probability of exceeding the historically unprecedented hot event at three levels of forcing.

    Maps show the median value of the bootstrapped return interval ratios between the lower and higher forcing. (Full distributions for all grid points are shown in Fig. 1). For ratios reported as “relative to Natural,” the lower forcing is that for a world without human influence; for ratios reported as “relative to Historical,” the lower forcing is the combined human and natural forcing that occurred during the historical period (see Materials and Methods). (A to C) Median return interval ratio for the hottest maximum daily temperature of the year (maximum TXx value; “hottest day”). (D to F) Median return interval ratio for the warmest minimum daily temperature of the year (maximum TNx value; “warmest night”). As described in Materials and Methods, the analysis is limited to the areas with observed values in the CLIMDEX data set (missing areas shown in white; fig. S1). See fig. S2 for regional boundaries used in the regional summary calculations.

  • Fig. 3 The change in probability of exceeding the historically unprecedented mild cold event at three levels of forcing.

    As in Fig. 2, but for coldest minimum daily temperature of the year (maximum TNn value; “mildest cold night”) and number of days with maximum temperature below 0°C (minimum ID0 value; “mildest freeze length”).

  • Fig. 4 The change in probability of exceeding the historically unprecedented wet event at three levels of forcing.

    As in Fig. 2, but for annual precipitation from days that exceed the 95th percentile (maximum R95p value; “wettest wet days”) and wettest day of the year (maximum Rx1day value; “wettest day”).

  • Fig. 5 The change in probability of exceeding the historically unprecedented dry event at three levels of forcing.

    As in Fig. 2, but for total annual precipitation (minimum PRCPTOT value; “driest year”) and longest consecutive dry spell of the year (maximum CDD value; “longest dry spell”).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/2/eaao3354/DC1

    fig. S1. Statistical comparison of observed and simulated climate indices during the historical period.

    fig. S2. Regions used in regional summary calculations.

    fig. S3. Frequency of occurrence of the maximum “HistoricalNat” hot event value in the CMIP5 RCP8.5 and RCP2.6 simulations.

    fig. S4. As in fig. S3, but for mild cold events.

    fig. S5. As in fig. S3, but for wet events.

    fig. S6. As in fig. S3, but for dry events.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Statistical comparison of observed and simulated climate indices during the historical period.
    • fig. S2. Regions used in regional summary calculations.
    • fig. S3. Frequency of occurrence of the maximum “HistoricalNat” hot event value in the CMIP5 RCP8.5 and RCP2.6 simulations.
    • fig. S4. As in fig. S3, but for mild cold events.
    • fig. S5. As in fig. S3, but for wet events.
    • fig. S6. As in fig. S3, but for dry events.

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