[JAKraig]

Yes, that’s what I was thinking of. Thanks.
I think we should shoot for the optimum, 1500 ppm.

Four-fold increase.

Nothing but the best for Mother Earth.

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I don’t know if your comment was meant to be tongue in cheek or not but there is nothing wrong with 1500PPM although most scientists think 1000PPM is where you start getting diminishing returns.

[Mrs. Don-o]

I am by no means an expert, but I went for 1500 ppm for two reasons.
First, you can get commercial plant nursery/greenhouse CO₂ generations (like the Johnson Gas Generator) which enriches to 1500 ppm output; and
second, when the earth's atmosphere was overall around 1500 CO₂, the earth supported vast forests of tree ferns averaging 15-30 meters tall!