Dachs left me morbidly curious in the Jared Diamond thread with this snippet:
So what is this theory about?
A load of horse dung.
About two and a half years ago, there was a spate of news articles about a short paper published in
Science analyzing tree-ring climatological data points for the late classical period. The authors correlated "wet and warm" periods with eras of Roman prosperity and "dry" periods with Roman decline, and suggested that these political changes could be explained by the effect of the altered climate on agriculture, which formed the foundation of the Roman economy.
The problems start to arise with an even cursory critical reading of the article. Firstly, the authors noted that the dry periods began in earnest in the third century, a period of political crisis in the Roman Empire. Problematically, the so-called Third Century Crisis has mostly been abandoned by classical historians over the last few decades for a variety of reasons. And insofar as there is still a sense of the Roman Empire undergoing serious conflict in the third century, there is almost nobody who believes that this was connected to
economic issues. So the correlation isn't even there in the first place.
Secondly, even if we agree that the climate changes might have had something to do with the Crisis of the Third Century - and there is no reason to do so - then one would have to explain why the Roman Empire underwent a period of economic and political prosperity in the
fourth century, a historiographical trope that by contrast is very well supported by archaeological evidence and textual data. The climate was, supposedly, getting less conducive to agriculture, but at that time, across the empire, there was an agricultural boom (esp. in North Africa, Britain, and Syria).
Thirdly, the authors' explanation is superfluous, because we
already have a perfectly sound explanation for the collapse of the unified Roman state in Western Europe. Even if there is considerable disagreement about the mechanics of this collapse (e.g. between the supporters of Walter Goffart and those of Walter Pohl, or between those who highlight "barbarians" as causative agents and those who describe "barbarian" migrations as effects), the fundamental consensus remains: the Roman Empire ceased to exist because of a myriad of civil wars that tore the country apart and left periphery and core fragmented and antagonistic.
There isn't really much room in that explanation for a climatological element, except possibly with the "barbarians". Sometimes climate changes are employed to explain migratory activity in history, but the mechanics of this are usually shoddy and unconvincing. Sometimes we will see claims that climatic optima cause population surpluses which cause political turmoil which causes relocation
en masse, but the individual links in this chain are not substantiated. To complicate matters, sometimes the
opposite argument is put forward:
poor climate conditions cause population collapses which are staved off by...relocation
en masse. Effectively, any climate whatsoever is used to explain migratory activity: good climate, bad climate, changing climate,
unchanging climate, and so on. This should set off alarm bells in readers' heads.
And then there are the issues with determining whether migratory activity even took place
at all but that's another story entirely...