2,500 years of tree rings reveals link between Europe's climate and social upheaval

By measuring tree rings, researchers have found striking chronological parallels between significant variations of climate and major historical periods.

The scientists reconstructed the summer climate in Europe over the last 2,500 years from the information provided by annual tree growth, and found that both the times when the Roman Empire and the Middle Ages were at their zenith, the summers were relatively humid and warm.

The team, consisting of climatologists and archaeologists, put together a complete history of rainfall and temperature over the past two and a half millennia in Central Europe. In order to do this, they analysed the annual growth rings of some 9,000 samples of sub-fossil, archaeological-historical and living wood originating from Germany, France, Italy, and Austria. The results were then compared with weather data in order to collate the findings with actual information on rain- and snowfall, and temperature variations.

This enabled the scientists to consider major historical events and epochs in the context of the fluctuations of the European summer climate in the period from the late Iron Age 2,500 years ago right up to the 21st century.

"During the Roman era, the climate was predominantly humid and warm, and also relatively stable," explains one of the authors of the study, Ulf Büntgen of Swiss Federal Institute for Forest, Snow and Landscape (WSL) in Zurich.

The decline of the Western Roman Empire coincided with a period after 250 AD in which it became much colder and climatically changeable. This phase of more marked climatic variation persisted for 300 years, accompanying the 'Barbarian Invasions' and the associated socio-economic destabilization in Europe that occurred roughly from 300 to 700 AD.

The cultural revival of the early Middle Ages occurred as both temperatures and rainfall began to increase with the dawn of the 7th century. It is also possible that climatic factors may have contributed towards the spread and virulence of the Black Death (most likely an outbreak of bubonic plague) after 1347.

In addition, the new findings suggest that a cold period during the Thirty Years' War in the first half of the 17th century could have aggravated the contemporary widespread famines.

The study, published in 'Science' (abstract) as 2500 Years of European Climate Variability and Human Susceptibility, also compares the climate of the 20th century and the changes (partly) attributable to human activity with the natural fluctuations of the past 2,500 years.

Recent warming is unprecedented – the summers of the late 20th and early 21st centuries appear to be have been unusually warm when considered against the background of natural temperature variation. On the other hand, there were also periods of very heavy precipitation in the past that, in terms of quantity of rainfall and duration, were more extreme than anything we are witnessing today.

However, the team of authors explicitly draws attention to the complexity of the relationship between climatic change and historical events, and warns of the dangers of drawing overly simplistic conclusions with regard to cause and effect.

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