A volcanic winter took place in the 530s CE, the most severe drop in temperature in the Common Era. An eruption of sulfate aerosols possibly in late 535 dropped summer temperature averages in 536 by at least 4-5 degrees Fahrenheit. In 539-540, a second volcanic eruption caused summer temperatures to drop another 5 degrees.
This was recorded in the Northern Hemisphere by contemporary writers in Constantinople. Procopius, whose writing revealed the secret of where silk came from, records of 536
...during this year a most dread portent took place. For the sun gave forth its light without brightness … and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear."
Cassiodorus, in a letter in 538, describes the sun's rays being weak, no shadows from people at noon, the sun's heat being feeble, the moon "empty of splendor," prolonged frost, unseasonable drought, frosts during harvest, the need to use stored food because harvests were so poor.
The Annals of Ulster mention a failure of bread in the year 536.
Dendrochronology (tree ring analysis) shows very poor growth in Irish oak in 536. Ice cores from Greenland and Antarctica show substantial sulfate deposits around 534±2 years, which offers evidence for the volcanic eruption. Which volcano was the cause, however, has never been agreed upon. The 536 event was worse than 1816, when the explosion of the Mount Tambora volcano caused the "Year Without a Summer."
As mentioned in the prior post, the line regarding the Battle of Camlann in the Welsh Annals that says of 537 "there was great mortality in Britain and Ireland" is likely a reference to the famine that resulted from the volcanic winter.
The 536 event was not the only severe weather crisis in the Middle Ages. Next time, let's jump forward to the Great Famine of 1315-17.
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