According to written records and supported by dendrochronology and archaeological evidence, for 12-18 months in AD 536-537, a thick, persistent dust veil or dry fog darkened the skies between Europe and Asia Minor. The climatic interruption brought by the thick, bluish fog extended as far east as China, where summer frosts and snow are recorded in historical records; tree ring data from Mongolia and Siberia to Argentina and Chile reflect decreased growing records from 536 and the subsequent decade.
The climatic effects of the dust veil brought decreased temperatures, drought and food shortages throughout the affected regions: in Europe two years later came the Justinian plague. The combination killed perhaps as much as 1/3 of the population of Europe; in China the famine killed perhaps 80% of people in some regions; in Scandinavia the losses may be been as much as 75-90% of the population, as evidenced by the numbers of deserted villages and cemeteries.
The rediscovery of the AD 536 event was made during the 1980s by American geoscientists Stothers and Rampino, who searched classical sources for evidence of volcanic eruptions. Among their other findings, they noted several references to environmental disasters around the world between AD 536-528.
Contemporary reports identified by Stothers and Rampino included Michael the Syrian, who wrote "the sun became dark and its darkness lasted for one and a half years... Each day it shone for about four hours and still this light was only a feeble shadow...the fruits did not ripen and the wine tasted like sour grapes." John of Ephesos related much the same events. Prokopios living in in Africa and Italy, said "For the sun gave forth its light without brightness, like the moon, during this whole year, and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear nor such as it is accustomed to shed."
An anonymous Syriac chronicler wrote "...the sun began to be darkened by day and the moon by night, while ocean was tumultuous with spray, from the 24th of March in this year till the 24th of June in the following year..." and the following winter in Mesopotamia was so bad that "from the large and unwonted quantity of snow the birds perished."
Cassiodorus, praetorian prefect of Italy at the time, wrote "so we have had a winter without storms, spring without mildness, summer without heat". John Lydos, in On Portents, writing from Constantinople, said: "If the sun becomes dim because the air is dense from rising moisture--as happened in [536/537] for nearly a whole year...so that produce was destroyed because of the bad time--it predicts heavy trouble in Europe."
And in China, reports indicate that the star of Canopus could not be seen in as usual in the spring and fall equinoxes of 536, and the years AD 536-538 were marked by summer snows and frosts, drought and severe famine. In some parts of China, the weather was so severe that 70-80% of the people starved to death.
Tree rings show that 536 and the following ten years shows a period of slow growth for Scandinavian pines, European oaks and even several North American species including bristlecone pine and foxtail; similar patterns of ring size decrease are seen in trees in Mongolia and northern Siberia.
But there seems to be something of a regional variation in the worst of the effects. 536 was a bad growing season in many parts of the world, but more generally, it was a part of a decade-long downturn in climate for the northern hemisphere, separate from the worst seasons by 3-7 years. For most reports in Europe and Eurasia, there is a drop in 536, followed by a recovery in 537-539, followed by a more serious plunge lasting perhaps as late as 550. In most cases the worst year for tree ring growth is 540; in Siberia 543, southern Chile 540, Argentina 540-548.
AD 536 and the Viking Diaspora
Archaeological evidence described by Gräslund and Price (2012) shows that Scandinavia might have experienced the worst troubles. Almost 75% of villages were abandoned in parts of Sweden, and areas of southern Norway show a decrease in formal burials up to 90-95%.
Scandinavian narratives recount possible events that might be referring to 536. Snorri Sturluson's Edda includes a reference to Fimbulwinter, the "great" or "mighty" winter that serves as a forewarning of Ragnarök, the destruction of the world and all of its inhabitants. "First of all that a winter will come called Fimbulwinter. Then snow will drift from all directions. There will then be great frosts and keen winds. The sun will do no good. There will be three of these winters together and no summer between."
Gräslund and Price speculate that the social unrest and sharp agrarian decline and demographic disaster in Scandinavian may have been the catalyst for the Viking diaspora.
Scholars are divided concerning what caused the dust veil: a violent volcanic eruption, a cometary impact, even a near miss by a large comet could have created a dust cloud made up of dust particles, smoke from fires and (if a volcanic eruption) sulfuric acid droplets such as that described. Such a cloud would reflect and/or absorb light, increasing the earth's albedo and measurably decreasing the temperature.