Did worldwide drought wipe out ancient cultures?
Monsoon records link demise of the Tang in China and Maya in Mexico.
They lived in resplendence, half a world apart, before meeting their respective downfalls within decades of one another. Now a new theory suggests that the decline of the Tang Dynasty in China and that of the Mayan civilization in Mexico may both have been due to the same worldwide drought.
Sediments collected from Lake Huguang Maar in southeastern China suggest that Asian summer monsoon rains were weaker during the eighth and ninth centuries AD, the time during which the Tang Dynasty faded from glory. And intriguingly, the same pattern is seen in sediments from Cariaco basin off the Venezuelan coast, suggesting that a similar drought might have been occurring in nearby Mexico.
The events may both be the result of a southward shift in rain patterns that deprived the entire northern tropics of summer rains, suggest researchers led by Gerald Haug of Germany's National Research Centre for Geosciences in Potsdam. The hardship caused by this drought could have been a key factor in the declines of the two cultures, they suggest.
At the moment, it is little more than a theory, admits Haug's colleague Larry Peterson, of the University of Miami, Florida. "The records are pretty intriguing," he says. "But it's really just a correlation in time."
Rain, rain, gone away
The parallels are nevertheless notable. The Tang Dynasty, regarded as a high point in Chinese civilization, began to wane in the mid-700s and ultimately fell in AD 907 after a string of rebellions. Similarly, the Maya, who produced the earliest known written records in the Americas, numbered some 15 million in the middle of the eighth century, but had declined by three-quarters by AD 830, and left the last of their calendrical carvings in AD 909.
Failing summer rains might have contributed to these precipitous declines, Peterson suggests. "The Mayan royalty were viewed as gods and were depended on to summon the rains," he says. Besides harming crops, the lack of rain might have damaged the leaders' credibility, he suggests.
The researchers estimated the strength of summer rains in China by studying deposits of titanium minerals in Lake Huguang Maar. These sediments are carried there by winter monsoon winds; stronger winter winds have been previously linked to weaker summer rains. The Tang decline coincides with a period of strong winds, and so probably also weak rains, they report in Nature1. The team has previously found the same pattern in Venezuelan sediments.
The climate change was probably linked to a wholesale shift in the intertropical convergence zone (ITCZ), a band of heavy tropical rain that also moves in response to effects such as the periodic El Nio events, which likewise weaken monsoon rains in Southeast Asia. Haug and his colleagues suspect that these rain patterns migrated southward en masse, reducing average summer rainfall throughout the northern tropics for some two centuries. Such a shift is certainly possible, although the exact reasons for why it may have moved then are unclear, and hard evidence is lacking.
The timing of the declines and the climate changes make for a neat coincidence, but other factors cannot be ruled out, says Patrick Culbert, an anthropologist at the University of Arizona, Tucson, and an expert on Mayan history. "The Maya overpopulated and overexploited their environment, and they died," he argues. "The population had been growing exponentially for 2,000 years."
Drought was unlikely to have been the only factor in the cultures' declines, Peterson admits. The Tang people, for instance, were hit hard by a military defeat by the Arab army in AD 751, which would probably have sowed unrest and rebellion.
"I'm not saying they were keeling over from lack of water, but if the cultures were maxed out in terms of their environmental resources, it could have been a trigger," Peterson says.
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- Yancheva G., et al. Nature, 445. 74 - 77 (2007).