So, while in your mind it could apply to the atmosphere, in the real world it does not.
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Jeez, it's an ANALOGY. The point is that 2 to 3% does make a difference. A serious cumulative difference.
If we were to plant 2-3% more vegetation, then we might be able to neutralize the damage we do to the atmosphere. But we don't. Perhaps we couldn't.
I suppose some plants absorb more CO2 and emit more O2 than others. Perhaps we should determine which those plants are for each climatic zone, and plant more of them.