Chronicler Theophanes Declarante stated that winter 763-764 was one of the coldest in history. Snow and ice occupied the Byzantine capital and also saw an iceberg in Bosforón.

Climate cooling has been considered to be due, among other factors, to the lack of volcanic activity. But researchers of the universality of Bern and the Austrian Academy of Sciences have just drawn the reverse conclusion from that period of ice because of the violent volcanic eruptions, namely the volcanic eruptions of Iceland.

Katla volcano was active between 751 and 763 and sulfur particles emitted into the atmosphere decreased the effect of the sun and reduced the temperature throughout the northern hemisphere.