Cold, dry snaps accompanied three plagues that struck the Roman Empire

For those who enjoy pondering the Roman Empire’s rise and fall — you know who you are — consider the close link between ancient climate change and infectious disease outbreaks. 

Periods of increasingly cooler temperatures and rainfall declines coincided with three pandemics that struck the Roman Empire, historian Kyle Harper and colleagues report January 26 in Science Advances. Reasons for strong associations between cold, dry phases and those disease outbreaks are poorly understood. But the findings, based on climate reconstructions from around 200 B.C. to A.D. 600, help “us see that climate stress probably contributed to the spread and severity of [disease] mortality,” says Harper, of the University of Oklahoma in Norman.  

Harper has previously argued that the First Plague Pandemic (also known as the Justinianic Plague), combined with declining global temperatures to weaken the Roman Empire (SN: 5/18/20). 

The new findings reinforce an idea that climate shifts can influence the origin and spread of infectious diseases, says Princeton University historian John Haldon. But it’s unclear whether a range of factors in the ancient Roman realm, including long-distance trade networks and densely populated settlements, heightened people’s vulnerability to disease outbreaks, says Haldon, who did not participate in the new study. 

To reconstruct the ancient climate, marine palynologist Karin Zonneveld and colleagues turned to an extensive sample of fossilized dinoflagellates. These single-celled algae had been preserved in radiocarbon-dated slices from a sediment core previously extracted in southern Italy’s Gulf of Taranto.  

Dinoflagellates live in the sunlit upper part of the sea. Different species of this organism assume signature shapes in the late summer and autumn before settling on the ocean floor. Some species live only in cold waters, others only in warm waters. 

In late summer and autumn, water temperature in the Gulf of Taranto closely aligns with southern Italy’s air temperature, says Zonneveld, of the University of Bremen in Germany. Her group tracked changes in the composition of dinoflagellate species in sediment slices to estimate late summer/autumn temperatures in southern Italy during the Roman Empire.  

The team also used dinoflagellates to gauge changes in ancient rainfall. Plentiful rainfall in central and northern Italy causes rivers to discharge nutrient-rich water into the Gulf of Taranto. Dinoflagellate species known to rely on plentiful nutrients thrive under those conditions and end up on the sea floor. Other dinoflagellate species prefer nutrient-poor water. Their preservation in underwater sediment reflects stretches of scant rainfall. 

The dinoflagellate analysis revealed that warm, stable temperatures and regular rainfall occurred from around 200 B.C. to A.D. 100, Zonneveld says. That time corresponds to the Roman Warm Period, a time of political and social stability for the Roman Empire. 

Then, phases of increasingly cold and dry conditions occurred shortly before or during three pandemics: the Antonine Plague, which spread from Egypt to Europe and the British Isles in the late 160s; the Plague of Cyprian, which struck during a time of Roman political turmoil in the mid-200s; and the Justinianic Plague, which reached Italy by 543. By the late 500s, average temperatures were about 3 degrees Celsius colder than the highest averages during the Roman Warm Period. 

It’s unclear how high death rates climbed during these disease outbreaks and how they might factor into the fall of the empire. The Roman Empire’s power and influence fell dramatically by around the time of the Justinianic Plague, though the eastern half of the empire lasted until the fall of its capital in Constantinople in 1453. 

And despite providing valuable new climate information from ancient Roman times, neither Zonnefeld’s team nor anyone else can say with certainty how temperature and rainfall shifts may have aided the spread of infectious diseases, says classical archaeologist Brandon McDonald of the University of Basel in Switzerland. 

While it’s known that the Justinianic Plague was caused by the Black Death bacterium Yersinia pestis, specific disease-causing agents for the Antonine Plague and the Plague of Cyprian remain unknown, McDonald says, further muddying attempts to explain how climate may have influenced those events.  

Economic and social historian Colin Elliott notes that many infectious microbes flourish under cold, dry conditions.  

In Elliott’s new book that focuses on the Antonine Plague, Pox Romana, he argues that grain production in Italy and other parts of the Roman Empire suffered during cold years. As a result, hungry people in the Italian countryside may have migrated to cities where imported grain was available, says Elliott, of Indiana University in Bloomington. “Diseases moved with migrants, but surges of malnourished and immunologically [vulnerable] populations into cities almost certainly increased pandemic virulence as well.” 

Intriguingly, the new study also raises the possibility that cooler and drier autumns reduced malaria cases, says Ohio State University historian Kristina Sessa. The milder climate may have impaired or killed temperature-sensitive mosquitoes that regularly transmitted the dangerous disease in southern Italy.