When the Black Death swept through Europe in the late 1340s, it reshaped the continent almost overnight. Entire towns were emptied, economies collapsed, and tens of millions died in what became the most devastating pandemic in recorded human history.

The biological cause of the plague is well established. The bacterium Yersinia pestis, transmitted by fleas and carried by rodents, drove the catastrophe. What remains less certain is why the disease erupted so explosively at that moment, and how it spread so rapidly across medieval Europe.

A new study suggests the answer may lie not only in trade routes and warfare, but also in the sky.

Climate Shock Before the Plague

Researchers writing in Communications Earth & Environment propose that a sequence of volcanic eruptions in the mid-1340s altered climate conditions across Europe and the Mediterranean, indirectly setting the stage for the Black Death.

Their analysis points to two consecutive summers of unusually cool temperatures in 1345 and 1346. These cooling events, likely caused by volcanic aerosols blocking sunlight, appear to have disrupted agricultural production across key grain-growing regions.

The resulting food shortages, the researchers argue, forced Italian maritime powers to make political and economic decisions that unintentionally reopened pathways for the plague to enter Europe.

“This adds a missing piece to a long-standing historical puzzle,” said historian Hannah Barker of Arizona State University, who was not involved in the research. “Climate hasn’t really been considered as a trigger in earlier Black Death explanations.”

Read more: How SAP Is Using AI to Advance ESG and Nature-Based Solutions

Evidence Written in Trees and Ice

The research team reconstructed mid-14th century climate conditions using tree rings and ice core data. Co-author Ulf Büntgen, a dendrochronologist at the University of Cambridge, observed stunted growth in trees from the Pyrenees during the summers of 1345 and 1346, a clear signal of cold stress.

These findings were mirrored in tree-ring records from multiple locations across Europe. At the same time, ice cores from Greenland and Antarctica revealed elevated sulfur levels, a hallmark of volcanic eruptions that inject sun-blocking particles into the upper atmosphere.

“The temperatures weren’t extreme, but they were persistently low for two summers in a row,” Büntgen explained. “That pattern is consistent with clustered volcanic activity.”

Harvest Failures and Rising Tensions

Historical documents align closely with the physical evidence. Chroniclers across Eurasia described prolonged cloud cover and dimmed skies during the same period. In southern Europe, crop yields declined sharply, and grain prices surged to levels not seen in decades.

By early 1347, food scarcity was becoming a political crisis. Italian city-states such as Venice and Genoa had built sophisticated grain procurement systems, but those systems were constrained by ongoing conflict.

Since 1343, both cities had been effectively cut off from Black Sea grain markets due to hostilities with Mongol powers controlling the region. As climate-related shortages spread across Sicily, Spain, and North Africa, Italy’s options narrowed.

Governments began scrambling for solutions.

Trade Routes Reopened at a Dangerous Moment

According to the study, mounting food pressure forced Venice and Genoa to reverse course. In 1347, they negotiated peace with Mongol authorities, reopening Black Sea trade routes that had been closed for years.

Grain shipments soon resumed from ports in Crimea and along the northern Black Sea coast. These cargoes likely carried more than wheat.

Historical evidence suggests plague outbreaks were already affecting Mongol forces in the region. The researchers propose that fleas infected with Yersinia pestis may have traveled alongside grain, living in the dust and debris of bulk cargo.

When the shipments arrived in Italian ports, the chain reaction began.

Ports First, Then the Interior

The spatial pattern of early plague outbreaks supports the hypothesis. Coastal regions dependent on imported grain, particularly Venice and Genoa, were among the first to report cases. Cities with stronger local food supplies, such as Milan and Rome, were affected later.

“It’s an early example of how interconnected systems amplify risk,” Büntgen noted. “Trade accelerates spread.”

Within months, the disease moved inland, erupting into the full-scale pandemic that would devastate Europe by 1348.

A Chain of Contingencies

To historical epidemiologist Timothy Newfield, the research highlights how finely balanced the origins of the Black Death were.

“This makes the pandemic look even less inevitable,” he said. “So many conditions had to align.”

The pathogen needed to be circulating in the Black Sea region. Climate disruptions had to persist long enough to destabilize food systems. Political tensions had to ease at precisely the wrong moment. And trade networks had to function efficiently enough to move massive quantities of goods quickly.

Remove any one of those factors, and the outcome might have been very different.

Explore OneStop ESG Marketplace: Monitoring and testing

Climate as a Catalyst, Not the Cause

The study does not diminish the role of disease, poverty, or urban crowding in shaping the pandemic. Nor does it suggest volcanoes caused the Black Death directly.

Instead, it reframes climate as an active catalyst rather than a passive backdrop. Volcanic aerosols cooled the climate, harvests failed, markets destabilized, and supply chains were reconfigured in ways that allowed a deadly pathogen to travel vast distances.

Lessons Beyond the Middle Ages

While tree rings and ice cores cannot identify the specific volcanoes involved or trace the exact path of the first infected flea, they can establish timing. That timing aligns closely with documented hunger, policy shifts, and renewed trade in 1347.

The broader message extends well beyond medieval history. Food systems, health risks, climate shocks, and global trade are deeply interconnected. Disruptions in one domain can cascade rapidly into others, transforming local hazards into systemic crises.

In the case of the Black Death, the sequence may have been simple and devastating: dimmed skies, failed crops, desperate trade, contaminated grain, and disease in the streets.

A reminder that sometimes, history turns not on a single cause, but on a fragile alignment of forces.

Explore ESG Solutions on our marketplace - OneStop ESG Marketplace.

Keep abreast of the top ESG Events on OneStop ESG Events.

OneStop ESG Educate: Your go-to source for top ESG courses and training programs tailored to your needs.

Stay informed with the latest insights on OneStop ESG News.

Discover meaningful career opportunities on OneStop ESG Jobs.