Background
The authors write that "climatic fluctuation may be a significant factor interacting with social structures in affecting the rise and fall of cultures and dynasties," citing Cowie (1998) and Hsu (1998). When the climate worsens beyond what the available technology and economic system can accommodate, for example, they say that "people are forced to move or starve." And they indicate, in this regard, that "climate cooling has had a huge impact on the production of crops and herds in pre-industrial Europe and China (Hinsch, 1998; Atwell, 2002; Zhang et al., 2007a), even triggering mass southward migration of northern nomadic societies (Fang and Liu, 1992; Wang, 1996; Hsu, 1998)," while noting that "this ecological and agricultural stress is likely to result in wars and social unrest, often followed by dynastic transitions (Zhang et al., 2005)." In fact, they say that "recent studies have demonstrated that wars and social unrests in the past often were associated with cold climate phases (Zhang et al., 2005, 2007a,b)," and that "climate cooling may have increased locust plagues through temperature-driven droughts or floods in ancient China (Stige et al., 2007; Zhang et al., 2009)."

What was done
In a study designed to further explore the subject, Zhang et al. employed "historical data on war frequency, drought frequency and flood frequency," all of which were compiled by Chen (1939), as well as "a multi-proxy temperature reconstruction for the whole of China reported by Yang et al. (2002), air temperature data for the Northern Hemisphere (Mann and Jones, 2003), proxy temperature data for Beijing (Tan et al., 2003), and a historical locust dataset reported by Stige et al. (2007)," plus "historical data of rice price variations reported by Peng (2007)."

What was learned
In analyzing the linkages they found to exist among these different factors, the international (Chinese, French, German, Norwegian) team of researchers concluded that "food production during the last two millennia has been more unstable during cooler periods, resulting in more social conflicts," while specifically noting that "cooling shows direct positive association with the frequency of external aggression war to the Chinese dynasties mostly from the northern pastoral nomadic societies, and indirect positive association with the frequency of internal war within the Chinese dynasties through drought and locust plagues," which have typically been more pronounced during cooler as opposed to warmer times.

What it means
Zhang et al. conclude "it is very probable that cool temperature may be the driving force in causing high frequencies of meteorological, agricultural disasters and then man-made disasters (wars) in ancient China," noting that "cool temperature could not only reduce agricultural and livestock production directly, but also reduce agricultural production by producing more droughts, floods and locust plagues," while stating that the subsequent "collapses of agricultural and livestock production would cause wars within or among different societies." Consequently, although noting that "it is generally believed that global warming is a threat to human societies in many ways (IPCC, 2007)," Zhang et al. come to a somewhat different conclusion, stating that "some countries or regions might actually "benefit from increasing temperatures," citing the work of Nemani et al. (2003), Stige et al. (2007) and Zhang et al. (2009), while restating the fact that "during the last two millennia, food production in ancient China was more stable during warm periods owing to fewer agricultural disasters, resulting in fewer social conflicts."

References
Atwell, W.S. 2002. Time, money and the weather: Ming China and the great depression of the mid-fifteenth century. Journal of Asian Studies 61: 83-113.

Chen, G.Y. 1939. China Successive Natural and Manmade Disasters Table. Jinan University Book Series, Guangzhou, China.

Cowie, J. 1998. Climate and Human Change: Disaster or Opportunity? Parthenon Publishing Group, New York, New York, USA.

Fang, J. and Liu, G. 1992. Relationship between climatic change and the nomadic southward migrations in eastern Asia during historical times. Climatic Change 22: 151-168.

Hinsch, B. 1998. Climate change and history in China. Journal of Asian History 22: 131-159.

Hsu, K.J. 19998. Sun, climate, hunger and mass migration. Science in China Series D -- Earth Sciences 41: 449-472.

IPCC. 2007. Fourth Assessment Report on Climate Change 2007: Synthesis Report. Intergovernmental Panel on Climate Change, Geneva, Switzerland.

Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 10.1029/2003GL017814.

Nemani, R.R., Keeling, C.D., Hashimoto, H., Jolly, W.M., Piper, S.C., Tucker, C.J., Myneni, R.B. and Running, S.W. 2003. Climate-driven increases in global terrestrial net primary production from 1982 to 1999. Science 300: 1560-1563.

Peng, X.W. 2007. History of Chinese Currency. Shanghai People's Press, Shanghai, China.

Stige, L.C., Chan, K.S., Zhang, Z.B., Frank, D. and Stenseth, N.C. 2007. Thousand-year-long Chinese time series reveals climatic forcing of decadal locust dynamics. Proceedings of the National Academy of Sciences USA 104: 16,188-16,193.

Tan, M., Liu, T., Hou, J., Qin, X., Zhang, H. and Li, T. 2003. Cyclic rapid warming on centennial scale revealed by a 2650-year stalagmite record of warm season temperature. Geophysical Research Letters 30: 10.1029/2003GL017352.

Wang, H.C. 1996. The relationship between the southern migrations of north nomadic tribes and climate change in China. Scientia Geographica Sinica 16: 274-279.

Yang, B., Braeuning, A., Johnson, K.R. and Shi, Y.F. 2002. General characteristics of temperature variation in China during the last two millennia. Geophysical Research Letters 29: 38-42.

Zhang, D., Jim, C.Y., Lin, C.S., He, Y.Q. and Lee, F. 2005. Climate change, social unrest and dynastic transition in ancient China. Chinese Science Bulletin 50: 137-144.

Zhang, D.D., Brecke, P., Lee, H.F., He, Y.Q. and Zhang, J. 2007a. Global climate change, war, and population decline in recent human history. Proceedings of the National Academy of Sciences USA 104: 19,214-19219.

Zhang, D.D., Zhang, J., Lee, H.F. and He, Y. 2007b. Climate change and war frequency in eastern China over the last millennium. Human Ecology 35: 403-414.

Zhang, Z.B., Cazelles, B., Tian, H.D., Stige, L.C., Brauning, A. and Stenseth, N.C. 2009. Periodic temperature-associated drought/flood drives locust plagues in China. Proceedings of the Royal Society B 276: 823-831.