125-year mini ice age linked to the plague and fall of empires
Winter was coming. In AD 536, the first of three massive volcanic eruptions ushered in a mini ice age. It coincided with an epidemic of the plague, the decline of the eastern Roman Empire, and sweeping upheavals across Eurasia. 
Now we have the first evidence that the disruption to climate continued a lot longer than a decade, as was previously thought. The extended cold period lasted until around 660, affecting Europe and Central Asia, and perhaps the rest of the world too.
The work builds on research that used ice cores to identify three significant volcanic eruptions in the years 536, 540 and 547. Now Ulf Büntgen at the Swiss Federal Research Institute in Birmensdorf and his colleagues have used tree ring data from Europe and Central Asia to show that decades of cooler summers – in some cases 4 °C cooler – ensued, probably caused by volcanic particulates in the atmosphere.
Over this time, average summer temperatures would have been roughly 2 °C below those from 1961 to 1990, the standard reference period for studies of this kind.
This long cold spell coincided with a period of widespread social turmoil across Eurasia, including the plague sweeping across Eastern Europe, Chinese dynasties changing, the Slavs expanding across Europe, and the transformation of the eastern Roman empire into the Byzantine empire.
“There was dramatic social, cultural, and political change in this period,” says Shaun Tougher, a historian at Cardiff University, UK, who was not involved in the research. “Perhaps aspects of the changes were exacerbated by a colder period.”
Stress on societies
“Suggesting climate caused complex events in human history like the fall of empires is controversial,” says geographer Francis Ludlow of Trinity College Dublin in Ireland. “Ultimately [though], there can be very little doubt that these sorts of abrupt climatic events place great stress on societies, and can sometimes tip them over the edge.”
This could have helped speed the demise of what remained of the Roman empire, by then restricted to the Mediterranean, which lost land and power during the mini ice age. The shorter growing season would have affected crops, and this could have led to famine and made people more vulnerable to disease.
“Such climatic disruption could have contributed to the movement of plague-bearing rodents into the empire,” says historian Doug Lee of the University of Nottingham, UK.
It wasn’t just the Romans who suffered – the eastern Türk empire around modern-day Mongolia and the Northern Wei and Sui dynasties in China also fell during this time.
Weather winners
This period is what historians refer to as Late Antiquity, and so Büntgen’s team named the cooling event the Late Antique Little Ice Age.
It could have been more severe than the later, better-known Little Ice Age. “Based on this study, we would say this episode was the coolest over the last 2000 years,” says Büntgen.
The period had its share of winners too. “In any period of changing climate, there will be some regions and societies that are better able to adapt,” says Ludlow.
The Arabian peninsula may have been one area that benefited, perhaps becoming less dry during this time, says Büntgen. “We argue that this was a time when increased vegetation in this area could have been useful for nomadic people or for feeding camels.” This could helped Arab peoples move into Europe and take land from the Romans.
Other winners during this period include the Lombards, who invaded Italy, and the early Slavic languages, which seemed to have spread across most of continental Europe at this time from an unknown homeland.
Nature Geosciences, DOI: 10.1038/NGEO2652
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