November 7, 2008
Dry Spells May Have Led To Fall Of Asian Dynasties
A new study suggests the demise of some of China's ruling dynasties may have been linked to changes in the strength of monsoon rains.
Researchers uncovered the findings in a 1,800-year record of the Asian monsoon preserved in a stalagmite from a Chinese cave.A US-Chinese team found that weak and dry monsoon periods coincided with the demise of the Tang, Yuan and Ming imperial dynasties.
Stalagmites told the history of strong and weak cycles in the monsoon - the rains that water crops to feed millions of people in Asia.
Chemical analysis of a 118mm-long stalagmite from Wangxiang Cave, in Gansu province, north-west China, showed that over the last 50 years, greenhouse gases and aerosols have taken over from natural variability to become the dominant influence on the monsoon.
Small variations in the forms, or isotopes, of the stalagmite's oxygen composition reflected variations in rainfall near the cave.
Researchers were able to date the stalagmite layers to within an average of two-and-a-half years using proportions of the radioactive elements uranium and thorium in the deposits.
"It was unexpected that a record of surface weather would be preserved in underground cave deposits," said David Verardo, of the National Science Foundation (NSF)'s Paleoclimatology Program. "These results illustrate the promise of paleoclimate science to look beyond the obvious and see new possibilities."
Pingzhong Zhang of Lanzhou University in China and colleagues compared the rain record with Chinese historical records and found that three out of five "multi-century" dynasties - the Tang, the Yuan and the Ming - ended after several decades of weaker summer monsoons with drier conditions.
Hai Cheng, co-author from the University of Minnesota, said summer monsoon winds originate in the Indian Ocean and sweep into China.
"When the summer monsoon is stronger, it pushes farther north-west into China."
Such moisture-laden winds bring rain necessary for cultivating rice. But when the monsoon is weak, the rains stall farther south and east, depriving northern and western parts of China of summer rains.
The researchers speculate this could have led to poor rice harvests and civil unrest.
The researchers wrote in the journal Science: "Whereas other factors would certainly have affected these chapters of Chinese cultural history, our correlations suggest that climate played a key role."
However, a weak monsoon could also be linked to changes further afield. A dry period between 850AD and 940AD coincides not only with the decline of the Chinese Tang dynasty but also with the fall of the Mayan civilization in America, the researchers said.
Subsequent strengthening of the monsoon may have contributed to the rapid increase in rice cultivation, a dramatic increase in population and general stability at the beginning of China's Northern Song Dynasty.
The natural archive shows that climate change can have devastating effects on local populations - even when this change is mild when averaged across the globe, the study showed.
The monsoon followed trends in solar activity over many centuries in the cave record, suggesting the Sun played an important role in the variability of this weather system.
It also followed, to a lesser extent, northern hemisphere temperatures on a millennial and centennial scale. As temperatures went up, the monsoon became stronger and, as they dropped, it weakened.
But over the last 50 years this relationship has switched. The influence of greenhouse gas emissions and sulphate aerosols released by human activities attributed to the change, researchers said.
Image 1: The Leshan Giant Buddha, 71 m (233 ft) high; begun in 713, completed in 803. Credit: James Spurrier - Wikipedia
Image 2: Asian monsoons, Northern Hemisphere temperatures and alpine glacier data across 1,800 years are compared. Credit: Zina Deretsky, National Science Foundation
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