Closed ecological systems

What would happen if you increased the oxygen level? Could we see giant insects again?

How long would it take to see any changes?

Since evolution isn’t deterministic, we really have no way to answer the question of whether there would be more giant insects if oxygen concentrations increased. It could depend on whether the ecological niches filled by such insects are currently crowded or nearly vacant or vacant - and whether whatever caused the oxygen concentration to increase also vacated those niches.

It could also depend on whether predators, parasites, or diseases would knock back such a population… for example, because insects are so widespread and numerous, it’s entirely possible larger insects could develop, but if the birds loved eating them and provided enough selection pressure to eliminate any advantage to increased size, you wouldn’t see those insects spread or be successful.

As for the length of time to see changes, it would be on evolutionary time scales - you’d have to wait for a useful mutation or recombination of genes, wait for that to establish itself in a population - probably several times because conditions do not exist that will support the 3-ft wingspan dragonflies of ancient eras, so presumably that variation is not currently out there in the existing dragonfly population waiting for the right conditions to take off.

Definitely not on the timescale of a human lifespan, but it’s hard to say beyond that how long it might take.

Oxygen is a very reactive gas; it has never had a higher level in the atmosphere than it has at present. It is estimated that, holding temperature and humidity constant, an increase of atmospheric oxygen from 21% to 25% would lead to destruction of almost all forests due to lightning fires.

(If my reading of Wikipedia’s tables is correct, a mere 5000 years of photosynthesis at present level produces as much oxygen as the entire atmosphere holds! Animals and other living oxygen consumers consume almost as much oxygen as plants create.)

The Wikipedia page you link to says otherwise (and I’ve heard the same claim made by a bio geochemist I know, as well as coming across it in scientific literature). Some scientists believe that the oxygen concentration reached as high as 30% or so at one point. (And yes, it’s been argued that fires would have been more common under such conditions).

Some plants actually maintain exceptionally high oxygen concentrations transiently in their leaves (these would be the CAM plants, which fix carbon at night and close their stomata during the day, which means that the oxygen generated by photosynthesis can’t immediately escape).

I think O[sub]2[/sub] concentrations were considerably higher at one point, but it has been far far more frequent in history for O[sub]2[/sub] concentrations to be lower than at present:

See section 6, pg. 911 for a brief discussion of the possibility that the development of vascular plants and their contribution of additional decomposition-resistant carbon to soils for some time may have raised O[sub]2[/sub] to as high as 35% of the atmosphere (although the real value may be lower.) This may also have been related to the development of large insects the OP mentioned.

The oxygen in our atmosphere is many times more massive than the total mass of the biosphere, so can’t have been produced by the plants now on Earth; conversely, if photosynthesis were to cease, the biosphere would not be massive enough to remove much of the atmospheric oxygen by respiration. Oxygen would be slowly absorbed by the crust over the next half a million years or so, but remember that the top surface of the crust is already quite well oxidised, since it sits in an oxygen-bearing atmosphere.