increasing oxygen in the atmosphere

The percentage of oxygen in the atmosphere is about 21% while the level of carbon dioxide is about 0.04%.
In past ages, levels of CO2 have dropped as plant life increased.
Did the levels of O2 increase likewise?
If we plant more green stuff will levels of CO2 drop and O2 increase?
Could O2 levels ever increase to the point where they would be dangerous to us?


Well, growing plants do reduce the amount of CO2 in the atmosphere and increase the amount of 02, all other things being equal. That’s only true as long as the plants are growing though. When they die, or bits of them drop off, those things eventually return carbon to the atmosphere and take oxygen out.

There are dangers to breathing air too rich in oxygen, but I don’t know how high the O2 levels have to get or how fast problems would develop. I don’t think converting all the CO2 in the atmosphere to pure O2 would be enough to make much difference. It would make a difference to the climate of course. Earth would turn into an ice cube.

I believe there’s a feedback mechanism which will take excess O2 out of the atmosphere. Too much oxygen and forest fires happen more easily, spread faster, etc. Minerals will oxidize faster. the current O2 partial pressure is probably within 50% of what would be a practical maximum.

Not really. Quite a bit of the carbon removed from the atmosphere by plants ends up as mineral: carbonate rocks (limestone, chalk etc.), coal, oil and the like (sometimes after having been eaten by animals, after having been first fixed by plants). Of course, even this carbon can and does get recycled into the atmosphere by natural processes, such as volcanism, but that takes millions of years. A lot of Earth’s carbon is locked up in rocks that were originally biologically produced.

Even the plant material that does get recycled into the atmosphere without going through a mineral phase (because it get eaten and metabolized into CO2 by microbes, fungi, or animals), is likely to remain in solid form for quite some time, and thus acts quite effectively to sequester carbon. This is why deforestation, and the like, is a major contributor to rising CO2 levels, and, thus, anthropogenic climate change. It is not just the burning of fossil fuels that is significant.

I think the answer to all the OP’s questions is “yes”, although the likelihood of the proportion of atmospheric oxygen ever rising to a level that would be dangerous, or even noticeably deleterious to humans is extremely remote. I doubt that it would even be possible solely as a result of increased plant growth.

Too much oxygen would of course also be terrible for plants, since oxygen competitively inhibits (in C3 plants, the majority) one of the key enzymes in photosynthesis.

Also, while plants do a one-for-one exchange of O2 for CO2, they make much less of a difference to the O2 concentration than to [CO2], because there’s so much more oxygen around to begin with.

Yes. Even if they converted all the CO[sub]2[/sub] to O[sub]2[/sub], it would only be a small rounding error in the oxygen concentration.

IIRC, was not the higher O2 content in the air back in the big boy times one of the reasons they could grow so big & tall? ( I shop there )

Or has that been proven wrong or fallen out of favor?

Too much O2 would cause a lot of problems. Fires would be far more destructive and spread much faster. Anything made of steel or iron is going to rust very fast.

Yes, much higher in carboniferous era. IIRC (from Neil Degrasse Tyson’s Cosmos), trees sequestered a lot of carbon back then, and decomposers had not yet evolved to digest wood.