Are modern droughts unprecedented?

Drought variability over the last millennium remains poorly understood in a global context--reflecting the sparsity of instrumental records and reconstructions from paleoclimatic proxies. Yet, a quantification of the long-term forced and unforced variability of droughts across regions globally is key for assessing the extremeness of currently observed drought events. Here, we use 20 ensemble members from a 600-year ensemble simulation (ModE-Sim, 1420 - 2009) and modern reanalysis data (ERA5-Land, 1950-2024) to place recent drought extremes in a multi-century historical context. For measuring drought magnitude and timing, we consider the annual maximum potential cumulative water deficit (PCWD)--a physically and ecologically-grounded metric, integrating atmospheric moisture demand and supply and the relative timing of daily precipitation, snow melt, and radiation. To investigate changes over the simulated period, we assess trends, low-frequency variability, and effects of external forcing. Our results show that internal hydroclimate variability differs across regions and can be of the same magnitude as forced events. Towards the modern era (2000-2024), PCWD exceeds the reference (1420-1969) with unprecedentedly high values in 13 out of 43 regions. Despite modern droughts not being unprecedented in most regions when considering a 600 year reference and unforced variability across 20 ensemble members, our results show that the statistics of droughts are widely shifting. Moderate (5-year) drought extreme events have at least doubled in frequency in modern times in 20 out of 43 land regions. The severity of modern severe (30-year) drought extreme events is unprecedented in 36 out of 43 regions with respect to the last 600 years. Depending on the reference period chosen to characterize modern extremes, the affected region and total number of regions with unprecedented annual record PCWD differs strongly. This study highlights a fundamental transformation in drought regimes, which is unprecedented over the past 600 years for a large fraction of global land regions, even as unprecedentedly strong droughts have been recorded to date in only about a quarter of regions globally. This highlights the importance of reference period and metric selection in characterizing modern extremes. Taken together, our findings demonstrate that modern droughts are not only more intense, but also more frequent than those of the preindustrial past in most regions globally. However, in many regions, drought extremes in the reanalysis period do not yet exceed the most severe events simulated across ensemble members over the past 600 years.