Most complex systems, whether they be physical, biological, or technological (and presumably, social) can absorb a certain amount of damage without undergoing catastrophic collapse. Increasing deterioration will reduce efficiency, but the system usually continues to operate at some lower level of performance. If the damage is severe enough, the system may undergo a change of state, a phase change, and still continue operating in a some new mode altogether.

Complex systems can be remarkably robust, absorbing enormous amounts of punishment and still continuing to function. But there will always come a point where some catastrophe will be so severe, so all-encompassing, that the system cannot recover, and it will simply cease to hold together and unravel completely.

Like the earth's biosphere or the climate system, human societies have shown themselves to be remarkably resilient and capable of recovering from profound shocks. Even major events, like the fall of the Roman Empire, have not occured overnight and actually affected relatively few people at any one time. Rome fell, the barbarians sacked it, but people all over Europe still knew how to plow the earth and work metals and keep livestock. Life went on. Even many Roman institutions, like its language and religion, survived.

But modern societies, highly dependent on technology, have glaring vulnerabilities which could bring them crashing down like a house of cards. It is hard to conceive of our civilization suffering a phase change and suddenly restabilizing at some lower level of organization, say an 18th or 19th century condition. If any of a number of key "single points of failure" occur, such as a global nuclear war, a sudden shortage of fossil fuels, or even a sudden shift in global rainfall patterns, a cascade of events could occur which would accellerate unstopped into an endless fall. Even a purely abstract and symbolic event could precipitate this collapse, like an economic meltdown.