This article says that anything below 200 PPM co2 results in all multicellular life being destroyed. Snow ball earth was low CO2. Obviously, we are up around 380PPM now, but the climate people say that it’s way up from the 270 pre-industrial era. So, they want co2 lower (I hope they aren’t shooting for less than 200).
This article gives the last 800k years of data via glacial ice cores. You might notice that over the course of modern humanity 12-15k years, we have been very close to the bottom 200ppm co2, or almost wiped out from another snowball earth.
If I were looking for a stable environment, with less risk to all multicellular organisms, I would think about 420-480 ppm would be the goal.
Just remember that environmentalists are mostly idiots...................