Researchers are debating whether droughts triggered unrest in late Roman Britain during the so-called Barbarian Conspiracy of 367 AD. A study using tree ring data linked severe summer droughts in 364-366 to poor harvests and rebellion, but historians say the historical sources have been misinterpreted. The dispute underscores challenges in combining climate data with historical records.
A team led by Ulf Büntgen at the University of Cambridge analyzed oak tree rings from southern Britain and northern France, reconstructing climate from AD 288 to 2009. They identified severe summer droughts between 364 and 366, coinciding with the Barbarian Conspiracy, when warriors from Britain and Ireland defeated Roman forces and kidnapped a senior commander. Büntgen's group argued that the droughts led to poor harvests, prompting local leaders to rebel against Rome, contributing to the empire's gradual withdrawal over the next 50 years. The study also found battles more likely after dry or hot summers across a dataset of 106 Roman Empire conflicts. The findings drew media attention from BBC News, The Guardian and The Telegraph. A critique published in Climatic Change by Helen Foxhall Forbes of Ca’ Foscari University of Venice, James Harland of the University of Bonn and Dan Lawrence of Durham University challenges these interpretations. They argue the sole primary source, Ammianus Marcellinus's Res gestae, does not support drought causing famine or rebellion. Ammianus describes the Britons' 'ultimam… inopiam'—utter helplessness—as a consequence of the unrest, not its cause, and terms like 'barbarica conspiratio' are ambiguous, possibly meaning raids rather than coordinated uprising. Büntgen responded in the journal, citing a 1984 paper interpreting 'inopiam' as famine, and called for constructive debate with re-analysis of data. Climate historian Dagomar Degroot of Georgetown University noted the tree ring data's value despite historical expertise gaps, highlighting tensions between minimalist historians focused on specifics and maximalists seeking broader patterns.