"I just don't even know where to start with that. You're a bright guy, so I know you are consciously and deliberately missing the point.

Your argument *seems* to be (correct me if I'm wrong) that as long as we don't move outside the parameters within which life is viable, it's ok for humanity to alter the environment all its little heart desires. SinceCO2 levels have been much higher in the past, you seem to claim, we needn't concern ourselves with a CO2 rise today (regardless, I assume, of the cause for that rise)."

As you know, global CO2 levels are around 385 parts per million and in terms normal human scale the total amount of in Earth's atmosphere CO2 is rather huge [more than a trillion tonnes]. The sky is probably more massive than most people realize: "Total mass of atmosphere: 5.1 x 10^18 kg"
Or 5.1 x 10^15 tonnes or about Five thousand trillion tonnes.
And CO2 is a about .038% of the atmosphere.

Some people might imagine that since CO2 is such a small percentage of the atmosphere and so many people are concerned about it, that "this must mean" that CO2 is some how very potent.
It's not.
Others might imagine since they know CO2 is not a potent greenhouse gas, that CO2 must be powerful in terms of "a long life"
Again, it's not something which permanent- the Earth "breaths" hundreds of billions tonnes of it, yearly. Though one could see it as permanent in the sense that in nature you never get CO2 levels below, somewhere around 150 ppm, because plants simply can't survive on such low concentrations of CO2.
Humans could make air with lower levels of CO2, but you should keep in mind that air which is
99.9 % free of CO2 [or "pure air"] is just normal atmospheric air. You need air 99.99% free of CO2 to be talking about anything. Though if you were using 99.9% pure Oxygen you would have less CO2 than is normal and possibly lower CO2 levels than could be found in nature.

[note: O2 (Gaseous & Liquid) Medical Grade(99.5%)- http://www.sharjahoxygen.com/medicalgases.htm
I don't know how much CO2 is part of that- as compared other possible elements, such as nitrogen and argon or other trace gases.]