Ker Than, National Geographic News 6 Nov 08;
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.
"Really Surprised"
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.
"Compelling"
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.
Continuous Effect
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."
Monsoon link to fall of dynasties
BBC News 6 Nov 08;
The demise of some of China's ruling dynasties may have been linked to changes in the strength of monsoon rains, a new study suggests.
The findings come from 1,800-year record of the Asian monsoon preserved in a stalagmite from a Chinese cave.
Weak - and therefore dry - monsoon periods coincided with the demise of the Tang, Yuan and Ming imperial dynasties, the scientists said.
A US-Chinese team report their work in the journal Science.
Stalagmites are largely made up of calcium carbonate, which precipitates from groundwater dripping from the ceiling of a cave.
Chemical analysis of a 118mm-long stalagmite from Wangxiang Cave, in Gansu province, north-west China, told the history of strong and weak cycles in the monsoon - the rains that water crops to feed millions of people in Asia.
It also shows that, over the last 50 years, greenhouse gases and aerosols have taken over from natural variability to become the dominant influence on the monsoon.
Death of dynasties
Small variations in the forms, or isotopes, of the stalagmite's oxygen composition reflected variations in rainfall near the cave.
Proportions of the radioactive elements uranium and thorium in the deposit allowed the researchers to date the stalagmite layers to within an average of two-and-a-half years.
By comparing the rain record with Chinese historical records, Pingzhong Zhang of Lanzhou University in China, and colleagues, found three out of five "multi-century" dynasties - the Tang, the Yuan and the Ming - ended after several decades of weaker summer monsoons with drier conditions.
"Summer monsoon winds originate in the Indian Ocean and sweep into China," said Hai Cheng, co-author from the University of Minnesota, US.
"When the summer monsoon is stronger, it pushes farther north-west into China."
These moisture-laden winds bring rain necessary for cultivating rice. But when the monsoon is weak, the rains stall farther south and east, depriving northern and western parts of China of summer rains.
This could have led to poor rice harvests and civil unrest, the researchers speculate.
"Whereas other factors would certainly have affected these chapters of Chinese cultural history, our correlations suggest that climate played a key role," the researchers write in Science.
But a weak monsoon could also be linked to changes further afield. The researchers say a dry period between 850AD and 940AD coincides not only with the decline of the Chinese Tang dynasty but also with the fall of the Mayan civilization in America.
Human influence
Subsequent strengthening of the monsoon may have contributed to the rapid increase in rice cultivation, a dramatic increase in population and general stability at the beginning of China's Northern Song Dynasty.
The monsoon record also matched up nicely with the advance and retreat of Swiss glaciers.
Scientists say the natural archive shows that climate change can have devastating effects on local populations - even when this change is mild when averaged across the globe.
In the cave record, the monsoon followed trends in solar activity over many centuries, suggesting the Sun played an important role in the variability of this weather system.
To a lesser extent, it also followed northern hemisphere temperatures on a millennial and centennial scale. As temperatures went up, the monsoon became stronger and, as they dropped, it weakened.
However, over the last 50 years, this relationship has switched. The researchers attribute this to the influence of greenhouse gas emissions and sulphate aerosols released by human activities.
