Cool periods in China, and the resulting scarcity of resources, are closely linked with a higher frequency of wars over the past 1,000 years, according to Chinese researchers. The research, published in the journal Human Ecology earlier this month, pointed out that between 1000 and 1911, there were six major cycles of "warm" and "cold" phases, and all cold phases had higher war frequencies.

Traditional wisdom would explain the fundamental causes of wars as economic, political and ethnic, but the climate change could also be an important but long-neglected factor, says David Zhang from the Department of Geography at the University of Hong Kong.

Through history, China's main source of livelihood was agriculture, which is very much dictated by the climate conditions. Yield reduction caused by a lower temperature would trigger famine, tax revolt and a weakening of State power, Zhang says. In addition, the deficit in resources was aggravated by the population expansion in the previous warm period. Rebellions and wars were more likely to erupt during the cold phases, he says.

The wars ushered notable social changes that eventually induced the collapse and establishment of dynasties. Of the six cold phases in the last millennium, five had experienced dynasty collapse.

However, according to the research, the impacts of cooling varied in the three major geographical regions of China. In North China, cooling would generate social unrest very quickly because of its dominant pastoral practice, which is sensitive to cooling.

War frequency in South China had little correlation with cooling due to its humid subtropical and tropical climate where temperature decline could hardly affect yields from the predominant arable agriculture.

Comparatively, the outbreak of wars in Central China responded closely to cooling, but with a very obvious time lag of generally 10-20 years, because surplus farm products could be stored to serve as a buffer in difficult times.

Zhang says the finding also applies to other countries in ancient agricultural society. For example, when the Ming Dynasty (1368-1644) collapsed, other populated areas like Europe, Japan, Korea and the Ottoman Empire were also experiencing their most turbulent times.

Wang Shaowu, professor of Peking University's Atmospheric Physics Department, notes that it was improper to contribute every dynasty switch and war to climate change. Wang believes that social and political factors also played important roles.

Zhang says that the world has gone through an unprecedented warming period in the last 2,000 years. Both cold and hot climates are extreme climate events that could cause disastrous results in the Earth's ecosystem, he warns. "The ecosystem and agricultural production adapted to lower temperature would surely be disturbed in a higher temperature. What the changes would bring to humans is rather uncertain, as it might increase production in some colder areas while other warm areas might witness output drop," Zhang says.

An Chengbang, a research fellow with the Institute of Geography of Lanzhou University, says warmer climate could have two impacts on the inland regions: more precipitation due to enhanced monsoon flow from the Pacific Ocean, and more evaporation due to an increased temperature. "It is hard to say whether the inlands will become moist or dry. As global warming is a long-term trend, the meteorological data collected during the past decades are not enough to accurately predict how global warming might influence agriculture," he says.

(China Daily 07/30/2007 page8)