In China, monsoons’ story written in stone

A giant 1,810-year-old limestone stalagmite discovered in northwest China offers clues on the strength of past Asian monsoons and how they may have affected China’s political history.

In an article in the journal Science, researchers said the limestone told of strong and weak monsoon periods coincident with the rise and fall of several Chinese dynasties. Oxygen and carbon traces in certain limestones can provide a valuable record of past precipitation, temperature and vegetation changes.

“During strong monsoon periods, dynasties such as the Northern Song (960-1127) enjoyed increased rice cultivation and a booming population,” the researchers said. “But weak and consequently dry monsoon periods coincided with the demise of the Tang (618-907), Yuan (1271-1368) and Ming (1368-1644) dynasties.”

The scientists found the yard-long stalagmite in Wanxiang cave, some 4,000 feet above sea level between the Qinghai-Tibetan plateau and Chinese Loess plateau in Wudu county in northwestern Gansu province, a semi-arid area.

The stalagmite started growing in AD 190 and was discovered in 2003 about a half-mile from the cave entrance.

“The cave is located on the fringes of the area currently affected by the summer monsoon and is thus sensitive to and ‘integrates’ broad changes in the monsoon,” one of the researchers, Hai Cheng, of the University of Minnesota’s department of geology and geophysics, wrote in reply to questions from Reuters.

“We have demonstrated that the cave records correlate well with many records, including the Little Ice Age in Europe; the temperature changes in China and Northern Hemisphere; and major solar variability. As such, we think this is a good and reliable Asian monsoon record over the past 1,800 years.”

The correlation between temperature and the Asian monsoon changed around 1960, suggesting greenhouse gases and aerosols may now be the dominant influences on the monsoon.