A climate catastrophe that forced a rethink in trade routes was probably responsible for introducing the plague to Europe and causing the Black Death in the 14th century, researchers suggest.

Published in the journal Communications Earth & Environment, the new study combined historical records with studies of polar ice cores and European tree rings.

Researchers Martin Bauch at the Leibniz Institute in Germany, and Ulf Büntgen at Cambridge University, UK, conclude that an unidentified volcanic eruption around the year 1345 pumped climate-cooling ash and sulfur into Earth’s atmosphere and caused crop failures across the Mediterranean.

This forced powerful port cities to open trade with the Golden Horde, which dominated Central Asia at the time. In doing so, it provided safe passage for Yersinia pestis, the bacterium that caused the Black Death, into Europe.

Italian city-states had developed successful food security strategies. But they were no match for the plague.

"By a combination of several coincidences at a time, you get a side effect that you were not expecting. You couldn’t — from a 14th century perspective — calculate and expect this to happen — the same system which successfully saves you from starvation, will lead to their mass death if the Black Death reaches your city," said Bauch, who is an environmental historian.

Origins of the Black Death

Plague is caused by *Yersinia pestis bacteria. *The Black Death is the name given to a major plague wave across Europe from 1347 to 1351.

If bitten by an animal, such as a flea or a rodent, that is infected with the bacteria, a person would develop symptoms of swollen lymph nodes — called "buboes" — and potentially a mix of fever, fatigue, vomiting, nausea, and aches.

If the lungs were infected, the bubonic plague became pneumonic plague — a type that spread faster and was always fatal.

Fortunately, the development of bacteria-killing antibiotics has largely consigned plague to the past. But it still exists as a problem in some parts of the world, especially Madagascar, DR Congo and Peru. Cases continue to occur in the western United States, parts of Brazil and Bolivia, South and Central Asia.

And it’s in Central Asia where the Black Death likely originated. In 2022, another group of researchers from Germany and the UK were able to pinpoint the origin of the so-called "source strain" of Yersinia pestis.

They connected evidence of the disease that tore through Europe to outbreaks in the Tian Shan mountains, which border modern day Kyrgyzstan, in 1338.

It’s likely that through trade and human movement, disease-carrying rodents and insects were transported the long distances into western Eurasia and Europe — bringing the plague with them.

The Tian Shan mountain region of Central Asia is considered the cradle of plague bacteria that spread across Medieval EuropeImage: Lyazzat Musralina/REUTERS

Environmental markers, historical implications, plague trade

How plague arrived to Europe has been widely debated by researchers. In this latest study, Bauch and Büntgen used a mix of scientific data and historical records to detail at least one, potential pathway for the disease to enter the continent.

They argue in their paper that tree rings from eight regions across Europe, and volcanic sulfur in polar ice cores, show a major tropical volcanic eruption in 1345 could have caused a climate cooling effect that impacted the Mediterranean crops, and drove famine in southern Europe.

Records show that major Italian port cities, such as Venice and Genoa, engaged with the Mongol Golden Horde near the end of this famine, to import grain via Black Sea trade routes.

Supplying the region with grain helped prevent starvation for locals but likely introduced plague that continued to move throughout Italian states when other cities acquired grain supplies the same way.

Using environmental references like tree rings and ice cores allows scientists and historians to work together to understand how environmental changes may have influenced social and public health events.

Studies of subtle changes to tree rings and other "natural proxies" help what is known as paleoclimatic reconstruction — the understanding of ancient climates.

"Only tree rings have the quality that really enables us to bring things together," Bauch said.

Once the scientific data is combined with historical records, researchers like Bauch and Büntgen, can begin to explain the factors that may have driven major events, including the Black Death.

Maria Spyrou, a paleopathologist at the University of Tübingen, who led the group that identified the plague’s origin in 2022, said the new Bauch-Büntgen study added another piece to the puzzle of how plague had infiltrated and infected medieval Europe.

"The [Bauch-Büntgen] study provides further support for a mid-fourteenth century emergence of the pandemic, and is in line with genetic data showing that the ancestors of Black Death strains in Europe existed in the Volga region as well as in the Tian Shan region," Spyrou told DW in an email.

But Spyrou also notes that while the journey of plague from the Black Sea to Europe now seems to taken had another potential route, it’s still unclear how it moved throughout Central Asia itself.

Bauch agrees with that assessment. He said their study provided only one of several potential explanations for how plague entered and spread across 14th century Europe.

Edited by: Zulfikar Abbany