Ed reviews a new paper on the fall of the Angkor civilization. He concludes:
Of course, a changing environment was far from the only reason behind the fall of Angkor. By the time the droughts kicked in, the city was already weakened by social, economic and political strife. Buckley simply thinks that the climate simply sealed the city’s demise. In fact, others have suggested that some force may have pushed the local people to move from inland agriculture to maritime trade. Buckley says that this transition coincides neatly with the aftermath of the first drought.
An economic historian might term the droughts which Angkor was subjected to an "exogenous shock." Basically an outside factor which slams into an equilibrium system periodically (I assume that super-droughts would exhibit a poisson distribution but readers more climatically savvy can correct me). On the other hand, there are parameters which are endogenous to the system; consider the institutional frameworks which regulate social relations and distribute economic surplus. Pre-modern societies often live on the Malthusian margin on a per capita basis. In other words, the average Chinese peasant was no more wealthy than the barbarian nomad to the north (in fact, a peasant may be less wealthy on a median basis than a nomad for a variety of reasons). Despite the greater sum total of wealth of pre-modern China, and so the greater surplus which the rentier elites could tax or steal, most of that wealth vis-a-vis Mongolia or Central Asia was realized in the form of people. That wealth was fundamentally based on primary production, agriculture, and when the environmental conditions for agriculture were less favorable then the wealth would decrease. Naturally you would then see a major contraction in the primary manifestation of that wealth, census size. If that environment once more became favorable toward primary production then there might be a transient where individual per capita wealth increased before census size "caught up" (e.g., the average English peasant in the century after the Black Death was healthier and wealthier than before because the population was so much smaller). Grand per-modern polities such the the Khmer confederacy of which Angkor was the apex rely on massive numbers of primary producers from which they can skim and squeeze just short of destitution. In other words the elites who produced high culture were parasites. Civilization was located for most of history in cities, and all cities before 1900 were demographic sinks due to the pervasiveness of morbidity and mortality. Environmental catastrophe which forces the primary producers to look to themselves and evade or dodge taxation or theft by the elites results in the collapse of civilization. This environmentally induced collapse was not limited to the Khmer confederacy in Southeast Asia, in Strange Parallels: Volume 1, Integration on the Mainland: Southeast Asia in Global Context, c. 800-1830 the author shows that the same dynamic of disruption and destabilization was evident to the west and east, from Myanmar to Vietnam, correlated with climatic variation in the region. But things are never always the same. Similar environmental catastrophes did not result in total dissolution of the political order after 1500 in mainland Southeast Asia, the institutional framework which generated some sort of equilibrium managed to withstand exogenous shocks because they had become more robust. The same is evident on a global scale in The Human Web: A Bird's-Eye View of World History and After Tamerlane: The Global History of Empire Since 1405; political institutions over the past 2,500 years are much more robust than they were in the first 2,500 years of civilization. And they have been much more robust over the past 500 years than the previous 2,000 years. There are long term institutional changes which occurred through which we must view the predicted impact of environmental catastrophe. I am not here even touching upon the rapid rise in economic growth which allowed much of humanity to break out of the Malthusian trap after 1850 (see A Farewell to Alms). The main caution from history would be supplied by Brian Fagan in The Long Summer: How Climate Changed Civilization. He admits that political institutions during the Holocene have become more robust, dampening the impact of local famines or disruptions. But, he observes that when collapse does happen it is all the more catastrophic as enormous interlocking social, political and technological systems may now unwind. In other words, we're trading short term small risks for longer term large risks. The analogy that seems appropriate here is that of earthquakes, whereby small quakes tend to release energy which might otherwise pour out in a super-quake. This is why I like to suggest that only technology will save us.