The story of a cave and climate change

Among the foothills of a vast mountain range, a mayor guides his nervous citizens into the dank darkness of a nearby cave. The two hundred people huddle together, scared by the thought of possible starvation during another year of drought. The group shifts unsteadily along the rocky terrain as they move farther beneath the hills until they reach a large room typically full of water during monsoon season. This year, only damp rocks greet them. A fortuneteller steps away from the crowd and prays for more rain for their village.

This vignette is not fiction but rather inspired by a recent discovery of inscriptions in the Dayu Cave in central China. The writings span four hundred years and reveal societies across eras that visited the cave during times of drought to pray or gather water stored in an aquifer. In addition to finding the inscriptions, a collaboration of Chinese and US researchers has matched these historical accounts with chemical proxies for rainfall to predict when similar droughts may occur in the future. Such a connection gives an urgent reminder that climate change has already disrupted societies in the past and that similar trends may soon be on the horizon.

A history of droughts

We are not the only civilization threatened by changes in climate. The pyramid-building Egyptian empire likely collapsed due to drought conditions² and the Mayan civilization may have fell due to only a slight change in precipitation patterns.³ But, until now, little quantitative evidence has been connected to these historical accounts of droughts to be able to compare them to current changes in rainfall or climate that will affect us.

A serendipitous discovery of multiple inscriptions in the Dayu Cave in central China (see map) provides a first hint at this otherwise unknown connection. Ranging from 1520-1920 CE, the more than 70 inscriptions reveal societies intermittently threatened by seven different droughts during the same period. Government officials, from mayors to emperors, came alone or with citizens to gather water from the supplying aquifer in the cave or to pray for more rain, indicating the continued importance of this particular water supply throughout Chinese history.

Over one meter of rainfall drenches the Dayu region each year. Most of this rainfall occurs during the monsoon summer season, which provides an important surplus in underground aquifers for use during the rest of the year. Societies became so dependent on this supply that droughts dramatically affected their way of life. Starvation and cannibalism occurred in 1528 during a severe drought, and a civilian uprising against the government occurred in 1890 due to poor living conditions. Vibrant descriptions in the cave writing reveal their intimate relationship with precipitation patterns as they describe how "mountains are crying due to drought". Even today, these aquifers are an important water source for Chinese projects that transport the resources to drier regions.

The oxygen crystal ball

Since the aquifers near Dayu Cave are still so important to China, it is imperative that we try to compare the stories of drought with quantitative measurements of rainfall from the past and present. To do this, the researchers measured the level of the oxygen isotope O¹⁸in stalagmites in the cave. O¹⁸is the heavier of the two most common isotopes, the other being O¹⁶. Freshwater contains more O¹⁶ because the isotope is lighter and more easily evaporates to take part in the water cycle. Therefore, during times of substantial rainfall, O¹⁶ levels will be higher than O¹⁸, whereas during drought O¹⁸ levels will increase.

By measuring isotope levels in stalagmite rock in Dayu Cave corresponding to different historical periods (based on the growth of the stalagmite), the researchers could correlate isotope levels (a proxy for rainfall) with the human records of drought. Using such an analysis, they found that low O¹⁸ levels indeed correspond with the drought periods described in the inscriptions. Surprisingly, even only moderate levels of drought, as measured by only small changes in O¹⁸, created substantial social unrest and difficulties based on the writing.

But what do these historical relationships tell us about our future? Using the oxygen isotope data, the researchers created a model to predict rainfall levels over the next fifty years or so. In addition to accurately predicting a drought that actually occurred in the 1990's, the model also indicates another drought around 2030.

Such results emphasize how important these isotopic measurements can be to understand what to expect from continued climate change. In particular, precipitation from the monsoons in the region is inversely related to sea surface temperatures in the tropics. Thus, as global warming continues to heat the seas, precipitation will continue to decrease and threaten the towns, water transfer projects, and endangered species in the Qinling mountain region. History is now helping to show us the potential consequences we face if we do not respond and adapt to the threat of climate change.

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