Climate change in the early Middle Ages triggered by volcanic eruptions in Iceland

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Reconstruction of medieval Constantinople. The sea around the city was frozen ov
Reconstruction of medieval Constantinople. The sea around the city was frozen over in the winter of 763/764. The climate had become colder due to volcanic eruptions on Iceland. Johannes Preiser-Kapeller/ÖAW, image created with the help of KI.

Icebergs on the Bosporus and a frozen Black Sea: an international study by the University of Bern with the participation of the Austrian Academy of Sciences shows how volcanic eruptions on Iceland influenced the European climate in the early Middle Ages and led to severe winter cooling anomalies.

It was one of the coldest winters the region has ever experienced: In 763, large parts of the Black Sea froze over and icebergs were sighted on the Boporus. Contemporary historians reported this unusual weather phenomenon in the winter of 763/764 in their records from Constantinople, now Istanbul. An international, interdisciplinary study by the University of Bern with the participation of the Austrian Academy of Sciences (ÖAW) shows that this extreme cold period in the early Middle Ages was triggered by volcanic eruptions on Iceland.

Previous estimates of the influence of volcanic eruptions on the global climate in the period between 700 and 1000 AD assumed a volcanic dormant phase. However, this assumption contradicts the geological findings from Iceland and sulphate concentrations in ice cores in Greenland, which researchers have now published in the journal Communications Earth and Environment, published in the renowned Nature portfolio.

Sulphur clouds drift over Europe

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

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

The early medieval series of eruptions is referred to in the study as the ’Icelandic Active Period’. It began with eruptions of the Katla volcano between 751 and 763, some of which reached into the stratosphere and coincided with strong winter cooling anomalies throughout Europe. These cold periods can be reconstructed on the basis of isotope data from dripstone caves (such as the Spannagel Cave in the Zillertal Alps) and historical sources from Ireland to the Mediterranean.

God’s punishment

Byzantine researcher Johannes Preiser-Kapeller from the Institute for Medieval Studies at the Austrian Academy of Sciences, who was involved in the study, describes how these historical climate changes affected early medieval society: ’The historical sources not only describe that it was very cold, but also that the extreme temperatures caused animals to die and crops to freeze. People not only suffered immediate hardship, but were also deeply shaken on various levels,’ reports the ÖAW researcher.

When a meteor shower - an impressive astronomical phenomenon that lights up the sky - occurred in March 764, many people thought the end of the world had come. This time of crisis also had an impact on the political weather situation. For the Byzantine Empire of the time, which Preiser-Kapeller is researching, it was a time of internal conflict that went down in history as the ’image controversy’. Preiser-Kapeller: ’People argued about how to properly worship the divine. From an image worshipper’s point of view, the emperor was to blame because he forbade the proper veneration of the saints. The crisis was therefore politically instrumentalized and interpreted as a punishment from God.

Volcanoes have so far been given too little consideration in climate models

The interdisciplinary approach of the study also illustrates the significant contribution of persistent volcanic sulphate emissions to pre-industrial atmospheric aerosol pollution, which has not been sufficiently taken into account in previous climate reconstruction estimates. And it underlines the need for further interdisciplinary research to better understand climate feedbacks associated with these phenomena in the past and present.

Source: Austrian Academy of Sciences (ÖAW)