In March of 536, the sky over Constantinople turned gray. It stayed that way.
The sun still rose every morning. It just stopped doing its job. Procopius, the Byzantine historian who wrote the record of the empire’s wars, watched it happen for a year. “The sun gave forth its light without brightness, like the moon, during this whole year,” he wrote, “and it seemed exceedingly like the sun in eclipse.”
A Roman official described a sky that gave no shadows
Two years later, in Italy, a senior minister at the Ostrogothic court named Cassiodorus tried to describe what people were seeing. The sun looked blue-white and weak. The full moon had lost its splendor. Cloud lay across the sky like a hide stretched flat. At noon, a person standing in the open cast no shadow on the ground.
“We marvel to see no shadows of our bodies at noon,” he wrote in a letter that survives today.
The seasons had come apart with it. A winter without storms. A spring with no warmth in it. A summer that never got hot. Frost killed the crops before anyone could harvest them. Apples went hard on the branch. The grapes stayed sour. Cassiodorus spent his next two letters organizing famine relief.
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Further east, a Syriac church chronicle later copied down by Michael the Syrian counted the days. The darkness ran eighteen months. The sun managed about four hours of light each day, and even that was faint.
Chinese court records describe hail, killing cold, and famine
Six thousand kilometers away, Chinese court scribes were recording the same year.
The Book of Wei, the official record of the Northern Wei dynasty, logs hailstorms across several regions in the autumn of 536. The Bei Shi, or History of the North, records “great cold” and famine that same autumn.
Guanzhong took the worst of it. The plain around present-day Xi’an fed northern China. According to the Bei Shi and the Zizhi Tongjian, the famine there drove people to eat one another, and seven or eight in every ten died. That ratio follows the style of Chinese court historians rather than any headcount, and both records agree on the scale.
In the south, the Nan Shi records that in the eleventh month a yellow dust fell out of the sky like snow. It lay thick enough to pick up by the handful. Writers in the Middle East, Europe, and China all described the same dense, dry fog that year, and the same loss of visibility.
Irish oak trees and polar ice remember the same year
The written records stood alone until scientists began testing wood and ice.
Mike Baillie, a researcher at Queen’s University Belfast, studies climate history through the growth rings of ancient wood. Trees put on a wide ring in a good year and a thin one in a bad year. In 536, Irish oaks almost stopped growing altogether. They picked up a little, then crashed again in 542.
Cores drilled in Greenland and Antarctica contain a thick layer of sulfate dated to within a few years of 534. Sulfate at that level means acid particles were floating around the upper atmosphere, which is what happens after a very large volcanic eruption.
European summers in 536 ran up to 2.5 °C (4.5 °F) colder than normal. Another eruption around 539 or 540 pushed them colder still. Together they started a cold spell scientists call the Late Antique Little Ice Age, which lasted until roughly 660. The Plague of Justinian, the first recorded outbreak of bubonic plague, arrived in the middle of it.
The cause was almost certainly volcanic, and nobody has found the volcano
The physics is not in dispute. Enough sulfate high enough in the atmosphere blocks sunlight and cools the planet for years.
Finding the volcano has been harder. Rabaul in Papua New Guinea was the favorite for a while, until radiocarbon dating moved its big eruption to somewhere between 667 and 699. Drilling in the Sunda Strait cleared Krakatoa. Ash in the ice dated the huge Ilopango eruption in El Salvador to between 429 and 433, a century too early.
The ash trapped in Greenland ice points to North America. The tiny glass fragments sort into at least three different chemical groups, which means at least three volcanoes went off within months of each other. One matches ash from the Mono Craters in northeastern California. The other two probably came from the eastern Aleutian islands and from volcanoes in Alaska and western Canada.
Norse myth may be a memory of that winter
At the close of the Migration Period, wealthy Scandinavians began burying gold in enormous quantities. Archaeologists have suggested the hoards were offerings, an attempt to pay the gods to give the sun back.
Some scholars think the myths kept the memory too. In Norse legend, the Fimbulwinter is three winters in a row with no summer in between. It is the sign that the world is about to end at Ragnarök.
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