By studying stalagmites and centuries-old writing on the walls of a cave in central China, researchers have pieced together a story about drought and its impact on society over the last 500 years. The findings, published in Scientific Reports this week, is the first time it’s been possible to conduct an on-site comparison of historical and geological records from the same cave. 

"There are examples of things like human remains, tools and pottery being found in caves, but it's exceptional to find something like these dated inscriptions," lead author Liangcheng Tan from the Institute of Earth Environment at the Chinese Academy of Sciences says. The two-kilometer-long (1.2-mile-long) Dayu Cave is located on the southern slope of the Qinling Mountains. Climate in the surrounding area is dominated by the summer monsoon: About 70% of the year's rain falls during a few months. "Combined with the evidence found in the physical formations in the cave, the inscriptions were a crucial way for us to confirm the link between climate and the geochemical record in the cave, and the effect that drought has on a landscape,” Tan adds. 

Slicing open cave formations called speleothems (pictured below) reveals a series of layers, a record of their yearly growth. The team removed sections of speleothems, including stalagmites, to analyze the isotopes and elements within. Climate changes and moisture levels affect the concentrations of various elements, and higher oxygen and carbon isotope ratios correspond with lower rainfall levels. 

^{Speleothems. L. Tan}

They found that concentrations of certain elements were strongly correlated to periods of drought. Dayu Cave’s chemical profile was then cross-referenced with the inscriptions on the walls. These inscriptions describe the impacts of seven drought events that occurred between 1520 and 1920. That’s when locals went to the cave to retrieve water and to pray for rain.

"In addition to the obvious impact of droughts, they have also been linked to the downfall of cultures – when people don't have enough water, hardship is inevitable and conflict arises," study co-author Sebastian Breitenbach of Cambridge says in a statement. "In the past decade, records found in caves and lakes have shown a possible link between climate change and the demise of several Chinese dynasties during the last 1800 years, such as the Tang, Yuan and Ming Dynasties."

For example, an inscription from 1528 reads: "Drought occurred in the 7th year of the Emperor Jiajing period, Ming Dynasty. Gui Jiang and Sishan Jiang came to Da'an town to acknowledge the Dragon Lake inside in Dayu Cave." This drought led to widespread starvation and even cannibalism.

An inscription from 1891 reads: "On May 24th, 17th year of the Emperor Guangxu period, Qing Dynasty, the local mayor, Huaizong Zhu led more than 200 people into the cave to get water. A fortune-teller named Zhenrong Ran prayed for rain during the ceremony." (This corresponds to solar calendar June 30, 1891, and this inscription is pictured above.) Famine and the resulting social instability eventually led to a major conflict between the government and civilians in 1900. 

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