Slowly built from the minerals in dripping water over 1,810 years, chemicals in the stone tell a tale of strong and weak cycles of the monsoon, the life-giving rains that water crops to feed millions of people.
Dry periods coincided with the demise of the Tang, Yuan and Ming dynasties, researchers report in Friday's edition of the journal Science.
In addition, the team led by Pingzhong Zhang of Lanzhou University in China noted a change in the cycles around 1960 which they said may indicate that greenhouse gases released by human activities have become the dominant influence on the monsoon.
The Wanxiang Cave is in Gansu Province, a region where 80 percent of the rainfall occurs between May and September.
Chemical concentrations in the stalagmite indicate a series of fluctuations lasting from one to several centuries and roughly similar to records of the Little Ice Age, Medieval warm period and Dark Age cold period recorded in Europe.
There were decade-long fluctuations between A.D. 190 and 530, the end of the Han Dynasty and most of the Era of Disunity, the researchers said. From 530 to 850 the monsoon declined, covering the end of the Era of Disunity, the Sui Dynasty and most of the Tang Dynasty.
The monsoon remained weak, with another sharp drop between 910 and 930, then it rose sharply over 60 years and remained strong until 1020.
The researchers found that after 1020 the monsoon varied but was generally strong until a sharp drop between 1340 and 1360: the mid 14th-century monsoon weakening. It stayed weak, with substantial fluctuation, until a sharp increase between 1850 and 1880.
According to the researchers, the 9th-century dry period contributed to the decline of the Tang Dynasty and the Mayans in Mesoamerica. It also may have contributed to the lack of unity during the Five Dynasties and Ten Kingdoms period, they said.
The following strengthening of the monsoon may have contributed to the rapid increase in rice cultivation, the dramatic increase in population, and the general stability at the beginning of the Northern Song Dynasty, they suggested, adding that the end of the Yuan and the end of the Ming are both characterized by unusually weak summer monsoons.
The research was supported by the National Science Foundations of the United States and China, the Gary Comer Science and Education Foundation, the Ministry of Science and Technology of China and the Chinese Academy of Sciences.
By AP Science Writer Randolph E. Schmid