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So here's the premise:

We have an Earth-like planet, with a civilisation at roughly the same state as ours today. Scientists have been monitoring the atmosphere for centuries, what with forecasting the weather and the whole debate about greenhouse gasses, and all that. So far nothing unusual.

But now they have suddenly noticed something strange, and quite scary: The levels of oxygen in the atmosphere are dropping.

At first the scientists don't make a fuss about it, just trying to work out what's going on, but as the levels continue to drop, the news becomes public and the population starts to panic. We're not at a stage yet where everyone's suffering breathing difficulties, but if things continue it will start getting serious within about a year.

Nobody knows what's causing this or how to fix it. It's only oxygen that's affected, no other gasses. There's no obvious changes in global plant activity, no unusual volcanism, no unexpected melting of the tundras... what can be going on?

So, what is causing it? My favoured cause is that the planet is passing through a belt of hydrogen gas in space. This is being collected by the planet as it moves through space and is reacting with the oxygen in the atmosphere to form water.

Is this plausible? If so how will civilisation save itself?

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See this meta post: meta.worldbuilding.stackexchange.com/questions/522/… – Tim B Feb 12 at 10:35
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Is oxygen simply out, or replaced? And if replaced, then by what? Because at 21% it creates significant part of atmospheric pressure. – Mołot Feb 12 at 11:14
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@Mołot - with my space hydrogen idea, it would be replaced by water. a lot of that would probably stay in the atmosphere, but you do have a good point. – Simba Feb 12 at 11:44
    
I've edited this to be a reality-check question and cleared the close votes. It should be fine now. – Tim B Feb 12 at 12:22
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To be clear, this is no longer a "How can I make this happen?" question, but a "Is my idea plausible" question? – Tophandour Feb 12 at 16:18
up vote 9 down vote accepted

Your premise with interstellar hydrogen causing decrease in oxygen in atmosphere is NOT plausible.

TL;DR: Forget interstellar hydrogen. There is plenty of hydrogen in solar system. For bonus oxygen elimination, you can burn some iron and sulfur, it is plentiful too. Anything which oxidizes would do. But you need to burn huge amounts of material, and the arrival of that material to atmosphere will be most likely visible.

Full answer:

Because of solar wind, no interstellar hydrogen will be able to reach the Earth orbit (even if there might be a cloud of hydrogen in interstellar space).

BTW interstellar space is extremely empty, see density of interstellar medium. "Dense" is 10^6 molecules in m3. Atmosphere on sea level is 10^19 molecules per m3. Good man-made vacuum is 10^10.

Voyager only now, after decades of flight, left outer area of solar wind influence and is really out in interstellar space.

Earth is safely tucked in heliosphere, no interstellar gas can reach us that easy.

As TimB calculated, you need a lot of hydrogen. To get such amount of hydrogen to planet so quickly, no obvious natural process is plausible. Geological changes work on geological timeline (millennia).

If you are in such a hurry, use aliens. They may hurdle small (hard to detect) balls of frozen hydrogen to planet's orbit, to eliminate oxygen which is poisonous to them. They are alien-forming this planet to their liking. Kuiper belt might have enough hydrogen-based materials for that. Build a factory on Sedna, extract hydrogen, ping it down the Sun's gravity wall toward Earth. Fun has been had by everyone involved.

Kuiper belt might have mass up to 10% of mass of Earth, most of it being hydrogen. Then there is more nearby hydrogen in Oort cloud - more than in any random interstellar hydrogen cloud, even if much farther and even more dispersed.

If you go this way (aliens alien-forming Earth), Jupiter's moons Europa, Ganymede and Callisto have lots of water too, and are closer to Earth. Io has lots of tasty sulfur which aliens prefer over the oxygen :-) But then they might just like Venus as it is now.

Of course hydrogen balls from moons would have to escape Jupiter's gravity, but because they are on the Jupiter's orbit it should not be too hard.

You would also have to slow down photosynthesis (which converts water back to oxygen). One way would be to put lots of sulfur to atmosphere (as I mentioned from Jupiter's moon IO), to start runaway greenhouse effect like on planet Venus, with high temperatures and dense clouds (preventing light reaching lower atmosphere) acid rains.

Another way to bind a lot of oxygen is iron. Iron is pretty common too. Small iron meteorites. You want them small, so they will burn in the atmosphere and do not fall down to the surface. Thousands of tonnes per day. This will be more obvious - planet's surface will be covered by red dust (rust).

Iron, hydrogen and sulfur bombardment will make beautiful night view: lots of falling stars. To avoid this (make it more sneaky), you can create iron and sulfur dust, mixed in frozen hydrogen for delivery.

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Okay. that's a definitive deal breaker for that idea, then. Thank you. – Simba Feb 12 at 16:25
    
I wonder if there's any way around this? Some kind of opposing force pushing the hydrogen in. eg if we get close enough to a nebula with star formation going on, the ignition of a new star would kick up a pretty major blast of stellar wind toward us. Could that be enough to counteract our own solar wind and push some of the hydrogen into our system? – Simba Feb 12 at 16:43
    
Such star formation nearby would fry life with UV radiation. – Peter Masiar Feb 12 at 16:46
    
This is a good point. If the hydrogen cloud were dense enough then it would still reach the planet - but you're going to need it to be even more dense than I already calculated. – Tim B Feb 12 at 21:32
    
@Simba, why do you need hydrogen to be pushed into the system at all? Your OP only said that only oxygen is being affected. Why not make it related to declining magnetic fields and/or solar wind or something. I guess I'm just wondering why the persistence of the hydrogen vs oxygen theme here. – fractalspawn Feb 12 at 21:59

Your problem is mass. There's a LOT of oxygen in the atmosphere.


Hydrogen needed

For each $O_2$ you need 4 hydrogen atoms.

Our atmosphere has a mass of approximately: $5.15×10^{18} kg$

By mass 23% of that is oxygen (by volume it's 21% but we're interested in mass).

That gives oxygen: $1.18*10^{18} kg$

Combining Hydrogen and Oxygen gives water. $\frac2{18}$ths of the mass of that is Hydrogen. The ratio of Hydrogen to Oxygen is $\frac2{16}$.

So: $$1.18*\frac{2}{16}{*10^{18}} kg = 1.48 × 10^{17} kg$$

of hydrogen would be needed to combine with all the oxygen.


Hydrogen available

The densest nebula (clouds of gas in space) have a density of $10^4$ particles per $cm^3$. Even if that was pure hydrogen that's a mass of $1.67 *10^{-23}kg$. Per $km^3$ that's $1.67*10^{-8}kg/km^3$

The earth has a radius of $6,371 km$. Even if we say it's gathering in hydrogen up to $10,000km$ away that gives a capture area of $3.14*10^8km^3$

Lets say this is an interstellar cloud, our solar system is moving at a speed of $220km/s$ relative to the galaxy.

So in that time earth covers a volume of $6.9*10^{10}km^3/s$

This means it gathers $$1.67*10^{-8} * 6.9*10^{10}kg/s = 1152kg/s$$

That's right, all that movement and density gives us just over a metric tonne of hydrogen every second. Sounds like a lot?


Conclusion

We need $1.48 × 10^{17} kg$ of hydrogen.

$$\frac{1.48 × 10^{17} kg}{1152kg/s} = 1.28*10^{14} seconds$$

That's $4~056~162$ years

So the earth flying through one of the densest nebula we know about with a very unlikely speed difference and the nebula being made purely of hydrogen...would need 4 million years to gather enough hydrogen to react with all the oxygen in our atmosphere.

In other words you need some way to make the nebula not just a little denser but a million times denser than any known nebula and you would then have enough hydrogen to convert 25% of our oxygen into water per year.

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Awesome. Thanks for the fact and figures. Just to clarify, I wasn't trying to say that all the oxygen should be gone in a year; just that it would be sufficient in a year to begin causing problems. So we can probably cut a digit or two off your final figure, as you're working on removing it all. I guess I do still need to change that parameters though -- clearly a year isn't anything like enough. But it needs to be short enough to cause the population to start getting panicked. Maybe a decade? Still a lot of orders of magnitude removed from your answer though. – Simba Feb 12 at 16:27
    
Yeah, basically you need a way to make a really heavy concentration of hydrogen for us to fly into. A nova close enough to still be that concentrated would have fried the planet so options are pretty limited. Maybe some gas giants colliding and throwing a huge amount of gas into space or something like that. – Tim B Feb 12 at 21:30
    
Heliosphere will protect inner planets from interstellar hydrogen as I explain in my answer. – Peter Masiar Feb 12 at 22:14

While plausible, the reaction between hydrogen and oxygen is somewhat violent, think rockets engine...

I'd go with a reverse cyanobacteria mechanism, but I don't know how or if a civilization can (with a really big if) save itself in such a short time. For humans to just go down from 21% to 18% had some consequences. If in a year the oxygen level goes down from 21% to about 10%, humans are dying in less than a year. This is assuming that the missing oxygen is replaced by a non toxic gas.

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Hmm, okay. I wasn't anticipating to too be too violent -- space gas is quite diffuse but envelops the whole planet, so I was thinking it would just merge into the atmosphere and react to form water molecule by molecule. Possibly some heat byproduct, but would it really be violent in this context? In any case, thanks for the answer; it seems like I'm probably going to have to adjust some parameters here. – Simba Feb 12 at 11:38
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the (observable) violence of H-O reactions depends n a number of factors. A really violent burning or explosion only happens when they are well mixed. if on the other hand you have only surface connections, and in a very thin atmosphere (stratospehre) to begin with, the violence should hardly be noticeable. – Burki Feb 12 at 12:09
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If we go from 21% to 10% free O2 content in the atmosphere in a year, I would guess that all life forms (even extremophiles) on Earth are going to struggle. – Michael Kjörling Feb 12 at 13:52
    
@MichaelKjörling probably, but maybe some bacteria can adapt this fast. Tardigrades can adapt pretty well, I suppose, while some other life forms that actually live at higher altitude can migrate to lower zone. But in the end they will doomed as well – Gianluca Feb 12 at 14:09
    
@burki: Unless there's some pretty serious mixing going on I'm not sure we can remove oxygen from the atmosphere fast enough to meet the OP's requirements. I haven't run the numbers though, so... – Joe Bloggs Feb 12 at 14:11

Without intelligent intervention of some kind, whether technologically advanced human, alien or divine, this simply isn't possible. Oxygen is highly reactive, and thus any substance that was available on the Earth to react with it and take it out of the atmosphere would already have done so. And there's a lot of oxygen in the atmosphere, so it will take a correspondingly large effect to remove enough of it on your desired timescale.

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