Little change in global drought over the past 60 years

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Drought is expected to increase in frequency and severity in the future as a result of climate change, mainly as a consequence of decreases in regional precipitation but also because of increasing evaporation driven by global warming1, 2, 3. Previous assessments of historic changes in drought over the late twentieth and early twenty-first centuries indicate that this may already be happening globally. In particular, calculations of the Palmer Drought Severity Index (PDSI) show a decrease in moisture globally since the 1970s with a commensurate increase in the area in drought that is attributed, in part, to global warming4, 5. The simplicity of the PDSI, which is calculated from a simple water-balance model forced by monthly precipitation and temperature data, makes it an attractive tool in large-scale drought assessments, but may give biased results in the context of climate change6. Here we show that the previously reported increase in global drought is overestimated because the PDSI uses a simplified model of potential evaporation7 that responds only to changes in temperature and thus responds incorrectly to global warming in recent decades. More realistic calculations, based on the underlying physical principles8 that take into account changes in available energy, humidity and wind speed, suggest that there has been little change in drought over the past 60 years. The results have implications for how we interpret the impact of global warming on the hydrological cycle and its extremes, and may help to explain why palaeoclimate drought reconstructions based on tree-ring data diverge from the PDSI-based drought record in recent years9, 10.

At a glance


  1. Global average time series of the PDSI and area in drought.
    Figure 1: Global average time series of the PDSI and area in drought.

    a, PDSI_Th (blue line) and PDSI_PM (red line). b, Area in drought (PDSI <−3.0) for the PDSI_Th (blue line) and PDSI_PM (red line). The shading represents the range derived from uncertainties in precipitation (PDSI_Th and PDSI_PM) and net radiation (PDSI_PM only). Uncertainty in precipitation is estimated by forcing the PDSI_Th and PDSI_PM by four alternative global precipitation data sets. Uncertainty from net radiation is estimated by forcing the PDSI_PM with a hybrid empirical–satellite data set31 and an empirical estimate. The other near-surface meteorological data are from a hybrid reanalysis–observational data set31. The thick lines are the mean values of the different PDSI data sets. The time series are averaged over global land areas excluding Greenland, Antarctica and desert regions with a mean annual precipitation of less than 0.5mmd−1.

  2. Trends in the PDSI and PE.
    Figure 2: Trends in the PDSI and PE.

    a, c, e, Non-parametric trends for 1950–2008 in annual average PDSI (averaged over the results using the four precipitation data sets and, for the PDSI_PM, also over the two net radiation data sets) from the PDSI_Th (a) and the PDSI_PM (c), and their difference (e). b, d, f, Non-parametric trends for 1950–2008 in annual average PE from the Thornthwaite equation (b) and the PM equations (d), and their difference (f). Values are not shown for Greenland, Antarctica and desert regions with a mean annual precipitation of less than 0.5mmd−1. Statistically significant trends at the 95% level are indicated by hatching. The difference in trends in e and f and its statistical significance are calculated from the time series of differences between the two data sets.


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Author information


  1. Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey 08544, USA

    • Justin Sheffield &
    • Eric F. Wood
  2. Australian Research Council Centre of Excellence for Climate System Science, Research School of Earth Science & Research School of Biology, The Australian National University, Canberra, ACT 0200, Australia

    • Michael L. Roderick


J.S. and E.F.W. conceived the study with inspiration from M.L.R. J.S. performed the analyses and mainly wrote the manuscript. E.F.W. and M.L.R. contributed to discussion and the manuscript.

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