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For a story I have, modern civilisation must be destroyed by a worldwide disaster.

The objective is roughly something that:

  • Could be man-made or aided (if not now, maybe in the future)
  • Decimates, but not eliminates, land-based life
  • Directly destroys most buildings and technology, traces ok
  • Allows survivors to rebuild over hundreds or thousands of years
  • Doesn't affect the inhabitability of large parts of the world long-term

Something like nuclear war would be inappropriate, as it have too much widespread long-term impact. However, if floods could have worldwide impact then that would work.

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  • $\begingroup$ Welcome to WorldBuilding.SE! I'd remove your follow-up question, as it's far too broad, but your main question seems fine. $\endgroup$ – F1Krazy Oct 17 at 13:02
  • $\begingroup$ Welcome to the Worldbuilding Stack Exchange! That is a good question! I would recommend you remove your last line in agreement with F1Krazy. $\endgroup$ – A Lambent Eye Oct 17 at 13:18
  • $\begingroup$ I'm pretty certain that nuclear war ticks all your boxes. What criteria were you expecting it to fail? $\endgroup$ – Starfish Prime Oct 17 at 13:20
  • $\begingroup$ It's the fallout that might mess things up, although small patches of irradiated land could work. Ideally you could forget that it happened after a few hundred years, unless you were testing the soil or fossils or something. $\endgroup$ – savo Oct 17 at 13:29
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    $\begingroup$ Do you actually mean 'decimates' (literally: 10% death toll) or are you actually envisioning Mass Extinction Event (70+% death toll). Are you worried about wiping out animal/plant life, or are scenarios that primarily affect humans specifically acceptable? $\endgroup$ – Morris The Cat Oct 17 at 13:39
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I can think of a couple options:

Gray Goo

An unstoppable swarm of nanites, or maybe even larger (e.g. insect-sized) creatures that destroy all non-bio-associated metal, would probably do the trick. Ancient stone buildings (and wood buildings with all wood joining) will mostly be unaffected, but many modern buildings will collapse, and the economic disruption (especially because this will take out pretty much all transportation) will lead to mass starvation. Wild animals, however, will be almost entirely unaffected, and the only plants that will be affected are those that were in human-built structures or otherwise depended on humans (e.g. for cultivation).

Massive storms

A biblical, world-wide flood most likely would have affected an Earth that is very different from today's. Much deeper oceans, much more sub-surface water, much shorter mountains. Repeating this on modern Earth is probably not practical (not to mention all the Christians you'll upset because it would be God breaking a promise). However, you might be okay with some sort of planet-wide storm. Buildings are built to withstand wind, but if you throw in enough debris that you're effectively sand-blasting everything over a long period of time, that might have the desired effect. If your "sand" (which can be any loose objects that break or wear down buildings) supports plant growth, the biome should be able to recover. Hard-core survivalists will probably be able to weather this, although I'm not sure how much plant or animal life you'll have left.

Plague

Any sort of medical what-have-you that kills off most people will satisfy some of your points. Whatever happens, you'll most likely have some survivors (at least, readers aren't likely to question that). The main problem here is it doesn't satisfy your criteria of "directly destroy[ing] most buildings and technology" (emphasis added). If you've killed off enough of humanity, nature will do the job for you in a few hundred years, but it sounds like this isn't what you want.


In any case, I suspect a key part of your regression is going to come from economic disruption. This works well, because it will tend to kill humans, and only humans. Most wild animals are not dependent on humans and would get along just fine without us.

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  • $\begingroup$ I think people can find a way to deal with "Massive storms" Maybe the poorer countries would suffer more. But can't we just build stuff underground and try to weather the storm there? Surely not all humans would survive, but enough would. Especially if this took enough time to happen so that people started seriously thinking about solving or dealing with the problem. $\endgroup$ – Seallussus Oct 20 at 0:11
  • $\begingroup$ We'd have to know they are coming well in advance; underground construction of that magnitude doesn't happen overnight. Anyway, the storms are mostly to destroy (surface) buildings; most of the dying comes from the massive economic disruption in the aftermath. You're also assuming there isn't enough surface disruption that the "survivors" are buried alive. (All that said, I tend to agree it's not the best scenario; I think "grey goo" seems more promising.) $\endgroup$ – Matthew Oct 21 at 15:01
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I'd go with the traditional option: bioweapon (natural or man-made). It would have following properties:

  • no symptoms until activation (so low probability of detection and no panic)
  • highly virulent even before activation (so you have most of the world population affected)
  • activates by a singular world-wide event (e.g. massive solar flare)
  • turns most of nitrogen in human body into nitric acid (so their corpses will corrode cement under them).

While this wouldn't destroy buildings that don't house a lot of people, most of those either require constant maintenance (e.g. dams, nuclear reactors), or would be impossible to destroy without a cataclysm that wipes out most life, human or otherwise (nuclear bunkers, autonomous lighthouses and retranslation stations, etc.).

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  • $\begingroup$ "Turns most of nitrogen in human body into nitric acid" is highly unlikely at best. $\endgroup$ – Mike Scott Oct 18 at 7:00
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A nearby supernova might well meet your requirements. Such an event occurring within 50-100 light years of Earth might well have adverse effects on Earth. If the distance of the supernova were just 30 light years the effects would likely be devastating. In fact the close the event the worse the effect, so you can dial up whatever level of destruction you wants. The Sun won't turn into a supernova but if it did it would sterilize the Earth and boil away the oceans.

There are no type II supernova candidate stars within 50 light years of Earth, but there are expected to be several hundred type I supernova candidates, however they are very dim and hard to find before they turn supernova so it is reasonable to speculate as to their location.

https://earthsky.org/astronomy-essentials/supernove-distance

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    $\begingroup$ You wouldn't even necessarily need a supernova. A really big Coronal Mass Event would have pretty catastrophic effects on human civilization just from the simultaneous global destruction of most computerized and electronic infrastructure. $\endgroup$ – Morris The Cat Oct 17 at 13:41
  • $\begingroup$ Yes true, although I doubt it would decimate land based life $\endgroup$ – Slarty Oct 17 at 13:47
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    $\begingroup$ At a scale where the macrofauna survives would harm some electronics, but not provide much destruction. Anything sufficiently powerful to reduce most buildings to rubble would blow away the atmosphere and leave the Earth rather lifeless. $\endgroup$ – pluckedkiwi Oct 17 at 13:48
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    $\begingroup$ I don't see this having the desired destructive effect without sterilizing the planet, which contradicts one of the OP's other requirements. $\endgroup$ – Matthew Oct 17 at 16:52
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    $\begingroup$ @Slarty, note I specifically mentioned "the desired destructive effect". Sure, radiation can kill everything, maybe even break electronics, but how is that going to also destroy buildings? It seems to me that by the time your supernova is close enough to cause physical devastation — and I'm not even sure you would get that unless the other star was practically inside our solar system — you'd be way past the point of sterilization. $\endgroup$ – Matthew Oct 17 at 17:49
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Some kind of super-hurricane would do the job. The wind plus flooding would wipe out most people, animals, plants and buildings. (Not sea life though, if that matters.)

Only those at high elevations would survive, and maybe some by pure luck.

You could create such a hurricane by heating ocean water which feeds hurricanes. You would probably want to have a "hurricane generator", which would create hurricanes with warm water, then send them in random directions.

Once you stopped and the water drained the earth would recover rather quickly, plants would grow from random seeds. Animals would slowly replenish, as would people. But all the buildings would be gone.

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In my mind the hardest criteria to meet in what you posted is wrecking all the buildings. It's actually a lot easier to kill off most or all of the human population and leave them standing.

The thing about the human population is that it's maintained through a really terrifyingly fragile infrastructure, and the more developed and urbanized the population is, the more true that becomes. Anything that sufficiently disrupts the developed world's ability to distribute resources the way it does currently for more than a few months could easily kill off most of the global population, but wouldn't affect the rest of the ecosystem all that much at all.

One simple way to do it that's been mentioned a few times is disease. Whether natural or artificial, if you kill off enough humans quickly enough, the infrastructure keeping most of the rest of us alive grinds to a halt and now we've got nowhere near enough food and clean water. The bigger the city, the worse the death toll would be. Populations in rural agricultural areas would make it through with the greatest survivors. Big cities would turn into mass graves that survivors would avoid (literally) like the plague.

The other way that's closer to Slarty's answer is a massive solar flare or Coronal Mass Ejection. You totally don't need another star going supernova, our own sun could do it with a big enough fart.

Something close to this actually happened in 1859. We got hit with a massive blast of solar radiation that caused northern lights to be visible as far south as Florida and put so much power through telegraph lines that they caught on fire.

Something on that scale today would be spectacularly destructive. The entire electrical power grid, everywhere, would be damaged or destroyed. Almost everything that RUNS on electricity that doesn't have mil-spec EMP protection would be damaged or destroyed. The solar flare itself wouldn't kill that many folks. Anybody on the ISS would be pretty screwed. Civilian aircraft in flight would be in pretty severe peril depending on where they were, how much fuel they had, and how skilled their pilots were.

The sudden destruction of the entire global economy would have a massive death toll though because most of the population of the developed world now only has maybe a few weeks worth of food, and no way to easily get more, and no way to rebuild the infrastructure quickly enough.

And that's assuming that some trigger-happy nutjob somewhere doesn't react to the business by starting a nuclear war on top of it, which WOULD get most of your buildings destroyed, but it's hardly necessary.

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I would go for massive earthquakes over just about all of the earth. You'd be guaranteed to destroy almost all infrastructures, a lot people will die, but a lot of data and ressources could be safe enough to start rebuilding soon after.

An interesting development after such a global scale natural disaster would be large floods, landslides, plagues etc that would slow down rebuilding. I'd expect some elites to form after acquiring left over weapons, but they probably wouldn't last for very long.

What caused these earthquakes? Perhaps a massive shift in our tectonic plates, that would also rearrange our continents and mountain ranges. Plenty to work with!

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Your buildings don't have to be consumed in the event, they can be dismantled by the survivors for their new homes like Rome experienced once it totally fell apart. Or else these buildings collapse through neglect as the reduced population rebuilds on what is local and after hundreds or thousands of years of neglect buildings that aren't stone or brick will be really bad off. Watching 1000 years after humans will help you visualize this.

Yes flooding could work but you need a Noah situation.

Flash freeze/mini ice age killing off many who are outside or you could combo this with an emp, famine, and then mini ice age.

Grey goo or rouge helper bots but this is cliché

The Happening movie could replay in your world aka the trees did it causing everyone to go psycho and commit suicide and the few that didn't die were in the right place at literally the right time to avoid catching what triggered everyone else.

Eating GMOs causes the cells to commit immediate suicide so now only a very very few people who avoided the GMOs like the plague or consumed 1% or less are the last survivors to them people just dropped dead so they didn't know what had occurred.

Technology did it take the Happening but do it digital. Hackers or a mass world wide government planned plan to add more spy ware on their own citizens from around the globe goes horribly wrong. The signal suppose to be some software hack program instead communicates directly to the subconscious mind in an unhearable wavelength that basically causes the death note to the body and everyone sitting infront of any smart device, TV, laptop, and ipad die with brain hemorging or such so the bulk of many nations are dead in one moment including the very government officials who signed the order to make live the secret spy program.

Super bugs/ Plague this is slower but if they can't find a cure either it will kill most people.

And finally the plants did it again! This time its soo hot due to man geoseeding their world that the plants and trees stop working and instead of pumping out CO2 they switch over to pumping out carbon monoxide and killing everyone survivors will need to be on oxygen already, not on the surface, or in say the artic and maybe select desert regions.

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"What disaster could wipe out civilisation worldwide, but allow for immediate rebuilding? What would be the side effects?"

Any disaster that would permanently alter the environment won't do. So no supervolcanos or asteroids. You have to keep the ecosystem intact, so the disaster has to target only the humans. The easiest disaster for it would be plague. A plague as deadly as the ones that killed the native americans, killing 80% of population, but saving the elite technicians and equipment. Due to fast communications the killing would be quite fast.

The survivors need transcontinental planes, ships and offroad vehicles to gather the survivors. 20% of population is still quite a lot of people and if you gather them in a smaller territory you can achieve the population density to keep industrial civilization working. Maybe gather the survivors around the North Sea (oil, iron, coal) or the United States (farmlands, almost all resources that a society needs). It will need planning but it is doable, i think.

In a generation the population will start growing and resettling the abandoned lands, resources will be abundant: many cities to scavange.

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