Did Volcanic Eruptions Unleash the Black Death?

The Black Death, a harrowing epidemic of bubonic plague that swept into Europe in 1347, first appeared in Mediterranean port cities before it surged across the continent. The disease marked its victims with huge black boils that oozed blood and pus, followed by fever, chills, vomiting, diarrhea and often, a swift death. By 1352, it had killed more than 20 million people—almost one-third of the continent’s population.

Through modern genetic analysis, scientists now know the Black Death was caused by the bacterium Yersinia pestis. A 2015 study suggests that climate swings in the Central Asian steppes may have disrupted local rodent populations, driving infected fleas from animals such as gerbils and marmots to new hosts, including camels and humans. From there, the theory holds, the plague moved slowly west along Silk Road caravan routes before reaching the Black Sea at the edge of Europe.

Researchers now believe climate changes may have also played a crucial role in driving the plague from the Black Sea region into Europe.

Intense volcanic activity around 1345 may have triggered a far-reaching chain reaction, posits researchers Martin Bauch and Ulf Büntgen, a professor in environmental systems analysis at the University of Cambridge. Volcanic ash and gases released into the atmosphere cooled temperatures and ruined harvests across parts of the Mediterranean. Italian maritime republics imported grain from the Mongols of the Golden Horde Empire around the Sea of Azov, which connects to the larger Black Sea.

“This emergency grain trade helped avert starvation in parts of Italy, but at the same time also created a pathway by which Yersinia pestis reached Mediterranean ports, most likely via infected fleas transported in grain cargo,” says Bauch, a historian of medieval climate and epidemiology at the Leibniz Institute. In Bauch and Büntgen's view, the Black Death wasn’t caused by any single factor but by a rare and dangerous mix of environmental stress, famine and human commercial response.

For their research, Bauch and Büntgen drew on two natural archives: “We turned to ice cores and tree rings because they allow us to reconstruct environmental conditions with annual precision, which is especially important for understanding a crisis that unfolded over only a few years,” Bauch explains.

The researchers found signs of major volcanic activity—either a single large eruption or several in close succession—around 1345. Ice cores from Greenland and Antarctica indicate an estimated 14 teragrams of volcanic stratospheric sulfur injection, a measure of how much sulfur from an eruption was lofted high into the atmosphere. That exceeds the climate-relevant sulfur injection associated with even the 1991 Pinatubo eruption, one of the most powerful eruptions of the past century. While the team does not identify a precise geographic location, they believe the volcanic activity was likely tropical because the sulfur signature appears in both polar records.

Tree-ring data gathered from eight regions in Europe then revealed the aftermath: a run of abnormally cold summers in 1345, 1346 and 1347. “Our evidence suggests that the cooling was widespread across much of Europe, with especially strong indications in the Mediterranean region,” says Bauch.

In the central Spanish Pyrenees, the researchers found two consecutive “Blue Rings”—a rare tree-ring anomaly linked to sudden summer cold—pointing to sharp temperature drops in 1345 and 1346. Taken together with historical reports of foggy skies and reduced sunshine, the evidence suggests that volcanic ash and gases lingering in the atmosphere may have contributed to a sudden climatic downturn.

That downturn appears to have had immediate consequences on the ground. Historical records point to widespread crop failures and food shortages across parts of the Mediterranean just before the plague arrived. Cereal prices spiked—including the highest price of wheat in eight decades—and the Italian maritime republics of Venice, Genoa and Pisa scrambled to secure supplies with grain producers farther afield.

“Venetian evidence is particularly explicit. In April 1347, the trade embargo on the Golden Horde was lifted, ships were sent toward Tana and Asia Minor to acquire grain, and by August 1347 Venetian authorities were ordering agents to obtain grain ‘at any cost,’” Bauch says.

With the embargo lifted, Italian ships could reach trade hubs around the Black Sea and Sea of Azov, along with grain-producing regions farther north that seem to have escaped the worst of the climate shock. The ships that returned later in the year carried back not only grain but possibly Yersinia pestis, carried by infected fleas hidden in the cargo.

From the main trading ports of Venice, Genoa and Pisa, the disease spread rapidly. Outbreaks were also reported by the end of 1347 in other Mediterranean harbor towns, including Marseille, France and Palma de Mallorca, Spain. Interestingly, the researchers note that self-sufficient Italian cities like Milan and Rome, which didn’t import grain from the Black Sea region in 1347 and 1348, largely escaped the first wave of plague outbreaks.

While Bauch is careful not to present the research as the final word on how the Black Death spread, he says it “challenges accounts that treat trade as a static background factor, because we argue that the change in grain-trade behavior during the famine was crucial.”