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We know the common tropes for an apocalyptic event: meteor/asteroid hits earth; crazy infectious virus/bacteria; AI takes over; we nuke ourselves; climate change; hostile aliens. But what are some other, less known but still plausible, ways an apocalyptic event could go down.

Requirements:

• Human populations must experience a devastating blow (say at least 75% dead, and does not need to be evenly spread out) or be completely eliminated


• The blow to humans can be direct (e.g. radiation poisoning) or indirect (e.g. starvation because no arable land)


• All other plants and animals may or may not be affected (this is unimportant)


• This has to be somewhat plausible according to today's science (e.g. no time-traveling humans from the future, no we all get sucked into a parallel universe)


• This can't be a super popular or well known apocalyptic premise (e.g. no hostile AI takeover)

I'm looking for things like: Phytophthora, a water-mould genus that affects a wide range of food crops (think potato famine) mutates and destroys crops worldwide. Or a rogue planet enters the solar system and takes earth out of orbit. Or the whole gamma-ray burst thing. Or a super-volcano goes off. Basically anything that is somewhat scientifcally plausible but not (very) common knowledge. Please also give evidence as to why this event could be feasible (though I would also be interested in well theoriezed events without as much evidence)

Here are a few:

1. Super massive solar flare removes most of our atmosphere/scorches the earth. Hard to balance between everything destroyed and nothing destroyed, the earths magnetic field stops most solar wind from doing damage, but it could be overwhelmed.




2. Cult like religion takes over with a goal of depopulation. No explanation needed.




3. Collapse of an overly centralized system. Say technology advances to the point of completely replacing human labour. To counteract the massive loss of jobs, the government takes control of production. If the government was then to collapse, the supply of everything could disappear. Even if the production mechanism is fixed, the complete disappearance of the food supply for example would do massive damage in just a week (though a couple months would be where the real collapse would probably peak).




4. Widespread use of a flawed genetic procedure. Say a gene editing program begins to give everyone a gene that prevents cancer. It turns out the gene is flawed and stops the ability to reproduce after a number of generations. It is likely that technology would also then be able to remove the gene, but this can be an odd one.

Artificially generated Peak Oil event.

One of the trickiest technological approaches to problem solving, and the one that has consistently generated the bulk of our unanticipated consequences, has been the introduction of biological controls into the ecosystem. That said, we continue to develop such controls because they tend to be quite effective, at least in the short term.

So; enter Alcanivorax borkumensis, a bacteria that literally eats oil and has been designed to help with environmental contamination scenarios when oil tankers & rigs spill massive amounts of oil into the ecosystem.

We are now working on a transition away from fossil fuels globally but setting up new infrastructure, bringing costs of new technology down etc. takes time and the world isn't ready for an instant peak oil event.

But, if this bacterium got out into the ecosystem in an uncontrolled manner and infected all the oil reserves globally at the same time, we could end up with oil depletions on a massive scale within months.

Believe it or not, this would be every bit as devastating as a human infection. For a start, your 75% death rate would be achieved relatively quickly by virtue of the lack of food getting into large cities. Oil (read as petrol) is now a critical part of the food chain because of the food distribution requirements of larger population centres. Large cities simply don't produce much food, but they consume massive amounts of it. Disrupt fuel supplies for a month, and you'll face a complete and total breakdown of law and order and city life, triggering massive unrest, deaths from starvation and civil disobedience, and a mass exodus from the cities into country locatinos without the infrastructure to support the survivors. Not to mention that most farms today are 'productive' thanks to fuel and technology that is no longer available.

Also, many modern farms contain genetically modified seed stocks that can't be replenished through 'seed cropping' and the fertiliser needs of the soil are massive, meaning that without a functioning distribution network the farms simply couldn't support the populations coming out of the cities even if they wanted to - farming looks too different from 100 years ago when all those people could have been put to work on the farms instead of machines.

This is not a total extinction event by any means, and certainly pockets of civilisation would survive because of the use of nuclear and renewable electrical grids and the uptake in certain areas of electric vehicles, but planes would be grounded, we don't really have a functioning electric heavy vehicle infrastructure in place and many societies still rely on petrol as a key driver of their infrastructure. As such, you'll find that your society would face massive losses in the short term, and significant losses over the medium term, if all the oil on earth just suddenly vanished.

In the webcomic Nine Planets Without Intelligent Life, humanity just felt in such an hedonic state that people stopped reproducing. They would have robots serving and pleasing them in all ways possible and lost interest in human reproduction. Eventually humans all died out.

This is tangentially aligned with the V.H.E.M.T., which is a thing in the real world:

The Voluntary Human Extinction Movement (VHEMT) is an environmental movement that calls for all people to abstain from reproduction to cause the gradual voluntary extinction of humankind.