The Roman Empire Endured Three Plagues Alongside Harsh, Arid Snaps

If you are fascinated by the rise and fall of the Roman Empire, consider the intriguing connection between vintage climate changes and outbreak of diseases. There's indeed an observed correlation between periods of drastic decrease in temperature and rainfall and three pandemics that the Roman Empire experienced, as historian Kyle Harper and his colleagues reported on January 26 in Science Advances. The understanding of why there's a significant connection between cold and dry phases and those disease outbreaks is still lacking. However, the findings drawn from climate reconstructions circa 200 B.C. to A.D. 600 suggest that climate stress might have played a role in the proliferation and intensity of disease-related deaths, according to Harper from the University of Oklahoma in Norman.

Harper in the past has advocated for the theory that the heavy combination of declining global temperatures and the First Plague Pandemic weakened the Roman Empire. This new data reaffirms that climate changes can indeed affect the beginnings and spread of infectious diseases, adds Princeton University historian John Haldon. But many factors in the ancient Roman world, such as expansive trade networks and densely populated settlements, may have heightened vulnerability to disease outbreaks, says Haldon, who wasn't involved in the new study.

The team led by marine palynologist Karin Zonneveld, to resurrect the ancient climate, used a vast sample of fossilised dinoflagellates. These mono-cellular algae were conserved in slices from a sediment core which were carbon-dated previously, extracted from the Gulf of Taranto in southern Italy.

Dinoflagellates reside in the sea's sun-lit upper part. Different species of this organism adopt distinct shapes in late summer and autumn before they rest on the ocean floor. Some species exist only in cold waters, others only in warm.

Water temperature in the Gulf of Taranto closely coincides with southern Italy’s air temperature during the warm segment of the years – late summer and autumn, says Zonneveld, based at Germany's University of Bremen. The team tracked alters in the dinoflagellate species composition in sediment slices to estimate the late summer/autumn temperatures in Southern Italy during the time of the Roman Empire.

Furthermore, the group used dinoflagellates to study changes in weather conditions of the past. While numerous species thrive in nutrient-rich water conditions provided by rivers discharging water into the Gulf of Taranto due to sufficient rainfall in central and northern Italy, others prefer nutrient-deficient water. So their preservation in underwater sediment is reflective of periods of scarce rainfall.

As per their analysis, Zonneveld's team discovered that between roughly 200 B.C. to A.D. 100, there were uniform warm temperatures and regular rainfall, coinciding with the so-called Roman Warm Period – a politically and socially stable era for the Roman Empire.

Subsequently, cold and dry phases began increasing shortly before or during three epidemics - the Antonine Plague, that traveled from Egypt to Europe and the British Isles in the late 160s; the Plague of Cyprian that occurred during mid-200s, a chaotic political phase for Rome; and the Justinianic Plague that reached Italy by 543. By the late 500s, average temperatures had dropped about 3 degrees Celsius compared to the greatest averages during the Roman Warm Period.

The exact extent of death tolls during these disease outbreaks and their potential impact on the collapse of the empire remain uncertain. By the time of the Justinianic Plague, the Roman Empire's power and influence had significantly diminished, even though the eastern part of the empire survived till the fall of the capital Constantinople in 1453.

Although the new findings offer valuable insights into the ancient Roman climate, neither Zonneveld's team nor anyone else can conclusively define the possible influence of changes in temperature and rainfall on the spread of infectious diseases, admits Brandon McDonald, a classical archaeologist at Switzerland's University of Basel.

While the Justinianic Plague is known to have been caused by the bacterium responsible for the Black Death, Yersinia pestis, the specific pathogens for the Antonine Plague and the Plague of Cyprian are still unidentified, McDonald adds, which further complicates the task of explaining how climate could have influenced these events.

Finally, Colin Elliott, an economic and social historian, points out that many disease-causing microbes seem to thrive 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.