The historian Procopius, writing from Constantinople in 536, recorded something no one around him could explain. The sun, he wrote, gave its light without brightness — like the moon. It stayed that way for the better part of a year and a half. Harvests across the known world failed. Temperatures dropped. People starved.
Medieval chroniclers in Ireland, China, and Mesopotamia recorded the same thing independently: a dimming of the sun, a cold that came at the wrong time of year, snow in summer. They had no common language, no way to compare notes. They were all describing the same event.
The Coldest Decade in 2,000 Years
Dendrochronologists — scientists who study tree rings — found the evidence in Irish oaks. The rings from 536 and the years immediately following are among the narrowest in two millennia, indicating almost no growth. The trees barely survived. The same signature appears in bristlecone pines in California and in samples from Scandinavia and Siberia.
Climate reconstructions place the decade following 536 as the coldest ten-year period in the last 2,000 years. Average temperatures in Europe dropped by 1.5 to 2.5 degrees Celsius. In some regions, summer temperatures fell enough that crops could not complete their growing cycles.
The Timeline
536 AD — Sun dims. Temperatures fall. Famines begin across Europe and Asia.
540 AD — A second eruption darkens the sky again.
541 AD — The Plague of Justinian emerges in Egypt.
543 AD — The plague reaches Constantinople and spreads across the Roman Empire.
Death toll from the plague: estimated 25–50 million across the Eastern Roman Empire.
Then It Happened Again
In 540, a second major eruption sent another pulse of sulfate aerosols into the stratosphere. The cold deepened. And then, in 541, a plague emerged from Egypt — later named the Plague of Justinian. It killed somewhere between 25 and 50 million people across the Eastern Roman Empire over the following two centuries. It was the first recorded pandemic caused by Yersinia pestis, the same bacterium responsible for the Black Death.
Historians now call the period from 536 to roughly 660 AD the Late Antique Little Ice Age. It contributed to the collapse of the Eastern Roman Empire, the decline of the Sasanian Persian Empire, and the reorganization of societies across Eurasia.
The Answer in the Ice
For 1,300 years, the cause remained unknown. Then in 2015, a team of scientists drilled ice cores from a glacier in Greenland. At the layer corresponding to 536 AD, they found a massive spike in sulfate deposits — the chemical signature of a large volcanic eruption. Sulfate aerosols ejected into the stratosphere scatter sunlight, cooling the planet without blocking it entirely. The sun would have appeared dim, pale, and cold for months to years afterward.
A second spike appeared at 540, confirming the second event. A third appeared around 547.
The source volcano has never been conclusively identified. The leading candidate is Ilopango, a caldera in El Salvador that produced one of the largest eruptions of the last 10,000 years at some point in the mid-sixth century. Ilopango is now a lake. The eruption that may have helped end an empire left no mountain behind — only a hole filled with water.
"The sun gave its light without brightness, like the moon, during this whole year, and it seemed exceedingly like the sun in eclipse." — Procopius, History of the Wars, 536 AD