Did a relatively short and sharp mini ice age contribute to the fall of great civilisations? New research suggests it could have been a factor.
In a paper published in Nature Geoscience this week, researchers from Sweden recorded a period of "pronounced cooling" from around 536 to 660CE which coincided with major upheaval across Europe and Asia.
The authors, including Ulf Büntgen from the Swiss Federal Research Institute, have called the time period the Late Antique Little Ice Age.
Using tree ring width measurements from the Russian Altai mountains and the European Alps, the authors show the temperatures during the era were markedly cooler compared to the rest of the past 2000 years.
It coincides with massive societal struggles and changes across the two continents including the Justinian plague, which hit the Eastern Roman Empire around 541CE, the collapse of the Sasanian Empire, political change in China, the spread of Slavic-speaking people and the development of the early Arab-Islamic Empire.
The authors attribute the mini ice age to a series of large eruptions and suggest the sudden cooling should be considered an environmental factor which added to crop failure and societal change.
The Justinian Plague is thought to have been responsible for as many as 25 million deaths during its initial outbreak. It is thought to have been carried into Constantinople by infected rats on grain boats arriving from Egypt.
The plague followed widespread food shortages which was right after the mini ice age, the researchers have identified. They suggest it led to the reduction of the Roman Empire.
Meanwhile, shorter growing seasons and subsequent nutritional deficiencies in people and livestock "probably initiated large-scale pastoral movements towards China".
In an accompanying editorial, John Haldon, Professor of Byzantine History and Hellenic Studies at Princeton University, says the findings are as important for historians as it is for climate change scientists.
"No one, I think, can now doubt that climatic and environmental changes are causally linked to societal development, albeit in complex ways," he says.
"In the case of the Late Antique Little Ice Age, we seem to have a pretty clear a priori case for assuming that a dramatic period of cooling, that can be dated reasonably precisely and that coincided with substantial societal change across a large part of the Earth's surface, had a causal impact."
Prof Haldon says each region has its own environmental sub-systems based on geography and weather and have "dramatically different impacts" from area to area on societal arrangements.