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A bit of a whimsical question, or a few questions really.

What would happen in a world with no air - no oxygen (plant life stop producing oxygen).

Specifically, let's say the oceans shut down oxygen production due to [insert reason here]. For that matter, what could cause the oceans to do this? I think I read that global warming would effect algae production, and overheated oceans aren't good for most life.

Then let's say someone manufactured a nanovirus that affected all plant life, removing their ability to create oxygen, while keeping their lives intact (absorbing carbon or some other evolution). Some sort of mutated photosynthesis where only half the equation was in place.

How long would it take before we ran out of air? Would fires greatly affect this, or would they not be able to burn? Would people suffocate, or just slowly blackout? What kind of timelines are we looking at - how long before the 7 billion people on the planet (plus animals) ran out of air/sufficient oxygen? Would humans be able to manufacture their own oxygen (I know they could, but at what levels)?

What kind of differences in timelines would there be if the ocean remained suitable but all land-based plants stopped producing oxygen?

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    $\begingroup$ There are some interesting concepts here, but what, exactly is the question? Try editing a bit to make the question more clear. $\endgroup$ – Thucydides Nov 3 '15 at 1:08
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    $\begingroup$ Do you mean no air as in just no Oxygen, 100% Nitrogen? Or do you mean no air as in total vacuum, no anything? $\endgroup$ – John Robinson Nov 3 '15 at 1:19
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If I am not mistaken, your question is what will happen if oxygen production ceased completely. As oxygen is a highly reactive gas, obviously it would be depleted eventually, as you predicted.

The Earth's atmosphere currently contains about 21% Oxygen. This is about $10^{18}$ or a quintillion kilograms. Since 1 atm is 101 kPa, the partial pressure of oxygen at sea level is about 21 kPa. Modern day humans are unlikely to survive when the partial pressure falls below 8 kPa

Oxygen can be consumed by many different methods. One is to bind oxygen to biomass in order to produce $\text{CO}_2$, water and energy. With no plants to reduce the $\text{CO}_2$ back to biomass, there is not only an oxygen problem, but a food problem. If plants continue to reduce $\text{CO}_2$ but do not release the oxygen, they would have to keep the oxygen in oxidising agents, separate from their energy stores, which would presumably trigger, in the long term, animals that can process this.

In any case, with no recycling, the oxygen is consumed by biotic processes at the same rate biomass is oxidized, I'll assume that the production of 100 billion tonnes C/yr, according to wikipedia, also ceases, because I don't think plants can keep large amount of oxygen in them, so they die of the free radicals eroding their DNA. There are only 560 billion tonnes of biomass apart from bacteria though, so they'd only sequester around 1.5 trillion tonnes (1.5 quadrillion kilograms) of the 1000 quadrillion kilograms of Oxygen. Just 0.1%, not much.

Where did all the rest of the carbon go? Well, some of it is in bacteria, but a lot of it is in the stuff we're currently digging up and burning. If all the plants die, it'll take a while for people to realize that burning all those fossil fuels isn't such a good idea, but it will happen. We'd likely switch to carbon free fuels like Uranium and Plutonium (in fission) or, eventually, hydrogen and helium isotopes for fusion. The problem of feeding ourselves is the immediate one. Happily though, humanity has stored some seeds in seed banks. They won't be modified. We plant those and the ecosystem will eventually find itself on its way back to normal. Might take a while though.

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  • $\begingroup$ Won't the pressure be raised by the outputted $\text{CO}_2$? $\endgroup$ – HDE 226868 Nov 3 '15 at 2:14
  • $\begingroup$ @HDE226868 Good point, but only converting 0.1% of the oxygen, which is about 0.02% of the mass of the atmosphere, from all the animal and plant biomass, isn't going to change things that much, IMO. Perhaps there's a lot of bacterial biomass? $\endgroup$ – timuzhti Nov 3 '15 at 2:30
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For an Earth with other parts of air, but no oxygen production, things would get real bad for living things:

How oxygen would deplete

  • First off, the factories would consume a large amount of oxygen very quickly.

  • Another mega consumption would come from vehicles running on fossil fuel.

  • Then add in the consumption by aerobic organisms like all mammals, birds, reptiles, fish etc.

  • Forest fires.

How the world would respond

The revelation, that the oxygen content of atmosphere is decreasing, would come very soon. I would say within 3-4 days at most, if any science research team happens to be involved in a relevant research.

By that time, the oxygen content of atmosphere would be depleted by about 0.5% (rough estimate). With immediate research on the cause of oxygen depletion, the revelation that plants are not releasing oxygen, would come as a mind-blowing shock.

An urgent meeting of all UNO members would immediately be arranged. All factory production units around the world would be shut down and the vehicles stopped in an effort to preserve as much oxygen as possible, until a solution is found.

Huge plants of thermal decomposition of seawater would be quickly built and started all over the globe in an effort to replenish the oxygen being consumed.

A research team would begin investigating on means to come up with a solution for the plants inability to release oxygen in the atmosphere. If large reserves of oxygen are found trapped in the trees, research would be initiated in the direction of releasing that trapped oxygen without having to kill the plants.

The Result

Without any durable means to release the oxygen trapped in plant tissue, it would be impossible to sustain all modern architecture and lifestyle with simply decomposing seawater to replenish consumed oxygen in the atmosphere. We might be able to produce enough oxygen for the humans, but considering that there are so many other players in the oxygen consumption (plants too, for example, consume oxygen besides producing it), the aerobic world is doomed sooner or later.

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  • $\begingroup$ Thanks, that has given me some good perspective, I had no idea about thermal decomposition! $\endgroup$ – Thomas Wilson Nov 4 '15 at 2:18
  • $\begingroup$ It takes more energy, but gives much faster results than electrolysis. $\endgroup$ – Youstay Igo Nov 4 '15 at 3:23
  • $\begingroup$ By architecture do you mean our buildings would collapse? $\endgroup$ – Thomas Wilson Nov 4 '15 at 6:06
  • $\begingroup$ No, not the buildings but the scientific and social lifestyle progress we have made so far. When factories are shut down and all motor vehicles are garaged, you get no production of anything. Cellphone, computer, even fries and doughnuts won't be available in standard packing. $\endgroup$ – Youstay Igo Nov 4 '15 at 6:29
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    $\begingroup$ I am sorry but... did you do any numbers on that?! Also when you say "Huge plants of thermal decomposition of seawater"... where do you think you will get that energy from?! 85% of the world's energy today comes from fossil fuels and biomass, the extraction of which requires oxygen. And even if we somehow managed to quickly build lots of nuclear plants, what would you do with the leftover hydrogen? You cannot release that into the atmosphere too because then you make the whole thing explosive. $\endgroup$ – MichaelK Oct 8 '17 at 19:01
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the Oxygen in air is in fact, far more than that would be explained by photosynthesis alone. in fact, if you were to burn every last bit of fossil fuels and every organism in the world, only 12.5 percent of all atmospheric oxygen, or 2.5 percent of atmosphere worth of oxygen will be consumed.

This surprising result comes from the fact that: Water vapour photodisassociates in the upper atmosphere, the hydrogen escapes, and the oxygen remained, forming 60 percent of all oxygen in the atmosphere https://thesis.library.caltech.edu/3940/; Magnetite or iron(iii) is turned into fayalite or iron(ii) near the core mantle boundary of earth, releasing oxygen in the form of o2 and h2o/co2 while consuming carbon and hydrogen gas, this accounts for 30 percent of all oxygenin the atmosphere. The rest is dynamic oxygen, the oxygen generated from recent photosynthesis of which the resulting fossil fuel is still in the crust. this is just 10 percent, while the rest is turned into ferrous ions in the mantle or iron in the core. https://3g.163.com/news/article/6VP08M7N00014AED.html

The total lifetime of oxygen, given respiration and combustion power generation, is arount 4000 years for the modern concentration, half if the lowest survivable concentration is concerned.

However, without influx of new organic material, all sources of carbon will be depleted by respiration and combustion within 250 to 300 years, rendering the leftover oxygen without fuel to be burned up with.

for this scenario though, there will be no significant difference within 1000 years.

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