Dec. 4, 2025, 11:00 AM ESTBy Evan BushWhen the Black Death swept through Europe beginning in 1347, the plague wiped out more than half of the continent’s population, upending societies and interrupting wars. New research suggests that a volcanic eruption or multiple eruptions, unknown to Europe’s inhabitants, most likely catalyzed the pandemic’s arrival on the continent’s shores.The theory, described in a study published Thursday in the journal Communications Earth & Environment, suggests the eruptions set off a series of events that enabled the fleas that spread the plague to proliferate in Europe. The eruptions dimmed global temperatures for a few years, causing a sudden climate shift that affected harvests in Europe. With crops failing and fears of starvation rising, some wealthy Italian city-states like Florence and Venice imported grain from elsewhere in the world. And on those ships most likely came plague-infected fleas. The actions of Florence’s leaders prevented mass starvation — tens of thousands of famine refugees migrated there, and the city was able to feed them in addition to its own citizens. But the imports unwittingly ushered in a pandemic.City leaders were proud of their accomplishment in providing enough food for so many people, said Martin Bauch, an author of the new study and a medieval historian at the Leibniz Institute for the History and Culture of Eastern Europe in Germany. “They couldn’t have an idea of what danger was there,” he said. The research offers a historical example of the way that changes in the climate can alter human societies and animal ecosystems in hard-to-predict ways and with incredible downstream consequences. Researchers have debated and chased details of the plague’s origin and spread for decades, but this study is the first to outline in detail the potential role of a volcanic eruption. Previous research has suggested climate shifts could be responsible for introductions of the plague at various points in history, but most studies were vague about it, according to Henry Fell, a postdoctoral researcher at the University of Nottingham and the University of York in England.“This paper, I think, is really good for being quite specific on the mechanism that’s driving it,” said Fell, who was not involved in the study but has studied the Black Death and climate change. “We can see grain trade increasing from these ports, and the cause of it is climate.”Volcanic eruptions can cool the planet by injecting forms of sulfur into the stratosphere, which reduces the amount of sunshine that can reach Earth’s surface.“It literally blocks out some of the sun that leads to cooling,” Fell said.The effect can last several years after significant eruptions. To understand 14th century volcanic activity for the new study, researchers reviewed tree ring records, data from ice cores and written historical observations. All three lines of evidence agreed: A cooling period and a Mediterranean famine from 1345 to 1347 preceded and coincided with the plague’s emergence in Europe. The team studied the chemical composition of ice cores in Greenland and Antarctica, since layers of ice in polar regions can trap chemicals from when snow originally fell, offering a kind of timeline. They found that the year 1345 had the 18th-strongest signal for sulfur in the last 2,000 years. The amount of material injected into the stratosphere that year exceeded the best recent example of that dynamic, the Mount Pinatubo injection of 1991. Meanwhile, tree rings dating to the same period have biological stains called “blue rings” that indicate stress and a likely cold spell. The blue rings appear consecutively, an extreme rarity.Finally, written accounts from the time in present-day China, Japan, Germany, France and Italy report less sunshine and an increase in cloudiness, the study says. The researchers could not pinpoint which particular volcano or volcanoes erupted in 1345. But they did determine the region of the world: “​This must be a tropical eruption,” Bauch said.The reason: Ice cores from both of Earth’s poles had roughly equal measures of volcanic sulfate. Historical records show reports of crop failures and high prices for wheat during that cool period, with severe famine in large parts of Spain, southern France, Italy, Egypt and the eastern Mediterranean, according to the new study. At the time, Italian city-states were wealthy, Bauch said, and had developed storage systems for grain and huge trade networks established over centuries. But as famine deepened, their options dwindled. “They really start to realize in 1347, ‘We have to import from the Black Sea. That’s the last place where they still have enough grain for our needs,’” Bauch said. “Of course, they’re not aware of how the plague gets to them,” he added. “I have records from Venice in 1349, and they’re really satisfied, and they say, ‘Look, in the last famine, the Black Sea grain really saved us, and that worked very well.’” Bauch said he suspects that the plague most likely would have reached Europe eventually but that the events initiated by the volcanic eruptions were likely to have accelerated the process. As much as 60% of the population died in parts of Europe from 1347 to 1353, making it one of the deadliest periods in history. Past research on the plague’s origins has relied on written accounts, archaeological evidence and even genetic clues. In 2022, scientists found DNA evidence of the bacterium that causes the plague in bodies buried in modern-day Kyrgyzstan — far east of Europe. Grave markers indicate that many of those killed were buried in 1338 and 1339 (about a decade before the plague hit Europe) and that they died of “pestilence.” Yersinia pestis bacteria, which cause bubonic plague in animals and humans, are usually transmitted by the bites of infected rat fleas.BSIP / Universal Images Group via Getty ImagesThe same research group also discovered that marmots living today in the Tian Shan mountain range near the burial site carry a closely related strain of the bacterium, which is a clue that the Black Death could have emerged in that location before it spread elsewhere. The new study could explain what happened next, Fell said, adding that he thought the authors made a convincing argument that a volcano was to blame. After the plague first exploded in Europe, it re-emerged for centuries, reshaping human history. “In a European context, it’s so important to our history,” Fell said. “Any study where you’re looking at a long time period across Europe, there’ll be a plague.”Evan BushEvan Bush is a science reporter for NBC News.