Tree-ring δ18O illuminates hydroclimatic context during the Medieval Climate Anomaly–Little Ice Age transition in central-western France

Most current knowledge of past hydroclimatic variability over the last millennium in Western Europe is derived from tree-ring records. However, only a limited number of these archives span the entire millennium, which limits our ability both to place recent climate change within a long-term perspective and to characterize past climatic periods with sufficient resolution. Consequently, climatic conditions during the Medieval Climate Anomaly (MCA; ~900–1250 CE), generally considered relatively warm, as well as the transition toward the cooler conditions of the Little Ice Age (LIA; ~1350–1850 CE), remain poorly constrained, in terms of their temporal and spatial heterogeneity across Europe. In France, a quasi-millennial tree-ring δ¹⁸O chronology for the Paris basin (δ¹⁸O_PB), spanning the periods from 1046 to 1240 CE and from 1306 to 2007 CE, has been developed. However, data remain lacking for the transitional interval between the MCA and the LIA, a period that may have been critical for past societies and for understanding the dynamics of long-term climate variability.

In this study, we use oak tree-ring cellulose δ¹⁸O, a robust proxy for hydroclimatic conditions in lowland regions. Five site-specific δ¹⁸O chronologies were developed: one based on living trees and four derived from oak beams from medieval buildings, all located in central-western France. Correlations with δ¹⁸O_PB over overlapping periods range from 0.52 to 0.72, allowing the central France chronologies to be merged with δ¹⁸O_PB to produce a continuous millennial δ¹⁸O record spanning 1046–2023 CE. The strongest relationships with instrumental climate data over 1901–2023 CE were observed for June–August SPEI (r = −0.71), maximum temperature (r = 0.65), and May–August precipitation (r = −0.57). The final reconstruction was calibrated against June–August SPEI, which showed the highest predictive skill (r² = 0.50) and the greatest temporal stability across the calibration/verification split periods.

Hydroclimatic conditions are characterized in terms of long-term trends, regime shifts, and extremes, with particular emphasis on the transition between MCA and LIA. The results provide new insights into past summer drought variability in the region, revealing that the most extreme events occurred toward the end of the MCA (e.g. 1222, 1252, 1287, and 1331 CE). In contrast, drought conditions in the last decade (2014–2023 CE) are unprecedented over the past millennium and occur within a broader, statistically significant drying trend that has developed over the past century.