Volcanic eruption led to the Black Death, new research suggests

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.

By looking at tree rings from across Europe to better understand 14th century climate, checking data against ice core samples from Antarctica and Greenland, and analyzing historical documents, researchers have constructed a “perfect storm” scenario that could explain the origin of the historic tragedy. They reported their findings Thursday in the journal Communications Earth & Environment.

The study authors believe an eruption occurred around 1345, about two years before the start of the pandemic, from either a single volcano or a cluster of volcanoes of unknown location, likely in the tropics. The resulting haze from volcanic ash would have partially blocked sunlight across the Mediterranean region over multiple years, causing temperatures to drop and crops to fail.

An ensuing grain shortage threatened to spark a famine or civil unrest, so Italian city-states, such as Venice and Genoa, resorted to emergency imports from the Black Sea region, which helped keep the population fed.

However, ships that carried the grain were loaded with a deadly bacterium: Yersinia pestis. The pathogen, originating from wild rodent populations in Central Asia, went on to cause the plague that devastated 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 from 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 killed at least 25 million people. The societal, economic and cultural consequences of the population loss lingered for decades across Europe and the rest of the world.

Anomalous years

Ships and the grain trade were already thought to have played a central role in how the Black Death reached Europe. However, this study is the first to suggest that a volcanic eruption may have been the first domino to fall in a cascade of events that led 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 said. “Why the Black Death arrives precisely in 1347 and 1348, in Italy at least, we can’t explain without having that climate-induced famine background.”

Bauch looked at historical documents including administrative records, letters, plague treatises, and even poems and inscriptions to build a picture of these crucial pre-pandemic years, but he needed scientific evidence from beyond his field to confirm his findings.

At a conference, he met Ulf Büntgen, a professor of environmental systems analysis at the University of Cambridge in England and coauthor of the study. The two discovered they shared an interest “in the same anomalous years,” according to Bauch, and decided to work together to study the topic.

“We are talking about something that happened 800 years ago,” Büntgen said. “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.”

Tree rings offer a “high-resolution paleoclimatic reconstruction,” Büntgen said, because as a tree grows, it’s constantly affected by things happening around it. If the growth conditions are favorable, the tree produces a wider ring and higher wood density, for example. The rings, which are formed every year, can be read like a book to understand if the climate around the tree was cold, warm, dry or wet.

Büntgen looked at thousands of tree samples from living and ancient dead trees preserved naturally from across Europe, which had been collected for previous research about historical temperature reconstruction. The rings, he noticed, indicated a cooling climate that matched Bauch’s hypothesis of a famine. “In the tree ring we see a climatic downturn, which means colder than normal temperatures for two to three years consecutively,” Büntgen said.

Büntgen also examined historical ice core data to look for chemical signatures that would corroborate the tree analysis. “At the same time, we found evidence for sulfur spikes in the ice core records, which are completely independent of the trees, and that would refer to a volcanic eruption,” he said.

Large, sulfur-rich volcanic eruptions are known to produce a cooling in the following summers, Büntgen added. The volcanic origin would help explain one of the enduring mysteries of the Black Death, which is why some parts of Europe lost up to 60% of the population while others remained unaffected.

“For example, the plague didn’t spread to Rome or Milan,” Bauch said. “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.”

Transmission of the plague via grain shipments would support the idea that the Black Death is a complex event, influenced by a vast array of natural, societal and economic factors. “A lot of things needed to come together,” Büntgen said, “and if only one of them wasn’t there, then this pandemic wouldn’t have happened.”

An interesting wrinkle

By making a strong case that the plague bacterium arrived through Mediterranean ports as the result of volcanic activity, the study adds another interesting wrinkle to scientists’ understanding of the intersection of climate change and disease dynamics, said Mark Welford, a professor and head of geography at the University of Northern Iowa, in an email. Welford was not involved with the work.

The new research also nudges forward the ongoing debate on how weather fluctuations might have influenced the start of the Black Death, according to Mark Bailey, a professor of late medieval history at the University of East Anglia in England.

“The authors recognise that an event as exceptional as the Black Death must have been due to an exceptional coincidence of natural and social forces, which is sensible,” Bailey wrote in an email. He did not participate in the study.

“Their novelty is to emphasise the link between volcanic activity, dearth and changing grain trade routes in the two years before the Black Death exploded across Europe: we already knew about harvest failures, and the grain trade with the east more likely intensified rather than changed in 1347,” he added.

Praising how the paper demonstrates the interconnectedness of the medieval economy, Alex Brown, an associate professor in medieval economic and social history at England’s Durham University, said in an email: “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 also was not involved with the research.

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