Isn't their point that too much CO2 causes an unhealthy imbalance? I'm probably on your side in general on this issue, but it's unhelpful to misrepresent the actual arguments. I've never heard anyone argue that 0% CO2 on the planet is the goal or even a good thing. There are a few anti-human nuts who actually do just want us all to die (still wouldn't take us to 0%), but I see that as an extreme minority. There is a point at which too much CO2 would kill us. You can cover your head with an airtight plastic bag and prove that. The question is whether current levels are as emergent as people claim (I have doubts).