The New Scientist magazine – recommended, by the way, to anyone who likes to keep up with developments across science – included a couple of linked articles by Deborah MacKenzie in its 5 April 2008 issue, concerning threats to our civilisation. Always an interesting topic to SF fans!
To summarise, the general thesis runs something like this: simple civilisations tend to be relatively unaffected by disasters, unless drastic environmental change (e.g. prolonged drought) makes their location uninhabitable. So although 14th century Europe suffered a one-third loss in population due to the Black Death, the civilisation continued. This was because it was essentially a rural, subsistence society, with urban populations being small. The survivors just carried on as usual, and all they needed could be grown or made locally.
Increasing the level of complexity of a civilisation increases its resilience, up to a point. Local disasters, which would previously have had major local effects, can be mitigated by rushing in aid. The threat of epidemics can be countered by the rapid development of medical countermeasures. However, as a civilisation becomes even more complex and tightly integrated, as ours is, its vulnerability increases. An economic shock affecting one major region affects everyone (as we are seeing now). Commercial economics drives efficiency, which leads to such changes as concentrating manufacture of a product in as few locations as possible; preferably only one, located in a part of the world where the labour is cheapest. Clearly such a system is more vulnerable to local disaster or transport interruptions than distributed production. This is exacerbated by the fashion for "just in time" deliveries, which means that only limited stocks are kept in warehouses; for instance, cities typically contain only a three-day supply of food.
This emphasises that we are highly dependent on the continuous functioning of our transport infrastructure. Not just to supply food, but just about everything we need, including medical supplies to hospitals and fuel to power stations and road transport filling stations. Power failures would cripple our ability to respond to crises, or even to find out what is going on. They would also lead to the rapid spoiling of refrigerated foods; in fact, the inability to buy anything (electronic shop tills wouldn't work). If transport breaks down, we will be plunged into deep trouble very quickly.
But what could have such a widespread effect on transport? MacKenzie suggests an international pandemic, such as the post-WW1 influenza outbreak which killed far more people than the war. She points out that this only had a 3% death rate, whereas in the cases so far of the H5N1 "bird flu" passing to humans, the death rate has been 63%. If bird flu (or some other virus) mutates to be highly infectious, our international air travel system could distribute it around the globe very quickly, perhaps more quickly than we could analyse the virus and devise, manufacture and distribute medical countermeasures.
The natural human response to a pandemic would be to stay at home; either due to sickness, or to look after sick relatives, or to keep from being infected. If enough transport workers are sick or stay at home, the transport system will fail. If enough refinery workers stay at home, no more fuel will be produced. If enough power station workers stay at home, the power will fail. If enough water supply workers stay at home, the water will fail. Very often nowadays, the continuous functioning of such essential services depends on a few key individuals. (A personal anecdote: I once attended a conference based in a college building. It started late, because the caretaker didn't turn up on time; and he was the only man with the keys.)
MacKenzie goes on to suggest that the increasing vulnerability and specialisation of complex civilisations may make their eventual collapse inevitable. This is not a new thesis; Jared Diamond covered such ground in his 2005 book 'Collapse', as did Joseph Tainter in his 1988 book 'The Collapse of Complex Societies', which examined the ways in which all previous civilisations have collapsed. We are not as immune from that as we might like to think (naturally, every civilisation has assumed it will last forever). Even without such a dramatic event as a pandemic, the increasing pressures on limited supplies of fresh water, fuel, food and other raw materials, are eventually likely to make our present way of life unsustainable. Climate change will exacerbate these problems, because our systems of agriculture are finely-tuned for our present patterns of rainfall, and much of our urban and transport infrastructure to existing sea levels. Any major international disruption, for whatever cause, will hit the confidence of the international financial markets on which the functioning of our civilisation depends.
What can be done? Those societies which would be least affected by major collapse would be rural ones with self-sufficient lifestyles (preferably located a long way away from densely populated urban areas) but it isn't feasible for most of us to live like that. Some measures will help stave off the risk of collapse, but they will not be easy to implement because they would be uneconomic if left to the market. More distributed production of essentials to spread the risk and minimise the importance of transport would certainly help. More use of distributed power and other supply systems would also add resilience. The local (even domestic) stockpiling of long-lasting foods and other essentials would be very prudent. So would disaster planning which takes into account the potential risks of a major pandemic and implements plans to minimise the effects (current planning tends to assume a death rate of no more than 3%).
MacKenzie points out that only one complex society has ever survived collapse, and that was by deliberate downsizing: the empire of Byzantium lost most of its territory to the Arabs, and responded by simplifying their society; moving out of most cities, switching to more of a barter economy, and changing their professional army to a peasant militia. Education levels declined.
A final thought: if the worst happens and our civilisation does collapse, it does not of course mean the end of humanity; although the world population would shrink drastically and revert to an earlier and much simpler form of existence. Recovery from such a disaster to anything like our present level of civilisation may not be easy. We have already mined out much of the easily-obtained fuel and other raw materials, so humanity could be caught in the Catch-22: before advanced technologies could be developed again, raw materials would be needed which could only be extracted by the use of advanced technologies, even if records of their locations had survived. What happens to our civilisation is therefore of critical importance to humanity's future.