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First question

What event would cause hydrogen to slowly replace oxygen in the athmosphere?

it can be something natural or artifial.

it doesn't have to cause too much mass extinctions. Some sacrifices are ok but not an apocalypse.

So it must be slow enough so living organisms adapt to it, but not slower than 1 billion years, it has to happen way before the sun enters its death phase and eats the solar system.

Second question

What would be the most evident and drastic changes in the environment and life on earth?

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  • $\begingroup$ The obvious problem with this is that hydrogen reacts with oxygen forming dihydrogen monoxide. Also known as water. $\endgroup$ – a CVn Aug 5 '16 at 23:18
  • $\begingroup$ don't hydrogen+oxygen only form whater when burned together? $\endgroup$ – user25001 Aug 5 '16 at 23:29
  • $\begingroup$ "Fire" is just a faster chemical reaction than that commonly seen between everyday substances at everyday temperatures. Checmical reactions happen all the time, and oxygen happens to be quite reactive. $\endgroup$ – a CVn Aug 5 '16 at 23:38
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    $\begingroup$ Another problem is how light hydrogen is when it isn't bonded with anything else, it's even lighter than helium. Not sure how long it would take for it all to escape into space since free hydrogen is one of the gasses Earth can't hold onto over short periods of time. $\endgroup$ – Stephanie Aug 6 '16 at 1:00
  • $\begingroup$ @Stephanie: Single hydrogen atoms might exist for short periods of time, but in general they will be bonded with something else; the most common version of hydrogen gas is two hydrogen atoms bonded together. When discussing gas densities, the molecular weight is what is important. Since helium is a noble gas it (unlike hydrogen) exists as a single atom. This means that the common form of hydrogen and helium gasses both have a molecular weight of (close to) 2 and thus have similar densities and chance of escaping into space. $\endgroup$ – Mark Ripley Feb 11 '17 at 8:42
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The problem here is the presence of an atmosphere consisting of much denser gases like nitrogen or oxygen. This causes hydrogen to be pushed far away from earth where Earth's magnetic and gravitational fields are weaker and hydrogen can escape more easily. Of course, this also means that life on the ground or near the ground doesn't come into contact with hydrogen even if there is little enough of an atmosphere consisting of much denser gases that the planet can actually hold on to a lot of hydrogen.

So you need to get rid of that atmosphere first. Because there temporarily needs to be no atmosphere at all (really hard with all that water which would start to boil and create a water atmosphere once the pressure of the atmosphere at sea level drops to about 1% of what it currently is and because all rivers would either evaporate of if you're lucky part of them will remain frozen), this is fundamentally incompatible with your requirement of it happening slowly.

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