Houshalter

If that happened in the modern world, technological civilization might end and never be restarted. The modern world depends on hugely complex infrastructure and tons of different industries and inputs. If we lose that, it would be very difficult to rebuild. We've already extracted most of the easy to get to minerals and fossil fuels. Much farmland has been degraded from overuse and depends on inputs of fertilizer, irrigation systems, and of course modern machinery which would be difficult to replace.

skeptical_lurker

I agree. The end of technological civilization is a different point from simple mass casualties - if 'only' 40% of humanity dies, but those 40% are concentrated in first world countries and urban centres, would the survivors be able to rebuild? Machinery would continue to work for a while, although the oil distribution chain would break for a while at least, but in the long run machinery would break. The factories tend to be in the first world countries that have been nuked, the universities in the cities have been mostly destroyed. Moreover there would likely be a general luddite tendency to blame technology for the crisis. Its probably easier to restablish resource extraction then to restart scientific research, and so we would be less likely to develop renewable energy before the fossil fuels run out. I suppose the end of technological civiliseation would reduce the population back to medieval levels, although this would be a long process of resources slowly running out and machinery slowly degrading.

I've often heard claims like these and wonder what the exact date of regression would be. Suppose the low hanging fruit have been removed for a number of modern resources (oil, helium, fissionables, rare metals). We still have quite a lot of coal (in the US and Russia), wind, and hydro power for energy. We also have abundant common metals, which might be more accessible than before if civilization collapsed and left a bunch of scrap around. My understanding is that coal and modern smelting techniques with common metals get us to at least 1850. Furthermore, modern scientific knowledge can't be significantly lost because this requires destroying virtually all books or other records. Hence I would expect at least some of humanity to never slip further back than this point.

I'm sort of nitpicking though. I agree that 40% dead could easily lead to 90% dead.