Ian Morris on Why Societies Collapse

Stanford's Ian Morris, a classicist by training, is one of 32 fellows in the inaugural class of Andrew Carnegie Fellows in the humanities and social sciences. Here's a passage from a recent interview published on the Stanford website:

A few years ago I designed a quantitative index of social development, allowing us to measure the ability of different societies to master their physical, intellectual and political environments. This suggested that social development has been rising for most of the 15,000 years since the end of the last ice age, but also showed that there have been several great collapses, plunging societies into dark ages that last for centuries.
The same five factors recur in all the major collapses: uncontrollable migrations, new epidemic diseases, state failures, famine and – always involved, but never in quite the same way – climate change. All five of these factors, of course, seem to be pressing on us in the early 21st century. The greatest past collapses began in Song-dynasty China about 1,000 years ago, and all the way from the Roman Empire to Han China 2,000 years ago. In both cases, the societies in question seem to have developed about as far as was possible in a purely agricultural system.
The only way to go further was by having an industrial revolution; but because neither the Romans nor the Song managed this, both stagnated and then fell apart. The lesson of past collapses seems to be that every economic system has its limits, and we may now be approaching the boundaries of what is possible in a fossil-fuel, industrial world. If that is right, then the 21st century will be a race between innovation, transforming our fossil-fuel economies into something entirely new, and stagnation, leading us into a new dark age.