Increasing drought under global warming in observations and models

Historical records of precipitation, streamflow and drought indices all show increased aridity since 1950 over many land areas1,2. Analyses of model-simulated soil moisture3,4, drought indices1,5,6 and precipitation-minus-evaporation7 suggest increased risk of drought in the twenty-first century. There are, however, large differences in the observed and modelsimulated drying patterns1,2,6. Reconciling these differences is necessary before the model predictions can be trusted. Previous studies8–12 show that changes in sea surface temperatures have large influences on land precipitation and the inability of the coupled models to reproduce many observed regional precipitation changes is linked to the lack of the observed, largely natural change patterns in sea surface temperatures in coupled model simulations13. Here I show that the models reproduce not only the influence of El Niño-Southern Oscillation on drought over land, but also the observed global mean aridity trend from 1923 to 2010. Regional differences in observed and modelsimulated aridity changes result mainly from natural variations in tropical sea surface temperatures that are often not captured by the coupled models. The unforced natural variations vary among model runs owing to different initial conditions and thus are irreproducible. I conclude that the observed global aridity changes up to 2010 are consistent with model predictions, which suggest severe and widespread droughts in the next 30– 90 years over many land areas resulting from either decreased precipitation and/or increased evaporation. Although the historical and future aridity changes have been discussed in previous studies1–7, there still is a need to validate the historical changes and reconcile them with model projections. Here I focus on synthesizing the observed aridity changes and comparing and reconciling them with model-simulated changes, thereby improving our understanding of global-warminginduced drought changes. Different drought indices can result in somewhat different change patterns, especially on small scales14. Here I focus on the large-scale drying trends in precipitation, streamflow and soil moisture fields, which are commonly used to quantify, respectively, meteorological, hydrologic and agricultural drought1. Because historical records of soil moisture are sparse, I also used the self-calibrated Palmer drought severity index (PDSI) with potential evapotranspiration estimated using the Penman– Monteith equation (sc_PDSI_pm; ref. 2). The PDSI is calculated from a water-balance model forced with observed precipitation and temperature and has been widely used in monitoring drought development over the USA, palaeoclimate reconstruction15 and studying aridity changes2,5,6,16. The revised sc_PDSI_pm has improved spatial comparability and uses a more realistic estimate