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Palaeoclimatic evidence is necessary to place the current warming and drying trends of the Mediterranean region in a long‐term perspective of pre‐industrial variability. Annually resolved and absolutely dated climate proxies that extend back into medieval times are, however, limited to a few sites only. Here we present a network of long ring width chronologies from Pinus nigra tree‐line sites in northern Corsica (France) that cohere exceptionally well over centuries and support the development of a single high‐elevation pine chronology extending back to 974 CE. We apply various detrending methods to these data to retain high‐to‐low frequency ring width variability and scale the resulting chronologies against instrumental precipitation and drought observations to produce hydroclimate reconstructions for the last millennium. Proxy calibration and transfer are challenged by a lack of high‐elevation meteorological data, however, limiting our understanding of precipitation changes in sub‐alpine tree‐line environments. Our new reconstructions extend beyond existing records and provide evidence for low‐frequency precipitation variability in the central‐western Mediterranean from 974–2016 CE. Comparison with a European scale drought reconstruction network shows that regional predictor chronologies are needed to accurately estimate long‐term hydroclimate variability on Corsica.

Investigations of the impacts of past volcanic eruptions on climate, environment, and society require accurate chronologies. However, eruptions that are not recorded in historical documents can seldom be dated exactly. Here we use annually resolved radiocarbon ($^{14}$C) measurements to isolate the 775 CE cosmogenic $^{14}$C peak in a subfossil birch tree that was buried by a glacial outburst flood in southern Iceland. We employ this absolute time marker to date a subglacial eruption of Katla volcano at late 822 CE to early 823 CE. We argue for correlation between the 822–823 CE eruption and a conspicuous sulfur anomaly evident in Greenland ice cores, which follows in the wake of an even larger volcanic signal (ca. 818–820 CE) as yet not attributed to a known eruption. An abrupt summer cooling in 824 CE, evident in tree-ring reconstructions for Fennoscandia and the Northern Hemisphere, suggests a climatic response to the Katla eruption. Written historical sources from Europe and China corroborate our proposed tree ring–radiocarbon–ice core linkage but also point to combined effects of eruptions occurring during this period. Our study describes the oldest precisely dated, high-latitude eruption and reveals the impact of an extended phase of volcanic forcing in the early 9$^{th}$ century. It also provides insight into the existence of prehistoric woodland cover and the nature of volcanism several decades before Iceland's permanent settlement began.