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In the late ninth century a disastrous harvest precipitated by drought brought famine to China under the rule of the Tang dynasty. By A.D. 907—after nearly three centuries of rule—the dynasty fell when its emperor, Ai, was deposed, and the empire was divided. According to the atmospheric record contained in a stalagmite, one of the causes of that downfall may have been climate change.
"We think that climate played an important role in Chinese history," says paleoclimatologist Hai Cheng of the University of Minnesota, a member of the scientific team that harvested and analyzed the stalagmite from Wanxiang Cave in Gansu Province in northwest China. The stalagmite reveals, for example, that the vital rains of the Asian monsoon weakened at the time of the downfalls of the Tang, Yuan and Ming dynasties over the past 1,810 years.
"The climate acted," Cheng says, "as the last straw that broke the camel's back."
Composed of calcium carbonate leached from dripping water, the 4.6-inch- (11.7-centimeter-) long stalagmite preserves a record of rainfall in this region, which is on the edge of the area impacted by the Asian monsoon. The region gets less rainfall when the monsoon is mild and more when it is strong, the researchers explain today in Science.
These periods of strong and weak rains, when compared with Chinese historical records, coincide with periods of imperial turmoil or prosperity, as in the case of the expansion of the Northern Song Dynasty—a time of abundant harvests. Further, the stalagmite record matches those of glacial retreat in the Alps, sediment records from Lake Huguang Maar in southern China and droughts from Barbados to Southern France.
In fact, the collapse of the Tang Dynasty coincides with that of the Mayan civilization—both due to extreme drought. "We have demonstrated that the cave record correlates well with many other records, including the Little Ice Age in Europe, temperature changes [across the] Northern Hemisphere, and major solar variability," Cheng notes.
Fluctuations in the sun's intensity in the past seemed to play the key role in determining the strength of the Asian monsoon. The record revealed over the past 50 years, however, paints a different picture, with man-made soot and greenhouse gases determining the rains' strength.
"It is likely that the current globe warming trend or anthropogenic forcing will be accompanied by a weakening trend of Asian summer monsoons, especially in northwestern China," Cheng says. Perhaps that's why China's present rulers have been eager to act on man-made climate change.