Neurodope Magazine

Could scientists be grasping at straws now by trying to attach the idea of “climate change” to the infamous “Black Plague” of the European Middle Ages?

According to a study published in PNAS, wet springs and warm summers followed by dry cold snaps in Central Asia killed off flea-carrying great gerbils (Rhombomys opimus), possibly leading plague-harboring fleas to switch hosts and travel to Europe.

The research provides evidence that repeated European plague outbreaks in the four centuries following the original Black Death (1347-1353) were due to reintroductions of the disease rather than a latent reservoir of the responsible Yersinia pestis bacterium in local rats.

Norwegian and Swiss researchers analyzed records of more than 7,700 European plague outbreaks between 1346 and 1837, finding 61 outbreaks in 17 harbors such as London, Hamburg, and Dubrovnik, which were likely imported with Asian shipments. In 16 separate years during that period, the disease’s emergence could not be explained by contagion from nearby areas, suggesting that the plague had been reintroduced.

To further investigate the reason for the sudden resurgence of the Black Death on multiple occasions, the scientists examined historical climate data from juniper tree rings in both Europe and Central Asia, where the plague was known to have originated. There was no apparent link between European climate and the emergence of an outbreak, but when the weather was warmer in Central Asia, followed by a cold spell, the plague showed up in Europe 15 years later. The balmy weather caused gerbil populations to flourish, and previous work had shown that a 1°C increase in average temperature doubles the prevalence of plague carried by gerbils, study coauthor Nils Stenseth of the University of Oslo told New Scientist. When drier weather set in, the gerbil population crashed, and their plague-carrying fleas were forced to find new hosts—possibly humans, camels travelling the Silk Road, or rats stowed aboard ships traveling to Europe.

Gerbils, Fleas, and a Chain Reaction

The core of the theory lies in the impact of regional climate conditions on flea populations and their hosts. Specifically, wet springs and warm summers, followed by cold, dry snaps, may have caused massive die-offs among the great gerbil (Rhombomys opimus), a rodent native to the Central Asian steppes. These gerbils are known to harbor fleas that carry Yersinia pestis, the bacterium responsible for plague.

When the gerbil populations collapsed due to the unpredictable climate, the fleas — still harboring the deadly bacteria — would have been forced to find new hosts. That shift, researchers argue, could have launched a deadly chain reaction, leading to outbreaks of the plague in human populations across Asia and eventually Europe.

While it’s long been accepted that the Black Death (1347–1353) originated in Asia and spread to Europe via trade routes like the Silk Road and maritime shipping lanes, this new research provides more granular insight into the specific ecological and climatological triggers that may have facilitated the pathogen’s repeated emergence.

Rewriting the Story of Plague Persistence

Perhaps the most startling aspect of the study is its challenge to a widely held belief: that recurring plague outbreaks in Europe over the following four centuries were the result of lingering local reservoirs of the bacterium in rats and other rodents.

Instead, the researchers — hailing from Norwegian and Swiss institutions — argue that Yersinia pestis was likely reintroduced multiple times into Europe via trade from Asia. Using data from over 7,700 documented plague outbreaks across Europe between 1346 and 1837, they found that at least 61 significant events occurred in port cities like London, Hamburg, and Dubrovnik. These outbreaks did not correspond with local transmission patterns and could not be explained by contagion spreading from nearby infected areas.

In fact, in 16 different years during that period, the emergence of the disease appeared entirely disconnected from surrounding epidemics, strengthening the case for multiple independent introductions from distant lands — likely facilitated by trade networks and shifts in host-vector ecology linked to climate events.

Climate Change: Convenient Scapegoat or Genuine Factor?

Skeptics may argue that trying to draw a direct line from climate variation to centuries-old pandemics is speculative at best. After all, records of climate patterns from the 14th to 19th centuries are patchy, and the ecological complexity of zoonotic disease transmission makes causation notoriously difficult to establish.

Yet this study is part of a growing body of work that uses paleoclimatic reconstructions — such as tree rings, ice cores, and historical harvest records — to build credible models of how past environmental conditions may have influenced disease dynamics. If researchers can identify correlations between climate anomalies and disease emergence with enough consistency, the argument becomes less about “grasping at straws” and more about uncovering forgotten dimensions of human history.

Furthermore, understanding how climate extremes can influence the behavior of disease vectors has immediate relevance today. As global temperatures rise and ecosystems shift, the risk of zoonotic spillover events increases — a concern that has become all too real in the aftermath of the COVID-19 pandemic.

The Plague, Reimagined

If the new hypothesis holds up to further scrutiny, it could dramatically reshape how historians and epidemiologists view the medieval and early modern plagues. Rather than being a persistent menace that smoldered within Europe’s rat population, the plague may have been more of an invasive threat — periodically rekindled by conditions thousands of miles away.

This view doesn’t just add a new chapter to the story of the Black Death; it reframes the entire narrative. It suggests that Europe was not a self-contained petri dish of infection, but rather part of a vast, interconnected biosphere in which environmental shocks in one region could have deadly consequences half a continent away.

Whether or not one sees this as a stretch depends on how willing we are to embrace complexity in historical causation. Are scientists reaching too far to tie everything back to climate? Or are they finally beginning to recognize the often invisible threads that connect ecological change, human behavior, and the arc of history?