A new high-resolution climate reconstruction from marine sediments in southern Italy, spanning 200 BCE to 600 CE, shows that pandemic outbreaks in ancient Rome coincided with an increasingly unstable climate and cold periods — offering a more vibrant picture of how climate variability intersected with periods of prosperity and hardship in the heart of the Roman Empire.
Although ancient climate records indicate that the Roman Empire experienced substantial climate variability during its centuries-long reign, a lack of local evidence has made it difficult to understand what these changes may have looked like on the ground. The findings, published in the January 26 issue of Science Advances, suggest that the powerful empire may have been more vulnerable to harsher climates than previously believed.
"The marine [sediment] core helps to clarify that there must have been considerable climate stress — severe and rather prolonged," said Kyle Harper, G.T. and Libby Blankenship Chair in the History of Liberty, professor of classics and letters at the University of Oklahoma, and study coauthor.
Reconstructing Ancient Roman Climate
The researchers analyzed sediments from the Gulf of Taranto, located at the "heel" of the Italian peninsula. Sediments there accumulate rapidly and continuously, originating from river discharge carried southward by way of coastal currents in the Adriatic Sea. These sediments contain fossilized cysts of dinoflagellates — plankton that thrive in surface waters, varying in species composition depending on environmental conditions related to local temperatures and precipitation. Such detailed fossil archives are exceptionally rare worldwide, but the Gulf of Taranto met all the prerequisites.
"At the core location, about one centimeter of sediment is deposited in only about ten years, which is extremely fast compared to other marine regions," said Karin Zonneveld, professor of micropaleontology in the Center for Marine Environmental Sciences (MARUM) at the University of Bremen, and lead author. "We could cut the sediment core in such small slices that each slice represented about three years of deposition." This roughly three-year resolution allowed Zonneveld and colleagues to trace climate through historical events with greater clarity.
"The Gulf of Taranto is also unique in that it lies near the area where local Italian volcanoes regularly erupt and bring large amounts of ash into the atmosphere," Zonneveld added, noting the famous eruption of Mount Vesuvius in 79 CE that destroyed Pompeii. After these eruptions, volcanic ash eventually settles onto the seafloor, preserved as tiny glass shards in the sediments that can be extracted and used to pinpoint precise dates in the record.
Such dates include volcanic eruptions that occurred in 536 CE and 540 CE, for example, which emitted aerosols that blotted out the sun, leading to extreme cooling and the beginning of the Late Antique Little Ice Age. The new record showed that in the following decades, Rome saw average temperatures drop to as low as 3°C cooler than the warmest temperatures during the Roman Climatic Optimum — a period of unusual warmth and humidity from around 200 BCE to 100 CE that coincided with prosperity and development in the empire's early days.
"Our record provides striking local confirmation for the suddenness and severity of the Late Antique Little Ice Age," Harper said, noting that most evidence of the Late Antique Little Ice Age's onset is only available in high-altitude Alpine regions, where the cooling effect from volcanic emissions was strong and immediate, but the area was absent of civilization.
Climate, Pandemics and Rome's Decline
The abrupt onset of the Late Antique Little Ice Age around 540 CE coincided with the Plague of Justinian — the first wave of the first documented bubonic plague pandemic, caused by the bacterium Yersinia pestis. The new record also documented increasing climate instability beginning after around 130 CE, and cold and dry periods corresponding with two lesser-known plagues: the Antonine Plague (~165-180 CE) and the Plague of Cyprian (~251-266 CE).
Although the findings suggest that climate may have been influential in making the population more vulnerable to disease, the causes of these outbreaks remain unclear, Harper said, largely because of incomplete written records.
"We have to piece together every bit of evidence we can — written texts and documents, archaeological patterns, and increasingly, biomolecular remains," Harper explained. "The recovery of ancient pathogen DNA is helping [us to] understand everything from the evolutionary history of the germ itself to its patterns of movement over time, to the reach of the pandemic itself."
Harper has spent years investigating how climate and pandemic outbreaks influenced ancient Rome's trajectory. "It is amazing how much we have learned even in the last few years, and our record is another piece of the puzzle," Harper said, describing how the new results "reinforced certain interpretations and challenged others" laid out in his 2017 book on the same subject, The Fate of Rome: Climate, Disease, and the End of an Empire .
"A few years ago, I was hesitant to emphasize the role of climate stress in the period before the Antonine Plague," Harper reflected. "But now, there is a stronger case to see climate change as a major ingredient leading to the pandemic."
Past and Future Human-Climate Interactions
Although modern societies may be more technologically prepared to handle variable climates today, investigating how humans responded to past climate change could help scientists understand the risks involved in future anthropogenic climate change, Zonneveld and Harper say.
"Obviously it is important to stress the differences between our society and theirs," Harper explained. "We have modern science and all that goes with it — germ theory, biomedicine, antibiotics, vaccines, clean water, and so on." Harper explained. But the historical record can be valuable in underscoring the link between climate and human health embedded in the planet's physical and living systems, he said.
Zonneveld agreed. "Investigating the resilience of ancient societies to past climate change … might give us better insight into these relationships and the climate change-induced challenges we are facing today," she said.