Droughts Destroy Dynasties

| Fri Nov. 7, 2008 12:51 AM EST

400px-Drought.jpg Chinese history is replete with the rise and fall of dynasties. New research identifies a natural phenomenon behind at least three of the downfalls—the weakening of the summer Asian Monsoon. The same problem may be afflicting northern China now.

Summer monsoon winds originate in the Indian Ocean and sweep into China. When the monsoon is strong, it pushes farther into northwest China. The new research found a strong summer monsoon prevailed during at least one of China's golden ages, the Northern Song Dynasty, when rice first became China's main crop and China's population doubled. Weak summer monsoons coincided with drought in the northwest and the increasing civil unrest that unraveled the Tang, Yuan, and Ming dynasties.

The droughts were deciphered from layers of stone in a 1,810-year-old, 4.5-inch long stalagmite from Gansu Province, China. Measurements of uranium and thorium revealed the date each layer was formed. Analysis of two forms of oxygen were used to match those measurements to low rainfall dates. Prior to this research no one expected that a record of surface weather would be preserved in underground cave deposits.

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The study published in this week's Science also shows a relationship between temperature and the strength of the monsoons. For most of the last 1,810 years, the summer monsoon grew stronger when temperatures rose. But that suddenly flipped around 1960—a sign this current monsoonal weakening and drought in northwestern China is human caused.

Only which cause? If CO2 is the culprit, as some think, the drying trend may continue in Inner Mongolia and northern China. If however the culprit is manmade soot (think coal), the trend could be reversed by reducing soot emissions.

The study also found the drought at the end of the Tang Dynasty coincided with the drought halfway around the world that brought down the Mayan civilization. Meanwhile ample summer rains of the Northern Song Dynasty correlated with the beginning of Medieval Warm Period in Europe and Greenland, when Vikings settled southern Greenland and North America. Weak monsoons associated with the end of the Yuan Dynasty in China correlated to the Little Ice Age that afflicted Europe and Greenland and wiped out nearly 500 years of Viking settlement.

Too bad they snapped off that little Rosetta Stone of history. What might it have foretold next?

Julia Whitty is Mother Jones' environmental correspondent, lecturer, and 2008 winner of the PEN USA Literary Award, the Kiriyama Prize and the John Burroughs Medal.