So that's ~490 GtC to 2100 keeps us below 450ppm?

How does that compare with the resource we have available to us?

Oil

This is the data form the latest ASPO newsletter available here (.pdf) table:

The important number is 1398 Gb all liquids future production to 2100.

1.4 trillion barrels of oil left to burn, that's 191 Gt (7.33 barrels per tonne) or 162 GtC (assuming an average carbon share of ~85% by mass) in the remaining oil.

Gas

Gas... is less. The 2005 BP Statistical Review lists world gas reserves at 179.83 trillion cubic meters. 1.0 cubic meter natural gas contains 0.49 kg carbon so we are left with 90 GtC in the remaining gas.

Total so far

Total carbon in remaining oil and gas is therefore 252 GtC or barely half the IPCC’s 490 GtC threshold. This supports James Hansen’s point I emphasised in the past about being able to burn the remaining oil and gas reserves without causing dangerous climate change (considered here to be +2C and 450ppm CO₂).

Assuming that all the remaining oil and gas will be burnt as fast the depletion curve allows, it is strangely addictive stuff after all, what does this leave us with? A remaining “allowance” of 490 – 252 = 238 GtC, a whole lot of coal and the significantly smaller anthropogenic CO₂ emissions from land use change.

Coal

The World Coal Institute lists 2005 world coal production as 4973 Mt hard coal and 905 Mt brown coal/lignite. Hard coal is approximately 95% carbon and lignite is around 35% so that totals to approximately 5.0 GtC in 2005 from coal. Not all the hard coal will be the "good stuff" though so this will be a slight overestimation. Coal burn has increased rapidly in recent years - I wouldn't be surprised if 2007 doesn't come in close to 5.5 GtC.

Taking our remaining budget of 200 GtC (leaving the other 38 GtC for land use change and other minor sources) would allow 200 / 5.5 = 36 years of 2007 coal burn. It is this statistic that shows us where to focus our attention. It appears the climate change problem is all about coal. One approach would be to stop coal expansion and new build within a decade (at least without proven carbon sequestration technology) and be off coal by the 2nd half of the century. If we do stop new build within a decade the old plant will gradually retire producing the desired result.

Is this even remotely feasible? I can't help thinking that it is easier to move away from coal than it is oil and gas leaving it unburnt in the ground and surely that is what is important, the easiest of a bunch of hard choices? I say easiest as we have many alternative (wind, wave, nuclear, tidal, solar, geothermal, biomass etc) ways of generating electricity and from where we stand today electricity consumption could be reduced significantly with minimal pain.

Previously on The Oil Drum

Peak Oil and Climate Change
Dr James Hansen: Can We Still Avoid Dangerous Human-Made Climate Change?
Greenland, or why you might care about ice physics
More Coal Equals More CO2
Climate Change and Electricity From Biomass