She blinded me with ... Wikipedia

[bungo]

The total mean mass of theEarth's  atmosphere is 5.1×1018 kg.
Fraction of this which is CO2: 400 ppm (by volume)
Assume ideal, perfect gases, etc, etc.
Mass of CO2 in atmosphere: 2×1015 kg.  (1)

Estimated (2005-06) total coal reserves: 905 GT = 9×1014 kg.
Multiply by (12/14) * (44/12) to convert hydrocarbon (CH2)n to CO2: 2.8 ×1015 kg

Comments

[reverancepavane]

Don't forget the ocean acts as a buffer to maintain that magic 400 ppm volume - except there is good evidence that the buffer is getting full (even without the increase in temperature reducing the carrying capacity of CO2 in the ocean). That's a major reason why the guesstimate is so varied. [Of course the acquatic ecosystem isn't coping with the increasing acidification and temperature of the oceans, not to mention the overfishing. Although the jellyfish apparently like it.]

[And that's not even counting the very heat sensitive reserves of methane located on the sea floor and in the permafrost (methane plumes have been spotted up to the thermocline in the summer Atlantic).

A few respected atmospheric and planetary scientists haven't been shying away from the term "runaway greenhouse effect" as much as they used to be.]

From memory there is some atmospheric chemistry stuff that will affect CO2/CO balance but that's really a minor fillip in the scheme of things, especially as a back of the envelope calc like this (and it's been two decades since I studied all this).

On the other hand you are quite right with how much hard work it is to actually affect a planetary environment. [We should build a memorial/warning about it to future space travellers who stumble across the Moon and wonder how a world located in this orbit became another Venus. Especially since it has a large moon (tidal mixing).]

And planetary inertia works in reverse too, so it will take a long time to reverse all this as well. It's not just flooding from ice melt either. The global ocean currents which are responsible for massive amounts of heat distribution (as any steam engineer will tell you [except for ones that work at Valve] water is an excellent material for transporting heat energy around the place) may end up in a metastable state that is local (which is scary since it will amplify the problem locally), and models seem to indicate that they are unlikely to drop back into the current transoceanic patterns until we get a slight amount of continental drift.

[Speaking of which I haven't updated my Continental Drift Cam picture in a couple of decades. I wonder if anyone has noticed.]

[bungo]

Thanks, Ian. You're right, I've forgotten about the known unknown extent of buffering. But I think the timescales are different enough that it will mainly be of relief to our grandchildren, or theirs. What surprised me a little was how this simple balance showed me that the claim "well, we can't get to the worst case IPCC scenarios even if we wanted to" isn't quite as far-fetched as I thought at first glance.

[reverancepavane]

What makes me sad is that we can get to the worst case scenarios. =8(