Hi gb

CO2 in concert with H2O keeps enough heat locked in to keep Earth warmer than the albedo temperature. That's proven science, so you're just being a curmudgeon to suggest otherwise.

The significance of that tiny CO2 fraction lies in its stability. Water vapour is a condensible gas across all Earth's temperature range, while CO2 isn't. And water that has snowed out causes an albedo rise and temperature drop at an effective insolation level just a bit lower than the present day. No CO2 would cause such a runaway albedo rise and temperature drop - that's the Icehouse Earth scenario which is solidly evidenced in the late Pre-Cambrian.

So gb present a real alternative model to back your BS or shut up.

As for CO2 and plant life most C3 plant life (i.e. lots of tree species) would drop dead if CO2 dropped below 150 ppm. C4 photosynthesis is sustainable down to ~10 ppm, while much of the oceanic photosynthesisers would get by on carbonate washed in from erosion. But land-based ecosystems would be long dead by that stage.

Adam