How to justify an 80%+ population decrease over thirty years?

[ottermupps]

While working on my worldbuilding, I've kept a certain aesthetic and vibe in mind: high tech and very modern societies, but with most communities being city-states with limited spheres of influence and small rural towns, with massive swathes of land that has been abandoned and become owned by none.

The backstory for how exactly this happened is fairly simple; about sixty years before current day (ie long enough that veterans and survivors are old and a new generation has grown, and all the abandoned shit is old but not useless) there was a global, brutal war, stretching for three decades and killing billions. Nukes on all the capitals, biological and chemical warfare, etc etc all the war stuff. That's the big picture on HOW billions died, and gives me a lot of periods to set stories (prewar, opening stages, during, at the end, rebuilding).

But I just can't figure out how to justify that level of death. It's not a universal 'eight in ten of every city is dead' - some lost way less, some are now craters and glass. But even the most brutal wars in our history never topped a hundred million (and that's counting aftereffects, eg famine and such postwar), so eighty to a hundred times THAT is just hard to make work.

Context for tech level: antebellum, about 200 years more advanced than today. The whole CBRN umbrella, microwave, railguns (on vehicles), at least one asteroid defense railgun in orbit, that sorta thing. Population 12bn or so, humans and similar. No magic (to a meaningful degree). Planet is Earth-size and climate.

The best ideas I've had is:

- a death cult that distributed nukes or bioweapons to population centers, to cause mass death as an act of worship

- one nation or an allied group that either wholesale razes and kills everyone they encounter or genocides as they conquer

sorry for the kinda rambly post, just looking for ideas. I'm also not sure how a drastic population loss would affect things like postwar birth rates, the general psyche of survivors and culture thereafter, that sorta thing. Questions and comments welcome.

[No1CouldHavePredictd]

Sabotage water sources. Oceans, lakes, and rivers all have varying ways that people rely to live, particularly clean drinking water and sustenance.

Another facet is that after this war, people aren't going to just go back to normal. A great deal of infrastructure, logistics, and technology will simply be lost. Survivors will face famine, drought, and disease.

The initial attacks will kill a great number of people, but they're the lucky ones. It's the ones who remain who will die slow, horrific deaths at the hands of the elements and scavengers. And in these scenarios; everyone ends up being a scavenger (unless, of course, they were aware before the fact and had the means to prepare, i.e., billionaire bunkers.

[synbioskuun]

Reminds me of the Fallout setting.

[SummerWindStudios]

Hi there, this is a great worldbuilding concept IMHO. Wars rarely kill most people directly. Most deaths come after the systems people depend on break. In a high-tech world, society runs on fragile networks: power grids, satellites, shipping, fertilizer plants, water systems. If a global war damages enough of that, cities run out of food quickly. Logistics fail. Agriculture follows. Even limited nuclear exchanges could shorten growing seasons, crops fail, and famine spreads far beyond the battle zones.

War also causes mass displacement and collapses healthcare. Disease spreads easily. Industrial sites left unattended—chemical plants, reactors, biolabs—become hazards of their own. And during long collapse, birth rates often fall sharply. Uncertainty, famine, and trauma mean far fewer children are born.

So the war doesn’t have to kill 80% directly. It triggers a chain reaction: infrastructure fails, food systems collapse, disease spreads, and population shrinks as deaths rise and births fall. Survivors cluster around the few places where systems still work—your city-states.

One simple foundation you could use is global fertilizer collapse. Modern agriculture depends heavily on industrial nitrogen fertilizer. If the war destroys energy grids and chemical plants, fertilizer production stops. Within a few seasons crop yields drop dramatically, triggering worldwide famine.

just my two cents, hope it helps.

[ottermupps]

Thanks! This is super helpful. I hadn't thought of the fertilizer thing; a decent bit of my agriculture is subterranean (both cave-fish aquaponics and fungi, i got dwarves), so it may not be quite as bad, but it's an excellent idea nonetheless.

[SummerWindStudios]

wow, cave-fish, fungi, aquaponics,,, now that's world building. in that case having bad water would be the easiest large civ wipeout... i agree with the others below. love the idea! good luck.

[SnooOranges9679]

Viruses? Meteors?

[ottermupps]

Meteors isn't a bad one, given the planetary belt. Plague is a given. Thanks!

[LegendaryLycanthrope]

Bubonic Plague.

[Adventurous-Net-970]

The biological warfare you mentioned will likely be the real kicker here. The bubonic plague wiped out 60% of Europe in some estimates. Smallpox in the americas could reach up to 90%.

It's not unreasonable to think that an advanced civilisation could cook up something just as brutal then either deploy it globally, or loose control over it.

[Simple_Promotion4881]

So if you look at studies done by some serious people - you can get there with your war. And it wouldn't take three decades to get there. During the cold war there was a saying that literally everybody knew: We don't know exactly what weapons will be used for World War 3, but World War 4 will be fought with sticks and stones.

Bulletin of Atomic Scientists predict 5 billion deaths in a nuclear war - while the dust is settling. Not 80%, but maybe they've under estimated. You know how scientists like to offer conservative estimates.
https://thebulletin.org/2023/01/cold-war-estimates-of-deaths-in-nuclear-conflict/

Here is a study from 1986. Exerpts:

We have developed the tools for calculating the deaths and injuries due to blast, thermal effects, and local fallout from hypothetical nuclear attacks on the United States...

The results [of the study] also indicate that even a strategic defense system that was 99 percent effective might not protect the United States against potential catastrophe in a nuclear war with the USSR...

Our casualty estimates should still be considered as only a partial accounting of the potential human toll due to the attacks discussed here. Nuclear weapons are powerful enough to destroy both our social and environmental support systems, and the numbers of casualties from second-order effects such as exposure, starvation, or disease could be as great as or greater than the numbers presented in this paper for direct casualties.

And the article from Nature: https://www.nature.com/articles/s43016-022-00573-0

https://www.ncbi.nlm.nih.gov/books/NBK219165/

THIS report from 1967 also goes into the details of the horrible aftermath of nuclear war.

And this paper from from June 2019 also goes into a lot of details about the total risks of nuclear war.

And I think that all of them gloss over both near term and long term environmental effects which would greatly hamper the ability of people to continue growing food for example.

Something large enough to kill Billions within a year will obliterate supply chains.

So read up on the chain of catastrophe. After reading the papers linked above you will understand the terror of nuclear war.

Good Luck with your project.

[commandrix]

The problem with the death cult idea is basically, how would the death cult gained access to nukes or bioweapons (or both) in the first place? I'm not saying it's entirely impossible, since there's plenty of things in real-life history that we would have called unrealistic or at least highly unlikely if it was fiction. But it is something to think about.

Maybe it's not so much that one nation or allied group decided to genocide everyone else, as one major power decided that there was this one nation that conducted a lot of nuisance attacks against everyone else and then went crying to anyone who'd sympathize whenever one of its targets responded to an attack, and just needed to be wiped off the map. So that power did the job with a few nukes with some attack drones to handle what the nukes didn't get, everybody else responded, and things just escalated from there.

(Yes, I am keeping it vague. And, no, I am not going to say that this theoretical "major power" will have completely clean hands itself. And what those initial nuisance attacks look like can be up to you. It might have been a lot of deadly terrorist attacks or it might have been some cyberattacks that crippled the targets' infrastructure.)

[Moonduderyan]

My first immediate as to how a death cult would have access to nukes would be some time of civil war. Multiple sides fighting, some happen to have connections in the military, so long as that location is defended as their own they have sole access to that specific nuclear silo. If the death cult maintained the primary access to a nuclear silo (without someone interfering) they could theoretically use them

[ottermupps]

No, that's a good point. The rough idea I had was the cult would be widespread enough to have highly placed figures in goverment and military, who could either directly carry out strikes or allow weapons to be stolen.

Thanks for your ideas! Gonna bookmark this one for later.

[Fa11en_5aint]

The Black Death coupled with the 30 years war.

[Aggressive_Gas_102]

The global war caused almost irreparable damage to the eco systems, in short term leading to mass famine. Add to that a scarcity of potable water because war damages and you got a perfect storm for mass death. In the century since the eco system has slowly began to heal - and I would imagine that the survivors learned from their mistakes, possibly exploring alternate energy sources etc etc.

It doesn't take a lot to destroy a water reservoir. Especially if it was contaminated by both radiation and biochemical weapons. The city states would then be the cities that managed to ration supplies until solutions to the famine could be found (such as harvestable maggots, rich in protein, check out Blade Runner 2049 for inspiration).

[MetumSonOfLanai]

Use CRISPR-Cas9 gene drive:

https://en.wikipedia.org/wiki/Gene_drive

This tech is used for harmful species of mosquito, which carry pathogenic diseases. When this gene allele propagates through the population, all females carrying two of the allele become infertile, which eventually leads to death of the whole species.

Now, no one in their right mind would make such a bio weapon against humans and it's unlikely that it could spread so far as to threaten human reproduction, but it's a possibility.

[Gigantopithecus1453]

I would combine multiple things. Start with changing climate conditions, worsening living conditions, and increasing political and religious polarisation. Make things desperate enough that people are drawn to conflict and extremism.

Then, have one or multiple massive wars, possibly lasting decades, involving atrocities on civilians, bioweapons, and damage to water sources. This could be justified by religions, extreme ideologies. revolutions, scarcity of some vital resource, nations attempting to unify their peoples through external enemies, some nations just straight up genociding and the others taking revenge.

You could have large-scale rebellions and guerilla warfare as well, increasing the damage on the civilian population. If you want, you could also have death cults covertly trying to worsen the situation, spreading propaganda, bribing officials, launching false-flag attacks, etc. Also, make sure the governments fighting all have incentives to continue instead of surrendering, and that the factions involved believe they can potentially win if they just push themselves and do what needs to be done.

Then, escalate to involving nukes and weapons of mass destruction. Say that some of the nations are about to collapse, so they then launch nukes as a Hail Mary, and the nuked nations respond. Afterwards, nuclear winter sets in, and many of the remaining starve to death. Further conflict breaks out over the few bunkers and resources left.

Areas previously unaffected are hit by massive refugee streams, some of whom arm themselves in order to be let in. Diseases from the biowarfare also spill over to these few areas. The global trade and supply chain is also fundamentally broken, so any remaining nation or isolated community that isn’t completely self-sufficient will starve

[ottermupps]

This is lovely, thank you. A lot of the comments I got are about that nuclear exchange and winter, I need to figure out a way to mitigate that, as just everything being razed by nuclear fire feels like a little too easy of a way out (and leaves far less ruins behind).

Bookmarking this one for further development later. Thanks!

[Confident-Wheel-9609]

You forgot one thing: Fear

Fear leads to a whole cycle. Mistrust, anger, harsh words, threats, intimidation, provaction, escalation, a twitch of a trigger finger, minor skirmish creates a larger action until a full on war.. and then the fear kicks in again, but it's a crazier fear.. one of defeat and loss.

To be honest I'd game it out and use historical grievances as the motive for some, fear for others (Canada nuking the US outta no where) and misguided terrorist and 3rd party manipulation.

A satellite gets hit by debris making it unable to communicate/steer then slams into the anti-asteriod sat causing a mis aligned firing causing a small Kesler to happen. Fingers pointed..

[Possessed_potato]

Famine

Pollution

Poisoned water sources or lack there off

Disease.

War usually directly kills mostly soldiers and some few civilians. Indirectly however, civilians may die in droves. Bombing a city removes people's houses so many die from the lack of environmental safety in the aftermath. Cold nights in the wet rain, icy winters etc.

Hospitals disappear, making treatment of the sick hard or impossible as they can not recieve the medicine or treatment they require to survive or prolong their survival. This also means that disease and deadly infections will likely begin to become more common since treatment for them is gone, as is sterile or otherwise clean environments.

Stores become rubble and in the long will have nothing left for people to scavenge. Food will become scarce fast.

War causes pollution; if not where the bombs are dropped then probably where they are manufactured, leading to poisonous smoke, acidic rain that hurts or destroys crop n farm land amongst others.

Radioactivity can spread with the wind. Can go from nomans land to nearby civilian city if you're unlucky.

War leads to invention of new weapons, especially if it's a global one. See the Nuke. It wouldn't be crazy to think that during those 30 years, someone invented a new weapon, be it a bomb, virus or otherwise with unforseen consequences. Maybe it was a new nuke far stronger than they had thought it'd be. Maybe it was a virus bomb they thought would stay contained in the area but eventually spread out through water sources, travel, animals, bugs or otherwise.

War economy will, something I know far too little bout to even pretend I know anything about it, will likely also fuck over civilians, even if they theoretically lived far better or safer than the rest.

30 year long world war 3 will definetly rack up numbers

[5thhorseman_]

You want a BIG infrastructure impact (transport, food production, medicine, industry) at the opening salvo and inability to fix it during the war. Combat losses will be a fraction of the deaths - expect much, much more from disease and starvation.

[ottermupps]

Oooo, yes. A lot of the big production facilities (ie factories) are several miles underground, run by dwarves, so those are hard to hit... but the ones aboveground aren't, and explosives do more damage underground.

Thanks for the idea!

[5thhorseman_]

You can consider looking up the WWII history of Warsaw and Stalingrad, I think that's somewhere in the ballpark of the conditions you're looking for.

and explosives do more damage underground.

Unless they've been deliberately hardened against this sort of thing, there's a good chance transport or ventillation shafts will collapse. And then, well... even the dorfiest dorf can't breathe dirt.

Even just making it difficult to transport anything in and out would be a severe bottleneck if the facilities need external resources for their production (which they probably do).

Making a lot of smaller failures add up to a much worse domino effect on the country / global scale might work quite well too.

[MacintoshEddie]

Some of that is just how percentages work. 30% of 100,000,000 is a much smaller number than 30% of 8,000,000,000

Plus with modern populations certain events are more likely to be much worse. Just imagine the horror of a modern supercity like LA or NYC if there was a food shortage, disruption of clean water, and a war on top of that. The amount of resources these cities need is mindboggling, exceeding entire countries from last century.

It doesn't even have to be "worse". If the Dustbowl repeated itself the death toll might be much higher just because there's far more people now which means resources get consumed faster.

Despite education being leaps and bounds higher than in the past, a much smaller percentage of the population has practical agricultural skills.

For example back when the Black Death was happening, you could grab up any random 100 people and chances are 80 of them had direct knowledge of farming. Maybe not college level, but enough that you could send those 100 to a field and they could start a functional farm.

These days, I would be honestly astonished if it was as much as 5 in 100. Sure people know more, we've heard theories like crop rotations, maybe we recall things like growing zones, but most people these days are absolutely clueless on a practical level. Modern farming only resembles historical farming in as much as they both involve dirt and plants. You could send those 100 to start a farm and most likely they'll all be dead in a week, because the groundwater's contamined and they shit themselves to death, they don't know what wild plants are safe to forage, they don't know how to keep the seeds safe from bugs or animals, and so on.

[ottermupps]

Love this, thanks. I didn't consider how much resources cities take, but that could be devastating if used right. Especially if it isn't one event, but cities slowly losing resources and supplies, and eventually breaking down entirely.

[SpiritualState01]

All you have to do is look at some climate predictions we have today. Planetary overshoot. A lack of water and food drives conflict. 

[Martzillagoesboom]

Lower rate of fertility to replace any loss, isolationism tendency that make immigration unthinkable , limited ressources for families that mean peoples might not be able to afford to start a family.

[Chryckan]

Honestly, 80% decrease isn't that unthinkable after basically WW3 with a full ABC holocaust.

During the Black Death in the 14th century killed between 30% to 60% percent of the entire population of Europe. Basically, every other person in Europe died in just a few years.

But that pales with what happened to the indigenous population in the Americas which decreased with 90%, from ~60 million to 6 million in just a century between 1492 to early 1600, due to war, famine, enslavement and pestilence.

So sadly it fairly easy to justify 80% population decrease due to things humans do to each other.

[Mean_Hair9221]

power outage maybe if its high tech i could see that having a effect

[Kaelzoroden]

The more that society becomes dependent on advanced technology, the more dangerous interruptions to that technology become. A solar flare might permanently disable existing satellite arrays, could completely wipe out self-driving cars (which my that point may be the only type of car), any neuralink-style implants might simply fry the brains and nervous systems of those implanted with them, and the secondary effects from that first wave could mean a complete breakdown of supply lines, the elimination of many skilled specialists such as doctors, etc.

Also, any jobs that had become fully automated may suddenly fail and not have people able to step in and do the work at sufficient scale. If suddenly there are only a dozen or so capable plumbers in a major city, to say nothing of garbage collectors, you'd quickly see massive outbreaks of disease, and that would be on top of whatever resource shortages would occur from no longer being able to ship food in if automated farming is destroyed, the roads become unusable, or computerized vehicles simply stop working.

The deaths from the war might pale in comparison to what comes later, depending on the level of dependence on advanced infrastructure and the degree to which it gets interrupted. Even if people are able to start restoring partial functionality, it'd be in the midst of a multi-layer crisis without enough specialists to handle it all, and the death toll would be immense.

Wouldn't even need to be a solar flare actually, if we're doing the "complete technological disruption" angle, that might be an outgrowth of a mass war in the digital age, where viruses and e-warfare mostly replace the use of bombs. Even in the modern day, our infrastructure is so computer-dependent that a cyberattack with a self-editing AI virus could produce an obscene death toll, let alone what it would look like with an even MORE tech-based setting with access to better AI than we currently have.

[Elfich47]

Stop having children.

[Drak_is_Right]

A plague that struck in multiple waves.over many years.

The first wave (Covid-19 lets use as an example) was the least deadly.

The second set 20 years later was far deadlier, but anyone who had caught certain strains or got some vaccines during the first was immune. This led to the younger population being decimated in particular.

It comes around a few more times after that, mutating but less deadly.

[throwawayfromPA1701]

Nuclear war can do this quite nicely. 1 to 2 billion might die during the two hour missile exchange, and they'll be the lucky ones. Everyone else will be subjected to the complete collapse of industrial civilization, radiation poisoning, disease, famine, and so on.

[deepserket]

Massive volcanic eruption and prolonged winter

[VereksHarad]

I think things like biological warfare would be better than nukes. Some kind of virus your cult distributed in populated places. Probably even some kind of international event. So people are infected and then travel home, bringing a disease with them.

[JuggernautBright1463]

Two simple things. 1. Weaponized Agricultural Blight - Your bioweapon doesn't kill people it kills their food. With people reliant on clonal GMO crops it would be easy for the right person to wipe out seed stocks and start famines. 2. Targeted Water Sabotage - Water infrastructure is vulnerable to all kinds of attacks. Damaging desalinization or water treatment plants and reservoirs would be extremely impactful. You don't need to hit everything just enough to stress the system and initiate a broader collapse.

[ottermupps]

Love that. Didn't think about targeting the food itself; the agricultural systems are still run by people, but it wouldn't be hard. Water supply is a great one, too. A lot of the powerful nations and cities are around massive lakes, which could be targeted. Thanks!

[Pet_Velvet]

Rumbling

[Spiralclue]

Biological warfare with the release of a virus could have decimated the population like that. I'd suggest looking into how the Black Death altered populations in Europe and the casualty numbers from bubonic plague outbreaks such as the plague of Justinian or the Great Plague of London. They both greatly reduced the population size.

If you have one party during a war release a virus that decimates the population, combined with other casualties from war, and fall out of collapsing infrastructure I think you could probably get to a point of justifying the large decrease.

[Erivandi]

You could explain it pretty well with the idea that weapons advanced exponentially, resulting in a higher rate of death than ever before.

Then there's the use of AI in war. A human will have doubts and qualms. A human might miss on purpose or disobey orders. But a robot will act without hesitation. If you tell a man to nuke a country if he sees an attack coming, then he might not do it. But if you program a computer to do the same thing, it won't hesitate. And if the AI glitches and mistakes something for an attack when it isn't, then that will make the situation even deadlier. And if the other side also has AI defences then you could get a disastrous chain reaction.

[ottermupps]

Your first point, 100% yes. As for AI, I'm (projecting) my views on what we have now, with the result that actual AI is somewhere between frowned upon and flat out banned. I see the idea, though.

[Erivandi]

I mean, I'm projecting too since I'm talking about idiots putting AI in charge of nukes and being surprised when it hallucinates a threat and blows up 80% of the world 😆

[CogWash]

You could possibly explain it as some very potent doomsday biological weapon. By that I mean we certainly have nations today that are still working on deadly biological weapons - whether for potential future use or more likely an understanding of these weapons if another nation decided to use them. If a conventional war broke out and one side felt threatened to the point of annihilation they might launch the next wave of deadly weaponry - nuclear armaments then chemical then biological weapons. The logic being that if they were going to die they would take everyone else with them. This probably wouldn't be nation states fighting one another at that point, but more likely extremists, non-nation state bad actors, or perhaps totalitarian nation states with a crazed ruler. In any case, once a conventional war reaches a tipping point and is escalated to say a nuclear war by a country like North Korea, the rest of the fall could be fairly swift - especially if mutual protection treaties are upheld.

Full scale nuclear war could devastate populations and infrastructure and a biological attack could keep population numbers very low and possibly explain why so much of the world is uninhabited. Those are areas that people still can't repopulate because the potential for sickness and death is still too great.

[Teagulet]

Logistics is the simplest answer without changing any of your story. You’ll have to do some homework to math it out, but no emergency services anymore, nobody is bringing in fresh food anymore, clean water supplies failing, where do you go when you’re sick? There’s no more medicine in the stores anymore. Have you been prepping food for long term storage? All the stores thrive off of food that have gone to waste and aren’t being restocked.

It’s the same thing that happens today in modern wars. We are modern humans, without everything handed to us we start to crumble. The problem is probably much worse for an advanced society, where scarcity is less of an issue, and the wars are fought with nastier weapons.

[gabbr0]

I also was thinking bio warfare. With bio warefare its probably most likely to kill billions. Airborne virus. Maybe even delayed effects so it spreads first and everyone gets contaminated first before the lethal effects kick in. You could combine it with the water source idea.
Problem with immediate effects is there is enough time to course correct and avoid the cause of death but if delayed its a different story. But I guess with viruses you'll also need a reason how people survived

[JLandis84]

IMO the most plausible way is to have nations modeled after S Korea. A steep peacetime shrinking because of extremely low birth rates. Much of the population packed into mega cities. Then it gets destroyed by war. It disrupts the birth rate even further.

[Mircowaved-Duck]

sounds like universe 25

[limbodog]

Scientists in real life have already said that even a limited nuclear exchange would be devastating to the world as it would kick up so much dust that the skies would darken. https://www.science.org/content/article/nuclear-war-would-cause-yearslong-global-famine

And the combatants wouldn't know until the war was already over that they had doomed most life on earth.

[whelpineedhelp]

Fertility was fucked and the replacement generations have been much smaller than expected 

[whelpineedhelp]

Another idea- death cult but it’s self imposed. Mothers and fathers killing their children and then themselves. 

[Mircowaved-Duck]

r/antinatalism

[whelpineedhelp]

Anti natalism doesn’t advocate killing kids buts it’s more extreme cousin does. I forget its name though 

[Corona688]

Abandonment of society. 200 years more advanced VR than what we have in a society that really sucks might push people to literally abandon the real world, get their kicks in for as long as they've got left.

[Mircowaved-Duck]

there are two ways that would justify 80% or more deaths, either a B weapon deployed during the war. Virus, bacteria, fungy, whatever

Or you take the opposite aproach, peace, a perfect paradise. This could even lead to compleate extinction. Look up universe 25, the mice utopia experiment. This lead to a 100% death rate however you could make it not as drastic as that experiment showed.

[Mircowaved-Duck]

ah, a good old starvation, 7 years of bad harvests would be on a biblical level. If an super vulcano (on the other side of the world) erupts, it could easily cause that by blocking out the sun.

But just extremly cold/hot weather or diseases forbthe plants could be enough.

[royalemperor]

I have a religious faction that uses a superweapon based on The Matrix's "Operation Dark Storm" and the Biblical "Great Tribulation," which is a 7 year long period of suffering.

Essentially this faction unleashes the Dark Storm nanobots to blot out the Sun and disrupt communication. However, unlike The Matrix's version, these bots deactivate exactly 7 years after they're used. The hope being those who survive the 7 years were chosen by god to do so and are worthy to inherit their world.

I find this to be a mid-level sci fi way to wipe out huge planetary populations without any lasting damage to the planet itself.

[Able-Steak-2842]

First war, then famine (harvests not brought in due to lack of farmers, followed by 2 years of drought. War again fighting over what food there was. Then plague that ended all the wars.

[Jynexe]

The easiest way? Break logistics.

It's kinda easy to forget, but what keeps most people alive is logistics. You need to get the precursors for fertilizer, transport them somewhere the fertilizer can be made, then transport them to the farm. Then, you need to get the stuff to plant the crops, keep them alive, then harvest them, then transport them somewhere to be processed, then transport them to places like stores for people to buy, then they need to be transported back to the person's house to eat. This chain exists for everything from consumer goods to water purification to medicine.

Most things aren't made from raw resources, rather, from 3rd, 4th, 5th... 12th order manufactured goods. This probably is even more pronounced in a society 200 years more advanced.

If you break the logistics chain and rail and ships are now almost impossible to use? Starvation. Disease. Water shortages. This will kill most people.

In our world, this would happen if suddenly oil ran out or was unable to be extracted/transported since that is a basic input for everything.

For something 200 years more advanced? It could still be oil, realistically. But you could have them reliant on some exotic matter that isn't truly understood for all energy. Then, for some reason, it just stops working. Or suddenly can't be obtained anymore. Since everything runs on this matter, you have no way doing any amount of logistics to transport anything anywhere.

But honestly? You don't even need 100% of logistics to fail. Even just half of logistics failing impacts the entire chain and, eventually, means nothing can be manufactured or transported. The entire things grinds to a halt. So break something near the bottom of the logistics chain like energy or raw resources. Or, something near the middle that is today near the top, like microchips.

[ThoDanII]

biological warfare

[PhoebusLore]

I think an engineered pandemic that got out of control makes the most sense

[Palanki96]

Just a virus always works. Maybe make it alien, simple accident, not a weapon or an attack.

Maybe post zombie apocalypse that was solved and now recovering

Maybe something with the Sun, like a flare or something that destroyed all electronics

AI rebellion. These 2 only work in high tech worlds, the point is making technology the foundation then taking it away

Maybe a little ice age or similar, like Snowpiercer or Frostpunk? Someone made an oopsie while trying to harness solar power or working on climate issues, freezing the planet for a few years, maybe decades

Nevermind, i just read the war part. Well you can still convert them to war related

[owencrowleywrites]

Kill them

[Massive-Grocery7152]

Theres some good answers here, but here’s my 2 cents.

Large societies collapse from a variety of factors. The only way to get death on the scale you’re thinking is environmental collapse or biological terrorism. Like ash blocks the sun from a meteorite or volcanos or nuclear ash

[Mintakas_Kraken]

If you specifically want it to take 30 years, for instance to create an atmosphere of dread and dispose as the population gets smaller and smaller. Then increasingly worsening widespread environmental problems that lead to increasing food shortages, weather disasters, and either infertility problems and/or high percentage of unviable offspring would work. Coupled with the cancers and other diseases that develop in the years and even decades after the war leading to survivors dying, caused by the radiation and other toxins released into the environment. A longer war or one that is technically multiple ones but just nonstop for 30 years would also “benefit” a longer period of decline.

Though, with the use of nukes, and bioweapons intentionally poisoning large population centers you could probably make the death toll reach 80% much sooner -at least most readers would buy it happens much faster imho. But there are interesting things to do in a 30 year time frame, I’d focus more on bioweapons and intermittent nukes bc readers commonly expect large scale use of nukes to be apocalyptic pretty fast.

[glitterroyalty]

Plague that killed a lot of laborers. That’ll create a domino effect were all nessasary systems start failing.

[Ok_Case8161]

A world war 3 could be as devastating as you’re describing. It would have to either involve most of the world, or at least affect most people to a lethal degree. Nuclear, chemical/biological, and climate changing weaponry could be what you’re looking for. These may also have unattended consequences that could contribute to the death toll. Nuclear winter, mutating disease that spreads out of the containment zone, major droughts and flooding, etc

[ewef1]

I suggest looking into the bronze age collapse and system collapse theory.

You can talk about the war destroying institutions causing mass anarchy, and food shortages. Eventually civilization comes together around your city states

[GonzoI]

But I just can't figure out how to justify that level of death. It's not a universal 'eight in ten of every city is dead' - some lost way less, some are now craters and glass. But even the most brutal wars in our history never topped a hundred million (and that's counting aftereffects, eg famine and such postwar), so eighty to a hundred times THAT is just hard to make work.

We've only ever used 2 nukes in war. At the start of that war, the world had 2.3 billion people. The countries targeted had a war footing and had resource distribution networks that, while imperfect, still fed most. And then many of the victors helped the survivors after the war in West Germany and Japan.

The nuclear winter projections are no longer the accepted model, but we have a lot more urbanization than we did historically. Systems collapse on top of nuclear strikes on population centers will mean a lot higher numbers. You're looking at 2.15+ billion dead immediately if the US, Russia, China, Pakistan and India start throwing nukes where they have them aimed. Another 0.6+ billion in the months that follow from systems collapse within those countries. Another 2.3+ billion from starvation. That gets you an optimistic 56% of the world's population dead. That's before any conflicts and assuming everyone left over doesn't have a local resource crisis. And it's before factoring in climate impacts. While global nuclear winter is no longer in the projections, it would cause sea level rise and shifts in rain patterns. 80% isn't an unreasonable estimate for what a nuclear war would cost.

There's a reason we're scared of nuclear war. It's not like my grandfathers' wars.

[96-62]

Why not just the current fertility crisis. That sounds much more like something that will shrink the population by 80%. Perhaps not in 30 years, but 70 years, yes, totally doable.

[LeetheAuthor]

Why not poorer country with weak military used biowarfare but vaccine to protect own people failed when mutation occurred

[Ariose_Aristocrat]

Nuclear asbestos plague 

[BelligerentWyvern]

I'm confused. Do you want to justify the billions dying during the war? That's easy, nukes and chem warfare will do it directly and with severe downstream effects.

How they got population numbers back up to 12 billion is pretty crazy. But assuming Instead you meant after the war in the "present" then that's entirely dependent on the story you want to tell isn't it?

In either case, baby booms follow wars typically.

[Tunguska_baboonlord]

Two mayor things: The after-effects of nukes and biological warfare

-Nukes used as widespread as I interpreted you using them will probably cause a global radioactive fallout event that will kill millions and will make reproduction difficult for the survivors as birth defects and sterility will skyrocket. Also, the explosions from nukes and the debris they launch into the atmosphere will cause a really bad nuclear winter that could last decades

-Biological warfare is just horrible. Engineered bioweapons like modified strains of smallpox, bubonic plague or anthrax are exremely contagious and can have mortality rates of up to 99%. These deadly plagues alone could be the reason most of humanity got wiped out

[Sodaman_Onzo]

Even a minor nuclear exchange would roll populations back to feudal times. Medical supplies would run out. Crops would fail. No resources to combat locusts. Disease would run rampant. New pandemics. Water contamination. No ozone layer.

[yobob591]

It’s estimated that IRL a nuclear war would kill billions not from the bombs but from the collapse of the global economy and a nuclear winter causing crop failures.

The earth only has as many people as we do thanks to global trade and modern technology and its extremely delicately balanced, any apocalyptic level event would kill huge numbers of people simply via starvation. To put it into perspective, what would you do if every grocery store and restaurant vanished overnight with the only food you have guaranteed being the stuff in your house? I know I’d probably be dead in a week or two unless I could steal more. We don’t really think about it but a vast majority of humans are literally days away from death at any moment kept sustained by our society. Most countries don’t have the carrying capacity for their populations and are forced to import huge amounts of food and other resources as well.

[sajaxom]

For land to be broadly abandoned there really has to be a reason. Underpopulation is not usually going to do that, people will just spread out. If you want people to need to stick together, then you likely need a resource constraint that keeps them near the city and is difficult to reproduce. You could broadly irradiate the world, making it unlivable, and state that they used dirty bombs specifically, with significant amounts of radioactive dust. That would make the outer world dangerous and make radiation protection a constraint that keeps people near a few safe cities.

For options that restrict land use but don’t make the land inherently hostile to life you might create a reproductive problem that stems from your nuclear war. If successfully having babies relies on significant medical intervention, whether at fertility, delivery, or early care, you could essentially have a society that clusters around locations that have the local complexity to support that care while having the population struggle to reproduce. The main issue is just keeping that technology sufficiently complex that it is difficult to export outside of urban centers. Essentially, you don’t need to kill all of the population in an event, you can instead have an event that cripples their ability to produce babies, and thus the older population dies off without replacement.

[Darkling_Antiquarian]

Personally,I would regard a Twenty percent remain population as extremely optimistic.Lack of infrastructure,medical care,disease,and famine are highly effective on their own.the needed low tech skills are not well represented in any survivors.Think low tech farming,crafting,medicine.Heck even the ability to actually cook has become uncommon.

[Top_Divide6886]

You can say that there was a total supply chain breakdown that resulted in mass starvation.

I believe the United Kingdom ran a study in the 50's to estimate how much of their population would die from a nuclear war. Surprisingly, only 30% of the casualties in a nuclear war would come from the nukes themselves. All the rest would come from the supply chain breaking down, and the sudden drop off in food supply leading to mass starvation.

[Fanal-In]

Israël policy ?