Climate variations have influenced the agricultural productivity, health risk, and conflict level of preindustrial societies. Discrimination between environmental and anthropogenic impacts on past civilizations, however, remains difficult because of the paucity of high-resolution palaeoclimatic evidence. Here, we present tree ring-based reconstructions of Central European summer precipitation and temperature variability over the past 2500 years. Recent warming is unprecedented, but modern hydroclimatic variations may have at times been exceeded in magnitude and duration. Wet and warm summers occurred during periods of Roman and medieval prosperity. Increased climate variability from ~AD 250 to 600 coincided with the demise of the Western Roman Empire and the turmoil of the Migration Period. Historical circumstances may challenge recent political and fiscal reluctance to mitigate projected climate change.
Exceptional climate variability is reconstructed for AD ~250-550, and coincides with some of the most severe challenges in Europe's political, social and economic history, the MP [Migration Period]. Distinct drying in the 3rd century paralleled a period of serious crisis in the WRE [Western Roman Empire] marked by barbarian invasion, political turmoil and economic dislocation in several provinces of Gaul, including Belgica, Germania superior and Rhaetia. Precipitation increased during the recovery of the WRE in the 300s under the dynasties of Constantine and Valentinian, while temperatures were below average. Precipitation surpassed early imperial levels during the demise of the WRE in the 5th century before dropping sharply in the first half of the 6th century. At the same time, falling lake levels in Europe and Africa accompanied hemispheric-scale cooling that has been linked with an explosive, near equatorial volcanic eruption in AD 536, followed by the first pandemic of Justinian plague that spread from the Eastern Mediterranean in AD 542/543. Rapid climate changes together with frequent epidemics had the overall capacity to disrupt the food production of agrarian societies.
Büntgen et al. "2500 Years of European Climate Variability and Human Susceptibility". Science, 2011.