Fair enough, lots of complexity and uncertainty. I'll go so far as to grant that we don't know squat about whether the current CO2 increase has shifted the climate.

But surely you don't disagree with these basic points:

1. Atmospheric CO2 is a "greenhouse gas" (i.e., allows shortwave solar visible, UV, etc through, but absorbs some of the re-emitted thermal IR)

2. Increased quantities of atmospheric greenhouses gases like CO2, H2O vapor, etc increase a planet's equilibrium temperature, all else being equal.

3. If the total carbon content of the Earth's crust -- limestone, etc. -- were oxidized and put in the atmosphere, you'd have a junior-league Venus instead of our green and pleasant planet.

4. Short of 3, there is some threshold level of CO2 you could suddenly inject into the Earth's atmosphere that would cause climate change that would be undesirable, unpleasant, costly, annoying, whatever you want to call it. Note that even if out of equilibrium and this CO2 will be reabsorbed over time, that "over time" is likely to be long on human scales, with lots of excitement on the way.

So my question, Tom, is this: what is the CO2 level where you think undesirable effects might begin?

Given your utter certainty on the subject I'm apt to conclude that you think that converting every last speck of fossil fuel into atmospheric CO2 would still fall under that threshold. That is a CO2 increase many times more than we've seen today, and if I recall, totally out of the range seen since the Mesozoic Era (back when even the poles were ice-free). What's your reason for being so confident?