From Nature:
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.
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