By studying geological records, the researchers found a correlation between climate extremes and the fall of the Tang dynasty in China and the Maya culture in America, NewScientist.com said.
Records showed that about the time these contemporary civilizations began to decline, they were experiencing stronger than average winter winds and weaker summer monsoon rains, which may have reduced harvests, said Gerald Haug of the GeoForschungsZentrum in Germany.
Haug's team studied monsoon trends in geological records by examining stalagmites in China. To get a reliable estimate of winter winds, the team studied sediments deposited at the bottom of Lake Hugauang Maar, also in China, because few streams fed into it.
The team also found that when warm temperatures in the Northern hemisphere indicated a northward shift of the inter-tropical convergence zone, summer monsoon rains were strong and winter monsoon winds were weak. Shifts in the zone simultaneously affected civilizations on the opposite sides of the Pacific Ocean, he said.
"I am not a historian," said Haug, adding, "there is a coincidence at least."
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