Everyone forgets about the Oceans and the green plants therein. About 70 per cent of the oxygen that you breath in every time you take a breath, comes from the Oceans. The oceans also absorb carbon dioxide, more when their temperature is lower.
Another fact that is conveniently ignored is that CO2 is FOOD for plants, and not poisonous.
Oxygen, in large quantities, is toxic and destructive.
Most of the ‘air’ you breathe is made of Nitrogen. Oxygen is only about 20% of what we breathe.
Photosynthesis provides the equilibrium. It is the equalizer.