Climate fluctuation was a contributing factor alongside political failures and barbarian invasions, says Ulf Büntgen of the Swiss Federal Institute for Forest, Snow and Landscape Research in Birmensdorf, Switzerland, who led the project.
Büntgen used tree rings to build up a history of European climate. Using nearly 9000 samples from oak, pine and larch, Büntgen and colleagues were able to reconstruct how temperatures and rainfall in western Europe changed over the last 2500 years.
Climate flips and Black Death
From AD 250 to 550, the climate flipped, from one decade to the next, between dry and cool, and warm and wet. "Such decadal changes seem to have the most impact" on civilisations, Büntgen says, because they harm agriculture but are not prolonged enough for people to adapt their behaviour.
The climatic turmoil coincided with political upheaval and waves of human migrations. By AD 500, the western Roman Empire had fallen.
In other notable periods, the relatively stable medieval society was characterised by more constant climatic conditions. But the Black Death coincided with a wet spell and the disease spreads faster in humid conditions
Cold wars
"Relatively modest changes in European climate in the past have had profound implications for society," says Michael Mann of Penn State University in University Park, Pennsylvania.
Other studies have shown how war and climate are often intimately tied. For example, periods of unusually cold weather in China during the last millennium are thought to be linked to major bouts of warfare.
That said, it is difficult to draw conclusions for the present day from studies like Büntgen's. As Halvard Buhaug of the Peace Research Institute Oslo in Norway points out: "Modern societies are not nearly as dependent on the climate, because trade and technology can mitigate its effects."
Whether or not African civil wars today can be linked to modern climate change is the subject of intense debate.
Huge sample size
Büntgen and his colleagues used over 7284 oak tree samples from low-lying areas of France and Germany to obtain a record of spring rainfall, and 1089 Stone pines samples and 457 larches samples from high in the Austrian Alps to determine summer temperatures.
Others, including Mann, have used similar methods to put together detailed reconstructions of global temperatures during the last 1000 years. Going back 2500 years is "a very substantial contribution," says Mann.
Journal reference: Science, DOI: 10.1126/science.1197175
Ever since the Climategate revelations, any climate research or report to which the crooked Professor Michael Mann of Pennsylvania State University contributed is suspect. It's scandalous that such a charlatan has not been banished.