Does anyone have explanation, or can point me to papers explaining this thread title? Personally, I did some reading through years, but I could never find an answer to this question. In short, here is what I learnt so far: Oxygen: - in free form, is a product of photosynthesis of green life on earth, from combination of CO2 and H2O source. - is short lived, about 50 thousand years in atmosphere, then it's gone due to oxidation. - some of O2 is a product of H2O breakdown, when hit by high energy photons, but probably not much. - 21% of atmosphere is oxygen by volume - there are some claims that current level of oxygen can be regenerated in 2,000 years! Carbon-dioxide: - comes mostly from original earths atmosphere, which probably was composed mainly of it. - in last 500 million years CO2 level is dropping slowly, from about 20 times current level to current level. - the drop in CO2 level is explained by continues sequestration of it from air to the ground. - 0.04% of atmosphere by volume The dilemma is with short lived oxygen in atmosphere. If all current O2 is younger than 50 thousand years, then there wasn't and isn't enough CO2 to recreate it, not even close. Other words, if all oxygen is gone now, all the green life would be able to produce only about 0.05% O2 from all the CO2 available, only a fraction of 21% we have right now. There is also a claim that states that O2 level was fluctuating between 20-35% in last 500 million years. How is it possible if CO2 volume fluctuated from 1-0.04% of atmosphere? ...or what am I missing? I can understand CO2 level and depletion through ages. I can't understand O2 level in a light of it's rather short life span in free form.