Triumph of Death Wall Painting, ca. 1448, Palazzo Abatellis, Palermo, Italy.
The black death killed thirty to fifty percent of the people of Europe in six years starting in 1347. Plague, caused by the bacterium Yersinia pestis, ravaged Europe in wave after wave for over 300 years, then slowly faded to black after the Great Plague of London in 1665-1666. This, the second plague pandemic, has long been blamed on rats harboring fleas which were the cause of the third plague pandemic in the nineteenth and twentieth centuries. However,
new research published in the Proceedings of the National Academy of Sciences found that climate changes in Asia preceded plagues in Europe by 15 years. The researchers then tracked the progression of plague on the Silk road and other trade routes from Asia to Europe.
Plague, triggered by droughts in a wet climate period in central Asia, was carried to Europe by traders on the Silk Road.
The scientists first tried to find a relationship between plague outbreaks in Europe with weather and climate events in Europe. If European rats were the plague vector then conditions that favored growth in European rat and rodent populations would be expected to correlate with plague outbreaks, but no correlations were found. The cool wet climate in Europe from 1350 to 1700 was unfavorable for maintaining large rodent populations. The warm and dry steppes of central Asia are known today as a plague reservoir. From 1350 to 1750 these steppes went through cycles of wet and dry, which caused cycles of boom and bust in local rodent populations. When the rodent populations collapsed in the drought years, the fleas jumped from dying rodents to anything that moved including humans. Traders then inadvertently transported rodents and plague bearing fleas along the silk road towards Europe.
Plague was then spread to the ports of Europe by maritime trade.
"This chart shows these climate fluctuations in Central Asia preceded the Black Death in 1347, the Italian plague of 1629, and the Great Plague of Marseille a century later, but notably not the London plague of 1665 or the outbreak in Vienna the following decade."
Modern climate change, driven by rising levels of greenhouse gases associated with human activities, will cause droughts and wet-dry climate fluctuations. These droughts will affect populations of disease carrying rodents, as did the natural droughts that triggered plague outbreaks. Moreover, droughts may make wars more likely and cause millions of people to be displaced from their homes. The "Arab spring" and ongoing wars in Iraq and Syria were preceded by extreme drought. Drought caused wars and human migrations are ideal conditions for triggering disease outbreaks. However, with modern rapid transportation, outbreaks may spread at a much higher speed. We need a strong public health system that is capable of rapidly responding to disease outbreaks that develop in drought and war affected populations.