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I am interested in events that would so radically change Earth (for the worse) that it would be near uninhabitable for humans. The event may or may not be deadly to humans, but the world it leaves behind should be a very hard one to live in.

Note constraints:

  • Anthropogenic events must be based on current science/technology (e.g. no AI overlords making humans wash their metallic feet)
  • Natural events can be unlikely, but must be possible (e.g. "The Earth spontaneously blows up" is not a valid answer)
  • The event must be survivable both immediately and long term, even if the chances are very slim. It's fine to get creative but the means of survival must fit present or near-present technology levels.

I would expect a good answer to consider what would be entailed in surviving on Earth during/after the given event and how unlikely success is. There is certainly more than one answer to this, but the one that leads to the lowest chances of survival for humanity will be the correct one.

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  • $\begingroup$ @Agrajag I mentioned present (or near-present) technology levels as a condition. It was intended more as an upper limit, so if an answer posits some lower tech level as being preferable for a given event I'm open to it. After all, one could live on modern-day Earth at Tudor Elizabethean tech levels if they desired. $\endgroup$ – ben Apr 18 at 2:40
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    $\begingroup$ Ok, somehow I missed that, appologies. Close vote retracted. Specsavers appointment thursday. $\endgroup$ – 011358 smell Apr 18 at 2:42
  • $\begingroup$ "Earth blows up" is completely possible it just needs to be hit by a big enough rock. $\endgroup$ – John Apr 18 at 4:37
  • $\begingroup$ Am I correctly understanding that your objective is that some portion of humanity initially survives the event, but then EVERYBODY dies due to the changed environment, or do you want there to be the possibility of tenacious survivors? $\endgroup$ – Morris The Cat Apr 18 at 14:02
  • $\begingroup$ @MorrisTheCat I can see where it is slightly ambiguous. There is meant to be a possibility of tenacious survivors. Editing to clarify a bit. $\endgroup$ – ben Apr 18 at 14:32
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Yellowstone Caldera Explodes

Yellowstone exploding has been featured in several movies as well as the cause in "Van Helsing"

It has the possibility of blanketing out the sun and causing another ice age causing famine and massive loss of life. It's not likely to wipe out humanity but wouldn't be a fun time to live through

See Yellowstone

Bio Weapons

The worst case would be the accidental or deliberate release of a bio weapon. A quickly mutating airborn virus that can infect other species such as birds would be troublesome.

People may have to survive in shelters until a cure is developed or the virus burns out but if it can affect other species and mutates quickly (like the flu) a cure would be difficult to find.

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  • $\begingroup$ Virus, that's intriguing. Let's say hunkering down in remote areas is not an option due to other species being carriers or the virus being airborne or both. Obviously a sealed closed-loop shelter would work provided nothing breaks down, but are there any other options for survival that don't require counting on luck? Just trying to gauge the lengths to which people would have to go. $\endgroup$ – ben Apr 28 at 21:02
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Collision

The dinosaurs were wiped out when a big meteor or comet hit the Earth (K-T extinction event). It screwed up every ecosystem, both in land and in seas, produced mass extinctions of many types of animals and plants and changed the climate. However, smaller animals, like mammals and birds, indeed survived, but it was a very catastrophic event nevertheless.

If that happened nowadays, humans would surely survive thanks to technology and intelligence, but it would be a very hard struggle. This would cause havoc in agriculture, fishing, foresting and many other essential activities, which would lead to an economical crash and famine.

If something like K-T is not enough, maybe a higher dose like the P-Tr event is enough. Quoting wikipedia's article, "It is the Earth's most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct." It triggered a sudden 8° C rise in temperature and an increase in CO2 levels by 2000 ppm. That was more than enough to mess up with Earth ecology everywhere. Nature took some millions of years to recovery from that. The cause of the P-Tr event is unclear, but a very large bollide hitting the Earth would do that.

Now, imagine a very large bollide hitting the Earth and obliterating a large area, say, half the size of Australia, part on sea and part on land. That area would then be turned into an open magma pit by exposing the mantle. It would also cause an earthquake and a tsunami as never seen before and would also release a lot of toxic gases from the exposed mantle, from vaporized crust and also those gases that were imprisoned in the crust. Also, a lot of seawater would be suddenly vaporized.

The atmosphere becomes severely dusty, and the blocked sunlight would be more than enough to create a winter that could trigger an ice age, if it were not the greenhouse gases preventing that. Since much less sunlight reach the surface, the life of many plants and animals both in land and in sea would be screwed. Also, since there is much more water vapour in the atmosphere, and then much more clouds, so there would be still less sunlight. From that, there would be horribly severe storms, hurricanes, floods and landslides. Also, the summers would be much more warmer and the winters much more colder. This would cause a severe death toll for land lifeforms and also for many aquatic forms. And oh, since the water in the rain will combine with the dust, the rain would be acidic and it could also acidify the seas.

The death toll would create a massive quantity of rotting organic matter (imagine something like the amazon forest suddenly dieing and rotting down all at once). A large part of that matter will be washed down to waters (seas, lakes and rivers), were it will tend to deplete water's oxygen in many places triggering anoxic events that will further extinguish many aquatic lifeforms. The rotting organic matter itself would release a lot of greenhouse gases and consume atmospheric oxygen. Needless to say, having less oxygen dissolved in waters and less oxygen in air would be no good for whatever ecology that are still resisting so far.

In the following years, as the dirty settles down, the greenhouse gases are still in the atmosphere and they will warm up the planet severely, further screwing up once again the already screwed up ecosystems. The removal of much of the previous forest cover would also create large desertic areas.

Very few forms of plants and animals are able survive that, but a few surely will. Among them, a few humans surely could do, but it would be no easy task.

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  • $\begingroup$ I appreciate all the detail in this one. Despite the major blows to ecology and the probable collapse of civilization, is this one very likely to kill all humans? Obviously life will suck immensely for survivors, but it seems to me that if the Earth finds a new equilibrium and any staple crops are growable (even if their range moves north/south) that humans are likely to survive. If I am overestimating the likelihood of that then by all means tell me. $\endgroup$ – ben Apr 28 at 21:08
  • $\begingroup$ @ben Surely, there is some dose of luck here. This could very much wipe out all the humans, but, as you said in the question, "The event must be survivable both immediately and long term, even if the chances are very slim.", and I think that this is very much the case here. Surely, the nature will eventually find a new equilibrium -- in a few millions years. $\endgroup$ – Victor Stafusa Apr 29 at 5:51
  • $\begingroup$ Oh this definitely hews to the requirements. I just wanted to gauge how hard it would be living in the aftermath (provided one survived the initial events and was in the right place to be able to e.g. grow food). I guess for a big enough rock humanity is certainly doomed, so an impact just below that level would prompt the setting I described. $\endgroup$ – ben Apr 29 at 18:07
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What survivable events would make Earth least inhabitable?

[emphasis mine]

As far as I know, there is only one survivable event that makes the Earth least inhabitable for humans, and that is a...

Gamma ray burst from a nearby binary star system.

Such an event could strip the Earth of its ozone layer which will cause the ultraviolet radiation from the sun to penetrate through the atmosphere with nothing to slow it down. According to NASA scientists, this would in short time almost entirely wipe out life on Earth's surface. The only surviving organisms would be underwater or underground.

[source:] Without ozone, the Sun’s intense UV radiation would sterilize the Earth’s surface.

It would take billions of years for Earth to regain an ozone layer capable of protecting land-dwelling organisms again (the absence of an ozone layer is part of the reason it took so long to see land-dwelling organisms on the early Earth in the first place).

How to survive?

Initially, this event wouldn't directly harm many persons, but the rates of cancer due to DNA damage would drastically increase and eventually no creature would be able to survive if it remained on the surface. Any human that wishes to survive would have to retreat to underground sanctuaries (or underwater ones if they can be fabricated fast enough). The mass migration of people attempting to occupy these few locations that would be available would cause mass rioting and plenty of death due to starvation as so many people travel the country-side in search of a new home.

Reclaiming the surface is largely out of the question with current technology. The blow to the resources of the planet is so great that it will be at least a few hundred years, but likely more than a millennium before the survivors could mass produce ozone and replenish the Earth's protective shell. Habitats would have to expand under the earth, hopefully expanding towards the oceans where larger settlements could eventually be constructed.

In the extreme long term, humanity could restore the ozone layer and attempt to repopulate the surface with genetically engineered surface-dwelling organisms and crops (as the originals would all be dead). The recovery from an event like this will take even longer as it will take some time before humans are in large enough populations to muster the resources required to do anything except struggle to survive. In short, this event would set humanity, and for that matter: all surface life, back to the beginning.

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  • $\begingroup$ Let's say a few humans manage to get underground/underwater. Am I correct in thinking that they can still venture to the surface but they risk major cell damage with time spent in sunlight? What about venturing out at night (e.g. becoming a nocturnal civilization that shelters underground during the harsh days)? $\endgroup$ – ben Apr 28 at 21:04
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The gmo food has had cumulative long term effects on the human population and suddenly 99.999% of the population has become infertile. It's hard to find fertile people and food that is safe to eat as nearly all the food has been modified. The food is safe to eat but you do become infertile. Well not infertile in the way that you can't have children but the children you do have are stillborn. So it's not really testable to see you are infertile. you just have to wait and see.

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For some old classics:

Nuclear Winter Most of the world is a radioactive nightmare and the dust clouds in the sky block out most of the sunlight killing off most plants. Most of the survivors are in bunkers and will die when supplies run out but a few are in the rare places that weren't hit as badly and where life is still possible(if with way more birth defects and less sunlight than in the past)

Global Warming It keeps going or possibly speeds up and ecosystems keep dying. Mass starvation starts and leads to widespread war which kills off even more. Eventually long after there stop being enough humans around to keep polluting things level off. Much of the planet could be uninhabitable at this point and what life remains will have adapted to much different circumstances and possibly seem quite alien.

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Luckily for you, Earth has already gone through 5 mass extinction events. This video should help with why/how/when:

https://www.youtube.com/watch?v=DmPgthajlh0

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