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I need an Earth where the atmosphere has become unbreathable for humans and animals because all plants have died. But everything else should stay pretty much the same.

So here are my questions:

  • What could kill all flora on Earth over a period of several years without humans being able to stop it? Maybe some very aggressive necrotrophic fungi, who can spread over the air and quickly mutate to attack different plant species?

  • What would happen then? Plants would stop absorbing CO2 and its concentration would rise. From my research there are huge quantities of O2 in the atmosphere, so O2-depletion won't be a problem. However, a rise of CO2 beyond certain level would make the air toxic for the fauna. How long do you think this process will take? Will something else happen - I read about ocean acidification and global warming? How severe would these be?

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    $\begingroup$ If you manage to kill all plants all animals will die out as well, even before CO2 levels become toxic (which would take quite some time) because they would starve to death. Herbivores would die first, carnivores immediatly after. $\endgroup$ – Annonymus Aug 14 '16 at 20:01
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    $\begingroup$ We have been putting a lot of CO2 in the atmosphere in excess of what plants can consume, the level of CO2 is increasing "a lot", yet we are no way near levels were the concentration becomes toxic. Given that animals and humans would die in a few years of starvation what is left would be a planet with only bacteria consuming cadavers, and I don't think bacteria will care too much about a bit more of CO2. $\endgroup$ – Bakuriu Aug 14 '16 at 20:20
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    $\begingroup$ Flora aren't actually the prime oxygen-makers of our planet. The phytoplankton is -- if you killed all the plants, there would just be a big plankton bloom to compensate for it. Meanwhile, if you killed all the phytoplankton.... RIP Earth $\endgroup$ – Tony Aug 16 '16 at 4:44
  • $\begingroup$ What about the rotting dead plants? This would release a lot of CO2 in the atmosphere. And the phytoplankton would need some time to "catch up". What do you think the time intervals would be? $\endgroup$ – yado Aug 16 '16 at 6:09
  • $\begingroup$ As long as phytoplankton are still around, we shouldn't have much of a problem with oxygen - once the oxygen level drops to the point where the zooplankton that eat them slow down their reproduction, the cyanobacteria population should bloom quickly and balance things out - single-celled organisms reproduce fast, the only reason they don't dominate the seas is because the organisms that eat them multiply just as fast. Food for land animals is still a problem though. $\endgroup$ – IndigoFenix Aug 16 '16 at 9:13
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It's quite unlikely a single cause could kill all plants (apart from the sun going out). However, killing the right family of plants is much more plausible and causes a lack-of-food apocalypse quite readily. John Christopher's novel No Blade of Grass deals with a virus that starts by killing rice plants, which is very serious by itself, and mutates into a form that kills all grasses, including wheat, corn, barley, etc.

If you kill all plants, then as per Annonymus' comment, all animals and people will die of lack of food well before the atmosphere gets to be a problem. After everything has died, the oxygen in the atmosphere will slowly be depleted by assorted chemical reactions, but this will take decades, rather than the few years it takes for everything living to die.

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    $\begingroup$ Cyanobacteria that caused first oxygenation in Earth history are still there. They still can, if given right conditions (no plants and animals to compete with) bloom and produce oxygen faster than geology can suck it up. Last time they topped at about 30% (compare with 21% today). $\endgroup$ – Mołot Aug 14 '16 at 20:30
  • $\begingroup$ I'll look into the book. I assume that some people can survive in specially constructed structures and produce their own food. If most plants are killed and start to rot lots of CO2 will be released in the atmosphere. How long will the cyanobacteria need to recover the O2 level? Why is then the current CO2 level rising? $\endgroup$ – yado Aug 16 '16 at 6:19
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Let us assume that by plants we mean multicellular photosynthetic organisms.

No natural cause seems to be able to kill all plants (unless it destroys all the life on Earth, like increasing the Earth's temperature).

Unless alien civilisations are involved, this could be either a technological catastrophe, or an act of terrorism.

Not all animals will die. Cyanobacteria will survive, and they may serve as a food for animals. There are also cells able to perform chemosynthesis, which will also survive, and animals may eat them. So on some small scale, the life on the earth will remain.

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  • $\begingroup$ This reminds me of situations where natural predators are thinned or removed. Prey population explodes until disease and famine occur or until another predator fills the role. If the primary oxygen providers are removed, wouldn't secondary providers, algae, kelp, moss, etc. explode in population? $\endgroup$ – Jammin4CO Aug 15 '16 at 20:47
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How realistic does your solution need to be? Could a cosmic event 'suddenly' shift the wavelengths of light into a separate band or filter out those wavelengths most plants need in their microbiological processes? If this happens 'sudden enough' evolution might be too slow and all photosynthesis-oriented organisms would die. I guess sooner or later there would be accommodation, but there are certainly a few decades of havoc and mayhem. However, a shift of wavelengths would certainly be noticeable by non-plants as well...

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  • $\begingroup$ I really want it to be realistic and somewhat science-based. Basically I need the atmosphere to become unbreathable for humans without any changes to the climate. $\endgroup$ – yado Aug 16 '16 at 6:31
  • $\begingroup$ @yado Just humans? Wouldn't some specialised virus / nano-robot / chemical do the trick more nicely then? $\endgroup$ – BmyGuest Aug 16 '16 at 6:36
  • $\begingroup$ Well, humans and animals. I need something more rudimental, a specialised virus / nano-robot / chemical is a little too exotic for me. $\endgroup$ – yado Aug 16 '16 at 6:48
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I'm not sure what could kill all flora without killing fauna, but as for what would happen:

Some people with stockpiled foods may survive for a pretty long time. There will be a fierce battle for the stored supplies of grains, etc. In some places, people with weapons will take control of places where much food is stored and protect them like fortresses. These "people with weapons" will often be governments, armies, etc.

It's therefore possible that some humans will survive for long enough to feel the oxygen becoming a problem.

But humans do have technology to synthesize oxygen. I therefore think some resourceful people will build some enclosures with machines that create oxygen. Sort of like a moon base.

Given that all of humanity now has a few months figuring out how to survive, I think some of the most resourceful and powerful people in the world would get together and figure out something. Oxygenated enclosures would be built, and artificial photosynthesis would be employed to create hydrocarbons for human consumption.

Eventually life on Earth would be sort of like living on a foreign planet. You'd need an oxygen mask to leave the enclosure.

I predict that the vast majority of humanity would die. But I think a few, privileged humans with access to advanced technology would survive. After that, human settlements would be quite vulnerable. There would need to be redundancy in the oxygenators and artificial photosynthesis machines.

The surviving humans will probably eventually try to figure out why the plants died, and if there's any way to make seeds sprout again.

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    $\begingroup$ I would like to see your third paragraph blown out of proportion. Imagine a society that discovers how to quickly and efficiently convert CO2 and CO into O2 cheaply via industrial means. "COOL, we can undo all the problems we've caused." Production ramps up and before we realize it, there is no more CO2 for plants and they all die off before we determine the problem. $\endgroup$ – Jammin4CO Aug 15 '16 at 20:41
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Wouldn't killing the Algae in the oceans break the Oxygen cycle? Here's a fun fact, we're on the way to doing that for reals. We might get to see what your apocalypse actually looks like, albeit for a very short time.

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  • $\begingroup$ Then I might have a possible solution :D How are we on the way to do that? What would kill the algae? $\endgroup$ – yado Aug 16 '16 at 6:30
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increased irradiance from the sun, atmospheric response to increased irradiance, surface temperature increase, volcanic and tectonic response to increased irradiance and increased surface temperature, fluid dynamic anomalies, bacterial and microbial increases due to all of the aforementioned conditions, soot and toxic gases increase due to all of the aforementioned conditions, water sources along with evaporative processes spread and increase the surface toxicity of said planet, plant and animals ingest said toxins, microbes, and pollutants, exstinction level event progresses to the "tipping point" and the the multiplicity and speed of the aforementioned catalysts cause vast and widespread planetary failure of all life supporting processes very swiftly, as the conditions progress, they are more erratic and more unstable, the planet would eventually be choked in a thick toxic soot, and everything natural to a necessary normal environment will die, throughout this process. this is the course of the present planetary condition. ....if the planet itself physically survives these processes, which is not likely, due to the tremendous explosive forces of dying stars, and their abilities to change drastically the vast space they have influence over around them, then eventually at some point the planet could return to a somewhat stable atmosphere of some variation of it's pre-affected condition, again, very unlikely, but if it were to, then it would appear as other "dead" planetary bodies do presently. most have vented their atmospheres, or their atmospheres are toxic and uninhabitable by natural means.

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Something like a third of Earth's annual oxygen budget is the result of oceanic phytoplankton blooms you could kill all the phytoplankton, with a ocean going blight, either a bacterium or a fungus, possibly even a virus, that attacks chloroplasts directly. If you do it fast enough land plants won't be able to compensate and the atmosphere will get pretty toxic in short order, not least of all because the dead plankton is going to rot, and then everything that eats them (which is ultimately all sea life) will go extinct too. Life on land will persist relatively unchanged for a year or so before there's not enough partial pressure of free Oxygen for animals to breath. Carbon Dioxide and Methane may become toxic before then but how long that will take is based on some really complex interactions between the atmosphere, seawater, and land and also on the breakdown pathways for all that dead sea life. I wouldn't like to try and guess how quickly this disaster might unfold and certainly I couldn't give you a confident science-y answer.

I'd have a look at John Christopher's The Death of Grass for a model of a progressive infectious agent destroying a vital part of the planetary life-web and some very starkly realistic ideas about probable governmental and personal responses to such a crisis. He doesn't go as far as you appear to want to take your story but it'll give you a stepping off point, be warned it's a really depressing read.

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