Obviously, current oxygen levels should last us years, so we are talking about a long-term solution.

Also, in this hypothetical scenario, plants have stopped releasing oxygen but haven't stop producing organics, so the food is not a problem. I understand it's a far-fetched scenario, but I don't know how else to focus the question on oxygen production specifically.


If plants stopped releasing oxygen, how could we produce oxygen for breathing, and how sustainable and effective would these methods be?

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    $\begingroup$ Answerers! to give you a hint. CO2 in the atmosphere is 0.039% vs. 20.95% for oxygen. The problem isn't making more oxygen so much as it's eliminating the buildup of CO2 (remember that great square-peg-round-hole moment from Apollo 13). You can't solve the OP's problem by making more oxygen. You need to remove the carbon from CO2 (which is what plants do). The solution should be planet-wide and sustainable. Cheers! $\endgroup$ – JBH Oct 9 '18 at 22:57
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    $\begingroup$ @JBH, the way I'm reading the question, there are no other environmental impacts than plants simply no longer releasing O2. -- To me, that doesn't mean that they're no longer using CO2, as they're explicitly continuing to grow, which means they're still photosynthesizing. -- Rather, the new form of photosynthesis would create monosaccharides that have a greater amount of oxygen than usual. $\endgroup$ – Ghedipunk Oct 9 '18 at 23:40
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    $\begingroup$ Hello Anton! The major question is where does the oxygen go? Photosynthesis, at the very high level, is simply $n\mathrm{CO}_2 + m\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{C}_n\mathrm{H}_{2m}\mathrm{O}_m + n\mathrm{O}_2$. There is no place for the extra oxygen in the sugar molecules; so, where does it go? If we knew where it went we could extract it from there. $\endgroup$ – AlexP Oct 9 '18 at 23:58
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    $\begingroup$ @AlexP, they'd also be useless to the plant as food, too. This is the handwavium that OP would have to either completely ignore or use technobabble to explain away... There's suddenly a very toxic, chemically energetic (explosive, not just flammable like wood) chemical in every plant... Or else there would have to be some other place for the O2 to go that isn't the atmosphere... So for the sake of the story, all life -- plants, animals, and other -- have their biology slightly altered overnight... and we now have exploding plants and we have to release O2 in creative ways. $\endgroup$ – Ghedipunk Oct 10 '18 at 0:25
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    $\begingroup$ Plants also need oxygen. plants still need to preform cellular respiration. if plants stop producing oxygen they won't live very long. $\endgroup$ – John Oct 10 '18 at 4:28

Simple electrolysis of water would be the first step, as simply adding an electric current to water starts breaking the water down into free hydrogen and oxygen.

Of course, free hydrogen is very dangerous. Without proper care, one spark and your electrolysis facilities will go up in flames. By capturing carbon dioxide from the atmosphere, you could free up even more oxygen, and safely store both the carbon and the hydrogen to be later reused as fuel.

These chemical reactions take extra energy to perform. Water and carbon dioxide are the lowest energy forms that molecules that are made up of oxygen, hydrogen, and carbon can take. Since human life now depends on extra oxygen in the air, burning anything will be strictly regulated, so our energy will have to come from renewable sources, such as geothermal, dams, wind, and solar.

  • $\begingroup$ Thanks for your response. But is pure carbon ever used as fuel? First time I'm hearing about it. I had the solution of extracting oxygen from CO2 in mind, but as far as I know it's problematic: if you extract only one O, you get toxic CO byproduct, but if you extract both O, then firstly it's very energy-expensive and secondly you get pure C as byproduct, which is fine, but plants can't consume it so in very long term (thousands of years) we'll be slowly stripping ourselves from organics. $\endgroup$ – A.V. Arno Oct 10 '18 at 7:55
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    $\begingroup$ Sugar charcoal is almost pure carbon and is often used for fuel. All forms of pure carbon can theoretically be used as fuel (since once ignited they will sustain their own combustion), but some (like graphite and diamonds) require a much higher oxygen concentration than Earth's atmosphere to continue burning. $\endgroup$ – IndigoFenix Oct 10 '18 at 9:54
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    $\begingroup$ Why would you use the byproducts as fuel? Then you are just reversing the process by combining the hydrogen you electrolyzed with the oxygen you made, giving you water. That goes against the whole idea of electrolyzing water in the first place, because you're back where you started. $\endgroup$ – John Locke Oct 11 '18 at 11:03

Obviously, current oxygen levels should last us years...

Yes. A few hundred millions, give or take, supposing Earth's orbit does not change much across the eons. But I am not taking leap seconds into account.

The atmosphere has a mass of about 5.15×1018 kg. That is an expletive lot. ~21% is O2, approximately 1018 kg.

According to NASA:

A man needs 0.63 kg of oxygen per day.

So for 7 billion pairs of human lungs, we should need something in the vicinity of ~4.2×109 kg of oxygen per day.

If we off every other oxygen breathing creature, stop burning stuff, and keep the population level constant, we could keep breathing for maybe five hundred million years. Even if we didn't care and didn't change anything else we could still live long enough to colonize the galaxy and import working, alien plants from some other planet.

You see, we take oxygen from the atmosphere, and the plants and bacteria put oxygen back into it, but both sides only ever handle a very tiny fraction of the O2 present in it. If plants went on strike, we would have more than enough time to find another free, green source. Meanwhile we could use the process from Ghedipunk's answer - electrolyse water to make some oxygen and get some nice rocket fuel on the side.

We could also find a way to turn silicon dioxide into metallic silicon and free oxygen - effectively using sand and quartz crystals as extra sources. The crust of the Earth is 46% oxygen by mass - enough to knock ourselves out and blow the atmosphere into space, should we ever get tired of living here. We might need a lot of acid though (this links to a question in chemistry.stackexhange.com, and the method in the accepted answer can separate oxygen from materials other than iron too).

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    $\begingroup$ Actually most atmospheric oxygen will be gone in a blink of a geologic eye. The biosphere uses about 3E14 kg of oxygen per year for respiration and such. The 1.4E18 kg of oxygen in the atmosphere will be gone in 5000 years. (OK, not really, because once the oxygen level goes down most of the biosphere will die, but anyway its thousands of years not millions.) And even the lithosphere uses 6E11 kg of oxygen per year, so it is capable of scrubbing the atmosphere clean of oxygen in 2.5 million years. Without constant replenishment oxygen simply cannot stay uncombined for long. $\endgroup$ – AlexP Oct 10 '18 at 1:53
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    $\begingroup$ "While normal atmosphere contains between 20.8 and 21 percent oxygen, OSHA defines as oxygen deficient any atmosphere that contains less than 19.5 percent oxygen" - That leaves us with only 1.3 ~ 1.5% of atmosphere mass available to use. This gives us less than 40`000 years and only if we kill everything that's oxygen breathing but humans and stop all combustion. In reality, it'll be much faster. $\endgroup$ – Mołot Oct 10 '18 at 8:46
  • $\begingroup$ If you burn the rocket fuel, the hydrogen recombines with the oxygen to produce water, so you are back where you started. You end up losing energy because the system is not 100% efficient. $\endgroup$ – John Locke Oct 11 '18 at 11:12

There seems to be two themes in the answers.

First, the current levels of oxygen would last a very long time.

Second, the problem of what to do with the oxygen from photosynthesis, if it is not released as oxygen.

I submit that there is a work-around plot mechanism for both issues.

Instead of releasing O2, the plants develop a mechanism to produce ozone, O3. Conceptually, I would suggest handwaving a new enzyme, spread by a virus, that infects all plants. It gets into the organelles, that allow photosynthesis. This enzyme modifies the photosynthesis equation enough for O3 to be formed.

Don't ask about the details of how, but given that biologists have had to resort to quantum tunneling to explain photosynthesis, I suggest that quantum tunneling would be a good mechanism to hand wave away the small details. It could plausibly explain how the electrons get from where they are in O2 to where they need to be in O3.

Once plants start to give off O3 instead of O2, we humans have a very big problem.

You see, O3 is toxic to us. Our lungs cannot use it. If we breathe pure O3, we suffocate.

Except that, for the purposes of the plot, we have solved the two issues.

Plants get rid of the oxygen, and the atmosphere becomes non-breathable to humans.

In which case, the solution is to convert O3 back to O2. This process occurs naturally in the upper atmosphere. Humans just need to speed it up, perhaps commercially.

  • $\begingroup$ Thanks. Actually, what I had in mind is that all O2 goes into organics rather than being released in any form. So a plant uses H2O and CO2 just to produce glucose (obviously, through a very different biochemical pathway), or maybe excessive oxygen is redirected to production of other organic molecules (acids or proteins, for example). After the organics are consumed (by us, for example) water and CO2 are re-released, so here is a cycle, just without oxygen. $\endgroup$ – A.V. Arno Oct 10 '18 at 7:36

"in this hypothetical scenario, plants have stopped releasing oxygen but haven't stop producing organics" -- okay, so they're now fixating carbon dioxide.

The big problem is that this produces extra oxygen and there aren't many places where it can go if we don't vent it into the atmosphere: - hyperoxygenated carbohydrates would wreck the plant's metabolism, and are poisonous. Plus they're flammable, explosive, or both. - compressed oxygen can be stored in lignin bubbles but not for very long - then it enters equilibrium and nothing has really changed - the soil contains phyllosilicates and hydrated silicates, and both are very near the maximum oxygen saturation possible.

However, the most obvious way of dealing with the oxygen scarcity would be

  • try and kill off all the non-venting plants and replace them with the old versions,
  • go live in arcologies, and supply oxygen from closed-cycle photosynthesis supplemented with chemical manipulation of whatever material the oxygen from outside gets stored into
  • extract oxygen from silicates (simple dehydration plus electrolysis would suffice).

So the oxygen just vanishes, this is the basis of this answer. No weird biochemistry to account for it, just gone. Plants still use oxygen for their own breathing, and capture CO2 like normal, thus also thriving and bearing fruit for the ecosystem to carry on as normal.

Currently, about 21% of the atmosphere is oxygen. 19.5% is deemed the lowest acceptable content, so let's say that this is with a little safety margin, and say it's 18%. So 3%-points, or about 15% of the current oxygen could be used up before bad stuff starts happening (this is excluding the ecosystem-wrecking (and by the power of decaying stuff oxygen-depletion-accelerating) side effects of a dropping O2 content). 1.16x10E18 kg of oxgen in the atmosphere total come to about 1.6x10E17 kg of depletable oxygen. User AlexP gave 3x10E14kg global biological oxygen consumption per year, and i'm going with that figure as it roughly fits to humanities part in total global biomass (1x10E-4) and human consumption as given by Shadowzee. The biosphere would consume the usable oxygen in about 5000years. Humans produce about 1x10E13 kg of CO2 per year, about 75% of that being oxygen, so burning uses 7.5x10E12kg - meaning about 1/50th of the amount used for breathing - we would not even need to stop burning stuff.

As mentioned, the above does 'The Martian'-like naive calculations. No positive or negative feedback was taken into acount, no buffering, nothing. 5000 years still is such a huge timespan compared to human life expectancy that i dare say it would give humanity enough time to die on it's own accord...

To remedy the magically disappearing oxygen, we would need to produce some ourselves to offset the 3x10E14kg O2 life on Earth needs yearly. Usually, splitting H2O would come to mind, but unless the resulting H2 can be sent into the same magic vortex the plant-O2 went, this would be a zero-sum game. Better get the O2 off something where the reduced product can be stored without immediately beginning to oxygenate again. Sand? Store the resulting Si as slag? You need about 8x10E3Wh to produce 1kg of O2 from sand, so 2.4x10E18Wh annually to even out the missing plant-output. That is about 20 times the global human energy consumption... Might take a while to scale up to these levels, though the circumstance that the other part of the equation, pure Si, can also be used in energy production through solar cells might help a bit. And us having 5 millennia to get up to speed. - The ~10 000 square kilometers of real estate to build on could be found.


We would all die, there is no way for us as a species to replace oxygen as fast as we use it with out current level of technology and reliance on fossil fuels.

Think about it like this... we have roughly 80% of the worlds energy as fossil fuels and generated via burning. Coal, Gas and Oil account for basically the entire 80%. Renewables only form around 10% with the large majority being hydro... not solar or wind.. HYDRO. The rest of renewables is roughly 2%. (https://en.wikipedia.org/wiki/World_energy_consumption) (I leave out nuclear because as always, fear mongering has basically put it into the trash can and the plants take way too long to build).

So not only are we going to need to basically increase our current renewable production, we need to do it by 5000% in a single year because we run out of oxygen. (We need this energy production because without it, we dont have the power to convert Carbon dioxide into oxygen or water into oxygen.) (I also ignore hydro because that is very dependent on the terrain, water availability and requires the construction of huge dams to properly harness)

The problem with renewables and current renewable production and placement is that we have no way of doing anything significant in so short a time frame. We are ramping up production right now, but when that 1 year ticks by, oxygen levels start to drop, everything relying on power stops working because all our energy production methods rely on oxygen. What ever oxygen production plants we do have quickly become over crowded and people starve to death, because there is no way for them to get food (cars run on combustion, your kidding yourself if you think we can increase ALL current renewables by 5000% while converting all cars into electric and building oxygen conversion centers in a single year). Its not about renewables not being able to replace fossil fuels, its just we can't physically produce the numbers necessary to replace fossil fuels and our reliance on them.

Edit: just realised you put years.. I doubt we would have enough production capabilities even after 10 years. Not with our population still growing and the amount of time it takes for governments to understand the consequences that scientists have been telling us for over a decade and a clear lack of funding and push. Heck we probably won't even realize plants stop producing oxygen until a year later.

Edit2: I know 10 years is short, and I might be underestimating our supply (But seriously, its not going to be millions of years, maths below). The bigger issue wouldn't be the oxygen we breath but the lack of CO2 absorption by plants (since you take CO2 and create glucose and O2). That combined with all the algae in the world suddenly not producing oxygen would kill off sea life.

Okay and here is the maths I did with the non google sources (I got lazy okay).

So wikipedia says the atmosphere is 5.15*10^18 and 20.95% is oxygen which is 10^18 Kg of oxygen. (plenty)

NASA says we need 0.84Kg per person and with 7 billion people it doesn't even get close to the ball park of being a worry (Another answer did this). https://www.nasa.gov/pdf/146558main_RecyclingEDA%28final%29%204_10_06.pdf

This article https://www.sciencedirect.com/science/article/pii/S209592731830375X is a bit better and goes into oxygen consumption in different industries. Basically they estimate a 0.1% drop by 2100, so I assume 0.1% per 100 years. Still plenty of time

The issue is we can't go to 0% oxygen. We breath in 20%, breath out 15% so we need 5%. Americas OSHA https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=25743&p_table=INTERPRETATIONS says an environment is oxygen deficient at 19.5%

so instead of having hunders of thousands of years for oxygen to drop to 0 we can only drop a single 1% before we are affected by it. Basically 1000 years. (16% and your out of breath while sitting, we if you want to push it, we can do the 5% drop). Still, thats plenty of oxygen, but no where near the figure that you guys have been saying. (To go on, 10% oxygen causes vomiting and 6% causes convolutions so 16% is probably the final limit we can hit before we need to recover).

The final pressing issue is that there is no oxygen renewal. Every single plant stops photosynthesizing. I assume they keep growing magically, but no oxygen comes out (and no oxygen goes in at night). Google said a tree produces 260 pounds of oxygen a year or 120Kg. Thats 120Kg per tree per year that isn't being produced anymore. Google also gives an estimate of 3 trillion trees, and that ends with roughly 2*10^14 Kgs of oxygen that doesn't get renewed per year and is removed from the cycle (I'm not sure if the science article references this or not). So we effectively lose 2*10^14 Kgs of oxygen as well as what we spend (I could be wrong in adding these... I didn't read the article in depth, only browsed through it).

And the final nail in the coffin is that algae, which produces 50%-85% of all oxygen (google again) stops. So at best, we lose 4*10^14 Kg of oxygen or 10^15 Kg of oxygen per year (Which is 25 to 10 years due to the 1% drop requirement). Algae also replenishes a lot of the oxygen in the sea, which is going to experience a sharp drop in oxygen levels and a huge increase in CO2 levels which will kill off that ecosystem (Algae wont be using that CO2 for the same reason plants don't. They get to absorb some to grow, but I assume the exact reaction to make oxygen no longer occurs).

  • $\begingroup$ Actually, I had hundreds or even thousands of years in mind:) But it's an interesting point nonetheless: I realized that getting oxygen from something would be energy consuming, but it didn't cross my mind that the very same oxygen is also consumed when we produce the energy. $\endgroup$ – A.V. Arno Oct 10 '18 at 7:46
  • $\begingroup$ @Anton I added some maths I did before. It could be wrong, but its worth the shot $\endgroup$ – Shadowzee Oct 10 '18 at 8:33
  • $\begingroup$ @Shadowzee The reason that OHSA put the lower limit of oxygen concentration at 19.5% is that sustained mental activity is not possible in a hypoxic atmosphere, that is, the brain does not get enough oxygen through lungs and blood and it cannot compensate by depriving other parts of the body from oxygen for long. First you become drowsy, then you fall asleep and finally your brain cells die in your sleep until no recovery is possible. And the best part is that you will feel slightly euphoric before the drowsiness sets in. So a 5% drop is not sustainable. $\endgroup$ – GretchenV Oct 10 '18 at 9:18

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