Weather and Steppe Invasions

“Our geldings are fat”, the Mongol warriors pointedly told Genghis Khan.  They were asking their chief, indirectly, when they would ride to war. The Mongols typically launched their campaigns in late summer, when their horses had had their fill of grasses, could survive long marches, and gallop quickly in and out of combat.  The more grass grew on the steppe, the more geldings the Mongols could ride to war.  A succession of good springs and summers would swell the herd with horses.

So, I was puzzled to read in “Climate Change and War Frequency in Eastern China over the Last Millennium”  by David D. Zhang, Jane Zhang, Harry F. Lee and Yuan-qing  He in Human Ecology (Aug., 2007),  that the authors identified a correlation between bad weather and the outbreak of warfare, the opposite of what I would have expected.

Today we are concerned about global warming, but our ancestors feared cold spells. Zhang et al. write, “Temperature is probably the most important climatic variable influencing human societies, which are particularly vulnerable to long-term temperature changes. Temperature fluctuations directly impact agriculture and horticulture, exacerbate natural disasters, and can adversely affect plant, animal, and human rates of disease.”   A relatively small drop in average temperatures can have a huge impact on crop yields, and even greater on animal survival rates.  During a zuid, or frost, herders can lose 90% of their herd, even if the average drop is just 2%.

They then turn to wars, using tabulations recorded by the Editorial Committee of China’s Military History.  This organ recorded 899 wars between the fall of the Tang and end of Manchus, almost one war per year.  The intensity of these conflicts in terms of duration and deaths yields a plot of peaks and valleys showing the deadliest decades. Not surprisingly, the highest peaks of violence accompany the fall of the Song, the Yuan, and the Ming.   Scholars often attribute these dynastic collapses to poor crops, famine and ensuing peasant revolts. In this respect, climate and warfare seem to correlate well.  A lot of wars in China consisted of revolts, often initiated by desperate and starving farmers.

However, when the authors analyze southern and northern China separately, the data shows something else.  In southern China war is highly correlated with cold weather, with 2.4 times more wars than in warm weather.  In the north the correlation is only 1.6.  Still,  cold weather in the north of China might have weakened the resistance of the sedentary populations, without devastating the pastoralists herds, making it easier for the steppe warriors to invade.  The fall of the Ming dynasty in 1644 is typical in this regard.  This decade was one of the coldest on record.  Peasant revolts wracked the dynasty. The pastoralist Manchus waited for their chance—the last Ming emperor committed suicide.

Zhang et al. argue that the Manchus were able to organized themselves into the formidable banner regiments, devise their own alphabet, raise their ruling chiefs to imperial rank, and invade China because they faced the same terrible climatic conditions as the Chinese. I find this argument unconvincing. It could well be that Ming China’s collapse harmed their own economic well-being. They had grown wealthy from selling horses to their Chinese neighbors, benefiting from the Ming’s reluctance to buy from their old enemies, the Mongols.  Perhaps the loss of this horse trade encouraged Nur Haci to act.  But if he had lost many of his horses to a zuid, he would hardly be in a position to defeat a Ming army of 100,000 cavalry.  There is no direct evidence that the Manchu herds had shrunken in size.  So, it is very likely that the cold weather did not affect the horses, and Nur Haci’s geldings were fat and ready to attack when word of the emperor’s suicide came.

Studies of average temperature probably do not capture micro-climatic events like zuids, while they are better at explaining bigger agricultural crises. The Little Ice Age in Europe, for example, contributed to the 30 Years War (1618-1648).  I do not find climate per se a convincing explanation for Genghis Khan or Nur Haci, although such findings are frequent in the literature.  We need more research on good growing years on the steppe, to see if these correlate with the success of the Mongols and Manchus, as anecdotal clues suggest.