The infamous Black Death—a pandemic that killed as many as one third to one half of Europeans within just a few years—may have been aided in its devastation by an unknown volcanic eruption.
That’s the hypothesis presented in research published December 4 in Communications Earth & Environment, which argues that the eruption triggered several seasons of climate instability and crop failures. That instability, in turn, forced several Italian states to import grain stores from new sources—specifically, from regions surrounding the Black Sea. Riding along on those grain stores, the researchers posit, were fleas infected with Yersinia pestis, the bacterium that causes the plague.
Martin Bauch, a medieval and environmental historian at the Leibniz Institute for the History and Culture of Eastern Europe in Germany, studies historic famine. While looking through historic records, he noticed a particularly serious crop failure beginning in northwestern Italy in late 1345 after serious rainstorms. Within just two years, the Black Death had begun, so he was curious whether there might be a connection.
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Analyzing records about the grain trade suggested some Italian cities exhausted their typical food supplies, forcing them to import grain from the Black Sea region. Although the measure kept people fed, it may have introduced the Black Death to Europe as a nightmare ride-along, the Bauch and his co-author suggest.
The researchers analyzed tree-ring data from across Europe to see how temperatures fluctuated across the century leading up to the Black Death to better understand the massive rainfalls Bauch had seen referenced in records. Then they consulted other scientists’ studies of polar ice cores, looking for signs of volcanic eruptions that injected sulfur high into the atmosphere, where they can affect climates distant from the volcano itself, including changing rainfall patterns. Here a powerful eruption in 1345 stood out.
The eruption itself remains mysterious. The researchers suspect it occurred relatively close to the equator because its debris is visible in ice caps from both poles. But it will take significant additional work to identify the culprit. “Nobody considers this eruption particularly interesting,” Bauch says. “We hope that changes.”
Historians once believed the plague to have been carried into Italy by sailors fleeing a battle in the east, but that theory has fallen apart in recent years. Meanwhile researchers have discovered increasing evidence that the disease had been circulating in Asia in the decades before the Black Death began, setting the stage for the pandemic. “None of that could have happened without a lot of other things happening first,” says Monica Green, a historian of medicine who works extensively on plague and the Black Death and who was not involved in the new research.
Even without a particular eruption identified, the new research offers a refreshingly tight hypothesis, says Henry Fell, a disease ecologist studying the plague at the University of Nottingham and the University of York in England, who was not involved in the new research. “Climate is regularly discussed as a driver in the Black Death; however, quite what mechanism links climate to plague is relatively difficult to unpick,” he says. That’s because plague is an intricate disease system, with people, bacteria, fleas and multiple species of rodents all interacting in time and space.
Although scholars of all stripes have spent centuries working to understand what caused the Black Death, the new study underscores how that research is not yet complete. And it remains vital even today, both as the plague remains in circulation in pockets of the globe and as COVID has reminded us of the price of pandemics.
“We have to have a better understanding of pandemics,” Green says. “That’s kind of the moral imperative.”