In article ,
says...
It seems that with higher CO2 levels,
these green things (we'll call them "plants") work overtime doing something
called photosynthesis, which releases oxygen.

That's a common misconception. Plants "inhale" CO2 and "exhale" O when
light is falling on their leaves. When it is dark, the process is
reversed. O in and CO2 out. That's why aquarium keepers like me, with
heavily planted tanks, install an air bubbler that comes on when the
lights go out and off when the lights go on.

It is true that deciduous trees have their leaves in seasons where
daylight hours exceed night hours, so they do produce a net increase in
O, but this does go down somewhat on cloudy days.

Since evergreens have "leaves" the whole year, their O vs CO2 tends to
be pretty much a wash.

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