Climatic fluctuations in last 2,500 years linked to social upheavals

For example, the summers at the times when both the Roman Empire and the Middle Ages were at their zenith were relatively humid and warm. Professor Dr Jan Esper of the Institute of Geography at Johannes Gutenberg University Mainz (JGU), Germany was involved in this interdisciplinary study, the results of which were recently published in the journal Science.

The team, consisting of climatologists and archeologists, managed for the first time to 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 analyzed the annual growth rings of some 9,000 samples of subfossil, archeological-historical and living wood originating from Germany, France, Italy, and Austria. The width of these tree rings was measured using dendrochronological techniques. The results were compared with weather data compiled by Central European meteorological stations in order to collate the findings with actual information on precipitation 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 the first author of the publication, 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 age of Migration and the associated socio-economic destabilization. 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 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 exacerbated the contemporary widespread famines.

The publication impressively 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. 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.