A record from Wanxiang Cave, China, characterizes Asian Monsoon (AM) history over the past 1810 years. The summer monsoon correlates with solar variability, Northern Hemisphere and Chinese temperature, Alpine glacial retreat, and Chinese cultural changes. It was generally strong during Europe's Medieval Warm Period and weak during Europe's Little Ice Age, as well as during the final decades of the Tang, Yuan, and Ming Dynasties, all times that were characterized by popular unrest. It was strong during the first several decades of the Northern Song Dynasty, a period of increased rice cultivation and dramatic population increase. The sign of the correlation between the AM and temperature switches around 1960, suggesting that anthropogenic forcing superseded natural forcing as the major driver of AM changes in the late 20th century.
Standard story, right? Here's the main figure:
Late Tang Weak Monsoon Period = LTWMP
Northern Song Strong Monsoon Period = NSSMP
Late Yuan Weak Monsoon Period = LYWMP
Late Ming Weak Monsoon Period, LMWMP
Tying Chinese dynastic cycles to climatic change is not new. Economic history tells us that the Chinese invested enormous labor inputs into their agricultural system and so attained a very high level of population density. But these populations were at the Malthusian limit. Theoretically one assumes that perturbations, whether it be climate or war, would result in peasant die-off because the agricultural yield would decline without ideal environmental conditions or reduced labor input due to disruption. The broad productive base of the Imperial Chinese system was the free peasantry (in contrast to Imperial Rome, where slave productive surplus was essential, if not dominant). Operationally the Mandate of Heaven suspiciously tracked the fortunes of the agricultural populace; e.g., the Yellow Turbans.
These data are interesting, but I have to be honest: I doubt that climatic changes are sufficient conditions. If you know Chinese history I think you'll find that eyeballing the shifts show many periods when the climate seems to have been volatile and it is known that the society was relatively quiescent, and vice versa. Additionally, one macrohistorical trend has been that inter-dynastic interregnums have become progressively shorter with each shift of the Mandate. That suggests structural changes in the character of Imperial Chinese system as power shifts were less traumatic or disruptive during later periods.
Finally, the ecologist Peter Turchin in recent work has show that models driven by endogenous parameters, those factors integrated into an internal system of variables which results in social evolution, predict the social-historical data better than the alternatives. Climate-driven theories are inherently exogenous, the sociological parameters have only a marginal affect on climatic regime (deforestation, etc., can cause changes obviously). But, Turchin's data sets are European-biased. Could it be that the Chinese, living closer to the Malthusian limit, had less buffer to absorb disruptions from exogenous climatic shocks? Or perhaps European capital intensive farming was more flexible in the face of changing circumstances?
I think the final answer is going to be complicated, with many moving parts. But I think we need to take a step back from climatic deterministic on this level of historical granularity.
Collaps of Han dynasty followed by FIVE BARBARIANS CHAOS(äºè¡ä¹±å) was in same time period as fall of Roman empire with barbarian invasion and dark age. Very suspicious for climate reason.
Can't comment now, but Elwin's "The Retreat of the Elephants" looks to be an excellent environmental history of China. It's long and pretty dense, but well worth taking a look at.
Still not able to do justice, but I'll make my canned speech on climate determinism.
1. Steppe peoples do not need an excuse or cause to invade the agrarian civilizations. Because of the steep wealth threshold between the pastoral and the agrarian world, pastoralists always want to invade, if they're not getting tribute. (Agrarian civilizations are not wealthier per capita, but the populations are larger and wealth is more concentrated).
2. The thing to look for is successful invasions. Invasions that are turn back leave less of a historical mark. Success of an invasion is often the result of weakness on the defensive side.
3. Steppe unity is another variable. "Divide and conque" (actually, more often "divide and neutralize") was a successful anti-nomad strategy.
4. Climatic explanations are over a century old, and there are a lot of them. They're "scientific" in the sense of materialist and reductionist, but rarely or never are based on detailed observations of actual climatic conditions. The data above are the first detailed long-term datasets I've seen in several decades of following this.
5. Besides actual climate observations, there should be clarity about what the dependent variable is supposed to be. This isn't always clearly stated either, much less measured. Steppe aggression? Sedentary weakness? Size of political units? Volume of trade? Wealth at the capital?
6. Likewise, the mechanism should be clearly stated. Do nomads attack when they're strong and numerous, or when they're poor and starving? The former is the more true, but many theories assume the latter. (One theory is two steps: nomads build up their strength during good years, but attacks are triggered during bad years. This is actually plausible.)
7. Another promising line of research: changes in rainfall convert pasture to agricultural land, or vice versa, changing the relationships between the nomads and the sedentary peoples in various ways.
8. Many steppe peoples controlled considerable agricultural areas and were thus mixed economies. When steppe peoples had access to agricultural products, they could maintain longer campaigns because their logistics was much better.