In the 14th century, the Black Death swept across Europe, decimating populations and upending the continent. The precise reason for the amplification of the plague is unknown, but new evidence suggests that volcanic eruptions enabled inflected fleas to invade Europe, thus initiating the plague’s destruction.

Around 1345 CE, a series of volcanic eruptions occurred in the tropics, causing a sudden climate shift in Europe, leading to colder weather. This shift affected harvests across the continent, initiating widespread crop failures and eventually famine.

To prevent mass starvation, wealthy Italian city states decided to import grains from the Black Sea region. While this mass importation saved thousands from famine, it doomed many more, as it ushered in the pandemic. 

Fleas, carrying Yersinia pestis, stowed away on ships carrying the imported grains, giving them the ability to infiltrate Europe and pass their disease onto rats, who would eventually spread it to humans. 

Researchers previously believed that climate shifts were the cause for the introduction of the plague, but there was never any concrete evidence to support it. Building off of this concept, researchers Martin Bauch and Ulf Buntgen looked at tree ring growth from eight different regions in Europe. When they studied the rings dating back to the 1350s, they discovered ‘blue rings.’ These stains appeared in all of the trees that data was collected from and indicate stress and cold spells.

In addition, these researchers looked at sulfur levels preserved in ice cores in Greenland and Antarctica. These layers of ice can trap in chemicals from when snow originally fell, allowing researchers to study chemical compounds within them. These scientists discovered that the ice from the year 1345 had one of the strongest signals for sulfur, indicating there was a lot of sulfur injected into the atmosphere. 

These two pieces of data point to a volcanic eruption since volcanoes release tons of sulfur into the atmosphere, which in turn, reduces the amount of sun that can reach Earth’s surface and thus cools the planet. Using this information, researchers were finally able to develop a series of events that theoretically caused the spread of the plague and devastated humanity for centuries to come.