Climate change may have helped to alter the course of history. A period of extreme and unprecedented cooling in the northern hemisphere 1,500 years ago may have reduced the eastern Roman empire, set the stage for a famine and a devastating pandemic that killed millions, and favoured the rise of the Islamic forces that created the Arab empire.
The so-called Little Ice Age of the 14th to the 19th centuries has been linked to political upheaval and plague in Europe. But now climate scientists are more confident that something that could be called the “Late Antique Little Ice Age” followed a series of three violent volcanic eruptions in the years 536, 540 and 547 AD.
These would have ejected vast volumes of sulphate aerosols into the stratosphere, darkened the summer skies, blocked sunlight, and lowered the temperatures across much of the northern half of the globe.
Within five years of the onset of this overall cooling, what became known as the Plague of Justinian − named after the Roman emperor then in Constantinople − swept through the Mediterranean region and killed millions of people.
But while famine hit Europe and Central Asia, the argument goes, the Arabian peninsula got more rain, more crops, more fodder for horses and camels, and more rations for the commissariat of the Arab forces that were to begin to advance on Constantinople.
History is complex, but climate often seems to provide either the conditions or the driver for political and social change. In the past three years, researchers have now linked climate change directly with:
- The collapse of the Bronze Age civilisation in the Mediterranean.
- The birth of a Chinese empire.
- Change in the Indus Valley civilisations in prehistory.
- The fall of the Assyrian empire in what is now Iraq.
- The rise of the Mongol Hordes of Genghis Khan.
Ulf Büntgen, head of dendroecology at the Swiss Federal Research Institute for Forest, Snow and Landscape, and colleagues now report in Nature Geoscience that a study of tree-ring measurements in the Altai mountains of Siberian Russia and the Alps of Europe confirms a long, cool interval in the 6th and 7th centuries, which they attribute to volcanic eruption.
The cooling contributed to crop failure and famine, and must have been a factor in the plague, the transformation of the Roman Empire, political change in China, the spread of Slavic-speaking peoples, and the development of the early Islamic empire.
Turbulence of history
The study, by international climate scientists, naturalists, historians and linguists within Future Earth’s Past Global Changes project (PAGES) – which recently concluded that Europe’s summer temperatures were now higher than ever before – matches the new climate information with the turbulence of history, and finds an imperfect fit, but nevertheless a fit.
And it parallels contemporary alarm about the potential of human-induced climate change – this time global warming – to amplify social conflict and large-scale migration of the kind that must have characterised what historians used to call the Dark Ages of Europe.
“This was the most dramatic cooling in the Northern Hemisphere in the past 2,000 years,” Dr Büntgen says. “With so many variables, we must remain cautious about the environmental cause and political effect, but it is striking how closely this climate change aligns with major upheavals across several regions.”