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I need an Earth where the atmosphere has become unbreathable for humans, but the climate should remain basically unchanged. The reason should be realistic and natural or caused by human activity.

As pointed out in What would kill all flora on Earth and what would happen with the atmosphere killing the plants wouldn't be enough and it would probably change the climate...

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    $\begingroup$ Interesting question. Anything that's unbreathable for humans is toxic to most animal life on the surface, and a good chunk of aquatic life as well. As you mentioned, killing off a bunch of species would probably have a deleterious effect on climate stability. Do you have any thoughts on this? Is it OK to kill most life on Earth as long as one can make arbitrary-but-realistic changes to keep the climate "basically" the same? $\endgroup$ Aug 16 '16 at 8:20
  • $\begingroup$ Nothing. Lethal for humans = lethal for most animals = major changes. $\endgroup$ Aug 16 '16 at 21:09
  • $\begingroup$ I think you are using the wrong word there. "What would make the atmosphere on Earth unbreathable for humans but leave the climate basically unchanged". Radioactive wasteland would qualify. So would massive release of nerve gas, that kills all cellular life on Earth. Neither if these would affect the climate. the ecosystem, yes. But not the climate. $\endgroup$
    – PcMan
    Jun 2 at 12:06
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Hatred.

We hated each other so much, that despite international agreements to the contrary, all of the "great" nations poured millions into biological warfare.

The dream of those demented scientists was a weapon which would kill off their enemies but not hurt their friends; a virus that was designed to skip one nationality, while killing all others.

One particularly "pure-blooded" nation realized that dream. They created a perfect killing machine; airborne, long-lived and highly contagious, yet selective enough to be considered a genuine racist. It spread fast but killed slow; allowing its victims one last day to clean up their affairs, shut down their societies, and make their peace.

More than a few chose to use that last day for vengeance.

5.9 Billion died from that plague. The rest died more brutally with plague-ridden hands around their throats. No one survived to greet the next dawn.

Yet our legacy, the virus, lives on.

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  • $\begingroup$ So, what actually happened? Did everyone start hating each other so much that their bodies gave out? Did they beat each other to death with plaques? Or was it a disease? $\endgroup$ Aug 16 '16 at 18:49
  • $\begingroup$ @XanderTheZenon, Okay, so I can't spell! I have raised the art of misspelling to such a level that my mistakes take the form of real words. As long as I write, human spellcheckers will always be able to find work. $\endgroup$ Aug 16 '16 at 19:47
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    $\begingroup$ Well, :D you said hate at first, then started talking about plaque victims and then mentioned a disease. What am I to think :) It's fine, really. $\endgroup$ Aug 16 '16 at 20:30
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As in What could cause toxic gas to shroud the world? most toxic compounds decay rather quickly. But a new species that pumps the toxic gas, say Tetraethyl pyrophosphate into atmosphere could work.

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Simple. An air-borne virus that only targets Human DNA.

Mechanism (A Dummies guide):

  1. A common bacteriophage (virus that feeds on bacteria) is genetically engineered to secrete endonuclease on coming in contact only with human DNA. It behaves like a normal bacteriophage otherwise.
  2. Let's call this engineered bacteriophage HMN-KILL-1
  3. Because bacteriophages are in symbiosis with most organisms on earth, they are present everywhere, in air, in ocean, in vegetation, in animals.
  4. HMN_KILL-1 secretes endonuclease only when in contact with Human DNA.
  5. Endonuclease digests DNA, instantly killing the cell, therefore the organism.
  6. Because HMN-KILL-1 behaves like a common bacteriophage when hosted by any other organism, it will replicate and spread to every ecosystem, without making any change to the ecosystem or climate.
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