There black plaguewhich devastated Europe in the late 1340s, was not only an unprecedented health tragedy, but also an event that halved entire populations in a very short time. Tens of millions of people died, leaving a continent exhausted and forever transformed.
For centuries we have known that the bacterium was directly responsible for that enormous massacre Yersinia pestistransmitted by fleas and rats. What, however, continues to divide scholars is the dynamics that allowed the pandemic to spread with impressive speed and, above all, the reason why it exploded at that precise historical moment.
Black Death and volcanic eruptions
A new study published in the journal Communications Earth & Environment proposes a key to understanding that has so far little explored: the basis of the diffusion of black plague there might have been too medieval volcanic eruptions and the resulting climatic consequences.
According to researchers, a series of eruptions particularly rich in sulfur would have released enormous quantities of ash and aerosols into the atmosphere, capable of shielding sunlight. The result was an anomalous cooling of summer temperatures for two consecutive years, between 1345 and 1346. A rare event, but sufficient to profoundly destabilize the delicate agricultural balances of the Mediterranean.
These unusually cold summers compromised harvests in large areas, prompting Italy’s large maritime cities to reopen, out of sheer necessity, trade routes to the Black Sea, just as the plague was already circulating in those regions.
The historian Hannah Barker from Arizona State University, not involved in the research, commented:
It’s a piece that was missing from the puzzle. The role of climate in the Black Death had never been analyzed in such a concrete way.”
Tree rings, polar ice and medieval chronicles
To reconstruct the climatic context of the fourteenth century, the research team combined natural data and historical sources. Ulf Büntgen, dendrochronologist at the University of Cambridge and co-author of the study, identified an unequivocal signal in the growth rings of trees in the Pyrenees: during the summers of 1345 and 1346, growth was slowed by unusually low temperatures.
The same “climate footprint” emerged from eight other European dendrochronological archives. In parallel, ice cores extracted in Greenland and Antarctica recorded peaks in sulfur in the same years, a chemical signature typical of large volcanic eruptions that project reflective particles into the stratosphere, as Büntgen clarified:
It is not an extreme cold, but a persistent cooling, for two summers in a row. It’s exactly what we would expect after a sequence of sulphur-rich volcanic eruptions.
The news of the time confirm this scenario. Evidence from eastern Asia to western Europe speaks of consistently overcast skies between 1345 and 1347. In Italy, harvests collapsed and the price of grain reached levels unseen in the previous eighty years. In early 1347, with supplies running low, social tensions erupted in the major city-states, as Barker observed:
From the sources, the panic of governments clearly emerges, committed to finding desperate solutions.
Venice and Genoa, maritime and commercial powers, had complex supply systems and strategic reserves. However, by 1343 they were involved in a conflict with the Mongol Empire which had cut off access to the vital Black Sea granaries.
From famine to the reopening of the routes
The worsening climate in the Mediterranean made the situation unsustainable. Regions such as Sicily, Spain and North Africa also suffered from reduced agricultural yields. The Italian maritime republics thus found themselves without alternatives.
In 1347, under the pressure of hunger, Venice and Genoa were forced to sign peace with the Mongols, reopening trade routes to the Black Sea, as explained by historian Martin Bauch, co-author of the study. Within a few months, galleys loaded with grain began sailing again from ports in Crimea and what is now Ukraine.
According to the researchers, something much more dangerous was also traveling together with the wheat. In fact, the plague had afflicted the Mongol troops in the region for some time. Fleas infected by Yersinia pestisprobably nestled in the cereal dust, thus found an ideal passage.
Once the bags were unloaded in Italian ports, the leap from warehouses to local rats and, finally, to humans was a matter of little time. It is no coincidence that the first areas affected were precisely those most dependent on wheat imports, such as Venice and Genoa. The inland cities, more self-sufficient from an agricultural point of view, such as Rome and Milan, experienced the epidemic at a later time, as underlined by Büntgen:
It is one of the first manifestations of the effects of globalization. Trade accelerates diffusion.
For the historian of epidemiology Timothy Newfield, this research highlights how the Black Death was the result of an exceptional combination of factors. The bacterium must have already been present in the Black Sea area. The climatic cooling, caused by volcanic eruptions, must have been intense and long-lasting enough to upset the crops.
Italian cities had to reopen trade routes at the worst time. Finally, maritime logistics needed to do what it does best: transport huge quantities of goods quickly. The result was a lethal chain of events: less sunlight, unproductive fields, failing grain markets, and ships that brought the plague back to the heart of Europe.
Scholars point out that the climate change induced by the eruptions did not directly cause the Black Death. Rather, it set in motion a series of “dominoes” that led to the most devastating pandemic in human history. The lesson, however, is surprisingly timely. Food, health and economic systems are deeply interconnected. A disturbance in one area can quickly spread to others, transforming a local risk into a continental-scale catastrophe.
