Hospital staff, volunteers, and apparatus during the Changteh cholera epidemic, Changde, Hunan, China, ca.1910-1919. Photo: Wikimedia Commons

Conventional studies and forecasts have connected climate change and warmer temperatures with more human epidemics. The range of pests and the disease they carry, from malaria to cholera and plague, spread with the new opportunity, according to these theories. The Zika virus’s spread north through the Western Hemisphere of the last few years has even been associated with the beginnings of climate change.

However, detailed meteorological records in China spanning thousands of years appear to show the opposite: cooler and drier temperatures over the long-term are what caused the major outbreaks of the nation’s history, according to a new study in the Proceedings of the National Academy of Sciences.

“We show that long-term trends of cold and dry conditions indirectly facilitated the prevalence of epidemics through locusts and famines,” write the authors, from China and Europe. “Nevertheless, temperature showed unstable associations with epidemics on a small time scale.”

The scientists took data from the book A Compendium of Chinese Meteorological Records of the Last 3,000 Years (the book was a product of 20 years of Chinese scholarship and involved more than 8,000 historical documents), they report.

The illness records were from 1 A.D. to 1367 A.D. (pre Ming-Qing period) and Ming-Qing (1368 A.D. to 1911 A.D.). The periods yielded 250 and 5,131 records of epidemics, respectively. The focus of the historical data was focused on the eastern mainland of China, especially the lower Yellow and Yangtze River regions, and along the coasts.

Those two data sets were log transformed, and correlation analysis was applied to the sets performed by an IBM program called SPSS Amos, they report.

The correlations were clear, they write. And availability of food – or lack thereof – was the main factor underpinning outbreak of disease.

“Our results show that climate change can increase the incidence of human epidemic events through an increased incidence of famine,” they conclude. “Famine has been found to be linked to the poor nutritional conditions of people in Europe; it may have reduced immunity, hence increasing the risk of infection from various diseases.

Famine induced by cool temperatures has been reported to trigger wars and large-scale immigration, further accelerating the transmission of disease,” they add.

However, there were limitations to the findings. The illness data focused mostly on the Ming-Qing period, since it had more records available. But those records mostly did not include information on the kinds of disease plaguing the countryside at any given time. Also, the authors concede that data has shown that warmer temperatures in China did correlate with increased prevalence of mosquito-borne disease like malaria and dengue fever, as well as the dreaded bubonic plague. They also admit that some disease rates in parts of the world have shown increased incidence in connection with the warming of El Niño periods.