O2 never will run out. But Why? The answer is: Although humans and animals are permanently exhaling CO2 (resp. are permanently attaching C-Atoms to O2-Molecules) - plants on the other side have to extract C-Atoms from CO2 and thereby have to to free O2-Molecules before (else plants could not grow and animals had nothing to exhale).
The “BEFORE” is the point of the matter.
An animal or a human can only exhale exactly theses C-Atoms a plant has taken up before. For every O2-Molecule that is used up by an animal, a plant has to set one free before. Understandable?
Imagine there were no plants on the planet at all. So there would be no food. So animals couldn’t exhale anything. CO2 couldn’t increase by exhaling.
If we want animals to exhale something, we had to produce food BEFORE. That means: If we want animals produce CO2, we necessarily had to decrease atmosphere’s CO2 before, because else (without CO2-reducing plant-growth) we couldn’t produce food.
Pappadave believes that plants are taking up C-Atoms and that these C-Atoms remains stored within dead plant material (i.e. compost) forever. But this can’t true! Because this would implicate that more and more C-Atoms accumulate in the soil (since a forest is producing plant material continuously e.g. foliage, dying plants in winter, old trees,…).
If C-Atoms indeed would remain in the dead plant material, the earth logically would be contaminated by C-Atoms quickly. So I wouldn’t believe him. Sooner or later compost moulders into earth and the C-Atoms move back into atmosphere.