4,000 ppm was a period far before humans or advanced primates, and very different flora.
200ppm is around the level humanity, and much of the flora we share the planet with, evolved. It's the level we started to develop from, to farm.
Nature and the planet has no ideal, it copes with whatever there is. Humanity on the other hand would appear best served by the level we evolved with - 200ppm or thereabouts. 400ppm causes problems for us, 4,000ppm would almost certainly be the end of us or as near as makes no odds.
200ppm is around the level humanity, and much of the flora we share the planet with, evolved. It's the level we started to develop from, to farm.
Nature and the planet has no ideal, it copes with whatever there is. Humanity on the other hand would appear best served by the level we evolved with - 200ppm or thereabouts. 400ppm causes problems for us, 4,000ppm would almost certainly be the end of us or as near as makes no odds.