Researchers from the international Past Global Changes (PAGES) project write in the journal Nature Geoscience that they have identified an unprecedented, long-lasting cooling in the northern hemisphere 1500 years ago. The drop in temperature immediately followed three large volcanic eruptions in quick succession in the years 536, 540 and 547 AD (also known as the Common Era or CE). Volcanoes can cause climate cooling by ejecting large volumes of small particles – sulfate aerosols – that enter the atmosphere blocking sunlight.
The findings of the study have been largely covered by international media, such as the New Scientist and the Washington Post. The latter points out that this might be some of the strongest evidence of a direct link between climate change and monumental shifts in civilisations across entire regions.
Within five years of the onset of the "Late Antique Little Ice Age", as the researchers have dubbed it, the Justinian plague pandemic swept through the Mediterranean between 541 and 543 AD, striking Constantinople and killing millions of people in the following centuries. The authors suggest these events may have contributed to the decline of the eastern Roman Empire.
"With so many variables, we must remain cautious about environmental cause and political effect, but it is striking how closely this climate change aligns with major upheavals across several regions," added Büntgen.
The multidisciplinary research team made up of climatologists, naturalists, historians and linguists mapped the new climate information against a particularly turbulent period in history in Europe and central Asia. The volcanic eruptions probably affected food supplies – a major famine struck the region at precisely this time followed immediately by the pandemic.
Further south, the Arabian Peninsula received more rain allowing more vegetation to grow. The researchers speculate this may have driven expansion of the Arab Empire in the Middle East because the vegetation would have sustained larger herds of camels used by the Arab armies for their campaigns.
In cooler areas, several tribes migrated east towards China, possibly driven away by a lack of pastureland in central Asia. This led to hostilities between nomadic groups and the local ruling powers in the steppe regions of northern China. An alliance between these steppe populations and the Eastern Romans brought down the Sasanian Empire in Persia, the final empire in the region before the rise of the Arab Empire.
Large volcanic eruptions can affect global temperature for decades. The researchers suggest that the spate of eruptions combined with a solar minimum, and ocean and sea-ice responses to the effects of the volcanoes, extended the grip of the freezing climate for over a century.
The research is part of the Euro-Med2k working group of the international Past Global Changes (PAGES) project. Last week (29 January 2016), members of the group published a comprehensive analysis of summer temperatures in Europe in the last 2000 years, concluding that current summer temperatures are unprecedented during this period. Future Earth supported the communication efforts around the publication of this study.
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Buentgen et. al, "Cooling and societal change during the Late Antique Little IceAge from 536 to around 660 CE", Nature Geoscience, published online: 08 February 2016, DOI: 10.1038/NGEO2652