2500 Years of European Climate Variability and Human Susceptibility
Ulf Büntgen et al. (2011)
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.
AMJ [April-June] precipitation was generally above average and fluctuated within fairly narrow margins from the Late Iron Age through most of the Roman Period until ~AD 250, whereas two depressions in JJA [June-August] temperature coincided with the Celtic Expansion ~350 BC and the Roman Conquest ~50 BC. 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. Most of the oak samples from this period originate from archaeological excavations of water wells and sub-fossil remains currently located in floodplains and wetlands, possibly attesting drier conditions during their colonization.
AMJ precipitation and JJA temperature began to increase from the end of the 6th century and reached climate conditions comparable to those of the Roman period in the early 800s. The onset of wetter and warmer summers is contemporaneous with the societal consolidation of new kingdoms that developed in the former WRE. Reduced climate variability from ~AD 700-1000, relative to its surroundings, matches the new and sustained demographic growth in the northwest European countryside, and even the establishment of Norse colonies in the cold environments of Iceland and Greenland. Humid and mild summers paralleled the rapid cultural and political growth of medieval Europe under the Merovingian and Carolingian dynasties and their successors. Average precipitation and temperature showed fewer fluctuations during the ~AD 1000-1200 period of peak medieval demographic and economic growth. Wetter summers during the 13th and 14th centuries and a first cold spell ~1300 agree with the globally observed onset of the Little Ice Age, likely contributing to widespread famine across CE [Central Europe]. Unfavorable climate may have even played a role in debilitating the underlying health conditions that contributed to the devastating economic crisis that arose from the second plague pandemic, the Black Death, which reduced the CE population after AD 1347 by 40-60%. The period is also associated with a temperature decline in the North Atlantic and the abrupt desertion of former Greenland settlements. Temperature minima in the early 17th and 19th centuries accompanied sustained settlement abandonment during the Thirty Years' War and the modern migrations from Europe to America.
Fig. 4. Reconstructed AMJ precipitation totals (Top) and JJA temperature anomalies (Bottom) (wrt 1901-2000). Error bars are +/− 1 RMSE of the calibration periods. Black lines show independent precipitation and temperature reconstructions from Germany and Switzerland. Bold lines are 60-year low-pass filters. Periods of demographic expansion, economic prosperity and societal stability, as well as political turmoil, cultural change and population instability are marked (green and grey text).