Emperor Gaozu, founder of the Tang Dynasty

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Weak winter monsoon rains may have contributed to the decline of both China's Tang Dynasty and the Mayan civilisation in Mexico, suggests an article published in Nature (4 January).

The authors speculate that global climate change, causing a tropical rain belt to migrate, may have been a factor — with the same system affecting monsoon rains in Southeast Asia today.

They link shifts in an area of low pressure that follows the equator, known as an intertropical convergence zone, with weakened monsoon rains in winters during the eighth and ninth centuries. They write that dynastic changes often involved popular uprisings during phases of crop failure and famine, and these are consistent with periods of reduced rainfall.

The same intertropical convergence zone shifts in response to periodic El Nino events, which weaken monsoon rains in Southeast Asia in the modern era.

A team of Chinese and German palaeoclimatologists analysed material from Lake Huguang Maar, a natural sediment trap in Southeastern China. They used the magnetic properties and titanium content of the sediment core, spanning a 16,000 year period, as indications of the strength of winter monsoon winds.

A comparable pattern to that found in China was seen in Cariaco basin sediment off the Venezuelan coast, indicating that a similar drought may have hit nearby Mexico in the late eighth century. This coincided with the downfall of the Mayan civilisation, around the same time period as the end of the Tang Dynasty.

According to Fang Xiuqi, a researcher in Chinese disaster history at the Beijing Normal University, historical documents suggest a series of dry seasons and bad harvests preceded a rebellion that is considered a turning point in China's political hierarchy.

He cautioned that there was no direct evidence of the rebellion having anything to do with the climate change. "But I believe the weather did play an important role in Tang's decline," Fang told SciDev.Net.

Link to the full article in Nature

Reference: Nature 445, 74 (2007)