Levels of Collapse (warning: may be disturbing)
1. Short term, basic infrastructure and money system remain operational, possible interruptions to electric, gas and water supplies. Less locally devastating than severe floods, earthquakes, storms etc but much more widespread. Many businesses cease operation, significant unemployment. Larger impact than anything in developed countries in last 50 years, worse than 'Great Depression' of 1930s.
2. Short term, considerable economic dislocation but basic infrastructure and money system (local at least, but probably not at 'normal' value) remain largely intact. Low die off (< 5 to 10% ?) unless widespread lawlessness when it could be higher, perhaps >25% in some dense population areas. Probable need to survive a few weeks or months without normal water / gas / electricity / shopping supplies for a significant proportion of population.
3. Short term, significant collapse of infrastructure and money but sufficient remains to re-establish pre-existing society if it does fragment and repair most critical damage within months or a few years. Electricity, water, currency value all largely absent for several months, maybe years. Low to medium die off for developed countries, perhaps 20% to 60%. Probably equivalent to go back 40 to 80 years. Most important knowledge probably preserved.
4. Medium term, most infrastructure, government, money systems fail. Most systems and infrastructure have to be rebuilt locally once the population has learned to survive and feed itself. Medium die off for developed countries, 40% to 80% overall, very variable between urban and country areas, could range from 0% to 95% for different localities. Probably equivalent to go back 100 to 300 years. Significant knowledge lost.
5. Long term. This is mostly differentiated from medium term by the amount of population, skills, knowledge, that are lost. Major die off for developed countries, 70% to 90%. Go back 500+ years. Most knowledge lost.
6. Very long term. 90% to 99% human population lost, survival and repopulation first priority. Go back 1000+ years, nearly all knowledge lost.
7. Re-evolve 1. Human experiment terminated. Go back 1+ million years, apes probably still best bet.
8. Re-evolve 2. Back to small mammals / reptiles / insects, back 50+ million years.
9. Unicellular / full restart.
The first two levels are insufficient, of themselves, of providing sufficient 're-adjustment' to solve the resource and other problems we will imminently face, thus it is very likely that further shocks / collapses would follow level 1 and 2 collapses.
A level 2 collapse might hopefully trigger a massive change in human priorities, behavior and intent such that we could avoid anything worse and buy us the time to find solutions - that is my best guess of our best hope. A level 1 collapse is unlikely to be sufficient.
Level 3 or greater collapses will disable countries as functional entities, mostly temporarily in the case of level 3. But local survival becomes the priority for years. Level 3 is the least level of collapse that, of itself, probably makes humanity sustainable beyond this century.