China's wars, rebellions driven by climate

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The collapses of the agricultural dynasties in China are closely associated with drops in temperature, say researchers.
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Two millennia of foreign invasions and internal wars in China were driven by climate change, a bold study released today argues. Food shortages severe enough to spark civil turmoil or force hordes of starving nomads to swoop down from the Mongolian steppes were consistently linked to long periods of colder weather, the study found.

In contrast, the Central Kingdom's periods of stability and prosperity occurred during sustained warm spells, the researchers say. Theories that weather-related calamities such as drought, floods and locust plagues steered the unravelling or creation of Chinese dynasties are not new.

But until now, no one had systematically scanned the long sweep of China's tumultuous history to see exactly how climate and Chinese society might be intertwined. Chinese and European scientists led by Zhibin Zhang of the Chinese Academy of Sciences in Beijing decided to compare two sets of data over 1900 years.

Digging into historical archives, they looked at the frequency of war, price hikes of rice, locust plagues, droughts and floods. For conflict, they distinguished between internal strife and external wars. At the same time, they reconstructed climate patterns over the period under review.

"The collapses of the agricultural dynasties of the Han (25-220), Tang (618-907), Northern Song (960-1125), Southern Song (1127-1279) and Ming (1368-1644) are closely associated with low temperature or the rapid decline in temperature," they conclude.

Move or starve

A shortage of food would have weakened these dynasties, and pushed nomads in the north - even more vulnerable to dips in temperature - to invade their southern, Chinese-speaking neighbours, the authors argued. A drop of 2.0°C in average annual air temperature can shorten the growing season for steppe grasses, which are critical for livestock, by up to 40 days.

"When the climate worsens beyond what the available technology and economic system can compensate for, people are forced to move or starve," they say. The study found more droughts and floods during cold periods, but the factors that contributed most directly to wars and dynastic breakup were soaring rice prices and locust infestations.

The Roman and Mayan empires, they noted, also fell during cold periods.

More: http://www.abc.net.au/science/articles/2010/07/14/2953298.htm