They came to the cave to get water—and in times of drought, to pray for rain. They documented their efforts with inscriptions they wrote on the cave walls. "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,” someone wrote on the wall in 1891.

More than 350 years earlier, in 1528, another water seeker wrote: “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.”

Their tone was matter of fact, but their situations were dire. The droughts that afflicted central China in the 1890s led to severe starvation, social instability, and eventually to a fierce conflict between government and civilians in 1900. And the drought of 1528 caused such widespread starvation that there were reports of cannibalism.

These inscriptions are just two of several on the walls of Dayu Cave in the Qinling Mountains of central China that describe the impacts of seven drought events between 1520 and 1920. As they recently published in the journal Scientific Reports, a team of researchers from China, the U.K., and the U.S. have been studying not only the inscriptions but the cave they were found in.

What makes this research unique is that the researchers have been able to piece together a record of periodic droughts in the region thanks to both the inscriptions and to a detailed chemical analysis of stalagmites in the cave. They say it's the first time it has been possible to conduct an in situ comparison of historical and geological records from the same cave.

The climate in the area around the cave is dominated by the summer monsoon, in which about 70 percent of the year’s rain falls during a few months. An ill-timed or unusually short (or long) monsoon makes a major impact on the region’s ecosystem. 

The locals were well aware of this, as the inscriptions show. They visited the cave at least 70 times between 1520 and 1920. Their inscriptions document seven major droughts, the first in 1528 and the last in 1894.

The researchers compared this information with a chemical analysis of the stable isotopes and trace elements contained within cave formations, or speleothems—especially stalagmites. (When cut open, stalagmites frequently reveal a series of layers that record their annual growth, just like tree rings.) Using mass spectrometry, the researchers dated the ratios of the stable isotopes of oxygen and carbon, as well as concentrations of uranium and other elements, all of which are affected by changes in climate, moisture levels, and vegetation around the cave. The researchers found that higher oxygen and carbon isotope ratios corresponded with lower rainfall levels, and vice versa. 

Based on these results, the researchers also constructed a model of future precipitation in the region, starting in 1982. Their model correlated with a drought that occurred in the 1990s—and suggests another drought in the late 2030s. 

Such climate changes have destabilized many cultures before, including in China, says Cambridge University's Sebastian Breitenbach, a co-author of the paper: “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.” 

"With direct historical evidences, our results suggest that droughts and even modest events interrupting otherwise wet intervals can cause serious social crises," the researchers write, adding "that future precipitation in central China may be below the average of the past 500 years. As Qinling Mountain is the main recharge area of two large water transfer projects and habitats of many endangered species, it is imperative to explore an adaptive strategy for the decline in precipitation and/or drought events."