Scholars have been searching for years to find out what caused a worldwide catastrophe in the middle of the 6th century. A cryptic entry in the Winchester manuscript of the Anglo Saxon Chronicle notes that on Feb. 15, 538 AD ?the sun grew dark from early morning until 9 a.m.?

There had been many solar eclipses in the previous half millennium, but this was the first to be recorded among the important events, such as battles and coronations. In the chronicles of Annales Cambriae of Wales, the entry for 537 AD records King Arthur?s death, yet it gives equal weight to a strange plague in Britain and Ireland. Ten years later, this ?yellow? plague was considered responsible for destroying the kingdom of Maelgwyn the Great.

These historical narratives are the main surviving threads in a literary record that suddenly became sparse and sporadic. Now, from these literary fragments and a growing body of physical evidence, scientists are piecing together the outlines of a global climatic catastrophe that occurred 1,500 years ago, shattering the existing world order, demolishing the most powerful empires on the planet and completely altering the course of human history.

In one slender document entitled The Ruin of Britain, a contemporary British monk named Gildas describes the abandonment of cities, the disintegration of civil society, gangs preying upon one another in the midst of famine and a time when ?every head is sick, every heart is sorrowful; from the sole of the foot to the crown of the head there is no health in it.? The aftermath of all this is hinted at by a surviving fragment of Anglo-Saxon poetry entitled simply The Ruin.

Before it trails off into illegibility, the poem describes one of the abandoned cities decaying around the elaborate Roman baths that survived at its center. Meanwhile, on the other side of the planet, a chronicle of the Sui dynasty reports that in 535 and 536 a strange yellow dust fell from the heavens, so thick it could be scooped up in handfuls, and that in 537 frost and snow in the summer destroyed crops and started a famine that lasted for 10 years.

One of the richest and most productive provinces of the Roman Empire faced famine, there was disease and civil war on the western edge of Europe, and a rich Chinese kingdom was collapsing on the eastern edge of Asia?all at the same time. Taxes were no longer collected, there were reports of people reduced to cannibalism and governments were destroyed by insurrection and civil war.

Britain disintegrated into kingdoms ruled by Anglo-Saxon warlords in the east, the descendants of Romanized British princes in the west and south, and Irish and Pict invaders to the north. China?s southern empire broke into three savagely warring parts. In India, the powerful Gupta dynasty fell into ruin and in Central America, the Mayan population collapsed.

In the Mediterranean, just as the reign of Justinian began, a devastating plague rolled across the Byzantine Empire. The power and glory of Rome was now a shadow of itself. People fled the cities, there was widespread famine, corpses were buried in mass graves by the tens of thousands and the deaths mounted up so swiftly that they soon overwhelmed authorities to the extent that they stopped keeping records.

All across central Eurasia, so-called barbarians were on the move, displaced by the pressure of other civilizations that had been forced to leave their own homelands or perish. Byzantine church historians were convinced that God had unleashed his final wrath to punish the world for its sins. All this lead to the period in history known as the Dark Ages.

It?s clear that something unimaginably dreadful happened to the world?s climate in the mid-sixth century?but what was it? David Keys, archeology correspondent for The Independent newspaper in Britain, began piecing together the simultaneous chronologies of all these bizarre events. Keys was on Dreamland a year ago, talking about his book Catastrophe. He made the convincing argument that the old world was destroyed by a combination of famines, plagues and political instability that were all rooted in a singular climatic event which occurred around the year 535 A.D.

Keys traced his evidence back to the massive explosion of a volcano not far from where Krakatoa blew up in 1883, causing 72 foot high tsunamis and killing more than 36,000 people around the Sunda Strait between Indonesia?s islands of Java and Sumatra. Keys believes that an earlier and much larger explosion, that was heard as far away as China, vaporized a land bridge between the two islands. Such an eruption would have blasted a huge amount of dust, ash, water vapor and gases into the atmosphere, creating the equivalent of a nuclear winter.

Dust blocking the sunlight would cause the summer snow reported in Asia and the darkening of the heavens reported in western chronicles. Subsequent disruptions in weather would have caused the floods and droughts that both ruined crops and precipitated an outbreak of rodent-borne plague in Africa. The plague was spread quickly by rats hiding in the cargoes of ivory that were shipped into Rome and around the world.

Keys?s theory is being challenged by a Belfast scientist who makes a compelling and dramatic case for a celestial origin for the climatic upheaval that turned the world upside down. Mike Baillie, studies what the growth rings in the world?s oldest trees can tell us about the past. He noticed evidence of a sudden onset of harsh climatic events recorded in ancient Irish oaks, bristlecone pines in the United States and ice cores from Greenland.

In the latest issue of Current Archeology, he says the volcano theory can now be set aside because a tell-tale layer of acid that identifies other known volcanic events is missing from the Greenland ice.

Instead, Baillie speculates that the dust came not from the Earth but from outer space. He noticed a 1990 paper by three British astrophysicists observing that in the period between 400 and 600 AD there were an abnormally large number of meteor showers. He began searching for corroborating evidence in the literary record, in both myth and history. He found references to a brilliant comet by the classical historian Procopius in 539 AD, a reference in Zacharias to ?a great and terrible comet? in 538 and 539, and a reference by 13th-century historian Roger of Wendover, citing a much earlier and now lost source, to the appearance in the year 541 of ?a comet in Gaul, so vast that the whole sky seemed on fire. In the same year there dropped real blood from the clouds and a dreadful mortality ensued.? So we have yellow dust falling like snow in China, blood-colored rain in Gaul, a ?yellow? plague in Wales ?running across the ground like a shower of rain,? Edward Gibbon’s conclusion that for 52 years after the outbreak of Justinian?s plague in 542 ?such was the universal corruption of the air that the pestilence which burst forth … was not checked or alleviated by any difference of the seasons.?

Baillie points to a curious consistency between Gibbon?s half century of calamity, 542-594, the collapse of the Mayan population, 536-596, and a major depression in the growth-rings recorded in the Irish oaks, 540-590. ?Perhaps, in the years around AD 540, a comet shedding dust vapor passed close to the Earth, or fragments of a comet exploded in the atmosphere and filled it with debris,? Baillie says. ?If so, the skies would have grayed, the sun grown dim and the people of the Earth have faced ?years without a summer.??

The question we can?t help asking is: Will it happen again?

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