Ancient tree rings have revealed that three consecutive years of severe drought weakened Roman Britain before the catastrophic “Barbarian Conspiracy” of 367 CE, when coordinated attacks by Picts, Scots, and Saxons devastated the province.
The Cambridge University-led study, published in Climatic Change, combined tree-ring records with Roman historical accounts to demonstrate how climate extremes triggered a cascade of societal collapse that ultimately contributed to Rome’s withdrawal from Britain.
“We don’t have much archaeological evidence for the ‘Barbarian Conspiracy’. Written accounts from the period give some background, but our findings provide an explanation for the catalyst of this major event,” said Charles Norman from Cambridge’s Department of Geography, the study’s first author.
By analyzing ancient oak tree rings, researchers reconstructed precipitation levels in southern Britain during the critical period. They discovered that the years 364-366 CE experienced rainfall at just 54-72% of normal levels during the crucial April-July growing season – three consecutive droughts of a severity that has never occurred consecutively in modern times.
“Three consecutive droughts would have had a devastating impact on the productivity of Roman Britain’s most important agricultural region. As Roman writers tell us, this resulted in food shortages with all of the destabilizing societal effects this brings,” explained Professor Ulf Büntgen, a co-author from Cambridge’s Department of Geography.
The timing proved disastrous for Roman Britain. Agricultural practices in the province relied on spring-sown crops that were particularly vulnerable to early summer drought. As harvests failed for three straight years, grain supplies to military garrisons dwindled. Roman chronicler Ammianus Marcellinus described Britain’s population as in the “utmost conditions of famine” by 367 CE.
This environmental crisis created perfect conditions for the coordinated assault that followed. Elements of the Roman garrison on Hadrian’s Wall, likely suffering from food shortages, rebelled and allowed northern tribes to pour into the province. The province’s military leadership was killed or captured, and some soldiers reportedly deserted to join the invaders.
The study also expanded beyond Britain to analyze climate patterns before 106 battles across the entire Roman Empire between 350-476 CE. They found a statistically significant pattern of battles occurring after periods of drought in the Western Empire.
“The relationship between climate and conflict is becoming increasingly clear in our own time so these findings aren’t just important for historians,” noted co-author Tatiana Bebchuk from Cambridge. “Extreme climate conditions lead to hunger, which can lead to societal challenges, which eventually lead to outright conflict.”
While Rome eventually restored order in Britain, the province never fully recovered. The last official Roman administration left Britain around 410 CE, just 43 years after the Barbarian Conspiracy.
The findings demonstrate how even powerful empires can be vulnerable to environmental shocks that cascade through agricultural systems and social structures, ultimately triggering conflict and political transformation.
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