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I've been thinking about this for a while, and was so glad to discover this forum exists!

Humans aren't going to be around forever. But how will we actually get wiped out?

Even the Chicxulub impact event that wiped out the dinosaurs ~66 million years ago only extincted ~75% of living species at the time. But humans have tools and technology. We have a huge number of underground bunkers, tons of preserved food and bottled water. We are resilient and adaptive. What kind of event could happen that would actually kill off all humans on the planet?

Pandemic: not a chance. Too many people live in geographically isolated regions and could wait it out.

Nuclear war: not likely. Still too many isolated regions (Pacific Islands, Iceland, Arctic Canada) would avoid the worst effects of initial strike + radiation and could wait it out.

Asteroid hit: this is the most likely extinction event. But even here, there are enough bunkers and preserved food that I'm sure a small number of hardened survivors could survive underground for a few years and make it through.

So what's the end game? How will we actually get wiped out?

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  • $\begingroup$ There is a lot of possibilities, it could be almost anything. $\endgroup$ – Vincent Mar 27 '15 at 4:16
  • $\begingroup$ Global warming. 500 years, give or take. See e.g. the Permian-Triassic extinction (aka the Great Dying), and note that one of the most plausible explanations is a large volcanic eruption setting fire to large coal beds. $\endgroup$ – jamesqf Mar 27 '15 at 4:53
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    $\begingroup$ Unfortunately, this question is too opinion based for this forum. While all world building questions have some level of opinion-based nature to them, at some point it becomes difficult to put answers into the voting based scheme used on Stack Exchange. You might be able to resolve this by adding some verbiage as to what sort of solution you are looking for (ex: simple, understandable by a YA reader, not reliant on ridiculous levels of random chance, and using as little magic or advanced technology as possible). That could turn this from an opinion-based question into an answerable one. $\endgroup$ – Cort Ammon - Reinstate Monica Mar 27 '15 at 4:56
  • $\begingroup$ ... with an addition such as this, it would be easier for people to vote for the "best" answer because we will have a good sense of what sort of answer you are looking for. $\endgroup$ – Cort Ammon - Reinstate Monica Mar 27 '15 at 4:56
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Life is very stable, mutating and adjusting to the changing parameters. That means no short term change can totally eliminate it. It is also possible that a very small percentage, say 0.001% of humanity survive catastrophe's like nuclear wars, huge comets hitting the earth etc, all of these will still leave livable patches on earth.

However, if the change is drastic and permanently increasing so that it propagates to all corners of the world and the effect is progressive in nature , it will be the ending factor.

Such drastic and ever increasing changes include the following possibilities :

  • Earth is dislodged from its orbit by an external hit or a huge gravitational pull of an external body passing through the solar system. As earth falls nearer to the sun, temperatures rise day by day, water becomes steam mostly , then the surface heats up to unbearable levels and gravitational pulls cause earthquakes of unimaginable levels , things start melting until the earth falls into the sun.

  • The above scenario can also happen if there is a black hole formed at the center of our galaxy and pulls the solar system towards it, there will be an eventual collision of earth with the sun or another body which inturn will go into a sun.

  • Earth is dislodged , this time, away from the sun into space, everything cools down, loss of power , plants die slowly due to diminishing sunlight, , the atmosphere is lost , major life forms dont survive after a century but minimal life forms can survive billions of years under frozen oceans powered by the earth's core as the planet is able to maintain livable temparature deep inside.

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Define Humanity No, I did not get crazy. No, I do not accept: "Look in the mirror, duh" answer. Will give you example:

Look outside window on a tree. See the little winged animal sitting on the branch, joyfully tweeting? What is it? It is dinosaur

Birds evolved from dinosaurs. And it is plausible to expect, that in several million years, we will evolve into something else. Will that species be defined as humanity? Or do you need just time, when last example of Homo Sapiens Sapiens will roam around this planet?

If you allow also our evolutionary descendant as "Humanity" then I put my bet on scenario, that some form of humans will be around till our Sun turns into red giant and heats up our planet to unlivable temperatures

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  • $\begingroup$ No, that tweeting creature is not a dinosaur - unless of course you consider yourself to be a reptile :-) $\endgroup$ – jamesqf Mar 27 '15 at 18:34
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    $\begingroup$ That creature is dinosaur the same way as we are apes. Thats why I want to define humanity. Birds evolved from dinosaurs and most proably, we will evolve into new species too. Will that be still humans? $\endgroup$ – Pavel Janicek Mar 28 '15 at 18:15
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As you point out, a single event is unlikely to get everyone. However, a combination of events can be much worse without becoming too implausible.

To get rid of all humans, you need to do two things:

  • make the entire planet uninhabitable without technology
  • get rid of technology

Some sort of climate change is probably the best way to make the surface uninhabitable. Overheating would be harder to deal with than an ice age - lots of people live in cold places, but there's not much you can do if it starts to look like the surface of Venus.

You can make this happen suddenly if you want - an asteroid impact or super volcano could plausibly affect the atmosphere enough to seriously unbalance the climate.

That will wipe out almost all life, but humans have the technology to survive in space - we don't generally build completely isolated habitats on Earth, but that would change as soon as we saw the asteroid coming. It could even be done to a lesser extent if there was no warning. Most places would survive the initial impact - the real problem is that crops will stop growing for longer than the stores food will last, which happens slowly enough to be potentially solvable.

To prevent this happening you need a different kind of disaster - a nuclear war or pandemic fits well here, since even though it would miss isolated areas, it would take out all the places capable of adequately preparing for the upcoming disaster.

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  • $\begingroup$ Humans do not have the technology to survive in space, without frequent resupply from Earth. $\endgroup$ – jamesqf Mar 27 '15 at 18:35
  • $\begingroup$ @jamesqf having the technology is very different from it being reasonable to use under current circumstances. We could build a farm in space, but unless you are feeding thousands of people or are much further away it is far cheaper to have the farm on Earth. $\endgroup$ – Quentin Clarkson Mar 27 '15 at 21:06
  • $\begingroup$ I think you misunderstand. It's not that we couldn't build a farm, or many farms, in space. It's that they, and the humans who depend on them, would not survive indefinitely as isolated ecosystems. We simply do not know enough to construct a permanent isolated ecosystem. See e.g. the multiple failures of the Biosphere 2 project. $\endgroup$ – jamesqf Apr 2 '15 at 21:13

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