The study, published in the Proceedings of the National Academy of Sciences, looks at climate data from the 14th century, and concludes that the notion that plague came about thanks to diseased fleas hosted by Europe’s rats is unfounded.
“For this, you would need warm summers, with not too much precipitation,” Nils Christian Stenseth, an author of the study, said. “And we have looked at the broad spectrum of climatic indices, and there is no relationship between the appearance of plague and the weather.”
The pandemic appears to be connected to Asia’s climate. Analyzing 15 tree-ring records for yearly weather patterns, the researchers noted that the outbreaks occurred only after central Asia had wet springs followed by warm summers. Black rats hardly thrive in such conditions, which however, are just perfect for Asian gerbils. The gerbils would have hitched a ride to Europe on Silk Road caravans.
Over 100 million people are estimated to have died in a series of epidemics that stretched from the 1300s to the 1800s. This also explains why the epidemics occurred only episodically, rather than being a permanent fixture in Europe which always had rats around.
Stenseth and his team plan to analyze DNA from ancient European skeletons, and will look for significant genetic variation across time, which will indicate that outbreaks were caused by new waves of the disease.
