As the Black Death swept through Europe beginning in 1347, the plague wiped out more than half the continent's population, destroying societies and interrupting wars.
A new study suggests that a volcanic eruption or multiple eruptions unknown to Europeans was likely the catalyst for the pandemic's arrival on the continent's shores.
Theory, described in a study published Thursday in the journal Communications Earth & Environment.suggests that the eruptions triggered a series of events that allowed the fleas that spread the plague to spread across Europe.
The eruptions lowered global temperatures for several years, causing a sudden climate shift that affected crops in Europe. With crops failing and fears of famine rising, some wealthy Italian city-states such as Florence and Venice imported grain from other countries around the world. And these ships most likely received fleas infected with plague.
The actions of the leaders of Florence prevented mass famine – tens of thousands of starving refugees migrated there, and the city was able to feed them in addition to its own citizens. However, imports unwittingly led to a pandemic.
City officials were proud of their achievement in providing enough food for so many people, said Martin Bauch, author of the new study and a medieval historian at the Leibniz Institute for the History and Culture of Eastern Europe in Germany.
“They had no idea what the danger was there,” he said.
The study offers a historical example of how climate change can alter human society and animal ecosystems in ways that are difficult to predict and with incredible consequences for subsequent generations.
Researchers have debated and studied the details of the plague's origins and spread for decades, but this study is the first to detail the potential role of a volcanic eruption. Previous studies have suggested that climate shifts may have been responsible for the emergence of plague at various points in history, but most studies have been inconclusive, according to Henry Fell, a postdoctoral fellow at the University of Nottingham and the University of York in England.
“I think what this paper is really good about is that it details the mechanism that causes it,” said Fell, who was not involved in the study but has studied the Black Death and climate change. “We are seeing an increase in grain trade from these ports, and the reason for this is climate.”
Volcanic eruptions can cool the planet by releasing forms of sulfur into the stratosphere, reducing the amount of sunlight that can reach the Earth's surface.
“It literally blocks out some of the sun, which causes cooling,” Fell said.
The effect can last for several years after significant eruptions.
To understand 14th-century volcanic activity for the new study, researchers analyzed tree-ring records, ice core data and written historical observations. All three lines of evidence agree: the cooling period and Mediterranean famine from 1345 to 1347 preceded and coincided with the appearance of the plague in Europe.
The team looked at the chemical composition of ice cores in Greenland and Antarctica, since layers of ice in polar regions can trap chemicals left behind by the initial snowfall, suggesting a timeline of sorts. They found that 1345 was the 18th strongest sulfur signal in the last 2,000 years. The amount of material ejected into the stratosphere that year exceeded the best recent example of such dynamics – Mount Pinatubo injection.
Meanwhile, tree rings dating from the same period have biological spots called “blue rings” that indicate stress and likely cold weather. Blue rings appear sequentially, which is extremely rare.
Finally, written accounts from the time in modern China, Japan, Germany, France and Italy report decreased sunlight and increased cloud cover, the study says.
Researchers have not been able to determine exactly which volcano or volcanoes erupted in 1345. But they identified a region of the world: “It must be a tropical eruption,” Bauch said.
The reason: ice cores from both poles of the Earth contained approximately the same amount of volcanic sulfate.
Historical data shows crop failures and high wheat prices during this cool period, as well as severe famines across much of Spain, southern France, Italy, Egypt and the eastern Mediterranean, according to a new study.
According to Bauch, the Italian city-states were wealthy at the time and had extensive grain storage systems and huge trade networks built up over centuries.
But as the famine worsened, their options dwindled.
“They really started to realize in 1347: 'We have to import grain from the Black Sea. This is the last place where they still have enough grain for our needs,” Bauch said.
“Of course they don’t know how the plague got to them,” he added. “I have records from Venice from 1349 and they are very pleased and say, 'Look, during the last famine it was actually Black Sea grain that saved us, and it worked very well.'
Bauch said he suspects the plague would likely have reached Europe eventually, but events caused by volcanic eruptions likely sped up the process.
Between 1347 and 1353, up to 60% of the population died in some parts of Europe, making it one of the deadliest periods in history.
Previous research into the origins of the plague has relied on written sources, archaeological data and even genetic data. In 2022, scientists found DNA evidence of the bacterium that causes the plague. in bodies buried on the territory of modern Kyrgyzstan — Far East of Europe. Headstones indicate that many of those killed were buried in 1338 and 1339 (about ten years before the plague struck Europe) and died of “the plague”.
The same research team also found that marmots living today in the Tian Shan mountain range near the burial site carried a closely related strain of the bacterium, a clue that the Black Death may have originated there before spreading elsewhere.
The new study may explain what happened next, Fell said, adding that he thinks the authors have made a compelling case that a volcano is to blame.
After the plague first broke out in Europe, it resurfaced over the centuries, changing human history.
“In a European context, this is very important to our history,” Fell said. “Any study in which you look at a long period of time across Europe is going to lead to the plague.”
