It was one of the coldest winters the region has experienced: in the year 763, large areas of the Black Sea froze, and icebergs were seen in the Bosphorus. Contemporary historians recorded this unusual weather phenomenon during the winter of 763/764 in their accounts of Constantinople, now Istanbul.

Now, an international and interdisciplinary study conducted by the University of Bern with the participation of the Austrian Academy of Sciences (ÖAW) shows that this period of extreme cold at the beginning of the Middle Ages was caused by volcanic eruptions in Iceland.

Previous estimates of the influence of volcanic eruptions on global climate between 700 and 1000 AD were based on the assumption of a phase of volcanic inactivity. However, this assumption contradicts geological findings from Iceland and sulfate concentrations in ice cores from Greenland, which the researchers have now published in the journal Communications Earth and Environment.

The new study uses analyses of so-called cryptotephra (volcanic ash remnants invisible to the naked eye), high-resolution sulfur isotope analysis, and other chemical indicators of volcanic eruptions from numerous Greenland ice cores to determine volcanic activity and the concentration of sulfur aerosols relevant to climate between 700 and 1000 AD.

Cooling anomalies in 762-764 AD
Cooling anomalies in 762-764 AD. Credit: I. Gabriel et al. / Communications Earth & Environment

The result: a prolonged episode of sulfur dioxide emissions between 751 and 940 AD was dominated by eruptions in Iceland. Until now, volcanic eruptions had been interpreted as a random climate forcing of short duration, effective for a maximum of 1 to 3 years, explain Imogen Gabriel and Michael Sigl, lead authors of the study from the University of Bern.

The series of eruptions from the early Middle Ages is called the “Active Icelandic Period” in the study. It began with eruptions from the Katla volcano between 751 and 763, some of which reached the stratosphere and coincided with significant winter cooling anomalies across Europe. These cold periods can be reconstructed from isotopic data from dripstone caves (like the Spannagel cave in the Zillertal Alps) and historical sources from Ireland to the Mediterranean.

Researcher Johannes Preiser-Kapeller from the Institute for Medieval Studies of the Austrian Academy of Sciences, who participated in the study, describes how these historical climate changes affected early medieval society: The historical sources not only describe extreme cold, but also that these extreme temperatures caused animal deaths and crop freezing. People not only suffered immediate hardships but were also deeply shaken on several levels.

When a meteor shower occurred in March 764—an impressive astronomical phenomenon that lit up the sky—many people thought the end of the world had arrived. This time of crisis also impacted the political weather.

For the Byzantine Empire of the time, which Preiser-Kapeller investigates, it was a moment of internal conflict that went down in history as the “iconoclastic controversy”. According to Preiser-Kapeller: People argued about how to properly worship the divine. From the iconophiles’ perspective, the emperor was at fault because he prohibited proper veneration of the saints. Thus, the crisis was politically instrumentalized and interpreted as God’s punishment.

The interdisciplinary approach of the study also illustrates the significant contribution of persistent volcanic sulfate emissions to atmospheric aerosol pollution in the preindustrial era, which has not been sufficiently considered in previous climate reconstruction estimates.

It underscores the need for continued interdisciplinary research to better understand the climate feedbacks associated with these phenomena in the past and present.


Sources

Austrian Academy of Sciences (ÖAW) | Gabriel, I., Plunkett, G., Abbott, P.M. et al. Decadal-to-centennial increases of volcanic aerosols from Iceland challenge the concept of a Medieval Quiet Period. Commun Earth Environ 5, 194 (2024). doi.org/10.1038/s43247–024–01350–6


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