Image of inside of Wanxiang Cave courtesy of Science/AAAS
A stalagmite found on the floor of a Chinese cave suggests that several Chinese dynasties may have been connected to the varying strength of the Monsoon, seasonal winds that bring heavy summer rains to much of Asia.
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. Variations in the Monsoon can result in feast or famine.
“The 1,810-year climate record gleaned from the Wanxiang stalagmite suggests that dependence on the Monsoon was no less critical hundreds of years ago,” Pingzhong Zhang of China’s Lanzhou University and colleagues reported in today’s issue of the journal Science.
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.
A 4.6-inch-long stalagmite found in Wanxiang Cave in Gansu province, north-west China, chronicles nearly 2,000 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.
This sample was collected in May 2003, about a thousand yards from the cave entrance. It has grown continuously from AD 190 to 2003.
Image courtesy of Science/AAAS
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.
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 researchers noted. “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.”
The entrance of Wanxiang Cave. The cave is located between the Qinghai-Tibetan Plateau and the Chinese Loess Plateau in Gansu Province, China. It is about 4,000 feet above sea level. The cave area is semi-arid with annual precipitation of 19 inches, 80 percent of which falls between May and September.
Image courtesy of Science/AAAS
Read more about this in the National Geographic News story Chinese Kingdoms Rose, Fell with Monsoons?
Related NatGeo News Watch entry: U.S. Droughts May Last for Centuries, Cave Find Shows