Quantifying Extreme Drought Risk using Palaeoclimate and Model Observations from the Last Millennium

Droughts are associated with widespread adverse impacts across social, economic and ecological systems (Wilhite, 1992; Wilhite et al., 2007). The frequency, severity and persistence of drought events have increased across much of the Earth’s surface over the 20th and 21st centuries, and increasingly droughts coincide with periods of elevated temperature generating compound hot-dry conditions (Dai, 2011; Mazdiyasni and AghaKouchak, 2015; Chiang et al., 2021; King et al., 2024; Gebrechorkos et al., 2025).  

However, drought is a complex natural hazard that can result from both natural climate variability and anthropogenic influence (Wilhite and Glantz, 1985; Wilhite, 2000). With less than 100 years of instrumental data across much of the Earth’s surface, and the most severe droughts developing over an extended period, extreme droughts are few (Brunner et al., 2021; Seneviratne et al., 2021; Williams et al., 2022). It is therefore challenging to characterise worst-case outcomes, which is crucial for informing adaptation efforts. 

Here we use the Palmer Drought Severity Index (PDSI) calculated from CMIP6 last millennium simulations and independent PDSI reconstructions from tree-ring width chronologies to characterise drought conditions over the last millennium and place recent extreme drought conditions within a longer-term context. The two datasets are used together to establish how the relative severity of extreme drought conditions varies between regions, the frequency with which these events occur, and the extent to which such events are generated by compound hot-dry conditions. Extreme value statistics are then applied to the three PDSI datasets to estimate the severity of 1-in-10, -50, and -100-year summer droughts and assess how these values vary between the datasets. The extremes of the last millennium are then compared to those of the instrumental period. Results highlight regions that are yet to experience droughts during the instrumental period that exceed those of the last millennium; these areas may be particularly vulnerable to future extremes, particularly where these are enhanced by anthropogenic warming.