Volcanic winter of 536

Contemporaries in the Mediterranean described the phenomenon with alarm. Procopius, then serving with Belisarius in the West, wrote that “during this year a most dread portent took place”: all year the sun gave its light without brightness, like the moon, as if in eclipse. Cassiodorus, praetorian prefect of Ostrogothic Italy, wrote in a letter preserved in his Variae (its date is disputed: T. Hodgkin and the English article place it in 538, while A. Arjava considers the autumn of 536 more likely) of a bluish sun, of bodies that cast no shadow at noon, of “a winter without storms, a spring without mildness, and a summer without heat”, and told his deputy to gather the surplus of the previous year’s abundant harvest for the months of scarcity to come. According to Michael the Syrian, drawing on John of Ephesus, the sun was darkened for eighteen months; the Irish annals record a “failure of bread” in 536–539, and Chinese histories record great cold, famine and a yellow, ash-like substance falling from the sky.

Sulphate layers in polar ice cores and tree rings have confirmed and dated the event: an eruption in early 536 (or late 535) was followed by another in 539–540, and summer temperatures in Europe fell by up to 2.5 °C below normal in 536 and by up to 2.7 °C after the second eruption. The volcanoes have not been identified with certainty. Volcanic glass from the layer of 536 points to at least three simultaneous eruptions in North America — one matching the Mono Craters tephra of California, the others probably in the eastern Aleutians and the northern Cordillera; the Icelandic source proposed in 2018 from glass shards in a Swiss glacier is doubted, since the ash of 536 is chemically distinct from Icelandic tephra. Earlier candidates — Krakatoa, Rabaul and Ilopango in El Salvador — have been ruled out by dating (Ilopango’s great eruption is now placed around 431). From tree rings, Ulf Büntgen and colleagues (2016) defined a “Late Antique Little Ice Age” lasting from 536 to about 660.

According to the Syriac chronicle of Pseudo-Zachariah, the sun and moon were darkened in Constantinople from 24 March 536, the month Pope Agapetus arrived there, until 24 June 537. How far the cooling shaped the following decades is debated: a link with the Plague of Justinian, which broke out five years later, has been suggested but not proven, and the event and the ensuing famine have been proposed as an explanation for the gold hoards buried by Scandinavian elites at the end of the Migration Period. The historian Michael McCormick called 536 “the beginning of one of the worst periods to be alive, if not the worst year”.