Scientists who have discovered ancient graffiti on the walls of a cave in central China and examined the chemical make-up of the cave’s stalagmites say a serious drought is likely to affect the region within the next few decades.
The international research team found a series of inscriptions on the walls of Dayu cave, in the Qinling mountains of central China, that describe the impacts of seven droughts between 1520 and 1920.
The team, which included Chinese scientists and colleagues from the University of Cambridge, UK, has published its findings in the journal Scientific Reports.
The authors say the information in the inscriptions and the team’s detailed chemical analysis of the stalagmites together show how societies are affected by droughts over time. They say this is the first time it has been possible to conduct an on-site comparison of historical and geological records from the same cave.
Their results cover nearly five centuries, but it is one of their conclusions that speaks most directly to the present day: they say their findings point to potentially greatly reduced rainfall in the region in the near future, underlying the importance of preparing for a world where droughts are more common.
The climate in the area around the cave is dominated by the summer monsoon, in which about 70 per cent of the year’s rain falls during a few months. So when the monsoon is late or early, too short or too long, it has a major impact on the region’s ecosystem.
Significantly, the team also found that the droughts they identified corresponded with the El Niño-Southern Oscillation (ENSO) cycle − the periodic weather disturbance centred on the eastern Pacific that can cause widespread disruption.
Because human-caused climate change will make ENSO events more severe, they say, this too could mean that the region faces more serious droughts.
The story of the last five centuries that the cave revealed is dramatic. “In addition to the obvious impact of droughts, they have also been linked to the downfall of cultures,” says one of the paper’s co-authors, Dr Sebastian Breitenbach, a research associate in the University of Cambridge’s Department of Earth Sciences. “When people don’t have enough water, hardship is inevitable and conflict arises.
“In the past decade, records found in caves and lakes have shown a possible link between climate change and the demise during the last 1800 years of several Chinese dynasties, such as the Tang, Yuan and Ming Dynasties.”
The droughts of the 1890s led to severe starvation and triggered local social instability, which eventually resulted in a fierce conflict between government and civilians in 1900. The drought in 1528 also led to widespread starvation, and there were even reports of cannibalism.
“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,” says the lead author, Dr Liangcheng Tan, associate professor at the Institute of Earth Environment at the Chinese Academy of Sciences in Xi’an.
“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.”
The researchers examined sections of cave formations − or speleothems − and found that concentrations of certain elements were strongly correlated to periods of drought, which could then be verified by cross-referencing the chemical profile of the cave with the writing on the walls.
When cut open, stalagmites frequently reveal a series of layers that record their annual growth, just like tree rings. The researchers analysed and dated the ratios of the stable isotopes of oxygen and carbon, as well as concentrations of uranium and other elements. Changes in climate, moisture levels and surrounding vegetation all affect these elements.
They then used their results to construct 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 will occur in the late 2030s.
Dr Breitenbach says: “Things in the world are different from when these cave inscriptions were written, but we’re still vulnerable to these events – especially in the developing world.”