Pan-alpine summer temperatures since 742 CE

Albeit labor-intensive, tree-ring maximum latewood density (MXD) has become a prime proxy to reconstruct inter-annual to multi-centennial climate variability. We here combine such data from five valleys in Austria, France and Switzerland to present a June-September temperature history for the European Alps reaching back to 742 CE. The pan-alpine record correlates at r_1880–2023 = 0.89 with observational data and provides evidence for a prolonged Little Ice Age (LIA) from the 1250s to 1850s CE, during which summer temperatures were 0.59 °C cooler compared to the preceding Medieval Warm Period (MWP) from the 880s to 1240s CE. Temperatures rose by 3.65 °C from the coldest decade in the 1810s, which includes the 1816 CE post-Tambora “year without a summer”, to the warmest decade in the 2010s. The warmest summer in our reconstruction occurred in 2003 CE (+ 2.71 °C) and exceeds the warmest naturally forced summer in 970 CE (+ 2.19 °C) by more than 0.5 °C. This difference is non-significant, however, if we consider the increasing uncertainties back in time when fewer sites and trees contributed to the reconstruction. The pan-alpine record is the result of conducting MXD measurements over the past two decades in Swiss and German laboratories and sets a new standard in terms of explained variance and pre-instrumental temperature variability estimation in the European Alps.