Why has atmospheric O2 stayed constant and not elevated or decreased in conjunction with CO2 increases?

Question

Does weathering show a net gain? We've had ~21% levels since the biological era. That was marked by the simultaneous decrease in atmospheric CO2 and the increase in O2 due to life processes. Shouldn't there be less O2?

Answer 1

This question makes me nervous, and I have a feeling there is no known answer. Weathering played a large role in O2 formation at the onset in the form of the atomic O2 molecule. My best guess would be that photosynthesis is a reversible process. Where respiration and decay, in organic matter take up O2 to form CO2. But like I said this is a scary question and really beyond me.

Answer 2

Oxygen levels have, in fact, declined as CO2 has risen. But the rise in CO2 has been about 100 parts per million (or about 0.01%), and Oxygen levels have fallen by the same amount. So it doesn't make much practical difference.

Answer 3

Because the amount of the CO2 increase is tiny.  About 0.01%.

One reason some people don't understand global warming is a lack of understanding how such a tiny increase can have such a big effect.  

But it's just a relatively simple physics calculation.  That much increase in CO2 holds in a small part of the Sun's heat that's not natural, causing temperatures to rise, as we've seen.

Simple for a scientist to understand, not so simple for someone who doesn't understand science.