Chinese Kingdoms Rose, Fell With Monsoons?
for National Geographic News
|November 6, 2008|
Throughout centuries, the fortunes of China's ancient kingdoms rose and fell with monsoon cycles, a new study suggests.
The discovery is based on a nearly 2,000-year-old record of monsoon activity recently discovered in a cave.
Monsoon winds carry rain-laden clouds through China every summer, providing nearly 80 percent of the annual precipitation in some parts of the country.
When the winds are weak, little to no rain reaches large expanses of China, often plunging those areas into drought.
The new study "is a brilliant analysis of the problematic coincidence of abrupt climate changes and changes in political organization," said Harvey Weiss, an archaeologist at Yale University who was not involved in the study.
In the Wanxiang cave in north-central China, researchers discovered a record of monsoon activity preserved in a stalagmite—a rock formed by mineral-rich waters dripping onto the cave floor year after year.
The rock had been growing continuously for 1,800 years, from A.D. 190 to 2003.
Like trees, stalagmites have annual growth rings that can provide clues about local environmental conditions for a particular year.
The team measured the amount of oxygen-18—a rare form of "heavy" oxygen—in the stalagmite growth rings. Growth rings with large amounts of oxygen-18 indicate years of weak summer monsoons and less rains.
Comparing the stalagmite record with Chinese history, the researchers found that a period of strong monsoons was associated with the "golden age" of the Northern Song dynasty.
During that time, improved rice yields allowed the population to increase from 60 million to as many as 120 million.
"I was really surprised," said study co-author Hai Cheng, a geologist at the University of Minnesota.
Furthermore, weak monsoon seasons coincided with droughts and the declines of the Tang, Yuan, and Ming dynasties.
Weak monsoons may have helped trigger one of the most tumultuous eras in Chinese history, called the Five Dynasties and Ten Kingdoms period, according to the study, detailed tomorrow in the journal Science.
During this time, five dynasties rose and fell within only a few decades, and China fractured into several independent nation-states.
(Read about the brilliant, cruel emperors of the Han Dynasty in National Geographic magazine.)
Peter deMenocal is a paleoclimatologist at Columbia University's Lamont-Doherty Earth Observatory in New York.
"The synchrony between these cultural events and climate change events is really compelling," deMenocal said.
DeMenocal's research has examined the role of climate change in the declines of ancient civilizations, including those of the Maya and Mesopotamians.
Throughout history, climate change has likely exacerbated already tense situations within empires caused by political upheavals or societal unrest, he said.
"Climate in many cases acts like the straw that broke the camel's back," deMenocal said.
The monsoon effect on China continues today, the study authors added.
Scientists have linked droughts plaguing large swaths of modern China to weakening monsoon winds during the past half century.
"The local government has sometimes had to move people out of some regions because they don't have enough water," said study co-author Cheng.
Monsoon variability in the past was driven by natural influences—such as changes in solar cycles and global temperatures. But today's waning monsoons are the results of human-driven greenhouse gas emissions, the new study suggests.
"I do think it's useful to look at this [study] as a lesson for our future," Columbia's deMenocal said.
"In their time, these ancient cultures were in many ways just as impressive as modern societies."
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