Wednesday, January 03, 2007

Climate shift helped destroy China's Tang dynasty

Climate shift helped destroy China's Tang dynasty:
scientistsJan 03 2:25 PM US/Eastern

The Tang dynasty, seen by many historians as a glittering peak in China's history, was brought to its knees by shifts in the monsoon cycle, according to a study.
Famed for a flowering of art and literature and for prosperity brought by trade with India and the Middle East, the dynasty spanned nearly three centuries, from AD 618 to 907, before it was overwhelmed by revolt.

Scientists led by Gerald Haug of the Geoforschungszentrum (GFZ) in Potsdam, eastern Germany, looked at sedimentary cores taken from a lake at Zhanjiang in coastal southeastern China, opposite the tropical island of Hainan.
The magnetic properties and content of titanium in these deposits are an indicator of the strength of the winter cycle in the East Asian monsoon system, they believe.
They found that over the past 15,000 years, there had been three periods in which the winter monsoon was strong but the summer monsoon was weak.
The first two periods occurred at key moments during the last Ice Age, while the last ran from around 700 to 900. Each of these monsoon shifts coincided with what was, relative to the climate epoch, unusually cold weather.
The twilight of the Tang began in 751, when the imperial army was defeated by Arabs.
But what eventually destroyed the dynasty were prolonged droughts and poor summer rains, which caused crop failure and stoked peasants' uprisings. Eventually, these rebellions led to the collapse of the dynasty in 907.
Haug's team suggests this shift in tropical precipitation occurred on both sides of the Pacific, not just in coastal East Asia.
The same migration of the rainband occurred in Central America and doomed the so-called classic period of the Mayan civilisation, at almost exactly the same time as the Tang era, they believe.
Comparison of the titanium records from the Huguangyan Lake, in Guangdong province, and from the Cariaco basin, in Venezuela, have thrown up striking similarities.
Both suggest a general shift towards a drier climate at around 750 and then, during these generally drier period, three-year cycles in which rainfall was very low.
The new study appears on Thursday in Nature, the weekly British science journal.

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