Extreme climates and current analogues : from the last millennia to the Holocene optimum (2)

Abstract

For more than a millennium, the history of the Roman Empire was complex, punctuated by political and military events that left a lasting impression. A period of territorial expansion up to 200 AD was followed by a phase of decline marked around 400 AD by the division of the empire into two parts, followed by numerous armed conflicts resulting in territorial losses, particularly after the abdication of the last Western Roman emperor (476 AD).

Historians are still debating the many causes of the collapse of an empire that was morally and financially decadent : barbarian invasions, social divisions, civil wars, religious discord and the rise of Christianity, major plague epidemics, budgetary crisis linked to the military costs of an expansion necessary to supply the empire with goods, food and slaves. To these must be added the often overlooked environmental constraints. Indeed, recent research shows the importance of climatic variations in the rise and fall of Roman civilization.

Although some ancient scientists took the first steps towards estimating temperatures (thermoscopes by Philo of Byzantium and Heron of Alexandria), there are no instrumental records of meteorological variations for the Roman period. Nevertheless, ancient texts contain numerous mentions of climate-related events: extreme heat, intense cold, extreme rainfall and drought, famines... For the eastern part of the Empire, we must add records of Nile flooding, crucial for promoting agriculture in Egypt, the granary of the Roman Empire since its annexation (30 BC). The statistics of meteorological events mentioned in Roman texts suggest that the climate was favorable until around 200-300 AD, and that this optimal period was followed by deteriorations with undeniable human impacts (e.g. famines).

To reconstruct the climatic variations of the Roman era, we also need to consider natural archives such as lake sediments, tree rings and stalagmites, as well as the ice of Greenland and the Alpine glaciers. The latter have been the subject of a great deal of research, revealing widespread advances and retreats, one of which corresponds precisely to the Roman climatic optimum (around 200 BC to 200 AD). This retreat of the glaciers was accompanied by a relatively humid climate, as indicated by several records (lake sediments, stalagmites and tree rings). After 250 AD, the climate deteriorated, culminating around 500-600 AD in what some authors call the Little Ice Age of Late Antiquity (LALIA). The coldest phase corresponds to volcanic eruptions on a global scale, the geochemical signatures of which are found in polar ice.

This climatic change is affecting a relatively large geographical area, with multiple evidences in the Northern Hemisphere. Moreover, dendroclimatic series suggest a clear correspondence between climate variability in northern Italy and that of Central Asia. Numerical simulations carried out with climate models enable us to identify the atmospheric dynamic mechanisms responsible for the spatial correlation of temperatures in the two regions.

Although simplistic determinism must be avoided, it is clear that climatic deterioration from 200-300 AD onwards was accompanied by societal unrest in the Roman Empire, notably conflicts linked to the migratory movements of the Huns and Germanic or Slavic peoples. A major plague epidemic began in 541 AD and was followed by some fifteen other outbreaks of the disease until 767 AD. This pandemic (known as the " Plague of Justinian ") hit the Eastern Roman Empire hard, preventing any possibility of reconquest or restoration of the Empire's unity. Recent research shows that plague contamination is modulated by the climate in Central Asia, which affects populations of large gerbils and fleas, vectors of the Yersinia pestis bacterium. Although it is as yet impossible to quantify this role, it is clear that climate change at the end of the Roman Empire had multiple indirect repercussions, going far beyond the simple effects on agriculture and the food supply of populations.