Hadrian’s Wall marked the northern border of Roman territory in ancient Britain HISTORIC ENGLAND/HERITAGE IMAGES/SCIENCE PHOTO LIBRARY
Researchers are at odds over a claim that droughts helped trigger conflicts in late Roman Britain. Climatologists identified evidence of drought coinciding with unrest and battles, but historians say they have misread key written sources.
The dispute highlights the ongoing difficulties researchers face when trying to integrate data on past climates into the historical record. 鈥淵ou see this time and time again,鈥 says , a at Georgetown University in Washington DC.
In published last year, a team led by , a geographer at the University of Cambridge, analysed tree ring data from oak trees from southern Britain and northern France to reconstruct the climate between AD 288 and 2009.
The researchers identified a series of severe summer droughts in southern Britain between the years 364 and 366. They linked this to the so-called 鈥淏arbarian Conspiracy鈥 of 367, when warriors from Britain and Ireland inflicted a series of defeats on the Roman Empire, including kidnapping a senior commander. Although the empire did reassert control, it gradually withdrew from Britain over the next 50 years. B眉ntgen and his colleagues argued that the drought caused poor harvests, provoking local leaders to rebel against the Romans.
They also expanded their findings to the wider Roman Empire. Using a dataset of 106 battles, combined with tree ring records from across Europe, they found that battles were more likely in the years following dry summers and exceedingly hot years.
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The findings were widely covered in the media, including by , and .
Now, though, another group of researchers has in the same journal, Climatic Change. 鈥淲e felt that there were so many issues with the paper that it shouldn鈥檛 go unchallenged,鈥 says , a historian at the Ca鈥 Foscari University of Venice in Italy.
She says the data on past climates is 鈥渧ery interesting鈥, but the team鈥檚 interpretations of historical and archaeological sources are often wrong.
The only source about the Barbarian Conspiracy is Ammianus Marcellinus, a Roman author who lived sometime between 330 and 400. Several decades after the events, he wrote a history of Rome called the Res gestae, only parts of which survive.
Some of the sections describing the Barbarian Conspiracy are 鈥渇ragmentary鈥 and even 鈥済ibberish鈥, says , a historian at the University of Bonn in Germany. It isn’t clear what Ammianus meant by 鈥barbarica conspiratio鈥: while a coordinated uprising is one possible reading, it could also mean raiding, social unrest or many other things.
Likewise, Ammianus describes the British as being in a state of 鈥ultimam鈥 inopiam鈥. This means something like 鈥渦tter helplessness鈥, which could refer to a famine or something else entirely. Crucially, Ammianus says the inopiam was a consequence of the barbarica conspiratio, not a cause. 鈥淭hey simply cannot argue that drought caused a famine, which, in turn, caused a barbarian conspiracy, if they鈥檙e relying on what Ammianus says, because that isn鈥檛 what he says,鈥 says Foxhall Forbes.
B眉ntgen and his colleagues have responded, . They cite that interprets 鈥inopiam鈥 as meaning 鈥渇amine鈥. However, Foxhall Forbes and her colleagues highlight , which reconsiders the language and historical context of Ammianus鈥檚 work.
A further problem is that the conflicts in the battle database aren’t alike, says , an archaeologist at Durham University in the UK. While some are pitched battles, others seem to be urban unrest. A food shortage caused by drought might well lead to riots if poorly handled and might even spiral out of control into war, but such narratives would need to be demonstrated somehow.
鈥淭hey didn鈥檛 have a historian on their team who could have told them some of this stuff,鈥 says Foxhall Forbes. While two of the authors are archaeologists, neither specialises in late Roman Britain.
B眉ntgen says he always works in multidisciplinary groups and his team did include archaeologists of the Roman Empire. He says he would like to see 鈥渁 constructive debate鈥 where other researchers re-analyse the data or add to it, building on his team鈥檚 work. B眉ntgen adds that most studies of climate and history, including his own, often leave out ecologists, who play a key role in understanding how climatic anomalies influence agriculture. 鈥淭hat鈥檚 where most of the studies are very vague,鈥 he says.
Degroot, who wasn’t one of the authors of the critique, says the lack of historical expertise is 鈥渁 real weakness鈥 of the study. However, he says the study鈥檚 core 鈥 the tree ring data 鈥 remains . 鈥淲e still do see, now, that droughts probably did happen,鈥 he says. 鈥淵ou can try and clarify in specific case studies whether drought really did have an impact on violence.鈥
There is always a tension between “minimalist” and “maximalist” interpretations of history, says Degroot. Minimalists tend to focus on the detail of specific events and are reluctant to generalise, so they struggle to generate larger narratives. “They’re better at saying what didn’t happen than what did happen,” he says. “That’s not very interesting.” In contrast, maximalists try to identify overall patterns in fragmentary datasets. “They can create these really impressive narratives, sometimes identifying forces that haven’t been considered before,” he says, but those ideas sometimes turn out to be “built on sand”.
Follow in the footsteps of the Romans on this immersive walking tour along Hadrian鈥檚 Wall, one of Britain鈥檚 most iconic, ancient landmarks and a UNESCO World Heritage Site.
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