The Black Death—one of the deadliest pandemics in human history, estimated to have killed up to half of Europe’s population—might have been set in motion by a volcanic eruption, a new study suggests, reports BritPanorama.
Researchers have examined tree rings from across Europe, correlated their findings with ice core samples from Antarctica and Greenland, and analyzed historical documents to construct a “perfect storm” scenario that could explain the origins of this historic tragedy. Their findings were published Thursday in the journal Communications Earth & Environment.
The study’s authors believe an eruption occurred around 1345, approximately two years before the pandemic began, potentially from a single volcano or a group of volcanos located in the tropics. The resulting haze from the ash would have blocked sunlight across the Mediterranean region for several years, leading to temperature drops and subsequent crop failures.
This situation led to a grain shortage that threatened famine or civil unrest. In response, Italian city-states like Venice and Genoa resorted to emergency imports from the Black Sea region, enabling them to sustain their populations.
However, the ships carrying the grain were also laden with a lethal bacterium: Yersinia pestis. This pathogen, originating from wild rodent populations in Central Asia, subsequently caused the plague that decimated Europe.
“The plague bacterium infects rat fleas, which seek out their preferred hosts—rats and other rodents. Once these hosts have died from the disease, the fleas turn to alternative mammals, including humans,” said study coauthor Martin Bauch, a historian of medieval climate and epidemiology at the Leibniz Institute for the History and Culture of Eastern Europe in Germany.
“Rat fleas are drawn to grain stores and can survive for months on grain dust as an emergency food source, enabling them to endure the long voyage from the Black Sea to Italy,” Bauch added. “After arrival in the port cities, the grain was placed in central granaries and then redistributed to smaller storage sites or traded further.”
Before the pandemic, the world population was less than 450 million. Between 1347 and 1351, the Black Death is estimated to have killed at least 25 million people. The societal, economic, and cultural consequences of this population loss persisted for decades across Europe and beyond.
Anomalous years
It was previously understood that ships and the grain trade played a significant role in how the Black Death reached Europe. However, this study is the first to propose that a volcanic eruption may have triggered a series of events leading to the pandemic.
“I found that the most pronounced famine in the 13th and 14th centuries is specifically in these years directly preceding the Black Death,” Bauch noted. “Why the Black Death arrives precisely in 1347 and 1348, in Italy at least, we can’t explain without that climate-induced famine background.”
Through his investigation of historical records, including administrative documents, letters, plague treatises, and poems, Bauch sought scientific evidence beyond his field to support his findings.
At a conference, he met Ulf Büntgen, a professor of environmental systems analysis at the University of Cambridge and coauthor of the study. The two recognized a shared interest in the same anomalous years and decided to collaborate on the topic.
“We are talking about something that happened 800 years ago,” Büntgen remarked. “Where do we get information about climate? We have a limited number of sources. One is historical or documentary evidence, that Martin deals with, and then there are tree rings.”
According to Büntgen, tree rings provide a “high-resolution paleoclimatic reconstruction” as they reflect environmental conditions affecting a tree throughout its life. If conditions are ideal, a tree produces wider rings and denser wood. By examining these rings, researchers can infer the climate conditions during the tree’s growth and identify periods of unusual temperature fluctuations.
Büntgen assessed thousands of tree samples from both living and ancient dead trees, previously collected for historical temperature research. He found evidence of a cooling climate that coincided with Bauch’s hypothesis of famine. “In the tree ring we see a climatic downturn, which means colder than normal temperatures for two to three years consecutively,” Büntgen explained.
Furthermore, Büntgen researched historical ice core data to find chemical signatures that would support the tree ring findings. “At the same time, we found evidence of sulfur spikes in the ice core records, which are completely independent of the trees, and that would refer to a volcanic eruption,” he stated.
Such large, sulfur-rich volcanic eruptions are known to cause cooling in the subsequent summers. This volcanic origin could help explain why some regions of Europe faced severe population declines while others remained largely unaffected.
“For example, the plague didn’t spread to Rome or Milan,” Bauch noted. “These are large cities, but they were surrounded by grain-producing areas, so they did not need to import as urgently as Venice and Genoa.”
The transmission of the plague via grain shipments underscores the complexity of the Black Death, influenced by a confluence of environmental, societal, and economic factors. “A lot of things needed to come together,” Büntgen said, “and if only one of them weren’t there, then this pandemic wouldn’t have happened.”
An interesting wrinkle
The study’s conclusion that the plague bacterium arrived via Mediterranean ports as a result of volcanic activity adds a significant new layer to the understanding of the interplay between climate change and disease dynamics, according to Mark Welford, a geography professor at the University of Northern Iowa, who was not involved with the research.
This research also advances the ongoing discussion about the role of weather fluctuations in the onset of the Black Death, as highlighted by Mark Bailey, a professor of late medieval history at the University of East Anglia in England.
“The authors recognize that such an exceptional event as the Black Death must have resulted from an exceptional coincidence of natural and social forces, which is sensible,” Bailey noted, reflecting on the implications of the study.
“Their novelty is to emphasize the link between volcanic activity, dearth, and changing grain trade routes in the two years preceding the Black Death exploded across Europe: we already knew about harvest failures, and the grain trade with the east was more likely intensified rather than changed in 1347,” he added.
Alex Brown, an associate professor in medieval economic and social history at England’s Durham University, praised how the study illustrates the interconnectedness of the medieval economy. “Bauch and Büntgen’s study demonstrates the significance of understanding the relationship between people, animals, and the environment, for both the study of historical pandemics and future pandemic preparedness,” he remarked, providing additional insight into the research’s implications.
