[mpg33]

What is the right amount of CO2 to have in the atmosphere scientifically speaking?

[averros]

Depends on what your utility function is.

If it's "as close to the nature right before apes became humans", it's easy to answer (though hard to justify on any grounds other than quasi-religous belief in Gaia or something). If it's "planet with more plants and animals" it could be quite different, and any honest scientist would answer it "we don't really know, but here are some considerations..."

[turndown]

An amount similar to levels before human impact.

[m0zg]

There have been times "before human impact" when atmospheric CO2 was _way_ higher than it is now, though. You'll have to be more specific.

[averros]

Non sequitur.