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Modern Earth; a series of connected events (natural disasters, mass migration, drug-resistant pandemic, collapse in biodiversity, armed conflict) have led to a significant reduction in human population over a single generation.

Does "civilisation" continue? Societies could certainly adapt to short-term disruptions—but if the complex, global, inter-connected supply chains upon which they currently depend do not recover sufficiently quickly, would they not collapse entirely?

For example, a number of contemporary industries (unfortunately) depend upon oil: not only is most transportation fuelled by petroleum; but most fertilisers, pesticides, pharmaceuticals and plastics are also derived from it. If oil extraction, refinery and distribution were to suddenly cease for a sustained period (before such dependent industries have identified and adapted to alternatives), oil reserves would deplete and those industries themselves could collapse. That in turn could lead to a collapse of further dependent industries, including the manufacture of machines used in virtually any given supply chain.

Do we have any idea how many people are required to keep "it" (the modern industrial world) functioning?

Please give due consideration to the fact that disaster-struck areas may be uninhabitable, with their populations migrating en-masse and seeking refuge in other areas—thereby placing further strains on the system.


Clarification

There have been some very probing and helpful comments, which have prompted me to spend some considerable time reflecting on exactly what my question is and this edit is an attempt to elaborate. My thanks to all those commenters who contributed to this discussion.

By "significant reduction in human population over a single generation", let's assume the reduction is unforeseen, uniform across all demographics and complete within 10 years. To be clear, it is an "unmitigated apocalypse".

By "maintain the modern industrial world", I meant maintain our current technological capabilities. It strikes me that a good measure of this is the smartphone—it requires mining and refining many minerals, including some that are so rare that viable ore has only been found in a few locations on the planet; even once refined, maufacture of some parts (e.g. 7nm-process semiconductor fabrication) is so specialist that the number of capable facilities can be counted on one hand; even once assembled and distributed, their function depends on an infrastructure of components (antennæ, switches, routers, servers, etc) that themselves require such parts and maintenance thereof. Other yardsticks of technological capability might include space exploration, subatomic research (particle accelerators, quantum computing, etc), artificial intelligence, nanotechnology, ...

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch Jun 18 at 3:41
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    $\begingroup$ A better -- but harsher, and completely inhumane -- way to ask the question is, "who can we get rid of and still maintain modern civilization? For example, how much to the Caribbean countries contribute to modern civilization? I don't know, but an economist would, or would know how to find out. Go through every country (and every political 2nd level of every large country) and ask that same question. That's your answer. $\endgroup$ – RonJohn Jun 18 at 21:13
  • $\begingroup$ @RonJohn My answer aimed at that point, with slightly more inhumanity and ruthlessness. Though I articulated it rather badly at first, so its quite down the list. $\endgroup$ – Lupus Jun 18 at 22:09
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    $\begingroup$ @RonJohn Just don't neglect the perils of disposing of the B-Ark. $\endgroup$ – J... Jun 19 at 18:42

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The New Scientist article "Why the demise of civilisation may be inevitable" from Issue 2650, which is based on the work of Jared Diamond, in particular Collapse suggests that killing as few as 2% of the workforce could initiate a cascade of failures in infrastructure and transportation systems sufficient to bring down modern western civilisation; if those who died were key staff with unique skillsets and/or knowledge in their vital workplaces. But you don't even have to kill them, prolonged (read two weeks or more) absenteeism of a larger fraction of the workforce (as little as 10% in some sectors, up to 30% in more robustly staffed organisations) would have the same effect.

As to the bare minimum population you would need to maintain what we have, that's not really about total numbers so much as population density. Our current society is pretty well as complex and advanced as it can be at current levels of population density, kind of by definition. Maximum population density is largely determined by the efficiency of the growing, storing, and transporting of food to urban populations. In order to maintain what the west currently has we actually need most of the world to remain intact and active in the world commodity trading market or the raw materials aren't available to the urban-industrial centres that need them.

As a note if we get a bit further with manufacturing miniaturisation this question becomes almost moot as manufacturing and mining facilities become one and the same and potentially you can put the basis of one in your backpack.

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    $\begingroup$ I'm accepting this post because it not only gives a quantified answer (as sought), but it is furthermore sourced from (and cites) academic research into the subject. It's a rather sobering reality to face, but I'm very grateful for it—as indeed I am for all the contributions people have made; it has been very interesting and thought-provoking. $\endgroup$ – user65791 Jun 20 at 17:01
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    $\begingroup$ Surprising if true. 3% of the world died in WW2, though globalization has certainly advanced since then. en.wikipedia.org/wiki/World_War_II_casualties $\endgroup$ – Cireo Jun 20 at 23:36
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    $\begingroup$ @Cireo: “if those who died were key staff with unique skillsets and/or knowledge in their vital workplaces”. I’ll wager the deaths in WW2 were rather more discriminatory, with such key people not only spared from front-line action but carefully protected. $\endgroup$ – eggyal Jun 20 at 23:47
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    $\begingroup$ @Cireo: I think you’re again overlooking that “the” 2% referenced in this answer are a very specific 2%—those who are “key staff with unique skillsets and/or knowledge in their vital workplaces”. 2% of the planet’s 7.7bn people is 154m; by contrast, globally, there are only about 5m people employed in the oil and gas industries... if just an important fraction of those 5m people were to suddenly die, it’s hard to see how the oil industry could maintain production at levels that sustain civilisation before total collapse ensued; add key people from coal and nuclear, and it’s a certainty. $\endgroup$ – eggyal Jun 21 at 5:45
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    $\begingroup$ @Cireo The main issue with modern society is the degree to which every job anyone does is dependent on jobs done by others, this interdependence means that fewer loses make a much larger impact on the overall economy, and that effect just gets worse as time goes on. $\endgroup$ – Ash Jul 19 at 14:07
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I'm going to respond to this with a framing challenge, because as it stands I think you're asking the wrong question.

The issue is that the modern industrial world is a product of population DENSITY rather than total population. This may seem semantic but in a post-apocalyptic environment it's very meaningful. Modern industrialism depends on and benefits from massive economies of scale that accrue from having lots of people all close enough together to quickly and easily exchange goods and services.

In a post-apocalyptic scenario, if you're envisioning two million survivors scattered across the entire planet, they're all going to be spending pretty much all of their time just keeping themselves fed and sheltered, and it's straight back to a hunter/gatherer economy worldwide.

If, however, you're envisioning two million survivors concentrated in a specific area that (for example) quarantined itself before anybody could be infected with the zombie virus, then you'd potentially have enough critical mass of different kinds of expertise to allow cooperative maintenance and task specialization: e.g. you have a bunch of people whose entire job is providing the food for everybody. You have another bunch whose entire job is maintaining power/water/electric. You have another bunch who kill the zombies before they swim across the river.

This is basically how urban societies developed in the first place, and the same rules would apply in an apocalypse.

To more directly answer your question: The larger an area your population of survivors is spread across, the more people you need. The more widely distributed the resources you require are (food, water, factories, sources of raw materials), the fewer people you need.

The number could potentially be as low as a few hundred in a commune out in the middle of Nowhere, Idaho that already has its own self-sufficient sources of food, water, and power, and a smattering of skilled electricians, mechanics, and engineers.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch Jun 20 at 17:35
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I've used a personal analogy for this, which I call "The brake pad problem." I estimate (without any serious research) that maybe a few hundred people around the world really know what it takes to manufacture an automobile brake pad. If you kill them all off, we're in serious trouble at least until a new process (or a reconstruction of the original) can be completed. The idea is that everyone is incredibly specialized, and our outputs are very interconnected. This makes the whole system unstable. My WAG, then, is less than 5% of the people in industrial countries/regions would cause a complete collapse.

You might want to read Lucifer's Hammer for more ideas.

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    $\begingroup$ The essay I, Pencil makes the startling claim that no one in the entire world knows how to manufacture an ordinary lead pencil. By that, the author means that there are people who know how to assemble all of the components into a pencil, but they are distinct from those who know how to process each of those components. $\endgroup$ – Monty Harder Jun 19 at 15:28
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    $\begingroup$ While it is true that only a small number of people know certain critical parts of technology, that knowlege does not solely lie in their heads. Barring newly discovered knowlege, it also exists on paper and can be found and studied. There are certain things that I am one of maybe 100 or so people in the world who have mastered, but there are millions who are aware of the basic concepts and could study and master them if needed. $\endgroup$ – Paul Sinclair Jun 19 at 16:31
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    $\begingroup$ @PaulSinclair How much time do people have to study something of which they have the most basic knowledge, when the lights have gone out, the fresh water no longer flows into the house, the sewage no longer flows out, and no more trucks full of food products are arriving at the local supermarket? You're too busy trying to live through the day to have any time left to study a new skill. And the people under whom you should apprentice to gain mastery, if they're even alive, live too far away. The instructional videos on YouTube are no help without power for your computer. $\endgroup$ – Monty Harder Jun 19 at 20:17
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    $\begingroup$ @MontyHarder: not to mention how you will even obtain such paper-based records when they’re probably stored in libraries in cities that have burned to the ground and you’re living somewhere rural with no transportation! $\endgroup$ – eggyal Jun 19 at 21:42
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    $\begingroup$ @MontyHarder - Not the mention the zombie hordes rampaging the land, the space elephants dropping all the rocks, and the rising of Cthulhu, or the price of tea in China. Extreme exaggerations do not counter my argument that our knowledge base is not as fragile as Carl Witthoft's post and your comment make it sound. Yes, it can be broken with enough violence, but it will take significantly more violence than the loss of just 5% of the population. Can't make high quality brake pads? Make low quality ones, halve your driving speed, and replace them more often. $\endgroup$ – Paul Sinclair Jun 20 at 0:33
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A modest shock would be enough to prevent maintaining "our current industrial world".

7nm process semiconductors would become economically infeasible with a sustained 33% global depression, let alone megadeaths.

Note, however, that "rare earth minerals" are not rare; they are rare as in "rarer than iron" or carbon or silicon. Rare earth minerals are processed where they are cheap to process, not because they are there.

And it isn't that Lithium or whatever other element is only found in a few spots, it is just amazingly cheaper to get from those few spots because there is lots of it and it is easy to get. Gallium is produced as the by-product of purifying other ores. Etc.

Our current industrial world is set up for a constantly growing set of "global middle class" to be economically feasible. If we get a significant shock, some pieces will shut down.

Something as basic as Oil or Potassium will continue to flow unless we are talking only trace amounts of humanity left. Random complex doo dads? Demand will collapse; even if people knew how to make it and had supplies, the massive number of microwave ovens sitting idle and survival being more important would make making them not worth it.

Supercolliders, well, it depends on how France is doing. That is where the best-in-the-world Supercollider is. AI? If things are bad enough, you stop caring, and you repurpose people working on it for something more important. Quantum mechanics? Barring quantum computers, not an industrial-civilization scale effort. Nanotech? Much of it has to do with developing bleeding edge materials science. If you have, again, more important things to maintain, you stop doing it. Space exploration? Ditto.

These are economic questions more than anything. You can get to space in a whole pile of different ways. Do you think it is worth it? Then sure, you can keep at it. Heck, we could sent 1000 people to Jupiter in 2 years if we seriously thought we had to with our current level of technology (Orion) with a non-zero chance of success. It just isn't worth the costs.

Collapse of global and internal trade flows would cause economic damage faster than it would make things impossible. That economic damage would make things infeasible before they become impossible.

Imagine 99% of the world population dying off except for one small region caused by neutron bombardment (so just dead people/animals/plants, buildings etc are fine). That 1% region would (a) be economically savaged, and (b) have a massive ROI from resettling/expanding far beyond doing nanotech research. So even if they had a nanotech research lab there and everything they need, they'd stop doing nanotech research and they'd start farming and clearing roads and getting gas supplies etc.

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    $\begingroup$ Nit: The LHC lies in France and Switzerland. $\endgroup$ – Martin Bonner Jun 20 at 17:12
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    $\begingroup$ @MartinBonner Sure, but everyone knows Switzerland is going to be ok in the apocolypse. They'll just enact emergency plan 172sb.1232.3. $\endgroup$ – Yakk Jun 20 at 17:41
  • $\begingroup$ @Yakk: ie, do nothing, stay neutral. $\endgroup$ – eggyal Jun 20 at 17:43
  • $\begingroup$ @eggyal No, that is emergency plan 172.sb.1232.3.1. 172.sb.1232.3 is "stay neutral, do nothing". $\endgroup$ – Yakk Jun 20 at 18:10
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    $\begingroup$ I stand corrected! $\endgroup$ – eggyal Jun 20 at 18:11
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It's a tough question.

Someone mentioned population density above, and that's definitely a major factor. But I'd also mention two more: logistics and specialisation.

Civilisation requires specialisation - if everyone's hunting or in the fields, they've not got the time or energy to focus on more abstract things. So you need a way to give people time off from basic survival activities and a way to recognise and reward their activities, ideally in a way that leaves them better off than if they were still out in the fields.

So at the very least, you need to make sure they have sufficient food and water. Which means you need more people who specialise in producing more food than they consume. And that requires large amounts of land.

(There's some interesting discussion here which suggests a square mile per 180 people in medieval times:

https://www.reddit.com/r/worldbuilding/comments/6qocm8/question_about_population_density_and_required/

Quite how much that remains true for modern civilisation is debatable; for all we've vastly increased yields and reduced manpower overheads, we also have much higher expectations and use a lot of resources (e.g. fertilisers, pesticides) which may not be readily available in the event of a population decimation...)

And that brings us to logistics, as loved by great generals everywhere. Because you need to have a way to quickly and efficiently transport said food. And all the other resources the city needs, many of which require large amounts of space and/or involve processes inimical to people's health.

Multiply all the above by a few thousand times to account for the complexities of modern civilisation and divide by a few hundred to account for the impact of modern technologies such as the internet. And that's what you need.

So I suspect the answer would be uncomfortably close to the populations we currently have, unless the decline in population was slow enough to be managed. Which in a way is what we're doing now, given the decline in Western birth rate...

It might actually be possible to quantify things by looking at recession and inflation data. After all, the former is when we have too much slack in the system and the latter is when we don't have enough; somewhere inbetween is the sweet spot for our current civilisation!

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The problem is that most of the work today goes towards maintaining a society that supports this many people. You would not maintain everything we currently have. Would you field a single aircraft carrier? Would you keep one airline running or would you optimize to horses and electric cars.

You wouldn't need or even want most of the stuff you mentioned (if any)

What you would do is re-apply and re-focus our existing knowledge:

Create cars and machines with lower tolerances so that they can use 3d printed parts.

Focus our 3d printer technology on metal fabrication.

Improve solar, wind and battery technology so that what little oil we need would be focused on lubricating.

Without so many people, why would anyone choose plastic? Wouldn't paper, glass and metal work better in most cases if they weren't so much more expensive (rare?) In this world they would be so plentiful as to be virtually free!

Optimize nearly all mass fabrication like cars for maintainability and easier changes to production with much smaller volume (No need to manufacture thousands of the exact same car, so you don't have to make special machines JUST to stamp out left fenders for a two year run of a single make of car).

I think the things that we consider "Civilization" from a normal person's point of view wouldn't change much from a small town today--you'd still have:

  • houses (Not as many high-rise apartments, if any)
  • restaurants
  • better food
  • transportation (Simpler cars, horses, small planes)
  • Lots of old movies
  • New entertainment (Movies, plays, youtube-style stuff).
  • Space flight (Probably, thanks Elon!)
  • Some form of computers
  • Huge amounts of excess electricity because of vastly reduced demand even if we lose the existing infrastructure.

We would not have:

  • giant buildings/boats
  • mass produced Salisbury steaks.
  • giant food chains
  • Space station would probably be unmaintainable
  • Such powerful/dense computers (We would probably have to recreate most of the technology based on something easier to fabricate)
  • the ability to provide much support (medical, emergency) outside of the civilized areas

With this in mind, civilization could probably scale down to a few thousand people pretty easily--it's just that the form of that civilization would change completely (and from the point of view of our future selves--nearly all for the better--they would likely call us completely uncivilized!)

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  • $\begingroup$ The premise of the question was: the situation on the ground changes in a decade; if sufficient adjustments are not made very quickly, could total collapse ensue? It is not at all clear to me that the sorts of adjustments you mention could be implemented in the relatively short time available: it strikes me that many would take considerable planning and potentially multiple generations to effect. $\endgroup$ – user65791 Jun 19 at 11:17
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    $\begingroup$ I think that if you took a group of a few thousand people, dropped them naked on a new planet and gave them access to all our currently recorded knowledge, they would have decent housing, food and other necessities within a couple years.. Without current tools it would take MUCH LONGER to get back to computers though without existing tools. the problem with the 10 year collapse is the chaos it would probably cause--that could take a while to sort out--but you'd also start with sewers, homes and other infrastructure that would last for a few generations. $\endgroup$ – Bill K Jun 19 at 23:56
  • $\begingroup$ I think this is a nearest to the right answer to the question. It seems to me that the function of technology level by population is in no way a linear function. A 5-10% loss, especially among qualified workers and scientists will, most likely, be more damaging to the global technological level then the worldwide reduction of population to 50% overnight. The second scenario may even cause a global boom similar to late-medieval leap after the Black Death, as the population pressure is reduced and resources are freed. $\endgroup$ – Cumehtar Jun 20 at 11:22
  • $\begingroup$ +1, with fewer customers we’d also have much less variety. Today we can buy hundreds of different cars, smartphones, drilling machines, cutlery, furniture etc. etc. With fewer customers you’d probably have only one or two types of each. $\endgroup$ – Michael Jun 20 at 14:53
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    $\begingroup$ @Michael Might go the other way... without mass production every device could be hand crafted or custom built. Consider a 20 person team manufacturing cars that only had to put out one or two a year--no need for a high throughput assembly line... But getting back to the component density of an iPhone would be impossible for a LONG time. $\endgroup$ – Bill K Jun 20 at 15:49
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At least 25 million people.

North Korea has a population of 25 million people and is somewhat able to maintain our current technological capabilities. It’s not top-notch and they do receive some imports from other countries. I imagine with better management they could do much better.

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    $\begingroup$ Note however that NK most likely does not have a microchip fab. There are not that many production facilities with current scales and densities - though early 1980s chips have been produced in every other Warsaw block country. The same goes with modern big airliners. We already got into the point where there can be only a few top product factories in the world, because they are extremely costly and depend on too much input and the design phase needs too many top ranking experts. $\endgroup$ – Radovan Garabík Jun 19 at 8:28
  • $\begingroup$ Surely, if anything, this argument would be for an upper (not lower) limit? $\endgroup$ – user65791 Jun 19 at 11:13
  • $\begingroup$ @user65791: Well, as I said, they are not completely self-supported and lack some technology, so I’d consider a country like North Korea the lower limit. $\endgroup$ – Michael Jun 20 at 15:00
  • $\begingroup$ @Michael: The implication then is that you believe everyone in NK is essential to maintaining its technological capabilities. Is there any reason for believing that? $\endgroup$ – user65791 Jun 20 at 16:31
  • $\begingroup$ @user65791: One would think that the regime has an interest in the highest possible output/production. On the other hand they are probably afraid to invest in the education of their population and their military costs a lot. It’s really hard to pin down. $\endgroup$ – Michael Jun 20 at 17:05
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Quick edited note here: My answer, as I understood the question, is aimed at what is required to maintain modern technological levels. I answered this way, because in my understanding, that is the required foundation for modern life and in my view civilization is too subjective and broad to be answered. For that, I could say US or Europe or Japan, with automatized colonies around the globe for rare resources.

What we DON'T need, to only maintain modern technology

Let's start the answer in reverse. Who we can dismiss? Who we don't need to maintain modern technological levels. A good part of humanity if I says so. No offense for anyone. I'm part of the "dismissed".

  • Employees of huge companies who provide service to the masses? They can be significantly down-scaled. If functionality, the maintaining of technology instead of profit the purpose, only their engineering team. You can scrap their HR, sales, and so on departments.
  • Most services and its workers too - you don't need 20 kind of chips and cars and so on, to maintain modern technology levels.
  • Any kind of Media related profession - SCRAP!
  • Governmental bodies - Significantly downscaled, based on the situation and type of government.
  • Homeless, unemployed, and so on... they don't contribute to modern technology, so they too can be dismissed.

Other factors: Human intellect, productivity, automation

  • Automation - we are heading towards a heavily automated world, most mundane task can be or will be replaced by robots.
  • Human intellect - Automation means, you can dismiss almost all the heavy-lifters. You will however need highly educated and intelligent people to operate and maintain the machinery.
  • Productivity - much depends on the disposition of the people. Are they lazy? Hard-workers? Do you need police to manage big masses of them and so on. This too will affect the final number.

The final factor to account in this scenario, do all human capable of reaching the same intellectual level? Do they born equal in that regard? Or for example every hundredth people has high enough intellect? I can't be trained to be a Quantum Physicist. People's view vary in this regard, so I will leave this as a simple multiplier.


They are pointers for a reasonably close estimate. My very rough guess, a few hundred thousand or most a million can maintain an inherited and hastily retrofitted modern technological world.

NOTE: In my view, they have to be highly specialized in various technological fields, and preferable the cream of the crop in intellectual ability, among other things. Mostly, because this will lower the necessary number. Not ALL of them have to be like that, but a significant percentage. Depending on the level of automation and mechanization, this percentage may vary. (Look at Factorio) Also, my assumption is that a hard-laborer can't be retrained (in reasonable time and capacity, views may vary) to be a rocket scientist, nuclear reactor operator and so on...

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    $\begingroup$ I would like a short note, comment on why the downvote, so I may look into the issue and improve my future answers. Thanks. $\endgroup$ – Lupus Jun 17 at 19:46
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    $\begingroup$ I think this missed the real facts of what is required to keep the 'wheels' of an advanced civilization turning. Even with today's levels of automation, to mine raw materials and refine refine them, transport them, for example iron and aluminium and glass and cloth, and silicon, rate-earths, etc - then manufacture the products these are used in, maintain power supplies, communication and transport networks, water supplies, sewer systems, waste disposal etc. society needs a vast number of people who don't fit into the "highly specialized ... intellectual" milieu. And farming, forestry and... $\endgroup$ – Penguino Jun 17 at 23:35
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    $\begingroup$ I have a feeling that you may be underestimating the value of the "less important" for modern civilisation. If nothing else, they are consumers. Hardly any innovation is driven by pure curiosity. Most things need a market of buyers to be developed to production readiness. Other than that, i don't think the downvotes are really justified, so have a +1 to counter. $\endgroup$ – Burki Jun 18 at 9:43
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    $\begingroup$ If you cut out everything that appears non-essential, what remains is not 'modern civilisation'. Art, justice, democracy and care for the sick, poor and elderly are what makes us civilized, not just the size of our economy. $\endgroup$ – Robin Bennett Jun 18 at 11:07
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    $\begingroup$ You can scrap their HR, sales, and so on departments. If it was possible to scrap all these departments without impact to an organization's ability to deliver, that would be done already. $\endgroup$ – dwizum Jun 18 at 16:20
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I'd like to suppliment all of the really good answers above, which collectively address the economic and industrial aspects of massive population decrease, and instead look at the psycological and sociological implications, because I think those put a much lower ceiling on the number of people you can lose and preserve civilization.

Simply put: when massive catastrophes happen, people panic. And when enough people panic, civilization falls apart.

(Warning: callous discussion of many people dying. Sorry, can't discuss this topic otherwise. Clearly the deaths of folks outlined here are of way more than academic interest to them and their loved ones.)

The kind of catastrophe you're talking about -- the loss of, say, 25% or 50% or 80% of the world population -- would be unprecendented in human history. Based on human history, lesser population losses have resulted in major or even total social breakdown. For example:

  • The Syrian Civil War has "only" killed 2.3% of the original Syrian population. Yet the majority of Syrians are now refugees, and large parts of the country are lawless and without an industrial base.
  • European diseases killed an estimated 25% of the native population of North America, which caused some tribes/nations to cease to exist (but, interestingly, not others), such as the cities of the Illinois tribes.
  • Relatively limited (no solid figures, but not more than 10%) deaths due to famine in the Mayan civilization helped trigger a cascading collapse which wiped it out.

Now, you can find plenty of examples of civilizations that survived large population losses. Europe and the Black Death is probably the paramount example; despite losses of around 1/3 of the population in many areas, European civilization and technology didn't regress by more than a century -- despite profound and far-reaching effects on economics, war, and religion.

So losing a large percent of your population isn't a guarantee of panic, destruction, collapse, and regression to the stone age. But sometimes it is. The key questions tend to be:

  1. How stable was your civilization in the first place?
  2. What is the nature of the population reduction, and how do most people feel about, and react to, it?
  3. Are there external or internal forces that would compound the effects of large-scale population loss?

If you're basing this on modern world civilization, there's ample cause to be pessimistic about (1) and (3), but I'll leave that up to you.

However, I cannot find a single example of a civilization that lost more than 60% of its population, over a generation or less, and continued to exist. So if you want to have an absolute numerical ceiling, I'd say that you need to retain at least 50% of your population.

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    $\begingroup$ The Black Death is thought to have removed many artificial barriers to technological, economical and societal growth. Imagine if you had a hundred farmers, together capable of feeding the rest of the population, but at the same time, burdened with having to spend 90% of their time on pointless rituals. When 90 of those farmers die, you've lost a huge load of productivity - but it will probably also cause those 10 survivors to drop those pointless rituals, and work full-time instead. Could something similar happen today? Many people think so, but I think it's mostly aivity. $\endgroup$ – Luaan Jun 20 at 12:00
  • $\begingroup$ Luaan: yes, historians attribute a bunch of benefits to the Black Death in terms of breaking Medieval Europe out of stagnation. One key part of that is that Europe was, at the time, overpopulated for its technological carrying capacity, so at least some of the population loss was a blessing in disguise. However, that leaves open the question of: why did the essential socioeconomic structure of Europe stay intact despite losing up to 30% of the population? $\endgroup$ – FuzzyChef Jun 20 at 16:10
  • $\begingroup$ One of my theories is: the Catholic Church. That is, the Black Death was the kind of disaster that the Church was prepared to handle, and channel the resulting panic into activities that reinforced the existing power structure instead of destroying it. $\endgroup$ – FuzzyChef Jun 20 at 16:12
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This answer might sound mean, but most people can be killed off without affecting the industries, if you choose the right people. You only need the people directly employed in the industries, the people supporting the cities and governments where those people live, the people providing supplies to those. Currently that's basically the West, factory places in Asia and some others scattered around the world. Probably around a billion people needed to keep everything running as it is, but could be less.

Although it depends where and in how many places you want to see modern civilisation. In all the places it exists around the world? In a select few nations? Or is one city enough?

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    $\begingroup$ "The West" - what, the whole West? I highly doubt it. $\endgroup$ – immibis Jun 18 at 1:55
  • $\begingroup$ No, not the whole West. $\endgroup$ – Curiosity Jun 18 at 4:03
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    $\begingroup$ You may want to look into supply chains of raw materials. That might alter your views. "The west", without africa, will quickly deteriorate back to the iron age. (exaggerating, yes, but you get the meaning). Also, no industry without buyers. Eliminate those, and nothing much will be produced, let alone invented. $\endgroup$ – Burki Jun 18 at 9:46
  • $\begingroup$ @Burki: But Curiosity makes a good point. You could go over exports or production output of countries, regions and demographics in the world and for each of them consider if they are really required for our current technological level. $\endgroup$ – Michael Jun 20 at 14:59
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Almost all of them

There are two problems here, one is the population required to maintain the modern world, but the other is the population required to have reason to maintain the modern world. The world isn't a museum, much of the technology and infrastructure exists only because it's needed to maintain the global population and respond to its demands.

As already been mentioned, you need population density, as you're already aware you need population numbers, but you also need population spread.

  • If everyone lives in the same city, is there any reason to maintain GPS?
  • If everyone lives in the same country, is there any reason to maintain global shipping and logistics?
  • For a mere 2 million people is there any reason maintain vast high production car manufacturing plants?
  • Large numbers of people have to have reason to travel long distances to maintain the aircraft industry and related technologies including air traffic control.
  • The need for high capacity mass transit systems is tied to the need for large numbers of people to travel.
  • The need for vast orange juice plants is tied to the global distribution of the product.

As soon as you start cutting into the global population, you start cutting into the requirement to maintain large swathes of industry and their related technologies.

Sooner or later even the slush machine in your local cornershop is declared a redundant technology, there just isn't enough demand to maintain it over something smaller and simpler, or doing the job by hand.

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  • $\begingroup$ Yes, I see this. But that's also why I tried to frame the question in terms of technological capability rather than specific technologies. It's one thing to scrap some industries; another entirely to lose the ability to recreate them. $\endgroup$ – user65791 Jun 20 at 13:18
  • $\begingroup$ @user65791, you probably will lose the ability to recreate the redundant technologies, it's not just knowledge but also expertise. At the very least you'll lose a lot of the quality of work, the efficiencies and refinements, because they're no longer important. Formula 1 cars, supersonic aircraft, so many things that will fall by the wayside as unnecessary to the new world. $\endgroup$ – Separatrix Jun 20 at 13:23
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Less than 500 million

Georgia Guidestones

The Georgia Guidestones were created by "a small group of loyal Americans" "which had been planning the guidestones for 20 years" and was able to pay "several times higher than any project the [construction] company had taken" earlier. If this information is true, then we can assume they had hired actual economists to give an accurate answer.

At the time of construction (1979) the world population was 4+ billion, so they claim that less than 12.5% of the population would be needed. This obviously assumes that the population number was based on the requirements of a modern (1979 era) level of technology.

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  • $\begingroup$ While this is an interesting monument, I'm not sure if this answer is applicable since it seems to be centered on the maximum number of people for an ecologically sustainable society using 1979 tech, not the minimum number to maintain 2019 tech. $\endgroup$ – Nosajimiki Jul 1 at 13:55
  • $\begingroup$ @Nosajimiki the number looks about right, heard a similar number/estimation about 600million as a requirement of keeping a compeition edge in the current world system(as of today) as some sort of political alliance. To keep a market system going,, have incentives for development(have consumers) and to have capabilities of development etc. $\endgroup$ – MolbOrg Jul 19 at 16:31
  • $\begingroup$ @MolbOrg Maintaining modern civilization and being economically competitive are also two different metrics. 600mil is because you need a % of the global market to hold influence. If you killed half the world's population then that number would drop to 300mil. $\endgroup$ – Nosajimiki Jul 19 at 18:13
  • $\begingroup$ @Nosajimiki agree, keeping and to compete isn't the same, and there are reasons to believe(musk believes in a million, I would say 20 million) that a system can keep modern tech and stuff but what a life it will be - like a milirary drill. Competition is part of what makes stuff fun and developing - just keeping and maintain not super great stuff. But yes, I meant some relaxed existence without significant changes of how we do stuff compared to what we observe today. I certain there are possible more effective systems which would do the same with fewer people and do not be like an army drill. $\endgroup$ – MolbOrg Jul 21 at 5:29
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I feel that we need a collection of people skilled to connect technology and create a formidable ecosystem. For eg, To create a smartphone we need people trained and skilled in a lot of processes. So, we'll need a collection of differently skilled people in a singular place where they quarantine themselves which would involve making a constitutional structure to envision the armed forces, leaders to create law, economists, technicians and hunters and gathers and this ecosystem needs to adapt to changes in an exponentially fast manner and hence would require brainstorming and control and a very constituted system. We need to set up an economy parallel to every other process. This society and the ecosystem around would need to mature in a specifically designed manner as to support the sustainable development. I further deduce that we would need to be aware of the world atmosphere and habitants and geographical abnormalities and a satellite system to track target systems. Hence, it's only possible if we presently device such a system and follow log book instructions with certain situational innovations in case of such events. Our best bet would be the US military budget allocate for such situations where they run simulations and devise plans to counter them.

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    $\begingroup$ ...so how many people is that altogether? I'm not so sure how well this answers the question. $\endgroup$ – F1Krazy Jun 20 at 8:53
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It is all on a sliding scale. Most of the basic knowledge to keep civilization alive will remain in the libraries of most major universities. If there are 10 plants than make computer chips and they are ALL gone it will take a while to build a new factory. If all the manufacturing plants are destroyed it will take a while to rebuild them. If all the refineries are gone they will need to be rebuilt. If shipping is mostly gone, getting the needed materials will be hard for many complex products.

I would expect the population to be in pockets where the survivors could defend themselves. Each pocket will rebuild as much as they can with what physically survived and the available expertise minus time and effort to stay alive. As the small pockets begin to interact the knowledge, expertise, products, etc. will become available to each other. I do not think population is as important as resources.

The population of Tonga is 108,000. They do not produce much and most modern technology would disappear if they were the only ones left. If there was a pocket of 20,000 professors and advanced students holed up at a major university they could do well for themselves.

One other factor. The knowledge is already out there. I have never built an airplane but know you need to deal with yaw, pitch, and roll and the shape of the wing adds lift and Jet engine thrust can exceed propeller thrust. I do not know how to make penicillin but know it was derived from green mold. I do not know how to make fertilizer but know that plants need nitrigen and tilled ground speeds root growth. I never built an engine but know basically how spark plugs ignite compressed gas and air to produce the power. Finding the specifics while knowing the basic concepts behind the inventions would make the inventions occure significantly faster than discovering them for the first time. What is missing would be fairly quickly recovered depending on the survival efforts of each pocket.

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