> It is found that anthropogenic fossil fuel combustion is the largest contributor to the current O2 deficit, which consumed 2.0 Gt/a in 1900 and has increased to 38.2 Gt/a by 2015. Under the Representative Concentration Pathways (RCPs) RCP8.5 scenario, approximately 100Gt (gigatonnes) of O2 would be removed from the atmosphere per year until 2100, and the O2 concentration will decrease from its current level of 20.946% to 20.825%.
> AFAIK our oxygen was not made recently. It accumulated over billions of years.
According to wikipedia [1], the atmosphere has 34e18 mol of oxygen, and photosynthesis produces 8800e12 mol/yr of oxygen, meaning the atmosphere turns over in a little under four thousand years. There's a separate estimate of atmospheric oxygen's "residence time" in the text, of 4500 years.
So although oxygen levels have been high for billions of years, there is a sense in which the oxygen in the atmosphere right now has only been there for thousands.
That calculation is simplistic as it is based on the current steady-state. The oxygen catastrophe was a massive change away from the previous equilibrium state which first had to deplete all the buffers. That's how we got iron ore veins, by oxidizing most of the iron out of the oceans. And there are other sinks besides iron.
Only if the reservoir is a fifo queue. :) Otherwise there's a distribution, with a tail. But oldest in atmosphere still looks (envelope scribbling...) order Myr? Rather than Gyr. Similar for breath?
AFAIK our oxygen was not made recently. It accumulated over billions of years.
https://en.wikipedia.org/wiki/Great_Oxidation_Event
This here tries to project oxygen levels:
> It is found that anthropogenic fossil fuel combustion is the largest contributor to the current O2 deficit, which consumed 2.0 Gt/a in 1900 and has increased to 38.2 Gt/a by 2015. Under the Representative Concentration Pathways (RCPs) RCP8.5 scenario, approximately 100Gt (gigatonnes) of O2 would be removed from the atmosphere per year until 2100, and the O2 concentration will decrease from its current level of 20.946% to 20.825%.
https://www.sciencedirect.com/science/article/pii/S209592731...