Close-up of tree ring samples taken in the Pyrenees, showing the characteristic “blue rings”.
Credit: Ulf Büntgen
Tree ring data allowed Büntgen et al. determine that there was a volcanic eruption (or group of eruptions) around 1345, specifically the so-called “blue rings,” which indicate unusually cold or wet summers—in this case, three years in a row (1345, 1346, and 1347). Textual sources also mention details such as unusually high cloud levels and darkened lunar eclipses, which are signs of the effects of volcanic activity.
This colder climate in turn led to widespread crop failure and associated famine, especially in parts of Spain, southern France, Egypt, and northern and central Italy. While Milan and Rome were largely self-sufficient, according to the authors, smaller urban centers such as Bologna, Florence, Genoa, Siena and Venice relied on a complex grain supply system to import grain from the Mongols of the Golden Horde along established trade routes along the Black Sea coast. This is supported by textual evidence showing a significant increase in grain prices and the introduction of grain trading regulations in 1346. This saved people from starvation, but it also attracted Y. pestis along for the ride, with devastating consequences.
According to the authors, although the factors that triggered the spread of the Black Death in Europe are unique, the study illustrates the risks of a globalized world and calls for a similar multidisciplinary approach to future threats. “Although the coincidence of factors that contributed to the Black Death appears to be rare, the likelihood of zoonotic diseases emerging under climate change and developing into pandemics is likely to increase in a globalized world,” Büntgen said. “This is especially relevant given our recent experience with COVID-19.”
Communications Earth and Environment, 2025 DOI: 10.1038/s43247-025-02964-0 (About DOI).
