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One rule: it cannot be caused by aliens or AI invading the planet. But it is okay if an alien race causes the destruction in any other way.

It's an Earth-like planet. I need the effects of the destructive event to be continuous there-after, i.e massive storms and natural disasters for years to come that threaten the small population that survived in bunkers.

I want the majority surface to get wrecked. I need all things that are required to rebuild a civilization to survive, i.e, an atmosphere, because I need characters to come up from the bunker and be able to live. I don't want them to be comfortable. Their lives are to be miserable. The planet should look like a hell-scape afterwards and they should desire finding a new home.

Hope you don't consider this a duplicate question because I feel my question is more nuanced than any others on this site.

Edit #1

This human race is quite advanced. They'd have technology to redirect asteroids, comets, etc. Unless an alien race were to guide some into the planet. The event also needs to be fast acting. So global warming is out of the question

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    $\begingroup$ Wrecking the surface isn't that much of a problem. The stumbling block is the idea that people could survive for a prolonged period in underground bunkers. Weeks or even months, sure, but not for generations. You need a biosphere for that. $\endgroup$ – jamesqf Jan 27 at 19:54
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    $\begingroup$ Actually, I want them to survive above ground in the world. They can retreat back to bunker during aftermath storms. They will aim to evacuate the planet eventually. $\endgroup$ – halp Jan 27 at 20:05
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    $\begingroup$ A note regarding your scenario: If they can redirect asteroids and whatnot they probably have some kind of space colonies. This means that the only survivors won't be on the surface and may be able to contact those on the surface and help evacuate an/or rebuild. $\endgroup$ – stux Jan 27 at 22:39
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    $\begingroup$ Asked 10 years ago by NealStephenson ;-) $\endgroup$ – Peter - Reinstate Monica Jan 28 at 9:58
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    $\begingroup$ Hugh Howey's "Silo" series revolves people living in bunkers not able to venture out into the outside world. I don't want to give any spoilers away, but if you haven't encountered his writing, pick up the first book, "Wool", and give it a spin. To be honest, your idea sounds like it would fit quite neatly into where his series ends off. $\endgroup$ – mccdyl001 Jan 28 at 13:13

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First, welcome to Worldbuilding, halp!

Let me preface this by saying that this answer @rek gives about volcanism is a good start, and it was my first thought, as well. But in considering the additional details you've given, you may want to go another route (that is, depending on how realistic you want to be). I'll expand on some of the finer points below.

Now, do you want the surface of your planet to get really hot, or really cold?

Cold

If you would prefer the surface environment of your planet to end up being quite cold, then (paradoxically) a good place to start is with increased volcanic activity. As mentioned by @rek, a volcanic event on par with the Siberian Traps would indeed be catastrophic. However, the focus then becomes: How fast do you want your extinction-level event to occur, versus how long before people can safely live on the surface?

If you want your people to see this catastrophe coming far in advance, giving them plenty of time to build bunkers and prepare, then one way to do this would be to start with a series of small eruptions (or a couple of large ones) that in turn give volcanologists indications that such activity is poised to increase in severity and frequency. Respiratory issues will occur in places, but by-and-large, they won't be a major problem by themselves in the short term.

Alternatively, if you want volcanic activity severe enough to cause a massive, unexpected extinction-level event quickly, then (without protective gear), the surface of your planet will be uninhabitable for a very, very long time. The P-T extinction event took millions of years to unfold. In order for your wasteland conditions to appear in short order, you would need something much more severe. For starters, there will be a lot of fire. Molten rock and burning ash will rain down over your planet like fireballs and flaming confetti. All the resulting ash and CO2 would certainly cause the atmosphere to become too toxic for oxygen-breathing lifeforms, nor would you be able to reliably grow anything using traditional methods due to the reduced levels of sunlight.

Gradual volcanic event:

Even some of the largest eruptions in history, while disruptive, didn't result in any significant long-term repercussions by themselves. The ash these eruptions threw into the atmosphere caused global cooling of a few degrees; winters were rougher for a couple of years, with record snowfalls and strong blizzards occurring. Naturally, a string of such eruptions in close succession would increase these effects.

So, if a) your eruptions are relatively steady and strong, and b) there aren't many of them in total and/or spread out over too long a time (say, a few months or less), then once the volcanic activity stops, the planet will return to normal over several years, or perhaps decades (again, depending on the exact details regarding the climate and topology of your planet). However, if your increased volcanic activity goes on for too long (say, months to years), you could plunge your world into an ice age. Once ice sheet growth and ocean temperature decreases become significant enough, then even without a single eruption, it'll be a cloud-covered, snow-smothered wasteland for centuries to millennia, at best.

Sudden catastrophic event:

If instead you want a single, massive incident of volcanic activity to wreak havoc on your world with repercussions felt for an extended period of time, then the ash itself becomes a more immediate concern. This would be the route to take if, say, you didn't want the inhabitants of your planet to see the event coming, or you want them to have the bare minimum amount of time to prepare. Portions of the surface near the eruption zone will be in flames from burning debris, and large portions of the human the population (if not all of it) will find difficulty breathing. If you also want marine life to die off, then there will need to be enough CO2 in the atmosphere to choke off nearly everything living on land (humans would find the atmosphere toxic in this case, as well). Then, cue the blizzards.

Hot

A severe increase in solar activity could compromise or overwhelm your planet's magnetosphere. The increased ultraviolet exposure would turn your world into a desert. People would be able to walk around on the surface and breathe just fine without assistance, but they'd want to stay out of direct sunlight as much as possible. In any case, the increased exposure to UV and cosmic radiation will undoubtedly result in a spike in cancer incidence.

Increased solar activity can occur naturally. It's up to you whether your scientists get any advanced warning or not; stars can be fickle. The upsides to this scenario are a) you have more leeway in how much time people have to prepare, b) people can come up to the surface whenever they want, but they will still rely on their tech below ground to survive.

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    $\begingroup$ Nice, it definitely should be fast acting. I got my method. This morning I found a cool way for the volcanic erputions to take place, n I'm pretty sure it has never been done. The hot and cold scenarios were big help, cheers. $\endgroup$ – halp Jan 28 at 18:37
  • $\begingroup$ @halp You're very welcome. Good luck! $\endgroup$ – Dan Jan 28 at 19:31
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    $\begingroup$ Copy... s/volcano/alien mass driver attack/g ... Paste... Thanks for writing the answer I was about to write! ;) $\endgroup$ – Harper - Reinstate Monica Jan 28 at 20:50
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    $\begingroup$ @Harper Well, in the future, if I see any questions involving "alien mass driver attacks," I'll be sure to defer to you. :) $\endgroup$ – Dan Jan 28 at 20:59
  • $\begingroup$ It's happened in fiction, one example is Babylon-5, the Centauri attack on the Narn. They got in trouble because mass drivers are forbidden weapons. The effect was very similar to volcanic eruption. $\endgroup$ – Harper - Reinstate Monica Jan 28 at 21:53
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/Their lives are to be miserable./

I know a recipe for miserable.

Flies everywhere.

flies https://en.wikipedia.org/wiki/Black_fly

The flies are a genetic modification attempt gone wrong. They are everywhere and they attack anything that moves, biting and chewing and drinking. They cause blindness and carry other diseases as well. Attempts to control them with pesticides wound up wiping out other aspects of the ecosystem - agriculture crashed and with it, civilization. The bunker was not necessary for the flies, but it was for the fall of civilization and the chaos that followed. The genetic germ weaponry used in some of these wars wound up getting picked up by the flies as well, and now those diseases are also transmitted.

The flies did fine with all that. They are still out there. It is their world now. The humans know they are out there, because somehow now and then one gets into the bunker.

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    $\begingroup$ lol, that does sound miserable. But I specifically need the environment to be miserable. Like the weather and storms. Unfortunately flies won't level cities and create a "hell-scape" or cause "massive storms and natural distasters for years to come" $\endgroup$ – halp Jan 27 at 19:20
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    $\begingroup$ Hitchcock missed a trick here. +1 $\endgroup$ – We are Monica. Jan 27 at 19:31
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    $\begingroup$ +1 Flies make the world miserable. They also have the potential to wreck the environment - imagine if they wiped out bees or the local equivalent, causing massive plant die-offs and desertification. (Bees are absolutely crucial to Earth's ecosystem.) This could lead to climate change with associated extreme weather similar to what is being observed today, but worse if that's how you want it. $\endgroup$ – KerrAvon2055 Jan 27 at 23:54
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    $\begingroup$ A few days out of the year, we already have this where I live. I can promise you that if we had to put up with this year-round, the entire state of Florida would be abandoned. $\endgroup$ – Dan Jan 28 at 1:10
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    $\begingroup$ In this vein, part of the background premise of the rather good Cory Doctorow/Charles Stross book The Rapture of the Nerds is that a super-colony of Fire-ants has essentially conquered the entire north american continent and pretty much destroyed conventional civilisation there. What's left of the population live in hermetically sealed domes and only venture out in toughened plastic environment suits. The Death Sentence as a punishment has been changed to "kick them out the airlock without a suit". $\endgroup$ – Ruadhan Jan 28 at 9:07
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Gray Goo

Nanobots that can break down anything and use the raw materials to make copies of themselves go out of control. They literally devour the surface of the Earth.

Things get back to normal when for some reason the bots decide to stop replicating and decay naturally.

IPv4 has only so many addresses

Source: https://xkcd.com/865/

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    $\begingroup$ @JohnDvorak nanobots are hardware, not software. $\endgroup$ – Renan Jan 28 at 7:13
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    $\begingroup$ @JohnDvorak technically, you're both right. They're physical entities, but on the level of a gray goo scenario, they'll most likely have at least rudimentary "programming" of some sort. What's nice about this answer is that it could be accidental; perhaps that would meet the question's conditions. $\endgroup$ – Dan Jan 28 at 7:29
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    $\begingroup$ OMG Munroe is just so brilliant it hurts. $\endgroup$ – Peter - Reinstate Monica Jan 28 at 10:01
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    $\begingroup$ The grey goo scenario already happened a few billion years ago. The evolutionary result calls it "bacteria." $\endgroup$ – Draco18s Jan 28 at 12:07
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    $\begingroup$ @Dan There does not need to be any AI involved. It could simply be drones with simple drones that destroy and replicated whose emergency shutdown failed and so they continue to destroy and replicate. $\endgroup$ – ArtificialSoul Jan 30 at 10:08
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A significant increase in global volcanism would satisfy your requirements:

  1. The devastation would be widespread, varied, and continuous. Think of something on par with the formation of the Siberian Traps. Such eruptions can last millennia.

  2. The resulting volcanic winter would wreak havoc on the biosphere and agriculture, necessitating greenhouses and artificial light, but the atmosphere would still be breathable over much of the planet within the expected lifetime of a human civilization. Weather nearest the eruption site(s) would be turbulant year-round, and if the eruption is near an ocean the interruption to currents would have chaotic butterfly effects around the globe.

  3. The cause could be entirely natural, from an abnormally large mantle plume, for example, or alien-induced (perhaps a microsingularity or other weaponized hypothetical physics).

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  • $\begingroup$ You got my attention. Would the affects worsen over time to the point of extinction, or could it eventually get better? Also, I'd appreaciate if you can give me some specific "alien-induced" methods in which global volcanism is the result $\endgroup$ – halp Jan 27 at 21:16
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    $\begingroup$ @halp The scale of the eruption would determine whether things worsened and over what timescale, but even the worst case scenario with short-term survivability would put extinction thousands of years in the future. The Siberian Traps formation is possibly the cause of the P-T Extinction, and the Deccan Traps formation probably contributed to the K-T Extinction (it certainly didn't help), but these Extinction Events took millions of years to run their courses. $\endgroup$ – rek Jan 27 at 21:21
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    $\begingroup$ @halp As for alien origins of the volcanism, anything that disturbs the planet's mantle and/or punctures the crust could be the culprit. An antimatter detonation, a relativistic kill vehicle impact, a micro singularity intersecting the planet at an oblique angle... Lots of possibilities. $\endgroup$ – rek Jan 27 at 21:32
  • $\begingroup$ I will read into it. You have given a lot to think about. Though, to everyone else, I'm still open to ideas. $\endgroup$ – halp Jan 27 at 21:42
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    $\begingroup$ Technically this would be a volcanic winter not a nuclear winter... :) $\endgroup$ – Tim B Jan 28 at 12:11
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Nearby Supernova.

Dr. Mark Reid, a senior astronomer at the Harvard-Smithsonian Center for Astrophysics stated:

… were a supernova to go off within about 30 light-years of us, that would lead to major effects on the Earth, possibly mass extinctions. X-rays and more energetic gamma-rays from the supernova could destroy the ozone layer that protects us from solar ultraviolet rays. It also could ionize nitrogen and oxygen in the atmosphere, leading to the formation of large amounts of smog-like nitrous oxide in the atmosphere.

Pretty unpleasant, no laughing matter - killing off huge swaths of animal and plant life some would recover in time but not without huge changes, the nitrous oxide would cause serious warming:

On a per-molecule basis, considered over a 100-year-period, nitrous oxide has 298 times the atmospheric heat-trapping ability of carbon dioxide (CO 2)

A hot Earth then, but what about the oceans:

....phytoplankton and reef communities would be particularly affected. Such an event would severely deplete the base of the ocean food chain.

It's clear that if you take away the base of a food chain then the whole thing falls flat.

Sure, deep ocean life feeding on bacteria at oceanic vents might be ok, but everything else would radically shift.

Given time, technology and resources to shift everything back and make new plantings and reseed the oceans with life - it could be done, but it would be harsh.

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  • $\begingroup$ Thanks. But that would affect all neighboring planets in the solar system which must go unharmed. I'll edit that in my post $\endgroup$ – halp Jan 27 at 20:07
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    $\begingroup$ @halp, edits shall not invalidate existing answers. $\endgroup$ – L.Dutch - Reinstate Monica Jan 28 at 14:17
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Gamma Ray Burst. It would have pretty much all the same effects as the supernova in Fay Sugger's answer, except it affects a narrow beam. Nobody really knows how narrow the beam is, but I don't know of any reason why it couldn't be small enough to only hit the Earth.

Fun fact, this may have already happened on Earth once. Gamma Ray Burst is a popular explanation for the Ordovician-Silurian Extinction Event.

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  • $\begingroup$ will look into it $\endgroup$ – halp Jan 28 at 4:30
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A chemical treatment for a global problem gone awry.

Imagine the planet was headed for a man-made disaster - pollution, starvation, disease, etc., and the proposed solution by one of the superpower governments was a chemical release of an antidote high in the air.

There was a second warring nation who wanted to be seen as the savior of the planet. They created a different chemical treatment, but had a similar plan to release theirs into the sky.

The rest of the world was opposed to any global releases of any kind of chemicals, but had no solutions of their own.

Because the governments were at war, cooperation with the enemy was treasonous. Neither side discovered until it was too late that their differing plans and chemistries were incompatible, and when combined produced destructively acidic solutions that eventually broke down into flammable gasses. Acidic fog followed by firestorms swept across all the habitable regions of the planet.

The acid in the air penetrated everywhere, corroding virtually every bit of iron and steel and structurally ruining most buildings, bridges, vehicles, and infrastructure, which then collapsed in the fires. Smoke clouds blotted out the sun, ushering in an ice age in all but a narrow band around the equator. The ash cover killed most remaining plant life. The seas are covered with algae, and few fish species survived. Molds and fungus cover most things. For some reason, some lichens adapted and are one of the few things that still grow.

The clouds have cleared and the atmospheric acid is mostly gone now, so the air is breathable. Fresh water stores can be replenished from rains, but groundwater is not potable. Some lichens and algaes turn out to be edible, but most are toxic. Machinery and vehicles were destroyed by corrosion. Animal life on the surface has been reduced to insects - no vertebrates survived.

Sound hellacious enough for you?

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  • $\begingroup$ It's very creative. Not gonna roll with it, mainly because it's too complex for me to keep rolling, but, you have given me some possible solutions n ideas. Thank you $\endgroup$ – halp Jan 27 at 21:23
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Get rid of the moon.


EDIT: As @PeterA.Schneider points out in the comments, this very much looks like the novel SevenEves by Neal Stephenson. Never heard of it, but it looks like an interesting read if you're interested in my idea.


Or have some aliens get rid of it. Perhaps they needed the materials present in it, and deemed its removal "compliant" with their code of conduct, which mainly dictates they can not directly cause the loss of other species' life.

-What about indirect loss of life - asked the captain.

-Nothing on the books, ma'am- replied her lieutenant.

Yes, shady individuals.

More on topic, the moon, or rather its more or less sudden disappearance, could have devastating results on our planet, as devastating as your feel comfortable handwaving them.

The moon gives our planet axial stability

The moon is what scientist believe made our Earth have a tilted spin (whith respects to our translational plane around the sun). They also strongly believe that it is what is keeping that tilt. If it were to disappear, that tilt could become much more erratic and cause much more sudden weather changes, and stronger ones.

Just imagine we go from 23º to, say, 50º, in the span of a couple centuries. The planet's cold and hot areas would change dramatically, and seasonally, which would make winds a nightmare.

The moon is (almost) the only thing that keeps our tides going

And along with their almost complete stop, ocean currents would greatly diminish (EDIT) tidal currents would greatly diminish, which is one of the primary factors in nutrient distribution for the ocean's ecosystem. They wouldn't stop, but it would be significant enough to cause massive loss of life in the oceans. This would be pretty much immediate, and would cause severe damage to the economy, particularly in already empoverished places, which would cause major political and economical unrest around the globe.

Increased volcano activity

Maybe not that much. Maybe a lot. Maybe just the oposite, less volcano activity. This last one is my favorite, because it would mean that presure would be building up under the Earth's crust. The thing with the moon is that it doesn't just move water, it also moves our tectonic plates. No movement (or far less) would make them stop releasing the presure they build up, eventually resulting in one biiiiig BOOM (exaggerated for dramatic purposes).

Heavy moonrock shower

This one depends on the need of the alien race. Maybe they don't need the entire Moon, just about 50% of it. The other 50% is left there as a cloud of rocks, dust and other debree. It would rain down as a fire shower for centuries, potentially making the surface of the planet too risky to live on - hence the bunkers. This and evrything else I already mentioned.

It would affect everyone's psyche

Last but not least, there would be major unrest due to the uncertainty that watching your moon disappear would create, and the sense of defenselessness against a much more advanced bully race that just takes what they need and leave you to die. Morale would be just on the floors, and I would assume many people would start acting very irrationally, to the point that governments wouldn't be able to control populations any more, plunging the entire Earth into an environment more tipical of a few thousand years ago.

Final thoughts

Basically, it would trully be hell, and I don't expect much to survive. Though I think there's a good chance that humanity lives through it, and the Earth would eventually find a new balance. But I would assume it would take thousands of years.

Unless another, now benevolent alien race takes pitty on the Earth and puts a new Moon there. Or the same race judges those shady individuals and condemns them to restore it. Or not, and make everyone suffer. Up to you :)

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    $\begingroup$ And again, that's SevenEves. $\endgroup$ – Peter - Reinstate Monica Jan 28 at 10:03
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    $\begingroup$ @PeterA.Schneider Wow, never heard of SevenEves before. That looks like a very good read! $\endgroup$ – tfrascaroli Jan 28 at 10:07
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    $\begingroup$ xkcd.com/1053 ;-) $\endgroup$ – Peter - Reinstate Monica Jan 28 at 11:47
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    $\begingroup$ @ben True, it's not so much that currents would diminish, but their effect would be drastically affected. Tidal currents, the ones that are responsible for gases mixing in the water and nutrients being carried would pretty much cease to exist, and this is a grave problem for the aquatic ecosystem. I'll add an edit clarifying this points, thanks! $\endgroup$ – tfrascaroli Jan 28 at 23:21
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    $\begingroup$ @ben to be perfectly honest I didn't either until very recently. A couple of sources I found: oceanservice.noaa.gov/education/kits/tides/tides09_monitor.html, thrillist.com/culture/… (second point). I know I heard of this somewhere else but can't find it right now. $\endgroup$ – tfrascaroli Jan 30 at 8:09
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They thought they had the technology to deflect an asteroid.

The impactor looked large and menacing, two or three orders of magnitude more than the Chicxulub impactor. This gave one little advantage - it was seen far enough out that they had the time to do something.

Small asteroids and comets they could deflect using Yarkowski-Sekanina effect masers mounted on the Moon.

Largish asteroids, even rotating ones, would be electrically coupled to a solar sail or fusion-torch probe.

This one, they needed something more, and they needed it fast. So they supplied it with self-deploying Orion engines. The plan was to use a string of nuclear bunker-busters to drill a crater in the asteroid. Then, at every rotation, one large fusion device would be detonated at the bottom of the hole, vaporizing the surface and ejecting the resulting ionized gases at enormous speed and accelerating it towards Earth. After several dozens explosions, the speed gain would have ensured that the asteroid would cross Earth orbit several days before the Earth, and be safely slingshotted away.

The third fusion explosion, unfortunately, fractured the asteroid completely and the tephra destroyed the launching probe. The asteroid became a cloud of several thousand smallish impactors, lazily bumping into each other, fracturing into smaller objects, surrounded by a cloud of dust and gases that efficiently deflected both visible light lasers and microwave masers.

There was no time for a second expedition to be arranged, and the bombs already in flight could not be repurposed and detonated inside the cloud to ensure a wider dispersal.

As a result, four months later the cloud hit Earth. A large part bounced on the atmosphere, most of it aerobraking and entering unstable orbits that would collapse in months, or years. The rest began raining on the surface, more than half impacting the oceans - and often causing disastrous tsunamis - and the rest on the ground.

Humanity space-sight was instantly blinded by the resulting Kessler cascade, so there could be no forewarning of incoming meteor strikes, most between 5 and 7 on the Torino scale. By the time the cloud of shrapnel had reached Earth, it was calculated to be composed of at least 1,000,000 rocks ranging from the size of a pebble to that of a large hill; more than a quarter of those would be Tunguska-sized or larger.

Massive ocean level increase, firestorms and a nuclear winter were the results; after the first years, even the oxygen content of the atmosphere decreased to an uncomfortable 15%, while carbon dioxide, nitrogen oxide and sulfurs reached an unsurvivable 2%, requiring filter masks, oxygen extractors and air regeneration systems.

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  • $\begingroup$ or, you can make the asteroid out of something like osmium or solid iridium, super heavy and dense, with enough kinetic energy to shrug off the first nukes, and make rocket engine based trajectory modifications pathetically inadequate, the massive nuclear explosion they finally use, with earths largest and most powerful weapon, breaks it into a few large and now very radiated pieces, some of which impact $\endgroup$ – Richie Frame Jan 29 at 1:10
  • $\begingroup$ @RichieFrame I think it would add improbability without any real change in the ultimate results. However, a solid, metallic asteroid would be more easily deflected, and it wouldn't fracture in too many pieces unless it was designed to do so, which would require to know much about the incoming nuclear device. It's doable, but looks overcomplicated. It would do as a plot device though - such an improbable impactor would have little to no chance of being natural. Someone out there smelted and refined a great many rocky asteroids into one single metallic planet killer. $\endgroup$ – LSerni Jan 30 at 7:44
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Nukes.

That is actually the plot of every Fallout game ever.

Russia and the US ha e enough nuclear bombs to strike the surfave of the whole world. For the classical atom bomb, with a ground-level burst, the area of impact will be dangwrously radioactive for one to five years. Some time later there will still be radiation, but won't be as dangerous.

If every city and town in the world is nuked, only people in bunkers would survive. They would come out to see nature having taken a major impact too, as the oceans and rivers will have taken a lot of radiation as well. Every ecossystem would be impacted, even if indirectly. The world will never be the same.

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    $\begingroup$ This has always been a consideration of mine, but for plot reasons it would be a little tough and or forced to pull off. This is definitely my go to if I cannot think of something more original or suited to my imagination/story. $\endgroup$ – halp Jan 27 at 21:32
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    $\begingroup$ @halp you could always use a gray goo scenario as well. $\endgroup$ – Renan Jan 27 at 21:34
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Here’s a solution attainable with minimal effort.

Just let your population continue normal behavior until global warming takes off significantly. then the change in weather patterns should increase both the number AND intensity of tropical storms until you reach a point of permanent “bad weather”.

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  • $\begingroup$ The weather change would also affect the oceans. E.g. the circular motion in the Atlantic. New Your would suddenly get less cold water from the north pole, while the European coast would not receive the warm tropical water. 10C -ish change near the coast might not mean much, but life in the ocean will certainly be upset with cascading effects. $\endgroup$ – Hennes Jan 28 at 16:06
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In a future world with advanced technologies, the government of a desert kingdom is experimenting with new climate technologies in the hope of terraforming the desert into arable lands.

Experimenting in these new technologies leads to catastrophic effects, pushing the global climate into a new state of much higher energy resulting in decades of destructive weather events. All efforts to adjust to the new norm prove unsatisfactory and after the global death toll reaches breaking-point, all those who survive retreat to their "bunkers" awaiting better days.

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Coronal Mass Ejection

I hope this isn't too technical, but a coronal mass ejection is when the sun spits sun stuff at the earth. They can be bad. One, the Carrington Event, happened in 1859 and took down parts of the first telegraph network--starting fires and shocking operators.

Robert Schoch theorizes that a big ejection event was responsible for ushering in the end of the last ice age. Schoch is a bit of a fringe scientist, but his ideas are fascinating. This page of his, describes how (he believes) a plasma ejection from the sun destroyed the advanced civilizations operating during the time of the last ice age. From that page:

Plasma hitting the surface of Earth could heat and fuse rock, incinerate flammable materials, melt ice caps, vaporize shallow bodies of water creating an extended deluge of rain, and send the climate into a warming spell. The release of pressure that follows the melting of thousands-of-meters-thick ice sheets can induce earthquakes and even cause hot rock under pressure to melt and erupt to the surface as volcanoes. The world was in chaos, and this is the event recorded by petroglyphs and the rongorongo texts.

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  • $\begingroup$ I'll check him out $\endgroup$ – halp Jan 28 at 17:36
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    $\begingroup$ Combine this with the inevitable reversal of the poles and the destabilization of the earths EM field, and this would really cause some serious damage $\endgroup$ – Richie Frame Jan 29 at 1:06
  • $\begingroup$ @halp This is more or less the plot behind the early Assassins Creed series, ending with ACIII. It even shows a probabilistic aftermath that resembles the hellish environment you want. youtube.com/watch?v=Zc_kPRDw0W0 (MAJOR SPOILERS ALERT!!!). Relevant bit at 11:00 $\endgroup$ – tfrascaroli Jan 29 at 8:52
  • $\begingroup$ "Plasma hitting the surface of Earth could heat and fuse rock" -- no, it cannot. Temperature is not enough to do that, you need heat, and to have heat you need density. This would never be a "coronal mass ejection", it would need to be a very dense coherent plasma projectile shot straight at Earth, massive enough to overcome deflection from the magnetosphere. There is no evidence that solar plasma can behave this way by itself (Ekhat ships in the Jao Empire series have the means to make it behave this way through electromagnetic confinement, to use it as a weapon). $\endgroup$ – LSerni Mar 19 at 6:43
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They thought they had the technology to deal with any impactor version 2.

They could deal with anything in their own star system. They couldn't deal with the intergalactic rogue moonlet coming at 1000 km/sec. The good news: It didn't hit them. The bad news: It hit their moon hard and retrograde.

The orbital space is full of debris from that event, still being nudged around by what's left of the moon that is now in an eccentric orbit that raises some very nasty tides at periapsis, coupled with the very frequent pieces of debris that come down.

Their defense system was very good, before it's ordinance was expended they were able to smash the bits that would have been cataclysmic if they came down but they didn't have enough to deal with it all.

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I would have the aliens release some kind of advanced explosives from a cloaked or otherwise hidden ship or ships, but they are not aimed at major cities or military outpost. Instead they are aimed at supervolcanos. Specifically Yellowstone in North America, Taupo in New Zealand, Tobo in Indonesia, Santorini in the Mediterranean, and the Pacana Caldera in Chile. The explosives should be detonated about 1/2 of a mile above the surface causing a shock wave that could in theory collapse the crust above the magma chambers causing almost immediate eruptions. It would need to be detonated in the air to ensure maximum downward pressure as a detonation at the surface would have a relatively small area of downward pressure.

The Pacana Caldera in Chile has not erupted in about 4 million years so you could have that one fail and thus giving your people a chance to figure out exactly what happened.

https://en.wikipedia.org/wiki/Supervolcano for the names and location of the volcano and https://en.wikipedia.org/wiki/Tunguska_event for information on airburst explosions.

Note, the Tunguska object exploded somewhere between 3 and 6 miles in the air so the damage is more spread out and less violent in scope than what a similar event happening only 1/2 a mile in the air would be.

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  • $\begingroup$ This could used as an extra measure. I may consider $\endgroup$ – halp Jan 28 at 18:31
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Shutdown of thermohaline circulation

Thermohaline circulation is the mechanism which moves ocean waters around the world. It takes warm waters to high latitudes and is what keeps the UK and France from freezing over.

It is also affected by global warming.

A shutdown or slowdown of the thermohaline circulation is an effect of global warming on a major ocean circulation.

Global warming could, via a shutdown of the thermohaline circulation, trigger cooling in the North Atlantic, Europe, and North America. This would particularly affect areas such as the British Isles, France and the Nordic countries, which are warmed by the North Atlantic drift. Major consequences, apart from regional cooling, could also include an increase in major floods and storms, a collapse of plankton stocks, warming or rainfall changes in the tropics or Alaska and Antarctica, more frequent and intense El Niño events due to associated shutdowns of the Kuroshio, Leeuwin, and East Australian Currents that are connected to the same thermohaline circulation as the Gulf Stream, or an oceanic anoxic event — oxygen (O2) below surface levels of the stagnant oceans becomes completely depleted — a probable cause of past mass extinction events.

The north would be cold for a few decades. When the oceans become stagnant, every ecossystem in the world will collapse. After the tundras and rainforests are all dead, the amount of methane in the atmosphere will increase ever faster and the climate on Earth will be venusian for millenia, maybe millions of years. You may adjust for lesser, less dramatic effects than this though.

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  • $\begingroup$ What mechanism could trigger said 'shutdown'? How is this accomplished, either by aliens or natural causes? I like the concept and potential, but what kind of event(s) could have such an impact? $\endgroup$ – Dalila Jan 30 at 18:30
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    $\begingroup$ @Dalila we are already going that way. All we need is for the average global temperature to go up another 3 or 4 Celsius and this doomsday scenario comes true. And policticians around the world are passing legislation that will lead towards that. $\endgroup$ – Renan Jan 30 at 18:46
  • $\begingroup$ I guess I meant a more specific and detailed chain of events. in the case of global warming being the initial trigger, what is the connection between average global air temperatures and a shut down of ocean currents? What are the steps in the progression from the first link in the chain, temperature, and the last link, currents? $\endgroup$ – Dalila Jan 30 at 22:06
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    $\begingroup$ @dalila just check the link in the post. $\endgroup$ – Renan Jan 30 at 23:32
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I would think that a civilization capable of redirecting asteroids could manage normal volcanic activity. You could have a close approach with a rogue plant passing through the solar system. Something as large as a planet could disrupt the orbit of your fictional world. It could also collide with a gas giant. Something as kinetic as that kind of impact could catalyze a thermonuclear reaction turning the gas giant into a second star that could burn indefinitely. Your world would experience rapid climate change and biosphere degradation forcing the people into subterranean bunkers, while still allowing them to travel with specialized equipment.

You could modulate the intensity and duration of the disaster by how large and close the gas giant was.

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  • $\begingroup$ Welcome to Worldbuilding, arsogio996! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun! $\endgroup$ – Gryphon - Reinstate Monica Jan 30 at 15:51

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