Science: Rise and Fall of Chinese Dynasties Linked to Asian Monsoon

Clues from a stalagmite found in a Chinese cave hint that the fortunes of several Chinese dynasties may have been connected to the varying strength of the Asian Monsoon, a regional shift in winds that brings much-needed summer rains to a wide swath of the continent.

The Wanxiang Cave stalagmite chronicles nearly 2000 years of fluctuations in the Monsoon, linked to changes in the Sun's radiation, the retreat and advance of glacial ice in northern Europe and changes in Northern Hemisphere average temperatures. But these natural drivers appear to be giving way to human influences, researchers report in the 7 November issue of the journal Science.

Since 1960, air pollution—mostly greenhouse gases such as carbon dioxide and soot particles—has become the dominant force affecting the Monsoon's peak and weak periods. The human influence is so great that rising temperatures in the Northern Hemisphere are now correlated with a weaker, drier Monsoon—reversing a trend that stood for centuries.

Sweeping up moisture from the Indian and Pacific Oceans, the Monsoon affects nearly a third of the world's people, particularly those in eastern and southern Asia who depend on seasonal harvests to make a living. The 1810-year climate record gleaned from the Wanxiang stalagmite suggests that dependence on the Monsoon was no less critical hundreds of years ago, said Pingzhong Zhang of China's Lanzhou University and colleagues.

For instance, the first several decades of China's Northern Song Dynasty, about 960 to 1020 A.D., were marked by a population boom and flourishing rice cultivation. At the same time, the stalagmite record indicates a particularly strong and wet Monsoon, the researchers found.

On the other hand, the waning days of the Tang (850 and 940 A.D.), Yuan (1350 to 1380 A.D.) and Ming (1580 to 1640 A.D.) dynasties all coincide with weak and dry Monsoon periods. And China's Era of Disunity (190 to 530 A.D.), an age of civil war and warlord rule, coincides with an equally unstable time for the Monsoon as it varied in strength from decade to decade.

As stalagmites form on cave floors from the drip of mineral-laden water, their growth is recorded in seasonal rings much like the trunk of a tree, and the climate conditions during the formation of each ring can be recovered by analyzing their chemical composition.

Becky Ham

6 November 2008