In the spring of 367 CE, Roman Britain was besieged from all sides and by all kinds of forces. Harvests shriveled. Soldiers and the security they provided vanished. Communities were starving. And when the Picts stormed Hadrian’s Wall from the North, the Scotti landed in the West, and Saxons arrived from across the sea, they didn’t encounter a robust imperial bulwark—they found an empire on its knees.
The chaos, long remembered as the “Barbarian Conspiracy,” was never truly clear in the way it unfolded. How did such a well-fortified province fall into anarchy? A new study, published in the journal Climate Change, led by researchers at the University of Cambridge, points the finger at a lesser-referenced candidate: the climate.
Three years of horrible drought
Using tree-ring data from ancient oaks, the team reconstructed rainfall levels in southern Britain during the years preceding the rebellion. What they found was a trio of consecutive droughts from 364 to 366 CE, each more ruinous than the last.
“Three consecutive droughts would have had a devastating impact on the productivity of Roman Britain’s most important agricultural region,” said Professor Ulf Büntgen of Cambridge’s Department of Geography. “As Roman writers tell us, this resulted in food shortages with all of the destabilizing societal effects this brings.”
A Province in Collapse
In most years between 350 and 500 CE, southern Britain saw an average of 51mm of rainfall during the April-to-July growing season. But in 364 CE, that figure plummeted to 29mm. The following year saw just 28mm. Even in 366 CE, precipitation barely recovered, reaching only 37mm—still falling short of the historical average.
These numbers might seem dry (pun intended) but their implications came like a tsunami (my editor will have my neck). Roman Britain’s staple crops, spelt wheat and six-row barley, depended on adequate early summer rain. Without it, the province’s granaries emptied. Roman chronicler Ammianus Marcellinus described the people as being in the “utmost conditions of famine” by 367 CE.
Food was also the currency of loyalty. Roman soldiers were partly paid in grain. If there was no bread, there was no army.
“Drought from 364 to 366 CE would have impacted spring-sown crop growth substantially, triggering poor harvests,” said Charles Norman, lead author of the study. “This would have reduced the grain supply to Hadrian’s Wall, providing a plausible motive for the rebellion there which allowed the Picts into northern Britain.”
From Failed Harvest to Failed State
Ammianus also documented how soldiers guarding the northern frontier mutinied and deserted. They opened the gates to the Picts. At the same time, attackers from Ireland and the European continent seized their chance. One Roman commander was slain. Another, Fullofaudes, was captured. A few soldiers may have even joined the invading tribes. For two years, imperial control disintegrated as raiders looted freely across the countryside.
Rome’s grip slipped. Britain entered what one archaeologist has called a “phase of anarchy.”
Over the next two years, Roman generals tried to restore control. They failed. Finally, the emperor dispatched Theodosius the Elder—father of a future emperor—with a fresh army. By 369 CE, order had been restored, but many towns and villas across the countryside were never reoccupied. A generation later, Rome withdrew from Britain entirely.
The question lingered: Why did this collapse happen when it did?
Most explanations focused on military or political decline. But the new study offers a model in which environmental shocks—particularly drought—amplify existing weaknesses. In this case, climate instability may have pushed an already strained province over the edge.
The Climate-Conflict Connection
The team extended their analysis to 106 battles fought across the Roman Empire between 350 and 476 CE. A statistically significant number, they found, occurred after dry years.
“The relationship between climate and conflict is becoming increasingly clear in our own time,” said Tatiana Bebchuk, another researcher on the project. “Extreme climate conditions lead to hunger, which can lead to societal challenges, which eventually lead to outright conflict.”
This isn’t the first time scholars have drawn such links. Climate has been implicated in everything from the collapse of the Akkadian Empire to the turmoil of the French Revolution. But few cases are as stark as Roman Britain’s collapse during the Barbarian Conspiracy.
Unlike many modern regions, Britain’s Roman provinces were especially vulnerable to drought. With most crops planted in spring rather than winter, they needed timely rainfall. When the skies failed three years in a row, even a well-oiled imperial machine couldn’t save them.
And while the Picts, Scotti, and Saxons have long been painted as savage and desperate invaders, the study challenges that image. The tribes likely weren’t fleeing famine themselves, although the drought must have affected their crops too. Rather, they moved in with precision to exploit a weakened and isolated province—one that Rome could no longer afford to defend.
“The prolonged and extreme drought seems to have occurred during a particularly poor period for Roman Britain,” Andreas Rzepecki, a co-author based in Trier, Germany. “These factors limited resilience, and meant a drought-induced, partial-military rebellion and subsequent external invasion were able to overwhelm the weakened defenses.”
The research underscores just how quickly a society—however complex—can fall apart when its food systems collapse. The Roman Empire, after all, didn’t lose Britain to some grand external foe. It lost it, in large part, to the weather.
