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- The planet Golgafrincham in The Hitchhiker's Guide to the Galaxy: after getting rid of those useless Telephone Sanitisers, its civilisation is wiped out by a virus contracted from a dirty telephone.
Anime and Manga
- Cowboy Bebop, in which the Gate Disaster takes out half the moon and makes Earth uninhabitable for all but the hardiest of humans. It should, however, be noted that the rest of the terraformed planets and moons in the system are okay; Earth is still in contact with the greater solar community, but is regarded as a backwater. This makes this Class 3 in theory, but it's really more a large-scale Class 0.
- The ultimate goal of the Dragons of Earth in X1999.
- What will be the outcome in Gantz if the Giant aliens succeed in their invasion. To make a long story short, they're turning us into snack food, like an alien equivalent to wings or potato chips.
- It becomes an important plot point in Utawarerumono that scientists accidentally roused a malignant god in the distant past and brought about one of these.
- The Tuffle race in Dragon Ball is wiped out by the Saiyans who become giant apes at the full moon. They work under Frieza's planet trade to do this as a career choice.
- Also done when Buu unleashes a Beam Spam of epic proportions from the Lookout and kills every human NOT on the Lookout, just because he could. He later went and killed the few remaining people that were on the Lookout by turning them into food and eating them.
- In Dragon Ball Super, Goku Black hunts down and kills off 99 percent of humanity in Future Trunks's timeline.
- Super Atragon: The Big Bad is attempting this to the Earth's surface by building a giant ring that—once complete—will cook the Earth's surface with radiation bombardment. Note that this method that would've actually caused a Class 5 if it was achieved.
- This is the Big Bad's goal in Gundam SEED. Due to his method of choice, it just might reach Class 4 or Class 5.
- After War Gundam X came very close to this. It is established that a massive Colony Drop killed 99% of the Human race at the end of the last war. It has recovered since then, however.
- Puella Magi Madoka Magica: It is explained in episode 10 that Madoka herself can cause this. After a few timeline loops, she begins to transform into the incredibly powerful Kriemhild Gretchen, who will wipe out all human life in a matter of days. It's a good thing that, when she finally makes a contract with Kyubey at the end of the series, her wish is to be able to erase all Witches in all timelines, including her own Witch (of course, this comes with the side effect of being written out of existence almost entirely, but for Madoka, it's completely worth it.)
- Sakyo's Evil Plan in the latter stages of the Dark Tournament in YuYu Hakusho is revealed to be to create a stable portal between the Earth and the Makai so that demons of every class can pass through, which would result in a Class 2 at the very best, though with the sheer power of demon fighters that we see throughout the series it is more likely to be in this class, possibly even a Class 4 or Class 5, depending on how much plant and animal life the ensuing demon hordes leave behind them.
- Part of the backstory in Attack on Titan. A little over a century prior, strange beings known as Titans appeared out of nowhere and began to devour any humans they could find. The survivors have spent the last century living within three 50-meter Walls, with all information about the Outside World banned and mention that mixed-race heroine Mikasa is the last surviving Asian suggest extinction of multiple ethnic groups. The surviving population numbers in the hundred-thousands, and is under constant threat of extinction from either Titans (if the Walls are broken again), starvation from famine and food shortages, or even civil war due to the corrupt and oppressive government. The series begins with 20% of the population being wiped out when the first of the three Walls is lost. However, almost everything just mentioned is later revealed to be a complete lie; while the island the main cast lives on is overrun with Titans, the rest of the world is filled with thriving human civilizations. In fact, the Titans themselves turn out to be just tools of the most powerful human nation.
- The Death Force of MD Geist was designed to completely annihilate human life on Jerra. Geist is tapped to help prevent its activation. Then he activates it himself so he'll always have something to fight.
- Evangelion has the Human Instrumentality Project which causes the entirety of humanity to become extinct and become one of the angels that they fought for the entire series. Subverted when Shinji rejects it, allowing humans to return as they choose.
- The main premise of Dr. Stone involves every human in the world being turned to stone, and finding out the cause of this is part of the major Myth Arc of the series.
- It's not explained what happened in Girls Last Tour, but whatever it was wiped out most of the life on the artificial world where the protagonists live. At this point, humanity's population appears to have dropped well below sustainable levels.
- Devilman sees the world slowly fall apart as a result of the machinations of Ryo Asuka, who is actually the fallen angel Satan. The war between demons and humanity eventually leaves humanity extinct, and the world is destroyed in the final battle between Satan and Devilman. The anime series Devilman Crybaby shows another version of this apocalypse: after the existence of demons becomes public knowledge, humanity becomes paranoid and largely wipes itself out in the world war that follows.
- In current Martian Manhunter continuity, this is what Malefic did to Mars, using a virus to kill all Green Martians.
- Post-Crisis. Everyone in the alternate Earth of the Pocket Universe were killed by that universe's General Zod, Faora (renamed Zaora), and Quex-Ul. They even destroyed the planet's biosphere so no one would survive without a spacesuit. Matrix (the Post-Crisis Supergirl) is the only survivor. Also, Earth and Krypton were the only inhabited planets, thanks to the manipulation of the Time-Trapper. Likely, any other Kryptonian criminals in the Phantom Zone remain trapped there. Following Zero Hour!, the lifeless Pocket Universe is wiped from continuity, making this a Class X-4.
- In The Jaded Eyes Series Harry/Tristan kills off all the muggles but for a few he leaves alive to be used as slaves or blood banks for his Creature allies.
- In Calvin and Hobbes: The Series, the Lightning Man aims to use massive amounts of electricity to kill everyone in the world via heart attacks. The heroes have a And Then What? discussion with him, and he lowers the death toll a bit.
- Creation in Conquest Quest seems to have experienced this, which kickstarted the whole plot.
- Three times in Tales of the Emperasque - the Emperor once cleanses a daemon world with his Breath Weapon and once causes continental shift from the orbit. The third time, it's the Imperial Navy that drops a cyclonic torpedo on an Orc-infested world.
- RWBY: Reckoning has the titular event. From what has been established, someone seeks a Soul Jar, and wishes to use it to grant the Creatures of Grimm with souls. From there, the now intelligent monsters will ravage Remnant until no life is left. And it's not just Remnant: apparently, they plan to unleash the soul-charged Grimm onto Earth soon after...
- In A New World, the remnant of Lunarian society is so scared of human expansion on the False Moon, they see fit to seize control of Earth's nuclear arsenals and trigger a colossal war. It turns out to be worse than useless, as it was exactly what Yukari wanted them to do. Earth does suffer greatly... but the Lunarians are left extinct and their worst nightmare, the complete fusion of the worlds of magic and science, is brought into reality by humans and the magic races.
- In Home Is Where Your Curse Is, a solar storm triggered by Gosunkugi and his cult exterminates most of humanity, sparing only Ranma due to his being trapped inside a magic artifact. Much later, he's polymorphed into a pony upon being released, effectively rendering his species extinct.
- I Am Legend revolves around one of these. Mankind is either dead or zombified to a man, except for Will Smith and a few other people. However, the animals are doing just fine, and in fact have escaped the zoos and colonised the streets of New York. (There are lions.) The original book, however, is a 3b.
- In the movie, dogs and rats are also affected by the virus, although it's not made clear how they're doing in the larger sense. One would assume they had a dieback/zombie rate similar to humans, at least in populated areas, but nothing's said about it one way or the other.
- It's mentioned at one point that dogs are immune to the airborne strain of the virus, though remain vulnerable to contact strains. The only "zombie" rats ever seen are those being experimented on, perhaps implying the same applies to them.
- In the movie, dogs and rats are also affected by the virus, although it's not made clear how they're doing in the larger sense. One would assume they had a dieback/zombie rate similar to humans, at least in populated areas, but nothing's said about it one way or the other.
- 12 Monkeys had a virus exterminate 99% of humanity and render the surface uninhabitable to them due to viral residues. No other lifeforms were affected, and the animals have reclaimed the surface.
- The plot of the James Bond film Moonraker is that the villain wants to use a Depopulation Bomb to kill all humans on Earth and repopulate it with a select group of people under his leadership.
- Planet of the Apes (1968) and its sequels.
- This is essentially what happened to the planet of Miranda in Serenity as the result of a botched Alliance experiment. The chemical that they created to try to curb people's violent impulses, the Pax, worked a little too well, causing nearly the entire population to simply lie down and die. The tiny percentage that survived had the exact opposite reaction to the Pax, becoming the psychotically violent and cannibalistic Reavers.
- X-Men Film Series:
- X-Men: First Class: Sebastian Shaw plans a Planetary-Species Extinction (humans die in a nuclear holocaust, mutants inherit the Earth).
- X-Men: Apocalypse: Accidental Pun aside, the Big Bad Apocalypse's goal is to cause an extinction-level event where the scope is Planetary and its severity is Societal Collapse in the form of wiping out all cities killing billions, followed by Mass Hypnosis of the survivors.
- In The Last Witch Hunter, this is the Big Bad's endgame - to bring about total destruction of human race and civillization so that the witches can thrive.
- Independence Day: the invading Aliens attempt this on the human race after that they planned on colonizing and draining the resources of Earth with their Queen and move on. Twenty years later they've become much more sophisticated by launching one giant ship to drain the Earth's core which will no doubt cause the same effect .
- The aliens (from another dimension) in Pacific Rim have essentially the same goal as those in Independence Day. Destroy humanity with kaiju, use up earth's resources until it's a barren husk, then move on to the next target.
- By the time of Resident Evil: The Final Chapter, the Zombie Apocalypse has caused the human population to dwindle from seven billion to less than five thousand. The Umbrella Corporation considers this disappointingly incomplete.
- The Big Bad of the disaster flick Geostorm wants to annihilate the United States' rivals and enemies with a giant Weather-Control Machine while leaving the US itself (and probably a few of its protectorates) largely unscathed. Thing is, he intended to kill the one person capable of stopping the titular geostorm as part of his plan, and considering how chaotic and unpredictable even our normal weather can be, chances are he would've doomed mankind as a whole rather sooner than later.
- Kurt Vonnegut's Galápagos. Okay, technically the human race does survive in the end, but you can't really consider them human anymore by the time they do get back on their feet. Their brains have atrophied and their limbs evolved into flippers. The ghostly narrator sees this as a good thing.
- Nick Sagan lampshades this in Idlewild and its sequels, saying that it wasn't really the end of the world because insects survived and thrived. The event itself could probably be called a 2.9 (10 surviving humans.)
- not to mention several thousand cryogenicaly frozen people.
- Cthulhu will do this when The Stars Are Right. Eldritch Abominations running rampant usually falls somewhere between here and Class 5.
- I Have No Mouth and I Must Scream by Harlan Ellison®. A giant supercomputer has nuked the Earth and kept the remaining few humans alive in order to torture them. Things get worse.
- In Tom Clancy's novel Rainbow Six, the environmentalist extremists plan this with an engineered disease to wipe the planet clean of all humans except for their own community.
- In Stephen Baxter's Titan, China drops an asteroid in the Atlantic to destroy America, but underestimates its force and causes The End of the World as We Know It.
- The basic backplot of Orson Scott Card's Homecoming Saga, although it takes four books before you actually find that out.
- The "Final War," a major thread in the Bolo series of short stories, is very aptly named: the Terran Concordiat and Melconian Empire pull a Class 3 or worse on every inhabited planet either government has. Usually from ground level, with huge tanks. The overall effect is effectively a Class 2 on a Galactic scale.
- Mephisto, a recurring Big Bad from Michael Slade's RCMP thrillers, plays Xanatos Speed Chess with this as his objective.
- Signal to Noise by Eric S. Nylund ends with the world as a barren wasteland due to deceleration - the engineered part comes from the alien that gave humans the technology that caused the deceleration in the first place - in the second book A Signal Shattered explains that Wheeler specifically caused the destruction of earth so that the protagonist would have something in common with a group of aliens on the run from Wheeler - to lubricate the brokering of a deal!
- Margaret Atwood's "Flood" trilogy (Oryx and Crake and sequels) had an engineered disease that was intended to be a Class 3 - wiping out humans and replacing them with a genetically modified humanoid species, but, as of the publishing of the first two books, it seems that it will be a Class 2 bordering on high Class 1.
- In Octavia Butler's Xenogenesis trilogy, the last few human survivors of a nuclear war are rescued by aliens; but, although they'll be returned to a fixed-up Earth, they won't be human any more. And once the bioships are fully grown and leave the planet, it will cause a Class 5 or possibly even Class 6 Apocalypse.
- Geologist Dougal Dixon wrote (and illustrated) the literally science fictional After Man, a really quite detailed description of the various ecosystems of Earth 50 million years from now, with the base assumption that humans go extinct some time "soon" in geological terms (i.e. within the next 100,000 years or so).
- Spiral, the second book in Koji Suzuki's The Ring trilogy ends like this. Sadako's curse will inevitably spread across the world, destroying the human species. That is, of course, until the next book turned everything on its head by revealing that it all took place inside of a computer
- In the short story "Dark Benediction" by Walter M. Miller, a mysterious disease called neuroderm causes society to collapse. It turns out the "disease" is a microorganism sent by another civilization to help humanity, not destroy it.
- In Stephen King's It, this is what Patrick Hockstetter thinks will happen if he dies, since he believes himself to be the only actual (real) being.He was a sociopath, and perhaps, by that hot July in 1958, he had become a full-fledged psychopath. He could not remember a time when he had believed that other people – any other living creatures, for that matter – were “real.” He believed himself to be an actual creature, probably the only one in the universe, but was by no means convinced that his actuality made him “real”....He didn’t want to die; as the only “real” person, he wasn’t supposed to die. If he did, everyone else in the world would die with him.
- It's implied that this is what's happening in Richard Matheson's two-page vignette "Lemmings." Two cops stand by a coast highway, watching people simply walk into the ocean, come-as-you-are, for apparently no reason whatsoever, in an unexplained phenomenon that has been going on for weeks. note
- The Longest Joke In The World: The lever. If pulled, it kills all humans. It's a safety measure, in case the species goes bad.
- In Primeval the not too distant future appears to be populated entirely by giant mutant bat things that we unleashed upon ourselves. The series is non-specific if this has wiped out humanity entirely in a full Class 3, or just mostly, but given the ferocity of the future predators and the abandoned state of cities it is at least a 2. The Big Bad does state we've wiped ourselves out, but we're not about to take a villain's word for it just yet and the series does fall on the idealistic side of the Sliding Scale so there may yet be hope. Given the geologic time-scales with which this series usually plays around, the Future Predators may not have originally played any direct role in humanity's downfall. It's just as plausible that they evolved naturally, long after we'd gone extinct, and that they would never have met humans if the Anomalies hadn't brought some into the present.
- In Star Trek: Voyager, the Krenim had developed a weapon to delete people from history. Any offworld colonies they would have made would be retroactively removed from history. Addendum: If these civilizations had Terraformed any uninhabitable worlds, those would become Class 5 as the habitable world would never have been made habitable in the first place (at least by the people who made them so in the original timeline). It might fit Class X-2 more, because it shows that it's able to erase inorganic objects as well (Chakotay's talk with Annorax regarding simulation of erasing a comet), but the main show of the weapon is to specifically delete races, but left the planet intact for the Krenim to get it in the new timeline.
- An environmentalist conspiracy called "The Dawn" was alleged to be plotting to achieve this, in the short-lived TV series Strange World.
- New Doctor Who series 1 ending: the Daleks are back, and their plan is pretty much this. The Doctor's solution to save humanity? Kill at least ALL intelligent life on or near Earth, Dalek and human (granted, he didn't have time to actually remove the 'and human' bit from the weapon). And he squicked at holding a gun on a guy. The Daleks talk him out of doing this because it'd give him ANOTHER genocide on his record, even though he points out there are humans elsewhere who would survive this. By the end of the episode the Daleks seem to have enacted this on Earth.
- The gaseous Gelth plan to kill all humans, and inhabit their corpses.
- In Lost, if the Man in Black manages to escape the Island after being stuck there for two millenia, he plans to wipe out all human life.
- According to Rich, In the Community episode "Epidemiology", a Class 3 would occur within a year and a half. However, seeing as everyone is cured by turning the thermostat down to 58 degrees, one suspects this virus might have a little trouble making it out of Colorado.
- In the 2000 two part episode of The Outer Limits (1995) "Final Appeal" it's revealed that all episodes are alternate timelines and futures and that while many of them actually are tied together into semi coherence. the episode ends with a time traveler with a grudge detonating a fusion bomb taking out most of the eastern seaboard, rendering their arguing about this timeline's rejection of tech stronger than a light bulb pretty much moot.
- Known to happen in Warhammer 40,000, if rarely; the setting's instruments of devastation tend to be far less discriminate than this type of apocalypse requires. Nevertheless, the Eldar, especially of Craftworld Biel-Tan, are fond of utterly excising any human/ork/anyone colonies on their Maiden Worlds and returning the planets to their pristine state, conveniently erasing any sign that the world is under Eldar protection.
- In R.U.R., the robots turn against their masters and Kill All Humans except for one. One is too few to reproduce, but the robots don't know how to do that either. So the lone survivor creates an Adam and Eve robot couple who will repopulate the world with their descendants.
- Little Shop of Horrors. The entire human race is eaten alive by giant bloodthirsty plants, and the sung narration explicitly points out that they have no one to blame for it but themselves.
- Chrono Trigger, at least after the Day of Lavos. It's clear that a few isolated pockets of humanity have survived Lavos's wrath, but it's also clear that those isolated pockets are screwed, no matter how much "hope" the main characters spread. Luckily, the theme of the game is Time Travel.
- The second and third endings of Drakengard fall into this sort of apocalypse. The third has a glimmer of hope that it can still be averted.
- The fifth ending causes this in another world.
- Persona 3's bad ending is strongly implied to be the end of all human life on Earth due to the collective nihilism of man.
- Star Control II has several variants. First, the Dnyarri (psychic slavers) ordered the destruction of the Taalo, who were immune to psychic control; it's implied that the Taalo may have slid to another dimension or universe, though. Much later, the Androsynth poke around that area of space, and are then replaced by the Orz, who don't like to talk about it. The Ilwrath will wipe out the Pkunk if you don't intervene, and the Ur-Quan Kohr-Ah — former slaves of the Dnyarri — will wipe out everyone if you don't solve the game.
- In the original Descent: FreeSpace, the Nigh Invulnerable Shivan mothership Lucifer blasted the Vasudan homeworld of Vasuda Prime to an uninhabitable ruin, killing four billion Vasudans (and considering that Vasuda Prime was largely an extremely arid desert, there could not have been much more than four billion people there). They almost did the same to Earth, but the Lucifer was destroyed in subspace, where its shields were down, mere minutes before arriving in the Sol System. Unfortunately, the only subspace node leading out from Sol was destroyed in the process, meaning that Earth was cut off from the rest of human civilization, and vice versa.
- The final foe in Tales of Vesperia, while mostly apathetic towards humanity throughout the game, attempts this when things hit the fan in an attempt to preserve the planet at the cost of those who have done the most damage to it.
- In Wing Commander III, Class 3 is the potential fate for Locanda IV (Robin "Flint" Peters's homeworld) due to Kilrathi bioweapons aboard missiles fired at the planet. As per the novelization, canonically the Victory's wing fails to intercept the bioweapons.
- The crux of Wesker's ultimate plan in Resident Evil 5, wherein he attempts "complete global saturation" of Ouroboros (as he puts it) to rid humanity of its weaker kin and leaving those who can endure its horrifying effects to inherit the land.
- The Combat Of Giants: Mutant Insects games has a meteor that annihilated humanity and civilization. The game itself took place three hundred years later where giant mutated insects rule the world.
- This is the goal of the archangel Malthael in Diablo III: Reaper of Souls. He wants to purge Sanctuary of all humanity for being the offspring of angels and demons, and because with humanity's growing power thanks to the destruction of the Worldstone, he does not trust humanity not to turn their new power to evil like so many humans are wont to do.
- This is the fate that would befall humanity in Eternal Darkness if Pious succeeds in summoning his Ancient; they would be enslaved and/or exterminated on a global scale, and Alex sees brief visions of it actually happening after she realizes that the Ancient she summoned to combat Pious's own would just do the exact same thing. And it's possible that Mantorok will be just as ruthless after his victory across all three timelines.
- The backstory to Find the Cure! Scientists from another dimension arrive on Earth and revolutionize civilization, but it turns out they brought a deadly plague with them that transmits through proximity and kills people exactly 25 years after contracting it. The player character is part of a search team who travels to the scientists' original dimension in hopes of finding a cure.
- The world of Strife is slowly descending into this as those infected with The Virus are rotting away and those who aren't are killed or experimented on by those who are. The player has the capability of stopping this as he wields the Sigil, but if he dies whilst in battle with the Entity, the Entity will gain the Sigil's full power, using it to infect the rest of humanity and then leave the planet once there is no living human left for it to feed on.
- Winning a round of Plague Inc with the nanovirus or bioweapon definitely qualifies as one of these. Any other pathogen could qualify as 3B if you think it's evolving naturally instead of being engineered.
- The monsters of Evolve inflict this on every planet they arrive on, destroying all sufficiently advanced tech and slaughtering anyone who interferes. While Word of God says they would leave the humans alive if they didn't try to stop them, they have so far wiped out the population of entire planets. However, there is information that implies this is merely a minor symptom of their true goal: a Class X-4 event.
- Metroid Prime has a meteor crash into the planet of Tallon IV. The impact releases and spreads a toxic substance known as Phazon, which corrupted a lot of the wildlife and completely wiped out the Chozo race living on the planet (who now appear as corrupted ghosts that attack anyone that enters their territory). In Metroid Prime 3: Corruption, it's discovered that Phaaze, the source of Phazon, is a sentient planet that sends out leviathan seeds to crash into planets in order to spread Phazon across the galaxy and Tallon IV was one of the unlucky targets. Lore data in the game imply that Tallon IV is slowly recovering after Samus destroyed the source of Phazon there.
- Randal's Monday: Thanks to the ring looping reality so much, reality itself eventually gave up and all hell broke loose. Quite literally.
- Beyond: Two Souls: The main character receives premonitions of this happening, and it does if you fail to stop the second Condenser after Nathan Dawkins goes kuku and shuts its forcefield down.
- Bob and George: We're willing to kill everyone on the planet to stop you! This being Bob and George, people at the site of the asteroid strike survive.
- Jack. The full story still hasn't been presented, but apparently humanity was killed in a jihad by genetically engineered furries. Although it's hinted that by reincarnation and God arranging for history to repeat itself, the furries basically became humanity and vice-versa
- In Homestuck, Gl'Bgolyb's death caused the Vast Glub, a psychic shockwave that killed every Troll in the universe. The 12 Players only survived because they weren't in the universe at the time. Her Imperious Condescension, who is immune to the Vast Glub, and The Handmaid, who was not yet in the universe at the time either, also survived.
- In the 1939 animated short Peace on Earth, a civilization of cute little animals springs up after the human race wipes themselves out in endless trench warfare. In the 1955 remake Goodwill Towards Men, naturally, it's the Bomb that did mankind in.
- Though it wasn't his intention, Vandal Savage did this in the Justice League episode "Hereafter (Part II)." Being immortal, he ends up very lonely.
- Adventure Time takes place over a millennium in the future after the "Great Mushroom War", and there are very few humans left in existence. Later on they find a city filled with humans, but it's unclear if there's enough of them left for humanity to make a comeback.
- While technically not a human example, the Martians in Invader Zim worked themselves to extinction turning their planet into a giant spaceship. "Because it's cool!"
- Another technically-non-human example in Steven Universe: about 5,000 years ago, Gems were the dominant civilization on Earth but in open revolt against their extraterrestrial masters. Eventually the Diamonds caused something, alternately described as a light or a song, that corrupted all unprotected Gems on Earth. While the corrupt Gems are still around, they demonstrate only animal-level intelligence, unable to form much of a civilization. The few Gems on Earth that are both uncorrupted and free have been hunting them down and capturing them ever since.
- Rick and Morty: In the episode Rick Potion #9, Morty asks Rick to make him a love potion so his crush Jessica will fall in love with him. Rick is indignant, but complies anyway. Unfortunately, due to Jessica having the flu, the love potion spreads and infects the entire town, who all madly lust after Morty. In his attempts to find a cure, Rick accidentally turns everyone (as in the entire human race aside from them and their family) into praying mantis hybrids who want to kill Morty after mating with him, and finally into creatures he calls Cronenburgs, who are Body Horror incarnate. In the end, with their world in total ruin, Rick decides to cut their losses and moves him and Morty to another universe. The only humans left are Jerry, Beth, and Summer, who are still managing to eek out a living.
- The worst case scenario of a full scale nuclear war, which could, by some predictions, be damaging enough to destroy the very biosphere that humans depend on to survive. The actual explosions are the least of our worries. Following that, fires would rage for weeks or possibly months on end, releasing great plumes of smoke that are believed to blot out of the northern hemisphere for an extended duration, causing the effect we call "nuclear winter". This would drop the average temperature significantly, with the sun being simply unable to shine on the Earth for a reasonable amount of time. Snow and ash would intermingle. Then there is the radiation. If you are lucky, you will get irradiated enough to die within a few days, but radiation sickness is unpredictable and horrible, but basically it results in the decimation of your very DNA at worst. The surviving population would see higher rates of cancer and mutation due to the aforementioned screwing up of their genes. This goes for all species. Water would be unsafe to drink unless filtered extremely well (as water is largely unaffected by radiation by itself, but radioactive particles in the water could still do some damage), food would be unsafe to eat, and in many cases, the air would be unsafe to breathe. Sounds horrible, right? Well, the reason this is a possible outcome is that many of the effects may not be as strong on the southern hemisphere, since a nuclear exchange under current scenarios would be largely restricted to the northern hemisphere. They would still get hit with climate effects, radiation, and other complications, and civilization would definitely not survive, but humanity may be able to ride out the storm in the south. The general belief is that Australia is the safest since there's nothing of worth to be hit, and most refugees escaping a nuclear war before it starts will be safe...if the venomous spiders don't get you first.
- On the other hand, breakthroughs in fields of cybernetics, biomechanical technology and genetic engineering could cause a "benign" form of this. Humans could alter themselves to such a degree, first physically (and maybe mentally as well) that they simply could not be considered human any more. It would start with life extensions, organ transplants, cybernetic body parts, mind enhancing substances, and could easily progress to biological immortality, uploading one's mind to an artificial body that is not anthropomorphic, grafting tissues and organs that bear little to no resemblance to anything human or even eschewing physical form altogether and copying/transferring one's mind to a computer network. Provided we are not knocked back a few centuries or destroy ourselves/are destroyed, this scenario will likely happen eventually.
- Total human migration off the planet would achieve this outcome for Earth, even if our species continued to exist elsewhere. The most benign Real Life option, albeit unlikely to occur unless it's to avert a more conventional Class 3a due to anticipated catastrophe.
- In general, should humanity Ascend to a Higher Plane of Existence, it'll cause this.