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The pandemic has demonstrated how even a small reduction in capacity can produce downstream disruptions in supply lines.

The standard model for an apocalypse is that mankind's population drops below a certain level. Please skip down if you're familiar with this story.

There aren't enough people to both transport parts and materials across the world, or make use of them when they get there. Our globalization bites us in the butt because no single area produces enough parts and materials to sustain technology.

Food, fuel, and parts would run out in pockets of isolation, especially in rural areas. No fuel means no crops, after which the cities would tear themselves apart, wiping out our manufacturing capacity and communications hubs. Eventually, even police and military can't maintain themselves, and most of the world collapses to whatever level of population can be supported by local (walking distance) food production.

Primary question: What percentage of people would have to die/cease to exist for this chain of events to come into being?

Presuming that a percentage of mankind would spontaneously vanish. Here are the spherical cow criteria.

  • One-time event
  • Purely random
  • Completely unexpected
  • Everyone vanishes all at once, with whatever they're wearing

I'd like to specify that the final collapse state would be pre-industrial, but theoretically there's a stable state where even medieval people would be able to figure out the next steps to rebuild from the artifacts. Asking what that steady state might be would be a second question.

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    $\begingroup$ VTC. Depending on your point of view, this is either a duplicate of What is the minimum human population necessary for a sustainable colony? or a duplicate of How many people are required for a healthy re-population of the Earth (Post-Apocalypse)? Or a duplicate of any of the other "how few people are needed to restart the world?" questions. $\endgroup$
    – JBH
    Aug 3, 2022 at 23:21
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    $\begingroup$ "How high a percentage could mankind sustain before civilization collapsed?" and "How far would it collapse?" are two separate question and the second is highly contingent on the first. $\endgroup$
    – Halfthawed
    Aug 3, 2022 at 23:34
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    $\begingroup$ It would be very dependent on a lot of factors. If, for example, the disappearance was expected, or recurring, then the remaining people would plan for the alien mass abduction every ten years. $\endgroup$
    – DWKraus
    Aug 4, 2022 at 0:04
  • $\begingroup$ How much of the population has died out? $\endgroup$
    – Crafter
    Aug 4, 2022 at 2:54
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    $\begingroup$ Another thing t consider, how many people can you lose before breakdowns in supply causes even more people to die, getting to that number may drive you to the collapse number. $\endgroup$
    – John
    Aug 4, 2022 at 3:17

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Compared to pre-industrial levels, on average people in the world are smarter, more educated, and the world would have ample resources - so very little amount of people need to survive in order to perpetuate the species

The minimum people required to populate a colony is 98 fertile individuals - however to prevent ongoing genetic diseases it is preferable to have more than 500 (some suggest 2500 to prevent some of the more rare conditions).

If assuming people snap out of existence randomly distributed globally, all the worlds infrastructure would likely still be in place. It is conceivable that the remaining individuals would be able to find a radio in the following months to contact the others in the world, and arrange a place to meet and grow a colony. There would be plenty of food in the form of cans and bottles in globally ubiquitous Supermarkets.

Assuming the following:

  • Current population of 7,846,000,000 people
  • Assuming 40% is of fit reproductive age
  • Assuming 20% of remaining individuals are unable to make it to an existing supermarket, make it to a transport to enable them to rejoin others, and/or are unwilling to provide offspring

The resultant available pool of survivors is 2,510,710,000. Still quite a lot.

If we need only 2500 of these as a minimum to survive as a species (putting up with minimal genetic defects over time) your Snap would only need to leave 0.0001% of people behind.

Randomly distributing using current global populations means the central 'meeting place' is likely in China or India, with a large portion of the 2500 survivors being from Asia.

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  • $\begingroup$ You need enough in a single area that they can meet each other for critical mass population within a reasonable time. $\endgroup$
    – ohwilleke
    Aug 4, 2022 at 2:02
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    $\begingroup$ people are smarter and more educated but there is also a lot more information needed to keep civilization running, do you know what alloys are needed to make an MRI machine, How about the chemical sequence needed to make diabetes medication from crude oil? How about the proper to prepare a nuclear powerplant core for maintenance? How many people on the planet do you think know each of these pieces of information, how many such pieces can be lot before civilization as we know it starts to break down. How far away is anther bronze age collapse. $\endgroup$
    – John
    Aug 4, 2022 at 3:14
  • $\begingroup$ @John Our civilisation would exist as long as we do - there are also many records to pick up any lost information as the population recovers, as these would later become necessary. The question did not specify the degree in which the civilisation must survive. If we interpret it as 'no loss or effect on our way of life', then just a handful of disappearances would do it. If we interpret as 'so our civilisation does not become extinct', then we need to talk only about minimums. $\endgroup$
    – flox
    Aug 4, 2022 at 4:11
  • $\begingroup$ @flox Are there? how much information would be lost is the internet collapsed tomorrow? worse knowledge is not the same thing as training, skills get lost and some skills are both rare and essential. And no civilization would not survive as long as we do, civilizations can and have regressed in the past, hell reading and writing almost disappeared during the bronze age collapse. $\endgroup$
    – John
    Aug 4, 2022 at 4:19
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While a few hundred people in one area could allow the species to survive (a minimal viable population), it might take centuries or even many thousands of years for them to approximate the pre-event level of civilization with such a small starting point.

The Americas or Australia had a starting populations of that order of magnitude, but it took them something like 10,000 years before they reached a reasonable level of technological civilization - longer than people who were just reproducing existing discoveries would take, but quantity matters, not just quality, when it comes to reproducing civilization's accomplishments.

You'd really like to have more like 100,000 working age adults to preserve enough of the right mix of specialized modern skills important to a modern civilization, and to have critical masses of people in each area (perhaps several dozen to a few hundred) sufficient to manage until they can meet each other.

This large number is necessary both because some modern technology skills like medical and scientific and engineering and cultural specialties are quite rare, and because lots of knowledge isn't present in any book or other fixed record and has to be passed on from one master to an apprentice to be preserved successfully.

No historical society starts to display any meaningful level of technological sophistication before they have cities with 100,000 or more people.

Also, some fudge factor is necessary above what a planned colonization maintaining modern levels of technology would require. This is because there will be huge language barriers involved in uniting the people that remain, because many people in rural areas will be lost, because many adults will be too traumatized to function even if they are otherwise skilled and able bodied, and because it is likely that the survivors won't be able to manage to immediately unite into a single global society so the critical mass needs to be reached with less than the total remaining global population.

People in very rural areas, while perhaps more self-sufficient, are already so thin on the ground that they barely have enough population density to function and a significant drop in population would undermine that.

At one person in 10,000 surviving, there would be 800,000 survivors, perhaps two-thirds to three-quarters of whom are adults capable of working, and at one person in 100,000 surviving, there would be 80,000 survivors.

In Tokyo, there are 37,274,000 people, so that would be 373 survivors in a one in 100,000 scenario, and 3,727 in a one in 10,000 scenario.

If it takes 20 survivors to last until connecting with other urban centers, that takes 2,000,000 people in a city in a one in 100,000 scenario and there are 252 cities that large - with people in less densely populated cities often not making it and reducing the global critical mass below a high civilization maintaining level in the early first couple of years before society could rebuild. You would probably lose more than half of the 80,000 survivors early on in that scenario, and it would take longer for them to establish coordinated groups of even tens of thousands of work capable adults.

In a one in 10,000 scenario, a city of 2,000,000 would have 200 survivors, and a city as small as 200,000 could still have enough survivors to make contact with other survivors (and there would be many thousands of such cities). You would lose a much smaller share of the initial 800,000 survivors and would get to a critical mass of people coordinated in a single cooperating society much sooner.

For the reasons discussed above, I think that with 1 in 100,000 survivors you are going to take many, many generations to rebuild and you will lose a much larger percentage of the initial survivors early on. But with 1 in 10,000 survivors, a larger share of the initial group of 800,000 will make it, and a group with a critical mass of about 100,000 adults will come together quickly enough to avoid excessive knowledge loss before this critical mass can be assembled.

So, something that was 99.99% lethal could be recovered from in a single generation, while recovery from something 99.999% lethal would take much, much longer to rebuild after the event, possibly centuries or thousands of years.

One in ten thousand is still a quite small survival rate.

For example, it is on the same order of magnitude as the number of people who are born with one of the thirty or so monogenetic disorders known as muscular dystrophy, or the number of people with IQs of 160 or more. It would be twevle or so times smaller than the number of elected officials (federal, state or local) in the United States relative to its population, and similar in number to the number of adults with net worths of $100 million or more in the United States. The tallest one in 10,000 women are about 6'2" or more (one in 1000 are 6' or more), and the tallest one in 10,000 men are about 6'8" or more (one in 1000 are 6'6" or more).

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If the survivors were a group in a single area then 600 breeding pairs can perpetuate a culture and have enough genetic diversity to keep the species viable.

If they were random people who survived in diverse locations, then civilisation collapse would be immediate.

If it's just a percentage, then historically the black plague killed 30-50% and civilisations in terms of tech etc,. survived (although very changed). With some regions having more than 80% dead.

In Polynesia up to a third of the people died in places due to introduced disease and rebuilt. Although again thats general stats, some places had such high mortality that it was probably 80-90% in some locales from oral tradition.

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    $\begingroup$ Six hundred breeding pairs can indeed sustain a culture. It would be a Stone Age culture, but yes they can sustain it. However, the question is asking about civilization not culture. There is no way that 1200 people can sustain even early Bronze Age civilization. $\endgroup$
    – AlexP
    Aug 4, 2022 at 3:24
  • $\begingroup$ @AlexP how do you work that out? 1200 modern people are unlikely to even know how to go stone age. The science is already documented and libraries and schools are still there. One generation messing around keeping things working then moving forwards from that. Life would be easy. Second generation 6,000 - 10,000 people with lots of resources, it would boom. No idea why ypu'd want to start living in a cave banging rocks together $\endgroup$
    – Kilisi
    Aug 4, 2022 at 4:41
  • $\begingroup$ After the big plagues life improved for common people (elites complained). They suddenly had a lot of resources and a much easier and more powerful life. $\endgroup$
    – Kilisi
    Aug 4, 2022 at 4:50

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