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.

But while the Colosseum might have been the grand daddy of arenas, it was almost matched by similar grand structures all around the empire. (The one in Nimes, for example, has stood since about 100AD in in one way or another.) And this wasn’t frippery – it was the way the empire showed its power and weight. It wasn’t possible to just build enough and stop, Homer-Dixon explains. (A parallel form of consumerism you might say.)

But, Thomas-Dixon says, the Romans had an impossible problem: “The Roman empire was locked into a food-based energy system. As the empire expanded and matured; as it exploited, and in some cases exhausted, the Mediterranean region’s best cropland and then moved on to cultivate poorer lands; and as its grain supply lines snaked farther and farther from its major cities, it had to work harder and harder to produce each additional ton of grain.” Giving this a technical label, he describes this as the “energy return on investment”. (And the comparison with our oil-based economies, and the increasingly difficulty of extraction, is pretty obvious.)

The bulk of the book explores the obvious problem here – increasing complexity requires more and more energy to maintain yet, yet energy is getting harder and harder to find – and finding it takes more and more energy in the mere process. Thomas-Dixon looks back to Rome to see how they handled the problem – which he sees as becoming all too evident first around 180AD: “Rome’s control of its frontier territories disintegrated and many were abandoned. Travel and trade became unsafe, and literacy and recordkeeping plummeted; commerce declined. Although tax revenues were static or declining, government costs continued to go up as emperors tried to secure their power by expanding the dole, increasing the size of the army, boosting soldiers’ pay and holding more games and spectacles.”

Thomas-Dixon credits Aurelian and Diocletian for arresting the slide by introducing “complex and harsh measures to extract more energy from the land”. A rudimentary budget was introduced, with the tax being set each year in grain and other produce according to the calculated needs of the state, while theland was closely surveyed and every potentially productive bit identified – and tied to a particular person whose activities were also closely tracked – and many essential occupations were made compulsory and hereditary. But this meant, eventually, more rising taxes, and farmers either starving or fleeing. “By the 5th century in the West, the empire was literally burning through its capital – its productive farmland and its peasantry. Peasants deserted their lands, so power and wealth were increasingly concentrated in the hands of large landowners, who then used their influence to evade taxes. …. As state finances deteriorated, public services like roads, bridges, aqueducts and the postal service broke down.” (Put your own modern parallel of choice in here….)

You might have noticed by now that this is all pretty gloomy stuff, while the title of the book is apparently upbeat. For Thomas-Dixon devotes the last third of the book for trying to imagine how we can minimise the damage, and grow something positive from the wreckage. This is his prescription:

“First, we must reduce as much as we can the force of the underlying tectonic stresses in order to lower the risk of synchronous failure – that is, of catastrophic collapse that cascades across boundaries between technological, social and ecological systems. Second, we need to cultivate a prospective mind so we can cope better with surprise. Third, we must boost the overall resilience of critical systems like our energy and food supply networks. And fourth, we need to prepare to turn breakdown to our advantage when it happens.”

Thomas-Dixon visits the Temple of Jupiter in Baalbek , and particularly the trilithon – an enormous, hugely energy-intensive, and to us wholly pointless structure. He walks down the modern, damaged street, past a shiny new cash machine, and “reflected on alternative value systems that could help us achieve different futures… our values must be compatible with the exigencies of the natural world we live in and depend on. They must implicitly recognize the laws of thermodynamics, energy’s role in our survival, the dangers of certain kinds of connectivity, and the nonlinear behaviour of natural systems like climate. The endless material growth of our economies in fundamentally inconsistent with these physical facts of life.”

When we discussed Thomas-Dixon’s work at my book club, you could easily divide the group in two, into the optimists and the pessimists. The former focused on Thomas-Dixon’s exploration of Holling’s panarchy theory. Based on observations of cycles in forests, which go through a cycle of growth, collapse, regeneration, and a return to growth. In short – the total biomass of the forest grows as it develops, the genetic diversity grows, it evolves to maintain a stable system, through connected mechanisms that keep temperature, rainfall and chemical concentrations within the ranges best for the life of the forest. The result is maximum efficiency, maximum biomass from the inputs of sunlight, water and nutrients. But this fine-tuning eventually means that the system is less resiliant, and can’t cope with an inevitable shock, a change in climate, a fire, or the arrival of a new invasive species. But, panarchy says, this means for the overall system – the world – the potential is introduced for creative change – new species, new ecological cycles, more overall diversity. So, from the ashes, comes something even better.

But the pessimists, however, among whom I’d have to count myself, focused on the destruction, and couldn’t see this conclusion in panarchy as much more than wishful thinking. As we’ve seen in too many real forests – from the Amazon to the Pacific – what’s more likely to replace the forest after the shock is a much less productive, low-level ecosystem of grassland or scrub. Something, in Thomas-Dixon’s terms, producing and storing far less, and far lower quality, energy.

Nevertheless, although Thomas-Dixon didn’t leave me in an upbeat mood, he offers an analysis that is not only interesting but also potentially productive. Combing the concerns of energy and complexity does allow an important focus on the need to talk about, to advance, to promote resilience. Today talk in the public and private sectors focuses on “efficiency”, on using the minimum resources and money to deliver a service. My favourite example of the dangers of this is the Auckland power crisis of 1998. It is clear from Thomas-Dixon’s account that in our increasingly complex world, we have to stop talking about building “efficient” systems as the great goal and start aiming for resilient ones.