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Roman Britain collapsed into chaos in the spring of 367 A.D.—the rival Picts attacked by land and sea, while the Scotti barged in from the west and Saxons from the south. Anarchy ensued in an event that’s now known as the Barbarian Conspiracy: The invaders captured and murdered senior commanders, and some Roman soldiers may have even joined in. It’s considered a pivotal event in the abandonment of Roman Britain.
Historians have surmised some of the potential causes behind this rebellion, like the Western Roman Empire’s fading control of the region and its growing financial troubles. But these circumstances don’t fully explain the abruptness with which Roman Britain fell to its foes, and the scarce written records from the time don’t offer solid explanations. For clearer answers, researchers are asking the trees.
“The Barbarian Conspiracy is an iconic event with a clear date, where we knew what happened—but not why,” says Ulf Büntgen, a geographer and ecologist at Cambridge University.
Büntgen and his colleagues decided to consult tree ring data, from oaks both living and dead, and found the record suggests that three summers of brutal drought preceded the Barbarian Conspiracy. Dry years yield thinner tree rings than wet ones. To reconstruct important details of the Barbarian Conspiracy, the authors consulted records written by Greek and Roman soldier and historian Ammianus Marcellinus. The team’s analysis suggests that the recurring droughts may have led to harvest failures and shortages of food, which can help generate instability and violence. The findings were reported in a paper published in the journal Climatic Change.
“We don’t want to say that climate caused the fall of the Roman Empire, that is too easy to say.”
In recent decades, scientists have increasingly merged evidence from environmental sources, including tree rings and ice cores, with surviving written sources to learn more about the role climate plays in conflicts, such as those among the Maya people and in ancient Korea. Researchers have studied these potential links throughout history and around the world, but the new paper offers one of the most detailed looks at antiquity.
This study “really reflects the direction of the field, where we’re getting some very high-resolution climate data and tying them to models of the ways that these societies work. I think that it’s one of the most focused looks we have at a really particular moment,” says Kyle Harper, a Roman historian at the University of Oklahoma who was not involved in the research. “The later fourth and mid fifth century is a really consequential period in human history, certainly in Roman history.”
After aligning the tree rings with the Barbarian Conspiracy accounts, Büntgen and his team broadened their analysis to the greater Roman Empire. They encountered similar findings among 106 battles fought between 350 to 376 A.D., where a statistically significant number of these battles unfolded after warm and dry summers.
Still, Büntgen says, these tree revelations have their limitations. While the information gleaned from tree rings is more objective than written accounts, they can only provide one snapshot per year—which is limited to the growing season during the spring and summer. This essentially paints a “smushed” picture of a given year’s temperatures, rather than highlighting the variabilities throughout the seasons, Harper says.
And Büntgen stresses that weather is just one piece of the puzzle. He’s careful not to attribute the Barbarian Conspiracy and other Roman quarrels solely to drought. “We don’t want to say that climate caused the fall of the Roman Empire, that is too easy to say,” he says, “but we don’t want to exclude environmental factors.”
On the flip side, climate may have also enabled the Roman Empire to make its mark. Harper and other historians argue that unusually warm, wet, and stable weather in earlier centuries helped the Roman Empire flourish. Some claim that this period of stable favorable weather, known as Roman Climate Optimum, which may have lasted anywhere between 550 B.C. to 350 A.D. (estimates vary widely), helped the empire excel in agriculture, build a thriving economy, and swell in size.
In future work, Harper hopes that research teams will try to tease out other factors, besides limited food supply, that can lead to clashes, and how some societies stay resilient in the face of climate shifts. “Why in some cases, does climate stress and drought seem to cause conflict or other social crises, and why in some cases does it not?” he says.
As climate change intensifies, studies are also examining the modern connections between weather and conflict. And while we’re not living in the Roman Empire, we can still take lessons as we endure a warming world.
“It’s important to know this past, that reminds us that environmental challenge is a fundamental feature of the fate of human societies—it certainly will be of ours,” Harper says. 
Lead photo by Alwin Johnson / Pexels
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