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

[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(