You might think that the world doesn't need another why-the-Roman-empire-collapsed theory, when there are already so many to choose from - according to one professor's count 210. Depending on your disposition you might like to cling to vintage Gibbon - moral decline, or prefer the technological theory of Richter - that the Germans invented the horseshoe, or the plague theory of McNeill. There's no shortage.
But the reason for such diversity of explanations is surely that the collapse of the Roman empire, at least in the Western world where theories of continual progress tend to rule (rather than the predominant eastern model of rise-collapse-rise, which regards such change as normal) is the exemplar of collapse - the very model of fear. If we can decode this, explain it, then maybe we can avoid going the same way ourselves.
So it makes perfect sense that Thomas Homer-Dixon in his The Upside of Down: Catastrophe, Creativity and the Renewal of Civilisation has a new theory- or at least an elaboration on some of the old economic ones that relates explicitly to our current world civilisation. And he's also pretty good at explaining why this is indeed a good model for us. A society's complexity can be measured by its level of urbanisation, he says, and at the height of the empire, the population of Rome probably topped 1 million, and may have reached 1.5 million. The empire's total population was probably around 60 million, between 15 and 20 per cent urbanised: perhaps 30 per cent in Europe, 20 per cent on the Italian peninsula. No European city approached 1 million again until London reached it in the 19th century. (And yet the level of complexity the Romans reached is many orders of magnitude lower than that at which we are today - where it is expected that by 2030 4 BILLION people will live in cities.)
You can't fault Homer-Dixon on the empirical, slightly mad but informative research that underlies The Upside - a calculation of the energy required to build the Colosseum. Energy here of course is the form of grain hay and oil - the fuel that powered the human and animal muscles doing the work. So it is, after all of the work, he and a research assistant conclude that: "the Romans had to dedicate, every year for five years, at least 19.8 square kilometers to grow wheat and 35.3 square kilometers - or almost the area of the island of Manhatten. And to capture the solar energy needed to extract, move, carve and hoist the single keystone ... they needed nearly 1,300 suqare metres of farmland." And that, he explains, is an under-estimate, for it doesn't count the land needed to feed the farm workers growing the stuff.
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