The transition from a warm climate to the Little Ice Age in the early 14th Century, marked by heavy precipitation, may have set the stage for a series of plagues, including the Black Death (1346-51), a Jet Propulsion Laboratory scientist said.
In a presentation before a session of the American Geophysical Union's Fall Meeting in San Francisco Dec. 10, Kevin Pang said an antique Chinese encyclopedia of historical events along with other physical records indicate that pestilence followed the radical climatic changes.
The increased rainfall year after year, beginning in 1315 "very likely raised biomass production in the Eurasian steppes and deserts, and that in turn fed a rodent population explosion," he said.
The cool, humid conditions also favor the breeding and survival of fleas. The climatic changes coupled with some unique historical events paved the way for a series of flea- and rat-borne epidemics that killed a third of the population of Europe.
A century of very destructive wars started by Genghis Khan laid waste to much of the farmland in northern China, central and western Asia and eastern Europe. Vast areas from the Pacific to the Black Sea were fallow and depopulated; herding and hunting were reduced.
With less competition for food from herds and fewer hunters, wild rodents spread from Eurasian plague reservoirs, infected domestic rats. Commerce and mass migrations helped spread the plague throughout the medieval world.
The Gujin Tushu Jicheng, a 1726 encyclopedia of historical events lists 25 great epidemics in the period 1344-80. Although it gives no clinical details, Pang said he concluded the epidemics were bubonic plagues since their geographical and seasonal patterns were identical to the modern pandemic of 1894-1950 in China.
"Simultaneous outbreaks in the east, middle east and west required profound ecological changes over Eurasia," he said.
Medieval physicians did not know that bubonic plague is caused by germs and spread by fleas and rodents, but noted in 1348-49 that "for a long time the seasons have not been marked by their proper weather ... summer has been much less warm," and "an abundance of rain has been present to a high degree for three years."
The climatic changes were confirmed by analyses of grape harvest dates, Alpine tree rings and weather diaries. In all but two years between 1343 and 1351, the Yellow River breached its dikes and followed a new course to the sea.
Pang said the records are consistent with 14th Century western U.S. tree ring width measurements. Chinese texts also recorded greatly increased dim and red suns in the 1330s and 1340s, suggesting volcanic clouds contributed to the cool and wet climate.
Large volcanic acid peaks found in Greenland and Antarctic ice cores for the period confirm it, he said.
The first known bubonic plague pandemic began in the year 541 A.D., known as the Justinian plague, it started in the port cities of Egypt, spread around the Mediterranean and decimated the Eastern Roman Empire.
The Black Death (1346-51) and subsequent plagues later in that century killed tens of millions of Europeans and Asians. The third plague pandemic began in China in 1894, spread to India, and lasted for some 50 years, killing about three million Indians and Chinese.
Many public health experts predict that climatic changes associated with global warming and ozone depletion could help spread parasitic, especially insect-borne, diseases to regions now unaffected, although the exact locations and degrees of severity are difficult to pin down.