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Human history is pockmarked with periods of death and destruction on unimaginable scales. Of these calamitous epochs, one stands out: The Black Death. The mid 14th century scourge killed tens of millions of people in Europe, Asia, and Africa and changed the course of history—marking the tail end of the Middle Ages and ushering in the cultural reawakening of the Renaissance by disrupting society, the feudal system, and economies across the continent.
Researchers have long known the Black Death’s central villain: the bacterium Yersinia pestis, which caused the bubonic plague that swept through towns and villages with a mortality rate of up to 60 percent. Experts also know this microbial agent was spread by fleas, borne on the backs of rodent pests and maybe domestic animals, and passed between humans through the air and bodily fluids. But historians have had a tougher time recreating the sequence of events that initially started the devastating pandemic.
Now, a pair of scientists have found new clues hidden in tree rings. By looking at these rings in the Spanish Pyrenees—as well as details in historical accounts of the time—they suggest that heightened volcanic activity sometime around 1345 may have sparked a famine, kicking off the sequence of events that eventually led to the Black Death raging through Eurasia from 1347 and 1353. They published their findings today in Communications Earth & Environment.
“We looked into the period before the Black Death with regard to food security systems and recurring famines, which was important to put the situation after 1345 in context,” said Martin Bauch, historian of medieval climate and epidemiology at the Leibniz Institute for the History and Culture of Eastern Europe and co-author, in a statement. “We wanted to look at the climate, environmental, and economic factors together, so we could more fully understand what triggered the onset of the second plague pandemic in Europe.”
Read more: “The Volcano That Shrouded the Earth and Gave Birth to a Monster”
Here is the model Bauch and his colleague Ulf Büntgen, a dendrochronologist at Cambridge University, propose. As yet unknown volcanic eruptions ejected huge amounts of ash and gases into the atmosphere around 1345, causing drops in annual temperatures that persisted for several years. The cross sections from living and relic trees that the researchers studied had “blue rings,” denoting abnormally cold and wet summer growth seasons, in 1345, 1346, and 1347. Additional accounts from the time considered by Bauch and Büntgen tell of abnormal cloudiness and dark lunar eclipses, further hints of volcanic activity. This sustained cooling could have caused widespread crop failure across the Mediterranean.
The resulting food shortages drove merchants in the maritime republics of Venice, Genoa, and Pisa to increase imports of grain from the Mongols living around the sea of Azov in 1347. Along with shipments of grain coursing across established trade routes came plague-infested fleas. Once Y. pestis and the fleas that carried it landed in Europe, the pathogen jumped to rats, mice, and perhaps domesticated animals. Eventually the disease hopped to humans, and people began transmitting it in densely packed population centers. The rest is a dark part of history.
“For more than a century, these powerful Italian city states had established long-distance trade routes across the Mediterranean and the Black Sea, allowing them to activate a highly efficient system to prevent starvation,” said Bauch. “But ultimately, these would inadvertently lead to a far bigger catastrophe.”
From this newly proposed picture of the Black Death’s roots, Bauch and Büntgen, suggest a lesson for current humans. “Although this unique spatiotemporal coincidence of many influences seems rare, our findings emphasise the increased likelihood of zoonotic infectious diseases to suddenly emerge and rapidly translate into pandemics in both a globalized and warmer world, with COVID-19 just being the latest warning sign,” they write.
Continuing to study the intricacies of history’s most devastating episodes could help us avoid repeating mistakes made in the wake of natural disasters. 
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Lead image: Fresco, known as the “Triumph of Death,” attributed to Buonamico Buffalmacco; photo by Martin Bauch
