Sudden cooling may have hastened decline of Eastern Roman empire
After a careful scrutiny of tree-ring records, scientists say they may have identified a link between a sudden shift in climate and geopolitical upheaval about 1,500 years ago.
The sudden drop in temperatures in the northern hemisphere followed a trio of large volcanic eruptions in the years 536, 540 and 547 AD, when sulphate aerosols emitted by the volcanoes may have cooled the atmosphere by blocking sunlight.
Within five years of the onset of what the scientists have dubbed the “Late Antique Little Ice Age,” he 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.
The study was done by researchers with international Past Global Changes and published in the journal Nature Geoscience. The multidisciplinary research team made up of climatologists, naturalists, historians and linguists mappin the new climate information against a particularly turbulent period in history in Europe and central Asia.
“This was the most dramatic cooling in the Northern Hemisphere in the past 2000 years,” said lead author Ulf Büntgen, a dendroclimatologist with the Swiss Federal Research Institute.
A later “Little Ice Age” between 14th and 19th centuries has been well documented and linked to political upheavals and plague pandemics in Europe, but the new study is the first to provide a comprehensive climate analysis across both Central Asia and Europe during this earlier period.
“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 sudden climate shift brought more rain to the Arabian Peninsula, so the researchers speculated that this may have driven expansion of the Arab Empire in the Middle East because an increase in 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.
“We can learn something from the speed and scale of the transformations that took place at that time,” said Büntgen, explaining that study serves as an example of how sudden climatological shifts can change existing political systems.
The temperature reconstruction, based on new tree-ring measurements from the Altai mountains where Russia, China, Mongolia and Kazakhstan meet, corresponds remarkably well with temperatures in the Alps in the last two millennia. The width of tree rings is a reliable way to estimate summer temperatures.
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.
The Euro-Med2k Working Group reconstructs and models past climate in the Europe and Mediterranean regions (including southern Europe, the Middle East and northern Africa) over the last 2,000 years. PAGES is part of Future Earth – a major international research program to study global sustainability.