Volcanoes may have helped spread the Black Death
A new analysis aims to answer a longstanding question about why the plague reached Europe when it did—and why it spread so quickly.
In A.D. 1347, the Black Death arrived in southern Europe. It swiftly spread across the Italian peninsula, killing half the population in some areas. Eyewitness accounts tell of entire households succumbing to plague, common graves dug for victims, and terror that overtook cities.
The plague’s impact is infamous today, and scientists have extensively studied the Yersinia pestis bacteria that caused it, as well as the rats and fleas that spread it. But a new analysis implicates yet another collaborator: volcanoes.
Why the disease, which likely emerged in human populations in the early 1300s and ravaged communities in Central Asia in the 1330s, didn’t arrive in the Mediterranean until 1347 hasn’t been clear. The new study in the journal Communications Earth & Environment links the timing of the Black Death and its explosion across the Italian peninsula with volcanic activity that cooled the climate, caused famine and spurred grain imports that may have also brought pestilence.
“There’s this series of factors that all lined up. And if you took any of them away, this wouldn't have happened,” says Hannah Barker, a historian at Arizona State University in Tempe who wasn’t involved with this work. It took a confluence of climate change, animal interactions and human actions to produce medieval Europe’s plague pandemic.
(Fast and lethal, the Black Death spread more than a mile per day.)
Clues from tree rings and ice cores
Geographer Ulf Büntgen of the University of Cambridge found new clues about climate’s role in the pandemic while working on a climate archive based on tree rings. He and his colleagues use tree ring rata to reconstruct temperature and precipitation records for the past 2,000 years. “The dating is so precise,” he says.
Looking at the climate record compiled from trees across Europe, Büntgen noticed that temperatures across the Mediterranean were slightly cooler than average from 1345 to 1357. “Nothing super striking,” he says. But Büntgen wanted to understand why the cooling occurred. He suspected volcanic activity, which spews climate cooling aerosols. So he conferred with experts who study ice cores that preserve a chemical record of Earth’s atmospheric conditions. Cores from Greenland and Antarctica contained elevated sulfur, which is emitted by eruptions, in layers dated to around 1345. That suggested there may been one or a series of eruptions, likely in the tropics, at the time.
Wondering about potential societal connections, Büntgen teamed up with medieval historian Martin Bauch to piece together more of the story. Bauch found hints of volcanic activity in the historical record. People in China and Bohemia reported lunar eclipses when, based on calculations of orbits, they should not have occurred. It’s possible that particle-laden skies altered the appearance of the moon leading to the strange reports, says Bauch of the Leibniz Institute for the History and Culture of Eastern Europe in Leipzig, Germany.
The yearslong cool spell likely also impacted crop harvests across the Mediterranean, prompting human actions that may have sped plague’s spread. The Italian city states put a great emphasis on food security, Bauch says. After famines in the century before the Black Death, they built up long-distance trade networks, so they could acquire wheat from places such as North Africa and the Black Sea region to the west.
Based on historical, administrative, and legal records, grain prices spiked, and worries over food security seemed most pronounced in 1346 to 1347. “Even these major players like Venice and Genoa suddenly feel an urgent need to import as much grain as possible,” says Bauch.
In the years prior, a trade war between these city states and the Mongols, who controlled much of the northern Black Sea, halted grain imports from the region, Barker says. But as the Mongols were dying of plague and Venice and Genoa became increasingly desperate for wheat, both sides became less invested in the trade war, allowing ships to bear their goods—and the plague—to the Italian city states. Pathogen-carrying fleas can live off both grain dust and the blood of rats and mice on grain ships. “Making peace and reopening the grain trade is what causes it to spread,” she says.
Overall, the maritime city states succeed at staving off starvation, Bauch says. “Yet they bring the whole the worst danger to their to their cities that you can imagine.” Venetians sent some of the grain they imported on to Padua and Trento, which may have triggered plague outbreaks in those cities. By the end of 1348, many locations in Italy and around the Mediterranean had experienced the Black Death.
(An abbey graveyard shows how the plague impacted rural communities.)
Climate connections
Barker, in a paper published in 2021, had previously linked the grain trade to the spread of plague. But the connection to volcanic activity hadn’t been known. “This is yet further proof that historians working alongside paleoscientists leads to good things,” says Timothy Newfield, a disease historian at Georgetown University in Washington D.C. who wasn’t involved with the work. Normally historians research, write and publish alone he says. The work “raises the bar for how we study past plagues’ relationship with climate.”
These sorts of studies, combining historical sources with climate records, can help researchers better understand what drives the emergence and transmission of disease, says Kyle Harper, a historian at the University of Oklahoma in Norman who wasn’t part of this study.
Even though human mortality from Yersinia pestis infections has plummeted, its scientific relevance persists. Overall, antibiotics, vaccines and an access to clean water have largely curbed the death rate from infectious disease, Harper says. So researchers need to look back in time for case studies to better understand the factors at play, and particularly the link between climate change and health.
The coincidence of events needed to produce plague’s “mind-boggling outbreaks” have occurred a few times over history, Harper says. So researchers should learn from them. “The Black Death is exceedingly unlikely,” Harper says. “If you're playing the odds, that should never happen.”
