The short version is that as the climate degrades and fossil fuels become simultaneously more expensive and less useable, each generation inherits from its more prosperous ancestors an infrastructure that it can’t afford to maintain. Society muddles through from year to year — sometimes even seeming to advance — until some part of that poorly maintained infrastructure snaps and causes major destruction. The destroyed area may get rebuilt, but not to its previous level. The resulting community has less infrastructure to maintain, but is also less prosperous, and so the cycle continues into the next generation.~ Doug Muder from, http://weeklysift.com/2014/09/15/infrastructure-suburbs-and-the-long-descent-to-ferguson/
He draws heavily from John Michael Greer’s The Long Descent.