Unexpected role of climate in bringing plague to medieval Europe

Bubonic plague, also known as the Black Death, wiped out an estimated 60 percent of the population of Europe after traders coming from Asia inadvertently introduced it in 1347. Now a study published Monday in the journal Proceedings of the National Academy of Sciences suggests that shifting climate patterns could have played a role in repeatedly reintroducing the lethal plague bacteria Y. pestis during medieval times.The plague, transmitted by infected fleas often carried by rodents, reappeared every few generations until the 19th century and was responsible for killing tens of millions of people.

Comparing the largest dataset of medieval plague outbreaks compiled to date (7711 outbreaks), against 15 tree ring records throughout Europe and Asia, the researchers found 16 instances where climatic changes in Central Asia - warming spring and wetter summers - corresponded with an upsurge in plague in Europe about 15 years later. Tree rings have long been used as a guide of sorts for correlating climate with historical events and for understanding the fluctuation of temperatures dating back hundreds of years. In the latest study, the researchers found outbreaks of plague from 1346 through 1837 which could be explained by changes in Asian weather.

"Our findings support a scenario where climate fluctuations that positively affect tree-ring growth in the juniper trees in the Karakorum mountain range [in the border region of China, India and Pakistan] also affect climate in a larger region in a way that can promote and synchronize plague outbreaks among the rodent populations of Central Asia," the researches wrote. "When the climate subsequently becomes unfavorable, it facilitates the collapse of plague-infected rodent populations forcing their fleas to find alternative hosts. Such large-scale wildlife plague outbreaks in Asia would, during the time of the second plague pandemic, frequently result in the arrival of plague to Europe harbors."

The lag in transmission for what the scientists called the second plague was due to the fact that increasing presence of the disease had to find its way by way via overland trade routes - possibly via camel caravans - to Russian ports and then on to Europe. The study also calls into question the long-held belief that the urban black rat served as a reservoir for the plague. Instead, it suggests that that new strains of the disease were repeatedly imported from Asia.

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