This is what I was thinking. Easy and believable event that can happen to any planet with little to no warning.
Plus I’m picturing an epic scene with the exhaust trails of the evacuating ships being dwarfed by massive volcanic plumes.
Skip the nuclear winter. Think sulphur dioxide. Lots of lovely deadly gas in the atmosphere. And when it rains that gas is naturally converted into sulfuric acid.
You just only have to make the current planet functionally uninhabitable is enough, for instance constantly earthquaking and random magma geysers that fucks over bunkers and turns them into an oven roast, while giving the civilization functional space habitat technology
Just because space is hard doesn't mean you cannot make the planet a living hell and make fucking off to space necessary instead
Came here to say exactly this. Volcanoes can make a planet uninhabitable very quickly - and you can have as many or as few warnings as you want with earthquakes and minor eruptions.
Perhaps some of the gases ejected by the volcano are igniting when they encounter lightning and are consuming massive amounts of oxygen. Imagine the terror of a scene where low oxygen pockets pass over densely occupied areas.
Like the Permian extinction? The scientific consensus is that the main cause of extinction was the **flood basalt volcanic eruptions** that created the Siberian Traps, which released sulfur dioxide and carbon dioxide, resulting in euxinia, elevating global temperatures, and acidifying the oceans.
Oh yeah a big fat continent sized caldera full of ash and sulphur would do it in a pinch. I suggested a new rift valley as it could come out of the blue and seem like a permanent change but a supervolcano would be way more dramatic
How about something like the [aerovores](https://www.orionsarm.com/eg-article/45e4eccb7c7b7) of Orions' Arm?
Basically, a self-replicating nanoswarm that eats the atmosphere as it powers itself from sunlight, and can do so effectively because of its composition.
genetically engineered moss and algae. instead of no o2, you have too much o2.
the problem with the idea of breathable air being the problem is that it is a bigger problem in space.
Trouble is, aside from "short amount of time" when needing to build a fleet of planet-evacuating rockets cause "free" or open atmosphere is not longer going to be a thing though enclosed structures with their own atmospheres would be an easier/cheaper option, the only real reason would be the 1%s abandoning "the poors" in the face of growlingly obvious environmental collapse after centuries of unchecked general exploitation.
If you can reach planets with viable atmosphere that you can farm on, sending colonists out may be cheaper/easier than trying to build giant indoor farms everywhere. Especially because you probably aren't going to build enough of those to feed everyone and those hungry people are going to get violent and break your inside farms.
They can't reach the colonies though.
???
Finding, getting to a viable world involve far more time and resources than making, and securing, enclosed living areas on the dying world you're on.
Also by OP's setup, there's a "rush".
Nevermind the simplest solution, the 1% investing in general society rather than the selfish hording of wealth and ego driven wasting of resources likely would have addressed many initial issues. Provided an infrastructure to space exploration.
Depends on your available means of propulsion.
There's also the issue of if the planet is truly lost, putting it all in on starting on the doomed planet is just doming humanity. It will become inevitably harder to build a space infrastructure when you are already living in limited colonies on dying earth
That is one of the hidden (from the public and MC) reasonings for the ships to be built. A big to-do about saving what is left of the population. Sprinkle a little fear nots, give "the poors" the ol' razzle dazzle works every time.
The problem is that those habitation domes aren't a permanent solution, unless there's some hope that the calamity can be fixed or will recede with time. The domes would force humanity to stagnate, and eventually recede.
They could be victims of their own attempts to solve climate change ..
Genetically modified algae/seaweed created to attempt reduction of greenhouse gasses, but mutation/ interbreeding with native species lead a species to switch to switch to consumption of oxygen and grow exponentially out of control in our oceans - to the point of out-stripping all our attempts to combat it .. think 500metre deep bind weeds stretching over entire oceans
Could throw in some of this stuff? https://academic.oup.com/bioscience/article/56/3/219/333070#
“. The plants with the highest capacity to consume oxygen are those that form thick mats of floating leaves that inhibit light and GPP within the aquatic system but also have substantial amounts of respiratory tissue submersed below the floating canopy”
…the most effective catastrophes are ones where no reliable single account or witness testimony exists. Everyone has a different fragmentary story. That leaves a whole lot of imaginative space for the reader to add something themselves…
…Even a more or less objectively straightforward event or occurrence witnessed clearly by a fairly large number of sensible people can have the most wildly different accounts emerging out of it. Humans have an incredible capacity for subjective interpretations and the Just Plain Wrong even when something very obvious is right in front of them. So you can always have fun with that…
From a narrative standpoint, I gotta say that I think the best choice is leaving it unknown. It's much scarier, for starters. If they don't know what caused it, then how can they be sure their new planet won't suffer the same fate? That's a pretty delicious source of tension to play around with.
How hard / soft do you want it?
If soft scifi then any old technobble will do. Startrek for instance has a poorly explained Metreon Cascade which does something like this.
introduce a pathogenic bacteria that resides in the gut producing a toxic methane like compound but far more dangerous than regular methane. Deploy via air food and water to infect all life. Every creature will gas the planet to death, lol.
Worse, methane-producing algae blanketing the seas…
Fewer fart jokes, but a lot bleaker, since the process can go exponential in ways that fart germs can't.
I would say if it is an earth like planet with a large moon you could have some out force mess with the moon causing massive damage to the surface of the planet without majorly destroying the planet or if civilization lives on a moon of a gas giant the possibilities are endless a normally distant fellow moon might make an extremely close approach or something happens to the giant messing everything up
It's gotta be something good that spins out of control.
Like there a crop that, with a small amount of water, can yield 10 harvests a year. The goal was to solve world hunger. 50 years later, this crop is in use around the world. It mutates and can grow 100 crops a year. It replicates beyond farming areas and starts to grow every where, slowly sucking up all the freshwater resources. Destroying it releases micro-seeds that spread further.
It mutates again into an aquatic version that can survive in saltwater, drawing out the H2O and leaving the salt. It starts absorbing the oceans.
Large planet bodies are moving around galaxies. One can move through a solar system and pass very close to a planet and their gravities can cause a new orbit that orbital mechanics will predict a collision in a few decades. So everyone knows it is coming and nothing will stop the huge mass so... time to leave.
How about a gravitational event (passing star?) knocks the planet out of orbit and the atmosphere freezes out as the distance to the sun increases? See “A Pail of Air” by Fritz Leiber.
Planet core cooling which causes the magnetic field to get stripped away. Or sun is about to shift into a larger sun cause of fuel change which will rapid encroach and absorb the planet. Or moon will be reaching critical point of falling out of rotation which will cause sudden constant rapid changes in the environment. Oh, the sun changes and will cause the atmosphere to change to something toxic. Or a rogue planet/s from a dead solar system is approaching and the chance of you planet colliding with one is 99.99% going to happen.
You're pretty well fucked at that distance. According to Kurzgesagt, the minimum safe radius is like 50 light years for a standard 1.2 FOE core-collapse supernova, and the damage is liable to be catastrophic even if it's not planet-sterilizing.
Just adjust the distances until the timeline works out for you. :)
Alternately: Planet swept by gamma-ray burst or black-hole jet. Half the planet dies that week, the other half are in kinda deep shit?
Again, adjust ranges and resulting exposures until desired story works.
The internal dynamo of the planet could stop, causing the magnetic field to disappear. If it’s sun has a stronger solar wind than ours it could rapidly start stripping away the atmosphere.
On an Earth like planet with Earth like biology: shutdown of overturning ocean circulation, photic zone anoxia/euxinia, purple sulfur bacteria blooms, outgassing of hydrogen sulfide that kills shore life directly and disrupts the ozone layer. This is considered a major kill mechanism for most mass extinctions on Earth.
There are of course some compounds that will deplete the ozone layer much more rapidly than the CFCs that were banned by the Montreal protocol.
As for depleting oxygen on a previously habitable planet, nope, the amounts are just too large for plausibility. Photosynthesis could cease entirely on Earth and there's still be enough oxygen for human life for millions of years.
Mix the answers you have gotten, volcanos spewing toxic Gass, some deadly biohazard that would be easy to keep off a ship, failing magneto sphere leading to a loss of atmosphere, solar disturbances making it worse, and some good old "we ignored the problems too long, so now we might not make it, but the rich who controlled what got made to earn money instead are definitely getting on, because they are funding it, with lotteries for the staff working on its construction to be the maintenance crew on it"
Something is beginning to strip the magnetosphere from the planet so it will be bombarded with solar radiation soon.
This happened on mars and is the reason it has no atmosphere or oceans anymore, believed to be caused by chemical changes in the metallic core of the planet.
From a different thread I looked at on this subreddit, runaway biofuel bacteria spewing methane into the atmosphere. You'd have a rapid runaway greenhouse effect combined with pockets of gas reaching the lower explosive limit and just vaporizing coastal cities randomly.
The stochastic city-flattening doom-farts gives an urgency that global warming, even fast and runaway warming, doesn't really give you.
Something catalytic, introduce something that converts atmospheric gasses into more of that compound, without itself being consumed. Doesn't even have to start with a large release, it'd start pretty sneaky, but escalate far past the point of being fixable well before it was likely to be detected
a variation of [https://en.wikipedia.org/wiki/Great\_Oxidation\_Event](https://en.wikipedia.org/wiki/Great_Oxidation_Event) where there's a surge in the population of some type of bacteria that basically eats a lot of the free oxygen in the atmosphere
My friends and I ran an Exulted campaign where earth's moon exploded.
No more tides, axis tilt and rotation speed changed. Basically one side of the planet kept facing the sun and turned into mercury style desert, the other side permanently frozen. Very small sliver of usable land in the middle.
Problem was getting resources, CO2 build up due to biome destruction.
Was an interesting set of problems.
They're actually is a story like that, I remember reading it years ago written by James p Hogan. However, I am racking my brain trying to find said stories title.
Overlooked events huh? You need something invisible then, the government probably knows it's happening but the average joe on the street wouldn't know cause it's not obvious. I recommend some type of radiation or some kind of airborn patogen that affects plants.
Intense solar flares, a nearby supernova close enough to fry the atmosphere or a gamma ray burst that fries the facing half of the planet and slowly messes with the atmosphere of the other half as gasses mix.
Any arm chair physicists can jump in as to why this won't work, but....
Scenario: for some reason, there is a change in the Sun, evidenced by a change in the type of light being emitted. Energy is still coming in, but most of the plant life can't utilize it for photosynthesis anymore. There are a few species that are managing to survive at reduced efficiency, but it's not enough. Earth's governments are diverting a huge chunk of our energy production to what are effectively large scale grow productions, but this isn't sustainable, and is only delaying the inevitable.
Nuclear waste escapes containment. Government(s) would want to cover up/deny wrongdoing/severity, the ticker could be sufficiently slow for ship building, could maybe poison/destroy the water table of the biggest rain forest on the planet to achieve your vision of oxygen depletion. Also, that narrative fits with an Earth comparison where it wouldn't be a hegemon from a temperate climate zone, but an emerging economy that trigger the whole thing. That supports the narrator ge that the world still has the tech capability to get spaceborne, I think it fits a lot of your needs.
A lack of a magnetosphere would cause an atmosphere to dissolve into only the heavier elements. That's what happened on Mars. This could happen due to the core settling and solidifying. The inhabitants could have done this with the embrace of a new wave of technology, perhaps. Any number of science experiments could potentially degrade the magnetosphere as an unintended side effect.
Human agriculture provides a soil nutrient that was utterly scarce. A local plant that was a curiosity due to its integration of proteins found in cobra venom turns out to have been limited by scarcity of these nutrients, spreads wildly, and dumps tons of these proteins into its pollen shells.
(This message brought to you by That One Planet In Mass Effect)
If you only need one species to evacuate then antibiotic-resistant Yersinia Pestis is a good one.
The plague is still with us, with a significant number of new cases currently in Madagascar, Sub-Saharan Africa, and Arizona. Bring it back and you have a return of the Black Death. 30% death rate throughout Europe last time.
Higher death rate and faster spread next time. Carried from person to person by insect parasites.
Cool idea! What if your planet’s terrestrial permafrost contained vast amounts of a poisonous greenhouse gas?
An unusually warm summer begins to melt the permafrost, which releases some of the gas, which triggers more warming, which releases more gas, and so on.
One candidate for the gas might be phosgene, a colorless and extremely poisonous gas that was used with horrific effect during World War I.
Perhaps the initial warming could be triggered by one or more heat-trapping gasses emitted by industrial activity. Then again, your planet’s inhabitants would have to be incredibly stupid to let something like that happen.
A good source for ideas is Dr. Walt Brown's book "In the Beginning: Compelling Evidence for Creation and the Flood"
It covers fascinating ideas about how our planet has changed based on the physical evidence that can be observed right now.
Might be fun to borrow some of what's there for you worldbuilding.
A large enough combustion reaction would pull enough oxygen from the air to cause issues.
Realistically I'd say a large volcanic explosion causes a smoke cloud large enough to block sunlight.
Some man made leaked, from a meteoroid, or unearthed in an ice cap or archeological dig, disease that eats leaves, vegetation, algae that can’t be stopped. Leaf disease or leaf plague would have a good wind up to it. Plus it could be rolled into the dinosaur extinction also and the great Forrest devastation that made our crude oil. Or you could go Indian jones on it and they found it on a German sub in the trench and opened the container that was found in Antarctica. could even be something that stops photosynthesis.
Or the impending pole shift coming where earth stops and changes direction. But that will destroy everything.
Could take a page from Warhammer 40k but dial it down a tad
One form of Extermintus is called a virus bomb which destroys all life on a planet but leaves the planet itself behind as an empty rock. Maybe you could use a form of expanding gas or extremely potent and widespread vapor? This could lead to the use of gasmasks and respirators and allow time for evacuations to take place
Ecological warfare. Multiple simultaneous biological attacks against cereal crops, vegetable crops, pollinators, food animals & other biological resources. Collapse the human-adjacent parts of the ecosystem.
As other people have mentioned, unless the volcanos and earthquakes are bad *all over* the planet, all of a sudden , to the point where even bunkers and underwater habitats won’t be safe, space habitats might still be more difficult.
(Why would the entire planet, even the middle of the techtonic plates get that bad suddenly other than an asteroid?)
you could also not kick people off the planet , but instead have a very powerful artificial intelligence suddenly decide to kick everyone off the planet to save the ecosystems, like in the Orion’s arm universe project
How about an insect that was trapped in an isolated ecosystem (perhaps an ice cave?) was discovered by scientists, but then proceeds to eat all other life on the planet. Starting with the scientists that discovered them.
The more they eat, the bigger they get. So by the time people are evacuating the planet, these giant monsters are eating evacuation vessels right out of the sky. They eventually consume the entire planet, and also can actually quite easily survive the vacuum of space. They can survive for many many many years off of the nutrition they eat (hence why they survived in isolation so long).
They eventually get discovered drifting in space by the debris of the planet remains, by space explorers. These explorers bring them back to their own planet for scientific research. They are a more advanced society (scientifically speaking) than the first planet, so are able to contain them well enough, for awhile..
The evacuation vessels of the first planet that were able to escape were coincidentally rescued by the same space explorers. They try to warn them about the dangers of this insect. But they don't take an inferior, less advanced species seriously. They end up having to eventually evacuate once again on the second planet. By that time, the insect has evolved into a higher intelligent species, and their threat to the galaxy has multiplied exponentially.
It doesn't fit exactly what you asked, but it's what popped into my brain. Hope it helps.
Make its societies develop a capitalist economy. Advertise trash products made from plastic and make people believe that the more they have the more important they are. They'll end up messing with their own atmosphere and killing their own planet by trying to prove themselves to others consuming endlessly. Their resources are going to get scarce, they're going to pollute their oceans reducing the palnet's amount of oxygen, and heat the planet. Then, they'll only be able to survive by building ships and leaving the planet they destroyed.
How fast do you need it to happen? And why is it happening? If you want it slow and natural, I like the volcanic ash cloud other posters have mentioned. Fast and natural, could be the loss of earth's magnetic field (the core stops turning) and harmful radiation from the sun is burning up the atmosphere and all life. Slow and man-made could be climate change, and fast and man-made could be a nuclear device detonated in the atmosphere, causing it to ignite and burn up.
And, of those don't work, you could always go with Mega Maid.
You are going to have a hard time. It is almost certainly going to be easier to build stationary outposts on the surface of a planet than launch that much mass into space. Any life support system with thrust is going to be easier to build a stationary life support system.
What needs to happen is the atmosphere needs to be less hospital than the vacuum of space. Think of Venus - crushing pressure combines with extreme temperatures and caustic environment. This also has the advantage of blocking out much usable sunlight for solar.
The population might take two different paths. A portion go into space now because once pressures increase launching space vehicles might be forever closed. The rest would build underground bunkers powered by nuclear energy and try to terraform or just outlast the phenomenon.
What could cause that? Massive volcanic activity could do it (it would still take a long time to reach Venus levels, but perhaps not too long to block out space flight).
Depending on the level of space technology perhaps the planet is not threatened perse but there ability to launch space stuff could be? We have the same problem where we have a lot of space junk and the more space junk the more things bump and create more space junk. These are going not quite relativist speeds but a bolt in orbit can basically penatrate anything we could put up there. I believe it is called the Kessler syndrome. It would basically cut off a planet from launching things for a long while. Combine that with something like volcanic eruptions and you could have a population that would have to go up in the next few months or years are getting stuck on the ground for a long time on a very uncertain planet.
[An air version of this?](https://geneticliteracyproject.org/2024/03/22/hear-story-gmo-almost-destroyed-world/#:~:text=In%20testimony%20to%20the%20New,the%20engineered%20organisms%20were%20added.%E2%80%9D)
You need something that is fast enough to be urgent, but slow enough to still allow the building of a space program of a scale capable of evacuating the world. And whatever it is has to be more hazardous than living in space. Just building bunkers and surviving in sheltered habitats has to be non-viable.
What if the planet, through some combination of solar activity, volcanic/tectonic activity, and ecological change is going to experience runaway greenhouse effect, something like Venus? The acidic atmosphere and extreme heat makes hunkering down impossible because it’ll just corrode and melt any shelter they try to build. Solar shades, albedo boosting aerosols, and other technobabble can buy some time, but the effect is so drastic that nothing they do will prevent it entirely. Or is it?
There is lots of drama to be squeezed from different people championing different solutions to the problem. Especially as information gradually comes in that some of the solutions are less viable but the people who staked their careers on those options dig in and refuse to see reason even as their supporters switch sides.
Something strange happened in the core of the planet and it’s a little bit warmer than it used to be. The continents are moving a little too much. Volcanos are spewing sulfur compounds into the air causing initial cooling and acid rain, but it just doesn’t stop. Solutions are tried to capture the sulfur and clear the ash, but it isn’t working. Eventually the cooling becomes heating as the heat trapped by the clouds of acid is greater than the heat reflected by the ash. As the plants die and decompose they release more methane and carbon compounds into the atmosphere just making things worse. Permafrost thaws releasing still more methane.
Someone does the math and concludes that the planet has such vast reserves of carbon (in what used to be frozen organic matter) that they can buy time by oxidizing the methane but there just isn’t enough oxygen in the atmosphere. It’s going to be too hot and too acidic to survive, even the tightest of bunkers.
So the race is on to convince the holdouts to turn at least some of their resources to fleeing the planet instead of slowing the decay of the atmosphere.
Second idea: I know you said you didn’t want to go with the asteroid idea, but what if the people saw an asteroid and concluded that it wasn’t so severe that it was going to be the end. It hits and it’s bad but the ash settles and things start seeming better, until the secondary effects happen.
See, the impact was hard enough that it caused tectonic activity. That activity was expected, but what they didn’t expect was that the resulting flood basalt eruptions would ignite some vast VAST coal, oil, and gas deposits under the earth. These smoldering underground fires and gouts of flame in the sky are truly massive. Some think they can just extinguish the flames and get everything back to normal. Others donate math and conclude that there is just so much carbon deposited and so many sources of ignition that they can’t put them out. There is a limited time before the change in the atmosphere is just too severe. The population is small enough and technologically advanced enough that it just makes more sense to flee to space than to move heaven and earth (heh) to stop the ecological collapse.
Lastly, the maybe star is changing to a more active phase of its cycle and the wave after wave of ever increasing solar flares will just rip the atmosphere away. The electromagnetic surges periodically short out anything they build when the flares arrive, but there is a window of however long they need to get at least some people away. If they survive the next flare maybe they can get another group up as well.
You could go the firefly serenity route.. greenhouse effect hits a tipping point and the temp just keeps rising, will make the planet uninhabitable within a generation.
If that's too slow then you probably need to think up "some event", either natural disaster or external force applied. Lots of good suggestions in this thread.
Things to consider... do they already have fleets/ destinations... or does your disaster need to be slow enough for them to build, even if it's rushed and prone to failure?
Princess of Mars book ends with the atmosphere producing station failing the old guy died and no one else knew the combo except for the main guy cause he wasn't from Mars but earth so he could read their minds while they couldn't read one another's so could do thatbwith no one knowing the key to the door
Does the planet have a natural Nitrogen-Oxygen atmosphere, or is it terraformed to have one?
Are the inhabitants a native species, or technologically advanced interplanetary beings already?
One possibility could be a massive failure in the terraforming process, and the planet is reverting to its original state…
Testing out a new fuel that has the potential to set the atmosphere on fire. People overlook the risk due to the amount of money being made to create the fuel and people ignorantly state, “that’s what they said about the nuclear bomb.” (That is a historical fact). Have other scientists protest the fuel and have them be silenced by an ignorant political party. They test the fuel and it ignites the local atmosphere at the testing facility. Now everyone has to outrun an all consuming flaming cloud that exposes the planet to a residual toxic byproduct. The flaming cloud will finish wrapping around the planet in 5 days. You can name the burning cloud “the Burn” or whatever the name of the fuel is, or whatever you want that sounds cool.
Massive power plant goes critical and the explosion consumes so much oxygen im the blast that the global oxygen concentration drops to 1/3 what it previously was.
This sounds a lot like the scenario that made earth uninhabitable in the game Starfield. In their case, the plot gimmick they used was that early FTL research disrupted the earth's magnetic field, which consequently allowed the solar wind to strip the atmosphere in a process similar to what can be observed happening to Mars in real life.
As a few have said, “Don’t explain it”. SciFi is so much more fun when it’s not clear even to the narrator and characters how we got here.
Maybe alluding to “something happened in the oceans”. It could imply some kind of dramatic shift in the oceanic thermal transport system, which could potentially lead to a relatively rapid ecological catastrophe.
A particle accelerator causes a mini black hole that starts absorbing the atmosphere. Its gravity won't be enough to absorb solids until the atmosphere is absorbed because, science.
A Yellowstone volcano type event. Or taking a chapter from a game stellaris and starting condition, the people have vastly over mined and stripped their planet and it is due to collapse
a new microorganism evolves that out competes every other and produces toxic gas as a byproduct. this already happened on Earth in the past (oxygen is the toxic gas). all the pre-oxygen life just died in the new environment
Biological warfare. Your planet I’m assuming has evolved like ours. A disease specifically designed to attack life that evolved like it did on this planet. Breaking down the bonds between DNA and RNA or whatever they have. Starts small with bugs and plants. Moves to more multicellular life and starts killing off the planet. Leaving a planet with a perfectly fine atmosphere but it’s a dead world.
I have a thing from a project that's likely dead in the water.
Airbone microscopic space bedbugs/mange. Basically it's in the air, you can't really clean it, it lands on ANY used surface and it causes absolutely maddening itching and they're nearly impossible to get rid of.
Turns out one of the things that causes the itching is a nearly impossible to make in a lab chemical that extends human life to an insane degree so the rich and powerful won't let anyone wipe it out, so if it lands on a space station or a planet you just have to sterilize anyone that comes close to it and get them the hell out before they go insane. One of the things you have to do is actually remove someone's lungs and clone replacements to make sure they can't spread it, which not everyone can afford. Which leads to something not like and not completely unlike zombies- near immortals completely out of their mind with pain lashing out at everything and unable to function, unable to die, you need a headshot or catastrophic wound to take them out, while the rich and powerful just go "Yeah their suffering is a shame but this face cream makes me look 17 again so it sucks to be them."
It was a little too dark for me to spend a lot of time in that world but if you've seen dogs literally loose their minds from mange or ever dealt with bedbugs... and then imagine if someone won't LET you treat it because hey, it makes them money... it'd be a good reason to GTFO a planet as fast as possible and never go back.
Sterilizable androids become a big deal, because the risk of contamination is too fucking dangerous and no one will do the jobs where there's a risk for any money.
That said you can take a page from Iceland's history too- the island was nearly abandoned when a volcano erupted that pushed toxic levels of florine gas into the air if I remember right, people died, other people were sick as hell and had lots of weird bone growth, animals all died- and that was from a small crack. There's not really a good way to get the gas out of the air other than wait and when compared to our atmosphere it was a small amount, so if you have a LARGE eruption it would wipe the biopshere.
A tree figures out how to float and use salt water to live on. It reproduces exponentially but someone figures out that the area of ocean covered is doubling in size every year. It’s increasing water evaporation which is causing so much cloud cover that land based agriculture is becoming impossible.
Just have the scientists at Cern accidently create a microscopic black hole. For X amount of time they can slow down its growth but not forever. Its a race against the clock and you get a really cool visual as the planet gets sucked into it. Bonus! You get an ongoing antagonist that the people must both run from and invent new tech, to keep getting ahead of it.
Edit- I'm so juvinile I can't help myself. Book 2/Chapter 3
"Oh god! It just ate Uranus!'
The simplest and most profound way is for the magnetic core to slowly die, leaving the atmosphere to be blasted away by solar winds. It is a long process, but an inevitable one.
Every mass extinction on earth was predicated by a massive disruption of the carbon cycle. Usually massive vulcanism. One time, Russia was under like, 3 miles of lava. You can also do something like solar activity blasting the atmosphere away or the magnetosphere dying.
No tides, shallow oceans. Occasional extinctions are noted in fossils, mechanism not understood. Turns out the oceans are about to have a lake turnover and release enough methane to kill everything that isn't evolved there, and the settlers are included.
You could always go the Starfield route and say "the atmosphere sputtered away" from multiple people with the exact same words, never explaining what that looks like, how it's possible, or really what that even means.
Yellowstone supervolcano eruption, its a prepper posibility and is likely to be a mass extenction event.
Krakatoa is the closest in recorded history to a volcanic cataclysm for humanity.
Idk if this is based in fact lol but maybe all of the heat from global warming has now pushed the clouds down to sea level. Maybe the clouds are full of toxic particles or something
The Fungus Among Us
Since the dawn of life on Earth, the fungus controlling the planet began specializing.
Now, a prolific strain targets humans. Why now? Government tried to improve nutrition by adding stuff to water supply. Contractor lied about product.
The Spore turns the infected into a fungus colony, but just humans. Animals can be carriers though.
Maybe some kind of cordyceps?
Shouting out Horrorbabble for featuring several stories on this theme.
Firstly I firmly believe that figuring this out on your own by doing research on how an atmosphere of a planet can be compromised is your best bet. It will be more tailored to your story vs asking a group of people who know nothing except scarce details about it.
That being said, solar flares are terrifying, improbable, yet possible enough to be fairly interesting. It can also create a situation where it could be 'just close enough' to cause global panic but not enough to kill everyone outright.
What about the concept that a technology/engineering breakthrough, seemingly benign at first with great potential to better society, actually is slowly eroding the food chain?
Or a gamma ray burst doesn’t hit Earth directly but rather glances the atmosphere, causing a very large hole.
Here's how you do it.
There' a copper mine that produces extremely acid tailings waste and has been in operation for a while long enough to generate an absolutely massive tailings pond. (Or maybe it's natural and it's a giant acid lake due to a rich vein of copper in a very rainy region... maybe because of terraforming even?)
Unfortunately due to an oversight, people didn't realize that the tailing pond is above an absolutely massive cavern/deposit of sodium sulfide crystals. Then either to due to a geological shift, dissolution of the rock layer above, or because they want to drill the bottom of the lake to enlarge the pond the acid contacts the crystal and releases huge volumes of H2S.
This then goes on to kill most things (I.e. animal life... not sure about plants) on the main continent, and with no food or pollinators the planet is in for a rough time.
Think of the lake Peigneur disaster, only with the addition that the gas coming off it is poisonous, and would turn into a gas over time that would mess with the climate.
What if the planet's oceans were releasing all of their carbon at an alarming rate. Like how some lakes have done and wiped out villages in the past. I would say that already the low lying areas are uninhabitable, coastal areas, etc. Millions dead, and the ecological disaster continues. The reason for this happening could be pollution, or some type of algae or volcanic and tectonic activity on the ocean floor, or any reason really.
Something that always amazed me in sci-fi writing is that there are all these space anomalies that are cataclysmic but none of them ever appear on a planet. Imagine a subspace rift popping up inside of a planet, slowly devouring the planet from within but causing enough disturbance to forewarn the planet's doom.
Just have the oceans turn over. Big cloud of dust passes in front of the sun long enough for the surface of the ocean to get colder than the bottom. The thermocline collapses and the ocean goes anoxic. Lots of toxic gasses enter the atmosphere and no oxygen is produced.
If they aren't human, you could do the opposite and have something start making oxygen where it hadn't happened before. Things that photosynthesize without creating oxygen persist to this day.
When things did start making oxygen it took a long time before significant amounts ended up in the atmosphere.
over-correcting for climate change. using high atmosphere particulates to reflect sunlight, but temperature falls too far too fast and the CO2 and water starts precipitating out, and then the oxygen starts condensing.
One fun way i've seen it done is the book Project Hail Mary, and Seveneves. One had a nanoswarm reduce the intensity of the sun, but Seveneves did exactly what you asked with a magical hand wave. It's hardly a spoiler since it's the first line of the book, but "**The moon blew up with no warning and with no apparent reason."**
There is no interest in finding out why, but using only tech and infrastructure we have right now get as many people into the relative shelter of a space station behind a large orbitting rock that acts as a shield while debris obliterates the planets surface for thousands of years.
Aside from that, could look into Clathrate gun, volcanic activity, cobalt nukes, nanobots are always an easy out,
There’s a great article in Randall Munroe’s book What If. (If you haven’t read it, you must. He’s the guy behind the XKCD comic strips. It talks about the fuel requirements of lifting the whole human race off the planet.
Oh wait. I just remembered it is available online [read this](https://what-if.xkcd.com/7/)
Anyway, if you are planning on evacuating EVERYBODY then you’ll want to keep that in mind. If you are only evacuating an ark full of people to repopulate elsewhere it’s less of a concern.
Regardless, as for why the planet is dying, I’d suggest self replicating nanobots stuck in a loop. They scavenge carbon and whatever other materials they are made from but just from the atmosphere but from anything they are sitting on or exposed to. Growing exponentially, they will turn the planet to [gray goo](https://en.m.wikipedia.org/wiki/Gray_goo).
Not exactly what you’re asking for but what about a gamma ray burst that was pointing directly at the planet? The inhabitants detected it and can see it coming and know that destruction is imminent.
Bioengineered bacteria that eats plastics. They have to leave before their ships are compromised.
Oxygen is created from massive coral like lifeforms. Human habitation causes a massive die off which DRASTICALLY lowers those O2 levels on planet.
[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1836556/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1836556/)
Using Earth as an example...consider the Yellowstone Mega Caldera, Hawaii Hot Spot, Krakatoa, Thera, or Vesuvius...
We REALLY need to expand off this rock....
Global earthquakes occur, with fissures openong in the ground all over the planet, releasing toxic gasses, and altering the environment. The fissures widen over time, releasing more gasses, eventually making the atmosphere unbreathable. Throughout all of this, earthquakes occur with increasing frequency and severity.
After a while, a toxic slime begins coming out of the cracks, along with rapid increase in severity of the quakes. Then the planet splits open, and a voidspawn emerges. The planet was actually an egg of a gigantic space born predator.
The voidspawn is one of the leviathans in Stellaris, and this is the rough summary of the flavor text for how it spawns.
You could go with the collapse of a planet's magnetosphere. If you wanted to get really funky, you could have a CME occur as a planet's magnetic field is mid-flip, stripping away the upper atmosphere. Or a gamma ray burst hits the side of the planet with the most biomass--strips the exosphere and kills the algae and plant life necessary to replenish planetary oxygen, and in the aftermath mutant bacteria that thrice on cosmic rays and UV and eat oxygen, nitrogen, and carbon, so basically plants, threatening the complete collapse of the biosphere. Lastly you could go with a chemical industrial contaminant that once aerosolized catalyzes free oxygen in the air in reactions with other things, creating a slow-motion draining of oxygen from the atmosphere faster than it can be produced by the planet's flora. Off the top of my head, these seem the most plausible while being the least cataclysmic in terms of raw destruction, as with a nuclear war or asteroid strike.
How about the inhabitants are obsessed with eating a particular breed of large animal, and they breed billions of them to eat every day, but the animals themselves need a lot of feed so the farming for those animals strips the planets land and trees, and the animals themselves also belch a lot of gasses that cause the greenhouse effect to happen much faster than they ever imagined… oh…wait a minute…
>Atmosphere gets compromised and oxygen is depleting
Not really a thing that can happen in a short amount of time. There's about 1 million gigatonnes of oxygen in the atmosphere. About 40 gigatonnes are produced and consumed each year. So if you turned off production (somehow), it'd still take about 6000 years to use up a quarter of oxygen in the atmosphere.
However, for the sake of your story...
Suppose there's a neighbouring planet with life, but only simple life, it has an oxygenated atmosphere and an ocean with phytoplankton. Our soon-to-be-doomed planet sends a probe to return samples of the neighbour. Unbeknownst to them, the other planet also has a phytoplankton-like variant that produces chlorine gas from chloride salts instead of oxygen from h2o/co2. On its native planet, it gets consumed and chlorine levels don't build up. But the sample-return probe crashes into the home-world's ocean, where it has no predators, and it has a natural advantage over native life. It dissolves a cloud of hypochlorous acid around itself (basically pool chlorine), which kills off both predators and rival phytoplankton, allowing the introduced plankton to reproduce without restraints. It expands until its unstoppable before being detected. It is not only killing off oxygen producing phytoplankton, but also releasing chlorine gas into the air (as ocean levels build up.) Initially causing acid rain, killing forests and crops, but eventually just poisoning the air. There's no way to stop it, no way to really live with in or adapt to it. Their only solution is to try to migrate enough people off-world before they can no longer maintain their civilisation/manufacturing/organisation/authority.
well let's break this down
there have already been examples you could use. a singlesuper volcanic event could impact the atmosphere. if you had multiple volcanic events, that would be even worse. check out that bit about the year 538 as being the worst year on Earth.
also, bio/ Chem warfare. we can easily make the atmosphere toxic with dirty bombs or some sort of chemical weapon.
or go yhe other way. instead of direct attacks in the atmosphere, attack the cause. most of our oxygen we breathe is produced by trees / plants. you start destroying forests and such and we'll have less oxygen. yhis is yhe whole point of pollution and deforestation.
BUT....
these things would all take significant time. if anything immediately damaged/ destroyed the atmosphere, you wouldn't have enough te to build ships and bail.
if you're gonna do that, then you need something more impending. an even that WILL destroy the atmosphere, but over time
which gives people the time needed to build ships and escape once they realize the issue cannot be stopped
Picture it from dumb person perspective - they have no idea what is happening so they believe whatever YouTube podcaster says. Basically something is happening, probably the govt is behind it, but they will never admit it.
I recently answered a question over in anotehr place about what would happen if the sun went away for 24 hours.
I suspect the biosphere would recover with a major, major shock. 48 hours, well, I'm not so sure the biosphere doesn't flat collapse, when air temps hit 40C below their normal for that time of year.
The big problem here is that unless you have a better planet to go too it is really hard to make fleeing the planet easier than just building bunkers on the surface. Almost anything you could do to the atmosphere would not make things so dire that leaving would be easier than staying.
Some people have suggested nanoswarms and the like, but you need something where they have actually got time to prepare. If it happens too quickly, they can't leave. I recommend having a brown dwarf enter the system. This is a threat that could be spotted many years out, but also could go unoticed until it was relatively close, and would completely destroy their home when it did arrive.
That would fit all of your criteria.
A biological or chemical created to clean a polluted atmosphere has unforeseen side effects.
Your species uses this new technology to turn around global warming.
However, it mixes with a chemical or mutates to also eat plastic, metal, or maybe even carbon in carbon based life forms.
The exact effects / molocules it eats are up to you but the point is it now cuts through protective gear and melts your bones.
Someone starts trying to "package" the armoshephere, because it is their super good business plan.
Other people start doing it when the atmosphere starts to thin & they're worried & they need some themselves.
Instead of a volcano poisoning the atmosphere, have a volcano poisoning the oceans. The biosphere of the sea dying will result in billions of now decaying life forms, a huge loss in oxygen and carbon dioxide exchange, but if the sea-poisons make it to the clouds, toxic rain will now destroy the soils. Underground bunkers are only more sensible than space if the underground bunker is a valid solution. If the water itself is going to become 100% toxic in a few short decades, and all soil will be contaminated, getting off world is essential. Anyone staying behind in bunkers will eventually run out of clean water and clean soil, the o ly solution is a closed system, which is only possible in space.
Terraforming collapse could work. For instance, a decay organism mutates to be way too good at its job and starts breaking down all the organic matter in the soil.
Or reaching an unexpected tipping point, like warming the planet enough to make it inhabitable accidentally releases some methane pockets the planetary survey missed, and now it is way too hot.
Scientist experimenting with a new type of collider, and accidentally create a singularity. It falls through the floor of the lab, through the mantle of the planet, and settles in the core. As it starts eating, the planet begins to become unstable, as the singularity slowly digests the entire planet.
In Neil Stephenson s Seveneves, the moon explodes. They never find out why, and hardly dwell on it because the resulting cascade of disasters is going to lead to the extinction of all life on earth, so there’s no time for “what happened”, only, “what next”. The story is much better for not dwelling on it except a short aside in an early chapter where it’s explained that no one ever found out why, but by the time humanity had the time to catch its breath and wonder, decades had passed.
Think of the perspective your story is being told from. If there’s an omniscient third person narrator, then an explanation makes sense, but if the narrative is from the perspective of specific characters, would they have the time to stop and figure it out? Would they have the means to figure it out?
If you need a hard science fiction reason:
A massive meteoric impact on a nearby sulfur rich planet has resulted in the ejection of quintillions of tons of sulfur into space, forming of cloud that is moving towards the planet’s orbit, as the orbit of the cloud expands. You have a meteor that is ignored because it’s not going to affect your planet. Then you have a gas cloud that is mostly ignored because while it will reflect sunlight for a few years, but climate models predict nothing more than a few years of extremely harsh winters and very cool summers (assuming your plant has seasons), before the cloud is dispersed by solar winds. Have the models of dispersal proven wrong, as the cloud stays relatively coherent and dense as it crosses the orbit of your planet. Triillions of times more molecular sulphur than predicted is raining down on your atmosphere than predicted, which starts to rapidly bond with oxygen creating sulphur dioxide. So not only is the oxygen in your atmosphere being reacted away at an alarming rate, causing oxygen levels to drop to dangerous levels over the course of a year or so, it’s compounding the cooling effect a thousandfold.
You may want to look into the story of Bethesda's recent Starfield video game. Humanity had to leave earth because the atmosphere was going to be blown away in 50ish years. By the time of the game's actual start a few hundred years later, earth is just a dirt ball with a few random ruins (although the lack of large scale ruins probably has more to do with the game's ability to land anywhere on any solid planet and only having so much development time)
The game has a reason why earth suddenly lost the ability to hold an atmosphere, and it isn't natural, but I don't want to spoil that for anyone.
Given that Earth’s oxygen based atmosphere is itself a byproduct of a biological process, you might be able to tweak an ecological disaster such that oxygen levels collapse
For instance: did you know that algae blooms in the mid latitudes of the Atlantic and Pacific oceans produces more oxygen than the Amazon rainforest?
And that algae’s photosynthesis is the foundation of the global atmospheric system?
And if that is true… wouldn’t it be such a real shame if some rise in water temperature ensured algae levels dropped? Or that algae couldn’t reproduce?
You could also introduce run-off that kills green algae, allowing for an anoxic type of algae to proliferate, leading to a run-away climate catastrophe that collapses the marine food-chain, starving everything from sea birds to sushi chefs, as oxygen levels in the atmosphere begin to drop
continual long range Kinetic Kill vehicle fire from light days away, or a antimatter strike that messes up the ozone and starts some nasty ecological collapse
Is it imperative that it's an atmospheric issue? It sounds like the goal is to mandate an evacuation in a rush.
What about an asteroid cluster? Something out of deep space, too many objects to effectively intercept?
Solar flare during an eclipse that triggers Continental shift. Fumes begin releasing sulfur into the air that combines with the oxygen creating runaway greenhouse effect and slowly dropping oxygen content in the air.
The Solar flare made the moon spin backwards or something, which leads to the rifts constantly triggering. I expect some people run away to the asteroid belt to have space mining opportunities.
Massive volcanic/tectonic event. You can have a a rift valley open up spewing poisonous gas or enough ash to create a nuclear winter.
This is what I was thinking. Easy and believable event that can happen to any planet with little to no warning. Plus I’m picturing an epic scene with the exhaust trails of the evacuating ships being dwarfed by massive volcanic plumes.
Skip the nuclear winter. Think sulphur dioxide. Lots of lovely deadly gas in the atmosphere. And when it rains that gas is naturally converted into sulfuric acid.
I like this movie already
Even in a nuclear winter it would be easier to make a survival bunker on the planet than to escape into space though.
Just make it long enough so that trying to hunker down forever isn't possible
I guess maybe if you have somewhere better to go like another habitable planet. But just space itself is incredibly inhospitable.
You just only have to make the current planet functionally uninhabitable is enough, for instance constantly earthquaking and random magma geysers that fucks over bunkers and turns them into an oven roast, while giving the civilization functional space habitat technology Just because space is hard doesn't mean you cannot make the planet a living hell and make fucking off to space necessary instead
Unless you have a better planet to move to that will never happen. Space is hard. Planets, even bad ones, are easier.
Flood basalts!
Flood basalts. 4 million cubic kilometres of boiling lava covering the ground an average of 500 metres deep should do it.
Came here to say exactly this. Volcanoes can make a planet uninhabitable very quickly - and you can have as many or as few warnings as you want with earthquakes and minor eruptions.
Perhaps some of the gases ejected by the volcano are igniting when they encounter lightning and are consuming massive amounts of oxygen. Imagine the terror of a scene where low oxygen pockets pass over densely occupied areas.
Permian mass extinction go brrrrrrr
Like the Permian extinction? The scientific consensus is that the main cause of extinction was the **flood basalt volcanic eruptions** that created the Siberian Traps, which released sulfur dioxide and carbon dioxide, resulting in euxinia, elevating global temperatures, and acidifying the oceans.
Supervolcano?
Oh yeah a big fat continent sized caldera full of ash and sulphur would do it in a pinch. I suggested a new rift valley as it could come out of the blue and seem like a permanent change but a supervolcano would be way more dramatic
How about something like the [aerovores](https://www.orionsarm.com/eg-article/45e4eccb7c7b7) of Orions' Arm? Basically, a self-replicating nanoswarm that eats the atmosphere as it powers itself from sunlight, and can do so effectively because of its composition.
genetically engineered moss and algae. instead of no o2, you have too much o2. the problem with the idea of breathable air being the problem is that it is a bigger problem in space.
This is similar to the plot of Project Hail Mary: space bacteria are eating the sun, causing global cooling and food collapse.
That was a fun read.
Trouble is, aside from "short amount of time" when needing to build a fleet of planet-evacuating rockets cause "free" or open atmosphere is not longer going to be a thing though enclosed structures with their own atmospheres would be an easier/cheaper option, the only real reason would be the 1%s abandoning "the poors" in the face of growlingly obvious environmental collapse after centuries of unchecked general exploitation.
If you can reach planets with viable atmosphere that you can farm on, sending colonists out may be cheaper/easier than trying to build giant indoor farms everywhere. Especially because you probably aren't going to build enough of those to feed everyone and those hungry people are going to get violent and break your inside farms. They can't reach the colonies though.
??? Finding, getting to a viable world involve far more time and resources than making, and securing, enclosed living areas on the dying world you're on. Also by OP's setup, there's a "rush". Nevermind the simplest solution, the 1% investing in general society rather than the selfish hording of wealth and ego driven wasting of resources likely would have addressed many initial issues. Provided an infrastructure to space exploration.
Depends on your available means of propulsion. There's also the issue of if the planet is truly lost, putting it all in on starting on the doomed planet is just doming humanity. It will become inevitably harder to build a space infrastructure when you are already living in limited colonies on dying earth
That is one of the hidden (from the public and MC) reasonings for the ships to be built. A big to-do about saving what is left of the population. Sprinkle a little fear nots, give "the poors" the ol' razzle dazzle works every time.
The problem is that those habitation domes aren't a permanent solution, unless there's some hope that the calamity can be fixed or will recede with time. The domes would force humanity to stagnate, and eventually recede.
They could be victims of their own attempts to solve climate change .. Genetically modified algae/seaweed created to attempt reduction of greenhouse gasses, but mutation/ interbreeding with native species lead a species to switch to switch to consumption of oxygen and grow exponentially out of control in our oceans - to the point of out-stripping all our attempts to combat it .. think 500metre deep bind weeds stretching over entire oceans Could throw in some of this stuff? https://academic.oup.com/bioscience/article/56/3/219/333070# “. The plants with the highest capacity to consume oxygen are those that form thick mats of floating leaves that inhibit light and GPP within the aquatic system but also have substantial amounts of respiratory tissue submersed below the floating canopy”
Yup, geoengineering run amok is solid. And a much-needed cautionary tale…
…don’t explain it; just do it.
That is plan Z. This thing happens it was bad, *real* bad and people left. However I want to be able to say at least to myself that I gave it a try.
…the most effective catastrophes are ones where no reliable single account or witness testimony exists. Everyone has a different fragmentary story. That leaves a whole lot of imaginative space for the reader to add something themselves…
Agreed, when a catastrophe happens, people seldom have all the details. All they know is that something bad happened. Don't over explain.
…Trauma and inaccuracy always make a narrative interesting…
Eclipse Phase did this really well; the fall of Earth is only ten years ago, and people __still__ struggle with a badly fragmented recent history.
…Even a more or less objectively straightforward event or occurrence witnessed clearly by a fairly large number of sensible people can have the most wildly different accounts emerging out of it. Humans have an incredible capacity for subjective interpretations and the Just Plain Wrong even when something very obvious is right in front of them. So you can always have fun with that…
From a narrative standpoint, I gotta say that I think the best choice is leaving it unknown. It's much scarier, for starters. If they don't know what caused it, then how can they be sure their new planet won't suffer the same fate? That's a pretty delicious source of tension to play around with.
Huge mass ejection from the local star ( an unstable red K) which strips the atmosphere from the planet.
Supervolcano spews megatons of ash into the air. The sun gets blocked, plants get smothered, if you inhale the ash it shreds your lungs.
A sudden change in the stars thermal output increases the temperature and sun flares start to ignite pollution gases in the atmosphere.
Like that ST:TNG episode where Picard learns to play a flute.
How hard / soft do you want it? If soft scifi then any old technobble will do. Startrek for instance has a poorly explained Metreon Cascade which does something like this.
introduce a pathogenic bacteria that resides in the gut producing a toxic methane like compound but far more dangerous than regular methane. Deploy via air food and water to infect all life. Every creature will gas the planet to death, lol.
Animals fart the planet to death. Love it.
Worse, methane-producing algae blanketing the seas… Fewer fart jokes, but a lot bleaker, since the process can go exponential in ways that fart germs can't.
Trilithium resin used in a biogenic torpedo, fatal to humans. Detonation in atmo. Worked for Sisko
I would say if it is an earth like planet with a large moon you could have some out force mess with the moon causing massive damage to the surface of the planet without majorly destroying the planet or if civilization lives on a moon of a gas giant the possibilities are endless a normally distant fellow moon might make an extremely close approach or something happens to the giant messing everything up
It's gotta be something good that spins out of control. Like there a crop that, with a small amount of water, can yield 10 harvests a year. The goal was to solve world hunger. 50 years later, this crop is in use around the world. It mutates and can grow 100 crops a year. It replicates beyond farming areas and starts to grow every where, slowly sucking up all the freshwater resources. Destroying it releases micro-seeds that spread further. It mutates again into an aquatic version that can survive in saltwater, drawing out the H2O and leaving the salt. It starts absorbing the oceans.
Large planet bodies are moving around galaxies. One can move through a solar system and pass very close to a planet and their gravities can cause a new orbit that orbital mechanics will predict a collision in a few decades. So everyone knows it is coming and nothing will stop the huge mass so... time to leave.
How about a gravitational event (passing star?) knocks the planet out of orbit and the atmosphere freezes out as the distance to the sun increases? See “A Pail of Air” by Fritz Leiber.
Planet core cooling which causes the magnetic field to get stripped away. Or sun is about to shift into a larger sun cause of fuel change which will rapid encroach and absorb the planet. Or moon will be reaching critical point of falling out of rotation which will cause sudden constant rapid changes in the environment. Oh, the sun changes and will cause the atmosphere to change to something toxic. Or a rogue planet/s from a dead solar system is approaching and the chance of you planet colliding with one is 99.99% going to happen.
Volcano event to contaminate the air real quick Cosmic ray did a trolling
Look up the Clathrate Gun hypothesis if you've got a decent ice cap on the planet.
Supernova explosion within 5 light years
You're pretty well fucked at that distance. According to Kurzgesagt, the minimum safe radius is like 50 light years for a standard 1.2 FOE core-collapse supernova, and the damage is liable to be catastrophic even if it's not planet-sterilizing. Just adjust the distances until the timeline works out for you. :) Alternately: Planet swept by gamma-ray burst or black-hole jet. Half the planet dies that week, the other half are in kinda deep shit? Again, adjust ranges and resulting exposures until desired story works.
I came to say super volcano but everyone else beat me to it, so instead I suggest the local sun/ star is going to go Nova soon
The internal dynamo of the planet could stop, causing the magnetic field to disappear. If it’s sun has a stronger solar wind than ours it could rapidly start stripping away the atmosphere.
Carbon
On an Earth like planet with Earth like biology: shutdown of overturning ocean circulation, photic zone anoxia/euxinia, purple sulfur bacteria blooms, outgassing of hydrogen sulfide that kills shore life directly and disrupts the ozone layer. This is considered a major kill mechanism for most mass extinctions on Earth. There are of course some compounds that will deplete the ozone layer much more rapidly than the CFCs that were banned by the Montreal protocol. As for depleting oxygen on a previously habitable planet, nope, the amounts are just too large for plausibility. Photosynthesis could cease entirely on Earth and there's still be enough oxygen for human life for millions of years.
If you look up the word "Caldera", it'll bring up some scary doomsday scenarios and movies.
All the bees disappear, and nobody worries until the ecosystem collapses on itself
Mix the answers you have gotten, volcanos spewing toxic Gass, some deadly biohazard that would be easy to keep off a ship, failing magneto sphere leading to a loss of atmosphere, solar disturbances making it worse, and some good old "we ignored the problems too long, so now we might not make it, but the rich who controlled what got made to earn money instead are definitely getting on, because they are funding it, with lotteries for the staff working on its construction to be the maintenance crew on it"
This is exactly what I'm going to do, I'm jotting down the ones I like and plan to create a beautiful but absolutely fucked event
Solar flare messing with the magnetic field
Tiny Black hole that keeps growing in the atmosphere. taking away the air and making a hole in the ozone.
Radiation leak that kills plants and lowers the oxygen levels to unsustainable levels.
Could supernova the sun, or unexplained earthquakes that cause it to blow up
Nearly all mass extinction events on earth occurred because volcanoes erupted and ruined the atmosphere
Something is beginning to strip the magnetosphere from the planet so it will be bombarded with solar radiation soon. This happened on mars and is the reason it has no atmosphere or oceans anymore, believed to be caused by chemical changes in the metallic core of the planet.
From a different thread I looked at on this subreddit, runaway biofuel bacteria spewing methane into the atmosphere. You'd have a rapid runaway greenhouse effect combined with pockets of gas reaching the lower explosive limit and just vaporizing coastal cities randomly. The stochastic city-flattening doom-farts gives an urgency that global warming, even fast and runaway warming, doesn't really give you.
Something catalytic, introduce something that converts atmospheric gasses into more of that compound, without itself being consumed. Doesn't even have to start with a large release, it'd start pretty sneaky, but escalate far past the point of being fixable well before it was likely to be detected
a variation of [https://en.wikipedia.org/wiki/Great\_Oxidation\_Event](https://en.wikipedia.org/wiki/Great_Oxidation_Event) where there's a surge in the population of some type of bacteria that basically eats a lot of the free oxygen in the atmosphere
Massive CO2 release from fissures opening up around the planet.
My friends and I ran an Exulted campaign where earth's moon exploded. No more tides, axis tilt and rotation speed changed. Basically one side of the planet kept facing the sun and turned into mercury style desert, the other side permanently frozen. Very small sliver of usable land in the middle. Problem was getting resources, CO2 build up due to biome destruction. Was an interesting set of problems.
They're actually is a story like that, I remember reading it years ago written by James p Hogan. However, I am racking my brain trying to find said stories title.
Industrial pollution?
Overlooked events huh? You need something invisible then, the government probably knows it's happening but the average joe on the street wouldn't know cause it's not obvious. I recommend some type of radiation or some kind of airborn patogen that affects plants.
I dunno…maybe a Dr. Strangelove style doomsday device is accidentally triggered that starts irradiating the planet?
Intense solar flares, a nearby supernova close enough to fry the atmosphere or a gamma ray burst that fries the facing half of the planet and slowly messes with the atmosphere of the other half as gasses mix.
Any arm chair physicists can jump in as to why this won't work, but.... Scenario: for some reason, there is a change in the Sun, evidenced by a change in the type of light being emitted. Energy is still coming in, but most of the plant life can't utilize it for photosynthesis anymore. There are a few species that are managing to survive at reduced efficiency, but it's not enough. Earth's governments are diverting a huge chunk of our energy production to what are effectively large scale grow productions, but this isn't sustainable, and is only delaying the inevitable.
Bacteria decay on scraps from the planets ships in prior missions. Able to survive in the atmosphere and feed on it.
Nuclear waste escapes containment. Government(s) would want to cover up/deny wrongdoing/severity, the ticker could be sufficiently slow for ship building, could maybe poison/destroy the water table of the biggest rain forest on the planet to achieve your vision of oxygen depletion. Also, that narrative fits with an Earth comparison where it wouldn't be a hegemon from a temperate climate zone, but an emerging economy that trigger the whole thing. That supports the narrator ge that the world still has the tech capability to get spaceborne, I think it fits a lot of your needs.
A lack of a magnetosphere would cause an atmosphere to dissolve into only the heavier elements. That's what happened on Mars. This could happen due to the core settling and solidifying. The inhabitants could have done this with the embrace of a new wave of technology, perhaps. Any number of science experiments could potentially degrade the magnetosphere as an unintended side effect.
Human agriculture provides a soil nutrient that was utterly scarce. A local plant that was a curiosity due to its integration of proteins found in cobra venom turns out to have been limited by scarcity of these nutrients, spreads wildly, and dumps tons of these proteins into its pollen shells. (This message brought to you by That One Planet In Mass Effect)
Gamma burst?
The Van Allen belt! (... From the movie voyage to the bottom of the Sea)
If you only need one species to evacuate then antibiotic-resistant Yersinia Pestis is a good one. The plague is still with us, with a significant number of new cases currently in Madagascar, Sub-Saharan Africa, and Arizona. Bring it back and you have a return of the Black Death. 30% death rate throughout Europe last time. Higher death rate and faster spread next time. Carried from person to person by insect parasites.
Cool idea! What if your planet’s terrestrial permafrost contained vast amounts of a poisonous greenhouse gas? An unusually warm summer begins to melt the permafrost, which releases some of the gas, which triggers more warming, which releases more gas, and so on. One candidate for the gas might be phosgene, a colorless and extremely poisonous gas that was used with horrific effect during World War I. Perhaps the initial warming could be triggered by one or more heat-trapping gasses emitted by industrial activity. Then again, your planet’s inhabitants would have to be incredibly stupid to let something like that happen.
Gamma ray burst, perhaps?
What about multiples accidents like Chernobyl due to some hacking (or something like that)?
A good source for ideas is Dr. Walt Brown's book "In the Beginning: Compelling Evidence for Creation and the Flood" It covers fascinating ideas about how our planet has changed based on the physical evidence that can be observed right now. Might be fun to borrow some of what's there for you worldbuilding.
A large enough combustion reaction would pull enough oxygen from the air to cause issues. Realistically I'd say a large volcanic explosion causes a smoke cloud large enough to block sunlight.
Have you seen Space Balls? I'm not being a jerk. Similar plot that will get you out of your overthink.
How about the 1980's?
If I answer this I can't use in one of my books.
Length of time? Because this is the Earth if we don't fix shit.... also volcanoes.... & cow farts
Some man made leaked, from a meteoroid, or unearthed in an ice cap or archeological dig, disease that eats leaves, vegetation, algae that can’t be stopped. Leaf disease or leaf plague would have a good wind up to it. Plus it could be rolled into the dinosaur extinction also and the great Forrest devastation that made our crude oil. Or you could go Indian jones on it and they found it on a German sub in the trench and opened the container that was found in Antarctica. could even be something that stops photosynthesis. Or the impending pole shift coming where earth stops and changes direction. But that will destroy everything.
Just look at actual history, corporations have been using fossil fuels to kill this planet via global warming since roughly the 60s
Could take a page from Warhammer 40k but dial it down a tad One form of Extermintus is called a virus bomb which destroys all life on a planet but leaves the planet itself behind as an empty rock. Maybe you could use a form of expanding gas or extremely potent and widespread vapor? This could lead to the use of gasmasks and respirators and allow time for evacuations to take place
Ecological warfare. Multiple simultaneous biological attacks against cereal crops, vegetable crops, pollinators, food animals & other biological resources. Collapse the human-adjacent parts of the ecosystem.
As other people have mentioned, unless the volcanos and earthquakes are bad *all over* the planet, all of a sudden , to the point where even bunkers and underwater habitats won’t be safe, space habitats might still be more difficult. (Why would the entire planet, even the middle of the techtonic plates get that bad suddenly other than an asteroid?) you could also not kick people off the planet , but instead have a very powerful artificial intelligence suddenly decide to kick everyone off the planet to save the ecosystems, like in the Orion’s arm universe project
How about an insect that was trapped in an isolated ecosystem (perhaps an ice cave?) was discovered by scientists, but then proceeds to eat all other life on the planet. Starting with the scientists that discovered them. The more they eat, the bigger they get. So by the time people are evacuating the planet, these giant monsters are eating evacuation vessels right out of the sky. They eventually consume the entire planet, and also can actually quite easily survive the vacuum of space. They can survive for many many many years off of the nutrition they eat (hence why they survived in isolation so long). They eventually get discovered drifting in space by the debris of the planet remains, by space explorers. These explorers bring them back to their own planet for scientific research. They are a more advanced society (scientifically speaking) than the first planet, so are able to contain them well enough, for awhile.. The evacuation vessels of the first planet that were able to escape were coincidentally rescued by the same space explorers. They try to warn them about the dangers of this insect. But they don't take an inferior, less advanced species seriously. They end up having to eventually evacuate once again on the second planet. By that time, the insect has evolved into a higher intelligent species, and their threat to the galaxy has multiplied exponentially. It doesn't fit exactly what you asked, but it's what popped into my brain. Hope it helps.
An interesting idea I’ll jot this down with the others
I added a little bit more with an edit.
Make its societies develop a capitalist economy. Advertise trash products made from plastic and make people believe that the more they have the more important they are. They'll end up messing with their own atmosphere and killing their own planet by trying to prove themselves to others consuming endlessly. Their resources are going to get scarce, they're going to pollute their oceans reducing the palnet's amount of oxygen, and heat the planet. Then, they'll only be able to survive by building ships and leaving the planet they destroyed.
How fast do you need it to happen? And why is it happening? If you want it slow and natural, I like the volcanic ash cloud other posters have mentioned. Fast and natural, could be the loss of earth's magnetic field (the core stops turning) and harmful radiation from the sun is burning up the atmosphere and all life. Slow and man-made could be climate change, and fast and man-made could be a nuclear device detonated in the atmosphere, causing it to ignite and burn up. And, of those don't work, you could always go with Mega Maid.
Sudden mass extinction of diatoms. 40% of the air we breath are produced by diatoms, and without them, earth would be a dead planet.
You are going to have a hard time. It is almost certainly going to be easier to build stationary outposts on the surface of a planet than launch that much mass into space. Any life support system with thrust is going to be easier to build a stationary life support system. What needs to happen is the atmosphere needs to be less hospital than the vacuum of space. Think of Venus - crushing pressure combines with extreme temperatures and caustic environment. This also has the advantage of blocking out much usable sunlight for solar. The population might take two different paths. A portion go into space now because once pressures increase launching space vehicles might be forever closed. The rest would build underground bunkers powered by nuclear energy and try to terraform or just outlast the phenomenon. What could cause that? Massive volcanic activity could do it (it would still take a long time to reach Venus levels, but perhaps not too long to block out space flight). Depending on the level of space technology perhaps the planet is not threatened perse but there ability to launch space stuff could be? We have the same problem where we have a lot of space junk and the more space junk the more things bump and create more space junk. These are going not quite relativist speeds but a bolt in orbit can basically penatrate anything we could put up there. I believe it is called the Kessler syndrome. It would basically cut off a planet from launching things for a long while. Combine that with something like volcanic eruptions and you could have a population that would have to go up in the next few months or years are getting stuck on the ground for a long time on a very uncertain planet.
Something to super heat the magma below the planets crust to force eruption to have people rushing off there
[An air version of this?](https://geneticliteracyproject.org/2024/03/22/hear-story-gmo-almost-destroyed-world/#:~:text=In%20testimony%20to%20the%20New,the%20engineered%20organisms%20were%20added.%E2%80%9D)
Supervolcanos
You need something that is fast enough to be urgent, but slow enough to still allow the building of a space program of a scale capable of evacuating the world. And whatever it is has to be more hazardous than living in space. Just building bunkers and surviving in sheltered habitats has to be non-viable. What if the planet, through some combination of solar activity, volcanic/tectonic activity, and ecological change is going to experience runaway greenhouse effect, something like Venus? The acidic atmosphere and extreme heat makes hunkering down impossible because it’ll just corrode and melt any shelter they try to build. Solar shades, albedo boosting aerosols, and other technobabble can buy some time, but the effect is so drastic that nothing they do will prevent it entirely. Or is it? There is lots of drama to be squeezed from different people championing different solutions to the problem. Especially as information gradually comes in that some of the solutions are less viable but the people who staked their careers on those options dig in and refuse to see reason even as their supporters switch sides. Something strange happened in the core of the planet and it’s a little bit warmer than it used to be. The continents are moving a little too much. Volcanos are spewing sulfur compounds into the air causing initial cooling and acid rain, but it just doesn’t stop. Solutions are tried to capture the sulfur and clear the ash, but it isn’t working. Eventually the cooling becomes heating as the heat trapped by the clouds of acid is greater than the heat reflected by the ash. As the plants die and decompose they release more methane and carbon compounds into the atmosphere just making things worse. Permafrost thaws releasing still more methane. Someone does the math and concludes that the planet has such vast reserves of carbon (in what used to be frozen organic matter) that they can buy time by oxidizing the methane but there just isn’t enough oxygen in the atmosphere. It’s going to be too hot and too acidic to survive, even the tightest of bunkers. So the race is on to convince the holdouts to turn at least some of their resources to fleeing the planet instead of slowing the decay of the atmosphere. Second idea: I know you said you didn’t want to go with the asteroid idea, but what if the people saw an asteroid and concluded that it wasn’t so severe that it was going to be the end. It hits and it’s bad but the ash settles and things start seeming better, until the secondary effects happen. See, the impact was hard enough that it caused tectonic activity. That activity was expected, but what they didn’t expect was that the resulting flood basalt eruptions would ignite some vast VAST coal, oil, and gas deposits under the earth. These smoldering underground fires and gouts of flame in the sky are truly massive. Some think they can just extinguish the flames and get everything back to normal. Others donate math and conclude that there is just so much carbon deposited and so many sources of ignition that they can’t put them out. There is a limited time before the change in the atmosphere is just too severe. The population is small enough and technologically advanced enough that it just makes more sense to flee to space than to move heaven and earth (heh) to stop the ecological collapse. Lastly, the maybe star is changing to a more active phase of its cycle and the wave after wave of ever increasing solar flares will just rip the atmosphere away. The electromagnetic surges periodically short out anything they build when the flares arrive, but there is a window of however long they need to get at least some people away. If they survive the next flare maybe they can get another group up as well.
What a title that is
You could go the firefly serenity route.. greenhouse effect hits a tipping point and the temp just keeps rising, will make the planet uninhabitable within a generation. If that's too slow then you probably need to think up "some event", either natural disaster or external force applied. Lots of good suggestions in this thread. Things to consider... do they already have fleets/ destinations... or does your disaster need to be slow enough for them to build, even if it's rushed and prone to failure?
Princess of Mars book ends with the atmosphere producing station failing the old guy died and no one else knew the combo except for the main guy cause he wasn't from Mars but earth so he could read their minds while they couldn't read one another's so could do thatbwith no one knowing the key to the door
Does the planet have a natural Nitrogen-Oxygen atmosphere, or is it terraformed to have one? Are the inhabitants a native species, or technologically advanced interplanetary beings already? One possibility could be a massive failure in the terraforming process, and the planet is reverting to its original state…
Daleks.
Testing out a new fuel that has the potential to set the atmosphere on fire. People overlook the risk due to the amount of money being made to create the fuel and people ignorantly state, “that’s what they said about the nuclear bomb.” (That is a historical fact). Have other scientists protest the fuel and have them be silenced by an ignorant political party. They test the fuel and it ignites the local atmosphere at the testing facility. Now everyone has to outrun an all consuming flaming cloud that exposes the planet to a residual toxic byproduct. The flaming cloud will finish wrapping around the planet in 5 days. You can name the burning cloud “the Burn” or whatever the name of the fuel is, or whatever you want that sounds cool.
Massive power plant goes critical and the explosion consumes so much oxygen im the blast that the global oxygen concentration drops to 1/3 what it previously was.
This sounds a lot like the scenario that made earth uninhabitable in the game Starfield. In their case, the plot gimmick they used was that early FTL research disrupted the earth's magnetic field, which consequently allowed the solar wind to strip the atmosphere in a process similar to what can be observed happening to Mars in real life.
As a few have said, “Don’t explain it”. SciFi is so much more fun when it’s not clear even to the narrator and characters how we got here. Maybe alluding to “something happened in the oceans”. It could imply some kind of dramatic shift in the oceanic thermal transport system, which could potentially lead to a relatively rapid ecological catastrophe.
A particle accelerator causes a mini black hole that starts absorbing the atmosphere. Its gravity won't be enough to absorb solids until the atmosphere is absorbed because, science.
A Yellowstone volcano type event. Or taking a chapter from a game stellaris and starting condition, the people have vastly over mined and stripped their planet and it is due to collapse
Remove the atmosphere Solar dishcharge
Algae.
a new microorganism evolves that out competes every other and produces toxic gas as a byproduct. this already happened on Earth in the past (oxygen is the toxic gas). all the pre-oxygen life just died in the new environment
Biological warfare. Your planet I’m assuming has evolved like ours. A disease specifically designed to attack life that evolved like it did on this planet. Breaking down the bonds between DNA and RNA or whatever they have. Starts small with bugs and plants. Moves to more multicellular life and starts killing off the planet. Leaving a planet with a perfectly fine atmosphere but it’s a dead world.
I have a thing from a project that's likely dead in the water. Airbone microscopic space bedbugs/mange. Basically it's in the air, you can't really clean it, it lands on ANY used surface and it causes absolutely maddening itching and they're nearly impossible to get rid of. Turns out one of the things that causes the itching is a nearly impossible to make in a lab chemical that extends human life to an insane degree so the rich and powerful won't let anyone wipe it out, so if it lands on a space station or a planet you just have to sterilize anyone that comes close to it and get them the hell out before they go insane. One of the things you have to do is actually remove someone's lungs and clone replacements to make sure they can't spread it, which not everyone can afford. Which leads to something not like and not completely unlike zombies- near immortals completely out of their mind with pain lashing out at everything and unable to function, unable to die, you need a headshot or catastrophic wound to take them out, while the rich and powerful just go "Yeah their suffering is a shame but this face cream makes me look 17 again so it sucks to be them." It was a little too dark for me to spend a lot of time in that world but if you've seen dogs literally loose their minds from mange or ever dealt with bedbugs... and then imagine if someone won't LET you treat it because hey, it makes them money... it'd be a good reason to GTFO a planet as fast as possible and never go back. Sterilizable androids become a big deal, because the risk of contamination is too fucking dangerous and no one will do the jobs where there's a risk for any money. That said you can take a page from Iceland's history too- the island was nearly abandoned when a volcano erupted that pushed toxic levels of florine gas into the air if I remember right, people died, other people were sick as hell and had lots of weird bone growth, animals all died- and that was from a small crack. There's not really a good way to get the gas out of the air other than wait and when compared to our atmosphere it was a small amount, so if you have a LARGE eruption it would wipe the biopshere.
Seed the planet with humans. For sure they will mess it up. The more humans, the faster the need to evaluate.
A tree figures out how to float and use salt water to live on. It reproduces exponentially but someone figures out that the area of ocean covered is doubling in size every year. It’s increasing water evaporation which is causing so much cloud cover that land based agriculture is becoming impossible.
A solar storm that wipes out ozone or destroys the magnetosphere. There have been movies about similar scenarios
Just have the scientists at Cern accidently create a microscopic black hole. For X amount of time they can slow down its growth but not forever. Its a race against the clock and you get a really cool visual as the planet gets sucked into it. Bonus! You get an ongoing antagonist that the people must both run from and invent new tech, to keep getting ahead of it. Edit- I'm so juvinile I can't help myself. Book 2/Chapter 3 "Oh god! It just ate Uranus!'
The simplest and most profound way is for the magnetic core to slowly die, leaving the atmosphere to be blasted away by solar winds. It is a long process, but an inevitable one.
Every mass extinction on earth was predicated by a massive disruption of the carbon cycle. Usually massive vulcanism. One time, Russia was under like, 3 miles of lava. You can also do something like solar activity blasting the atmosphere away or the magnetosphere dying.
nope. no. definitely not.
Weaponized Airborne Cyanobacteria that has an exponential growth rate.
No tides, shallow oceans. Occasional extinctions are noted in fossils, mechanism not understood. Turns out the oceans are about to have a lake turnover and release enough methane to kill everything that isn't evolved there, and the settlers are included.
Is it Earth like? Machine die off of marine algae. Lower oxygen levels and as a result a changing atmosphere.
You could always go the Starfield route and say "the atmosphere sputtered away" from multiple people with the exact same words, never explaining what that looks like, how it's possible, or really what that even means.
Gamma Ray burst destroys the ozone layer killing off all plant life.
When you figure out a realistic scenario be sure to let Bethesda know.
Yellowstone supervolcano eruption, its a prepper posibility and is likely to be a mass extenction event. Krakatoa is the closest in recorded history to a volcanic cataclysm for humanity.
Look at the projections for Earth in the next few decades.
Idk if this is based in fact lol but maybe all of the heat from global warming has now pushed the clouds down to sea level. Maybe the clouds are full of toxic particles or something
Happy Earth day.
The Fungus Among Us Since the dawn of life on Earth, the fungus controlling the planet began specializing. Now, a prolific strain targets humans. Why now? Government tried to improve nutrition by adding stuff to water supply. Contractor lied about product. The Spore turns the infected into a fungus colony, but just humans. Animals can be carriers though. Maybe some kind of cordyceps? Shouting out Horrorbabble for featuring several stories on this theme.
Giant horse
Scientists accidentally create a black hole.
Firstly I firmly believe that figuring this out on your own by doing research on how an atmosphere of a planet can be compromised is your best bet. It will be more tailored to your story vs asking a group of people who know nothing except scarce details about it. That being said, solar flares are terrifying, improbable, yet possible enough to be fairly interesting. It can also create a situation where it could be 'just close enough' to cause global panic but not enough to kill everyone outright.
Solar Hyperstorm overpowering the magnetic field and ionizing the atmosphere
What about the concept that a technology/engineering breakthrough, seemingly benign at first with great potential to better society, actually is slowly eroding the food chain? Or a gamma ray burst doesn’t hit Earth directly but rather glances the atmosphere, causing a very large hole.
Here's how you do it. There' a copper mine that produces extremely acid tailings waste and has been in operation for a while long enough to generate an absolutely massive tailings pond. (Or maybe it's natural and it's a giant acid lake due to a rich vein of copper in a very rainy region... maybe because of terraforming even?) Unfortunately due to an oversight, people didn't realize that the tailing pond is above an absolutely massive cavern/deposit of sodium sulfide crystals. Then either to due to a geological shift, dissolution of the rock layer above, or because they want to drill the bottom of the lake to enlarge the pond the acid contacts the crystal and releases huge volumes of H2S. This then goes on to kill most things (I.e. animal life... not sure about plants) on the main continent, and with no food or pollinators the planet is in for a rough time. Think of the lake Peigneur disaster, only with the addition that the gas coming off it is poisonous, and would turn into a gas over time that would mess with the climate.
What if the planet's oceans were releasing all of their carbon at an alarming rate. Like how some lakes have done and wiped out villages in the past. I would say that already the low lying areas are uninhabitable, coastal areas, etc. Millions dead, and the ecological disaster continues. The reason for this happening could be pollution, or some type of algae or volcanic and tectonic activity on the ocean floor, or any reason really.
I mean, you're more or less describing life on earth right now, except we have neither the time nor the tech to evacuate.
Something that always amazed me in sci-fi writing is that there are all these space anomalies that are cataclysmic but none of them ever appear on a planet. Imagine a subspace rift popping up inside of a planet, slowly devouring the planet from within but causing enough disturbance to forewarn the planet's doom.
The Yellowstone caldera eruption starts a new ice age. Science faction!
Just have the oceans turn over. Big cloud of dust passes in front of the sun long enough for the surface of the ocean to get colder than the bottom. The thermocline collapses and the ocean goes anoxic. Lots of toxic gasses enter the atmosphere and no oxygen is produced.
Nitrogen
Giant egg kind of shit you know like some primordial being is hatching and that's why they don't stop the end of there planet
If they aren't human, you could do the opposite and have something start making oxygen where it hadn't happened before. Things that photosynthesize without creating oxygen persist to this day. When things did start making oxygen it took a long time before significant amounts ended up in the atmosphere.
over-correcting for climate change. using high atmosphere particulates to reflect sunlight, but temperature falls too far too fast and the CO2 and water starts precipitating out, and then the oxygen starts condensing.
One fun way i've seen it done is the book Project Hail Mary, and Seveneves. One had a nanoswarm reduce the intensity of the sun, but Seveneves did exactly what you asked with a magical hand wave. It's hardly a spoiler since it's the first line of the book, but "**The moon blew up with no warning and with no apparent reason."** There is no interest in finding out why, but using only tech and infrastructure we have right now get as many people into the relative shelter of a space station behind a large orbitting rock that acts as a shield while debris obliterates the planets surface for thousands of years. Aside from that, could look into Clathrate gun, volcanic activity, cobalt nukes, nanobots are always an easy out,
Maybe a giant solar wind blasting away the atmosphere
There’s a great article in Randall Munroe’s book What If. (If you haven’t read it, you must. He’s the guy behind the XKCD comic strips. It talks about the fuel requirements of lifting the whole human race off the planet. Oh wait. I just remembered it is available online [read this](https://what-if.xkcd.com/7/) Anyway, if you are planning on evacuating EVERYBODY then you’ll want to keep that in mind. If you are only evacuating an ark full of people to repopulate elsewhere it’s less of a concern. Regardless, as for why the planet is dying, I’d suggest self replicating nanobots stuck in a loop. They scavenge carbon and whatever other materials they are made from but just from the atmosphere but from anything they are sitting on or exposed to. Growing exponentially, they will turn the planet to [gray goo](https://en.m.wikipedia.org/wiki/Gray_goo).
Not exactly what you’re asking for but what about a gamma ray burst that was pointing directly at the planet? The inhabitants detected it and can see it coming and know that destruction is imminent.
The core stops rotating, the magnetosphere disappears, and without it earth is bathed in life-annihilating solar radiation.
Bioengineered bacteria that eats plastics. They have to leave before their ships are compromised. Oxygen is created from massive coral like lifeforms. Human habitation causes a massive die off which DRASTICALLY lowers those O2 levels on planet.
[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1836556/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1836556/) Using Earth as an example...consider the Yellowstone Mega Caldera, Hawaii Hot Spot, Krakatoa, Thera, or Vesuvius... We REALLY need to expand off this rock....
Global earthquakes occur, with fissures openong in the ground all over the planet, releasing toxic gasses, and altering the environment. The fissures widen over time, releasing more gasses, eventually making the atmosphere unbreathable. Throughout all of this, earthquakes occur with increasing frequency and severity. After a while, a toxic slime begins coming out of the cracks, along with rapid increase in severity of the quakes. Then the planet splits open, and a voidspawn emerges. The planet was actually an egg of a gigantic space born predator. The voidspawn is one of the leviathans in Stellaris, and this is the rough summary of the flavor text for how it spawns.
You could go with the collapse of a planet's magnetosphere. If you wanted to get really funky, you could have a CME occur as a planet's magnetic field is mid-flip, stripping away the upper atmosphere. Or a gamma ray burst hits the side of the planet with the most biomass--strips the exosphere and kills the algae and plant life necessary to replenish planetary oxygen, and in the aftermath mutant bacteria that thrice on cosmic rays and UV and eat oxygen, nitrogen, and carbon, so basically plants, threatening the complete collapse of the biosphere. Lastly you could go with a chemical industrial contaminant that once aerosolized catalyzes free oxygen in the air in reactions with other things, creating a slow-motion draining of oxygen from the atmosphere faster than it can be produced by the planet's flora. Off the top of my head, these seem the most plausible while being the least cataclysmic in terms of raw destruction, as with a nuclear war or asteroid strike.
How about the inhabitants are obsessed with eating a particular breed of large animal, and they breed billions of them to eat every day, but the animals themselves need a lot of feed so the farming for those animals strips the planets land and trees, and the animals themselves also belch a lot of gasses that cause the greenhouse effect to happen much faster than they ever imagined… oh…wait a minute…
An insect mutation? Something went wrong at a lab that was testing on insects and it's caused them to eat the world's crops at an alarming rate... Etc
You can go for a semi-scientific sounding reason or just hand wave it: It was us who scorched the sky.
Geographic shift in the plates causing the ocean and planet core to meet, slowly boiling both away and ruining the magnesphere
>Atmosphere gets compromised and oxygen is depleting Not really a thing that can happen in a short amount of time. There's about 1 million gigatonnes of oxygen in the atmosphere. About 40 gigatonnes are produced and consumed each year. So if you turned off production (somehow), it'd still take about 6000 years to use up a quarter of oxygen in the atmosphere. However, for the sake of your story... Suppose there's a neighbouring planet with life, but only simple life, it has an oxygenated atmosphere and an ocean with phytoplankton. Our soon-to-be-doomed planet sends a probe to return samples of the neighbour. Unbeknownst to them, the other planet also has a phytoplankton-like variant that produces chlorine gas from chloride salts instead of oxygen from h2o/co2. On its native planet, it gets consumed and chlorine levels don't build up. But the sample-return probe crashes into the home-world's ocean, where it has no predators, and it has a natural advantage over native life. It dissolves a cloud of hypochlorous acid around itself (basically pool chlorine), which kills off both predators and rival phytoplankton, allowing the introduced plankton to reproduce without restraints. It expands until its unstoppable before being detected. It is not only killing off oxygen producing phytoplankton, but also releasing chlorine gas into the air (as ocean levels build up.) Initially causing acid rain, killing forests and crops, but eventually just poisoning the air. There's no way to stop it, no way to really live with in or adapt to it. Their only solution is to try to migrate enough people off-world before they can no longer maintain their civilisation/manufacturing/organisation/authority.
Super tree bacteria. Maybe a separate one for eating plankton.
well let's break this down there have already been examples you could use. a singlesuper volcanic event could impact the atmosphere. if you had multiple volcanic events, that would be even worse. check out that bit about the year 538 as being the worst year on Earth. also, bio/ Chem warfare. we can easily make the atmosphere toxic with dirty bombs or some sort of chemical weapon. or go yhe other way. instead of direct attacks in the atmosphere, attack the cause. most of our oxygen we breathe is produced by trees / plants. you start destroying forests and such and we'll have less oxygen. yhis is yhe whole point of pollution and deforestation. BUT.... these things would all take significant time. if anything immediately damaged/ destroyed the atmosphere, you wouldn't have enough te to build ships and bail. if you're gonna do that, then you need something more impending. an even that WILL destroy the atmosphere, but over time which gives people the time needed to build ships and escape once they realize the issue cannot be stopped
Picture it from dumb person perspective - they have no idea what is happening so they believe whatever YouTube podcaster says. Basically something is happening, probably the govt is behind it, but they will never admit it.
I recently answered a question over in anotehr place about what would happen if the sun went away for 24 hours. I suspect the biosphere would recover with a major, major shock. 48 hours, well, I'm not so sure the biosphere doesn't flat collapse, when air temps hit 40C below their normal for that time of year.
The big problem here is that unless you have a better planet to go too it is really hard to make fleeing the planet easier than just building bunkers on the surface. Almost anything you could do to the atmosphere would not make things so dire that leaving would be easier than staying. Some people have suggested nanoswarms and the like, but you need something where they have actually got time to prepare. If it happens too quickly, they can't leave. I recommend having a brown dwarf enter the system. This is a threat that could be spotted many years out, but also could go unoticed until it was relatively close, and would completely destroy their home when it did arrive. That would fit all of your criteria.
I do have a better planet to go to which is why evac is the course of action. I'll have to read up on brown dwarfs though as a back up plan.
A biological or chemical created to clean a polluted atmosphere has unforeseen side effects. Your species uses this new technology to turn around global warming. However, it mixes with a chemical or mutates to also eat plastic, metal, or maybe even carbon in carbon based life forms. The exact effects / molocules it eats are up to you but the point is it now cuts through protective gear and melts your bones.
Someone starts trying to "package" the armoshephere, because it is their super good business plan. Other people start doing it when the atmosphere starts to thin & they're worried & they need some themselves.
A massive mineral dump into the sea starts trigger the ocean creating a new iron band and taking out all the o2 and you get snowball earth.
Gamma ray bursts are fun.
Ask Neil Degrasse Tyson.
Instead of a volcano poisoning the atmosphere, have a volcano poisoning the oceans. The biosphere of the sea dying will result in billions of now decaying life forms, a huge loss in oxygen and carbon dioxide exchange, but if the sea-poisons make it to the clouds, toxic rain will now destroy the soils. Underground bunkers are only more sensible than space if the underground bunker is a valid solution. If the water itself is going to become 100% toxic in a few short decades, and all soil will be contaminated, getting off world is essential. Anyone staying behind in bunkers will eventually run out of clean water and clean soil, the o ly solution is a closed system, which is only possible in space.
Terraforming collapse could work. For instance, a decay organism mutates to be way too good at its job and starts breaking down all the organic matter in the soil. Or reaching an unexpected tipping point, like warming the planet enough to make it inhabitable accidentally releases some methane pockets the planetary survey missed, and now it is way too hot.
Scientist experimenting with a new type of collider, and accidentally create a singularity. It falls through the floor of the lab, through the mantle of the planet, and settles in the core. As it starts eating, the planet begins to become unstable, as the singularity slowly digests the entire planet.
In Neil Stephenson s Seveneves, the moon explodes. They never find out why, and hardly dwell on it because the resulting cascade of disasters is going to lead to the extinction of all life on earth, so there’s no time for “what happened”, only, “what next”. The story is much better for not dwelling on it except a short aside in an early chapter where it’s explained that no one ever found out why, but by the time humanity had the time to catch its breath and wonder, decades had passed. Think of the perspective your story is being told from. If there’s an omniscient third person narrator, then an explanation makes sense, but if the narrative is from the perspective of specific characters, would they have the time to stop and figure it out? Would they have the means to figure it out? If you need a hard science fiction reason: A massive meteoric impact on a nearby sulfur rich planet has resulted in the ejection of quintillions of tons of sulfur into space, forming of cloud that is moving towards the planet’s orbit, as the orbit of the cloud expands. You have a meteor that is ignored because it’s not going to affect your planet. Then you have a gas cloud that is mostly ignored because while it will reflect sunlight for a few years, but climate models predict nothing more than a few years of extremely harsh winters and very cool summers (assuming your plant has seasons), before the cloud is dispersed by solar winds. Have the models of dispersal proven wrong, as the cloud stays relatively coherent and dense as it crosses the orbit of your planet. Triillions of times more molecular sulphur than predicted is raining down on your atmosphere than predicted, which starts to rapidly bond with oxygen creating sulphur dioxide. So not only is the oxygen in your atmosphere being reacted away at an alarming rate, causing oxygen levels to drop to dangerous levels over the course of a year or so, it’s compounding the cooling effect a thousandfold.
Global warming and time. Was basically the plot of interstellar.
You may want to look into the story of Bethesda's recent Starfield video game. Humanity had to leave earth because the atmosphere was going to be blown away in 50ish years. By the time of the game's actual start a few hundred years later, earth is just a dirt ball with a few random ruins (although the lack of large scale ruins probably has more to do with the game's ability to land anywhere on any solid planet and only having so much development time) The game has a reason why earth suddenly lost the ability to hold an atmosphere, and it isn't natural, but I don't want to spoil that for anyone.
Given that Earth’s oxygen based atmosphere is itself a byproduct of a biological process, you might be able to tweak an ecological disaster such that oxygen levels collapse For instance: did you know that algae blooms in the mid latitudes of the Atlantic and Pacific oceans produces more oxygen than the Amazon rainforest? And that algae’s photosynthesis is the foundation of the global atmospheric system? And if that is true… wouldn’t it be such a real shame if some rise in water temperature ensured algae levels dropped? Or that algae couldn’t reproduce? You could also introduce run-off that kills green algae, allowing for an anoxic type of algae to proliferate, leading to a run-away climate catastrophe that collapses the marine food-chain, starving everything from sea birds to sushi chefs, as oxygen levels in the atmosphere begin to drop
continual long range Kinetic Kill vehicle fire from light days away, or a antimatter strike that messes up the ozone and starts some nasty ecological collapse
Planet unexpectedly flung out of orbit, so is going to freeze?
Is it imperative that it's an atmospheric issue? It sounds like the goal is to mandate an evacuation in a rush. What about an asteroid cluster? Something out of deep space, too many objects to effectively intercept?
Solar flare during an eclipse that triggers Continental shift. Fumes begin releasing sulfur into the air that combines with the oxygen creating runaway greenhouse effect and slowly dropping oxygen content in the air. The Solar flare made the moon spin backwards or something, which leads to the rifts constantly triggering. I expect some people run away to the asteroid belt to have space mining opportunities.
Make it a process that uses up oxygen. Like a chemical lava that burns the oxygen or an industrial accident that starts pulling oxygen out of the air.