Drought is a devastating natural disaster causing significant socioeconomic losses. Europe experienced a series of severe droughts in the 21st century (2003, 2010, 2013, 2015, 2018), often associated with hot summers. These events, particularly the 2003 and 2015 droughts, highlighted Europe's vulnerability. The 2018 drought in central Europe, characterized by exceptionally warm temperatures and rainfall deficits, was sustained by a long-lived blocking event. While anthropogenic climate change has been implicated in the severity of these events, the limited instrumental and satellite records hinder assessment of their unprecedented nature. Multicentennial temperature reconstructions show recent summer warming is unprecedented in the last 2500 years. The study aims to provide a millennial perspective on these events by using long-term hydrological and meteorological data alongside paleoclimate reconstructions of the Palmer Drought Severity Index (PDSI) from the Old World Drought Atlas (OWDA) for central Europe. A 1000-year tree ring reconstruction of summer drought is used to assess whether recent drying was unprecedented in duration and severity. Central Europe is the focus due to its significant impact in 2015 and 2018 and the availability of rich climate reconstruction data. Sea surface temperature (SST), salinity, and large-scale atmospheric circulation reconstructions and paleo reanalysis are also utilized to explore the underlying mechanisms of past dry periods.

Existing literature highlights the severe impacts of 21st-century droughts in Europe, often attributing their increased severity to anthropogenic climate change. Studies have used various datasets and methodologies to analyze these events, such as the European Drought Impacts Database, and have shown that these events significantly impact the hydrological cycle and cause socioeconomic losses. However, limited data availability makes it challenging to determine the uniqueness of these events within a longer-term context. Previous research has employed multicentennial reconstructions of past temperatures, revealing that recent summer warming is unprecedented in the past 2500 years. There's a need for longer-term perspectives to understand the role of natural variability in shaping these events and to contextualize the impact of human-induced climate change.

This study constructs a millennial view of drought in central Europe (3°E-20°E, 45°N-56°N) using multiple datasets. The primary dataset is the Palmer Drought Severity Index (PDSI) from the Old World Drought Atlas (OWDA), a 1000-year reconstruction based on tree ring data. This reconstruction's reliability was validated by comparing it to observed self-calibrated PDSI (scPDSI) from the Climate Research Unit's TS 4.03 data product (1901-2012). The correlation between the OWDA reconstruction and the instrumental data (r=0.71, p<0.001) indicates a strong agreement over the overlapping period. Uncertainty in the scaled reconstruction was estimated using the root-mean-squared error from residual fitting. Streamflow data from the German Hydrological Institute was used to further validate the reconstructed PDSI against observed low flow periods in major rivers (Rhine, Elbe, Weser). The analysis included investigation of two identified megadroughts (Spörer and Dalton Minimum) through comparison with proxy data, historical documents from Glaser (2013) providing decadal precipitation and temperature indices for the past millennium (covering winter, spring, summer and autumn), and reconstructions from additional sources. Sea surface temperatures (SST) from the HadISST dataset and atmospheric circulation data from the 20th Century Reanalysis data were used to examine large-scale climate drivers. Two last millennium reanalyses, PHYDA and Last Millennium Reanalysis (LMR), provided additional perspectives on drought variability and its relationship to oceanic conditions. Composite analysis was used to determine relationships between PDSI (or TSI) and atmospheric circulation patterns by creating composite maps when values were below 0.75 standard deviations. This study does not generate new datasets; all data are publicly available.

The reconstructed PDSI from the OWDA explains 52% of the observed scPDSI variability (1901-2012). The driest years over the last millennium were 1102, 1419, 1503, 1504, 1858, 1865, and 1921. Two prominent megadroughts were identified: one during the Spörer Minimum (1400-1480 AD) and another during the Dalton Minimum (1770-1840 AD). The 1400-1480 megadrought coincided with reduced solar activity (lowest TSI), several large volcanic eruptions, and extremely cold winters and springs in Western Europe, resulting in significant agricultural losses. Similarly, the 1770-1840 megadrought overlapped with the Dalton Minimum, characterized by low solar activity and multiple volcanic eruptions, along with below-average global temperatures. Both megadroughts showed drier conditions across winter, spring, summer, and autumn seasons over multiple consecutive decades. Statistical distribution analysis (Gaussian fitting) of the reconstructed PDSI reveals significant differences between megadroughts and 20th/21st-century drought events, indicating megadroughts were associated with anomalous climate regimes. The 2003, 2015, and 2018 droughts are not among the most severe in the last millennium. Numerical integrations suggest a causal link between solar irradiance levels and AMOC strength: high/low irradiance leads to weak/strong AMOC states decades later. The TSI minima before 1400, 1600, and 1800 were followed by weaker AMOC states, almost synchronous with TSI minima. Model simulations and proxy/paleo reanalysis data indicated that weak AMOC states resulted in high-pressure systems over the British Isles and western Europe, suppressing precipitation and enhancing drought conditions. This atmospheric pattern persisted seasonally. The 1971-1976 period, with extended dry spells in central Europe, also showed cold North Atlantic conditions and enhanced atmospheric blocking, mirroring the megadrought mechanisms. However, the absence of severe droughts during other solar minima (e.g., Maunder Minimum) suggests an interplay of internal and forced climate variability, potentially involving North Atlantic SST conditions and volcanic activity. While recent droughts are within historical variability, future climate projections suggest substantial drying even in less aggressive scenarios. The combined effect of natural and anthropogenic factors on future droughts warrants further investigation.

This study addresses the research question by providing a long-term perspective on drought events in central Europe. The key finding that recent droughts are within the range of natural variability over the last millennium challenges the simplistic view that all recent extreme weather events are solely attributable to anthropogenic climate change. The identification of two megadroughts linked to solar minima and cold North Atlantic conditions reveals important insights into natural climate variability's impact on drought frequency and severity. This understanding is vital for developing effective drought mitigation strategies, considering both anthropogenic and natural factors. The findings have significant implications for the field of climate science, highlighting the importance of considering both natural and anthropogenic drivers when predicting future drought risks. The study strengthens the understanding of the complex interplay between solar activity, ocean circulation, and atmospheric patterns in influencing European droughts. This detailed analysis underscores the necessity of incorporating long-term perspectives when assessing the impacts of climate change and formulating appropriate adaptation policies.

This research comprehensively analyzes past megadroughts in central Europe using diverse data sources, demonstrating that recent droughts are within historical variability. The identified megadroughts during the Spörer and Dalton Minima were linked to reduced solar activity, cold North Atlantic conditions, and atmospheric blocking. Future drought risk will depend on the combined influence of natural variability and anthropogenic warming. A potential decrease in TSI could increase drought frequency, adding to the drying effect of global warming. Future research should focus on understanding the complex interaction between these factors to improve drought prediction and mitigation strategies.

The study relies on proxy data for the pre-instrumental period, introducing uncertainties inherent in such reconstructions. The spatial resolution of some datasets may limit the ability to capture finer-scale variations in drought intensity. The focus on central Europe limits the generalizability of the findings to other regions. While the study links megadroughts to specific historical periods, the exact mechanisms and relative contribution of different factors (solar activity, volcanic eruptions, AMOC) need further investigation.