This might be an interesting read for those considering an apocalyptic world and is a revised memory log of a discussion on an discord channel, content is 60% by me, 30% by Chili the Sheep and 10% other people.
What is the minimum requirement to put a civilization like ours back into stone age?
To fully analyze the minimum requirements to push a civilization like our current one back into the stone age, it is helpful to first consider what scenarios would likely not achieve that outcome. Scientific analysis indicates that the following events, though catastrophic, would likely still leave remnants of our modern knowledge and technologies intact.
A limited nuclear war, even with 1000 to 2000 nuclear bombs detonating and killing approximately 10% of the human population while destroying major cities, would probably not be sufficient. Isolated small towns with populations of around 10,000 people would likely still retain early 20th century technology. They possess libraries, manufacturing tools, metalworkers, mechanics and teachers.
Asteroids impacting the Earth below the level needed to devastate the entire planet, the Yellowstone supervolcano erupting, and other potential natural catastrophes would also fail to reduce human civilization to stone age levels. While the local effects would be disastrous, an estimated 90% of the world's population and infrastructure would likely remain mostly untouched.
Region-wide famines, though kill many people until food supplies become less scarce, generally do not cause the loss of an entire civilization. With appropriate measures, modern civilization could likely recover within one to five years. Even if famines killed 90% of the population, there would still exist millions of educated people such as mechanics, teachers, etc who could help rebuild.
My personal estimate indicates that to completely reset human civilization to stone age conditions would require the death of at least 99.9% of the human population, the total elimination of all evidence of our past knowledge, and ensuring that nobody reinvents basic technologies for at least 4 generations. Such an extremely brutal break in civilization approaching 100% mortality would be a miracle if any humans survived at all. I call this the "99.9% dilemma": if an event can kill 99.9% of humans, it could feasibly kill 100%. In that case, we would all be dead and no stone age would exist.
So there isn’t a way to reset our civilization back to stone age?
Yes, there are combinations of events which could theoretically remove knowledge from past ages and push human civilization back into the stone or early bronze age.
A catastrophic event followed by a primitivistic-luddite religion could achieve this. For example, if a global nuclear war occurred where only 20% of humanity survived, and afterward a primitivistic-luddite religion spread around the world eliminating traces of past knowledge. If integrated deeply into society, that luddite religion could simply suppress the delivery of past achievements.
However, even then it would be extremely implausible. If even a single small nation or mediocre-sized city somewhere survived with knowledge of the old civilization, they would quickly surpass the luddite nations. Remnants of modern technology would dominate a world of "cavemen" with ease, likely without needing nuclear weapons. A few assault rifles and aerial reconnaissance would be enough. The European conquest of the Americas would pale in comparison
In conclusion, the requirements for a "decline to stone age" are very stringent. You would need an extreme event leaving no town of 10,000 people alive. But it cannot be so extreme that it kills 100% of humans instead of 99.9%. This scenario walks a razor's edge.
A decline to more modern civilizations though is easier, but still difficult. Even if only a small African village survived, they likely possess the knowledge to restart an early industrial civilization with farming, mining, and manufacturing on medieval levels if they utilize their entire labor force. A larger community, for example New Zealand or Island, could theoretically even sustain an mid-to-late 20th century civilization without help.
Given division of labor and education, humans are remarkably adaptable.
The most common counter-argument is that humans are "too stupid" to survive without modern civilization. However, most humans are not stupid. We are remarkably adaptable. If after an apocalypse power is down and farming machines do not work, most people are not "too stupid" to plant or pick potatoes under supervision of an experienced farmer. After harvest, engineers among them could build a wood-fueled generator and fix farming engines so next year labor is not by hand. Humans would improve rapidly where it matters and progress faster than before.
The only thing which could slow rebuilding civilization is not utilizing every able worker. For example, what still holds back Arab civilization? Not fully using females in the workforce. Maximum resource utilization, though sounding cold, provides huge gains. In a typical civilization without females working, income per worker may equalize a fully integrated workforce. However, only half of possible workers labor. And expenses depend on total citizens, not workforce.
A nation of 1 million people not utilizing women (or men) might produce 33 million wealth units per year, while a nation utilizing all workers might produce 65 million. Sounds twice as rich? No, it is much worse. Expenses remain around 25 million wealth units. The patriarchal nation retains 8 million wealth units, while the equal nation retains 57 million. The equal nation is seven times more productive. Oh, I just solved the third world problem.
Funny fact, "Stand your ground" is also the Law in Germany. And in 150 years of German Law there were exactly seven cases where "Stand your ground" was used to defend an violent act.