Evidence that human-induced climate change may be affecting the Asian monsoon cycle has been published by a Chinese-US team.
Zhang Pingzhong, of Lanzhou University in China, analysed a 1,800-year-old stalagmite recovered from the Wanxiang Cave in West China's Gansu Province, which lies on the current path of the Asian summer monsoon.
The team measured levels of the elements uranium and thorium throughout the stalagmite and analysed its oxygen isotope ratios — different forms of oxygen whose levels are linked to rainfall and thus provide a record of the climate at the time.
Records show that, before 1960, warmer years were associated with stronger monsoons, and the temperature decreased when the monsoon weakened. But the study found a reversed association after this date.
"The rising temperature now leads to less precipitation, which is not a natural pattern," said Larry Edwards, geologist at the University of Minnesota and co-author of the paper, which was published in Science (November).
"So we came to an important conclusion that the monsoon had started to be affected by man-made causes."
Using the literature, the researchers linked the changes to human-induced greenhouse gases and aerosols.
Their finding corresponds with previous work by Ding Yihui, a leading Chinese climatologist, on changes in China's rainfall pattern in the late 1960s.
"The fact that recent Asian monsoon decline has taken place in the global warming period rather than in cold periods," was an important finding, Ding told SciDev.Net.
But he said that more investigations were needed into its causes.
Ding said he is soon to publish another study in Science predicting Asian monsoon activity over the next 100 years.
The team also discovered that weak summer monsoons were highly correlated with the demise of major Chinese dynasties in the past four centuries.
Using the results from the stalagmite analyses, the team was able to match the amount of rainfall to the dates that China's Tang, Yuan and Ming dynasties rose and fell.
For example, the researchers detected weak summer monsoon periods between 850 and 940 AD. This coincided with the last six decades of the Tang Dynasty. Decreased monsoon strength reduced the rainfall and led to a poorer harvest, which may have sparked unrest and led to the downfall of Tang.