[Perhaps] the calculation was based on the trillionth tonne of “fossil” carbon – as opposed to including forest/ag/land use change emissions, because the ag/land use totals already have a (poorly defined) fossil fuel component. But split the difference and call it 30 years. I reckon the threshold that really matters is what happens over the next investment cycle for key infrastructure decisions. It used to be 100 months, but I reckon we’re down to around 82. Is that meme compatible with the trillionth tonne meme? Oh-oh, mixed memes.Another, Chris Goodall, wrote:
The numbers below may be helpful or not.Dave Frame responded directly to Hrynyshyn here. An extract:
I’ve taken them from AR4 for 2004 and rounded them crudely. (I haven’t gone back to the Nature papers to check how many gt they say we have left).
Annual emissions (Carbon equiv)
CO2 from man made sources – inc CaCO3 - 8gt
CO2 from ‘land use’ changes and rotting vegetation – 2.5gt
CH4 - 2gt
N20 - 1gt
Fgases – a bit
= circa 12.5gt
Expressed another way, CO2 from man made sources is just under 60% of Kyoto gases.
So if we have 400 gt left, there’s about 50 years. The inclusion of other Kyoto gases would take this down to about 30 years.
But I suppose you could say that F gases will stop soon (we hope) and that N2O will fall as we get better at managing fertilizer use and that CH4 will fall (perhaps) as we reduce cow numbers and wetland abuse. We should, with any luck, be able to reverse land use changes (??) probably by improving/restoring soil carbon.
Regarding the emissions scenarios - we treated the emissions of CO2 as net emissions (in our future scenarios) and didn't try to partition them by source. We tried to steer clear of the conversations regarding negative emissions - sequestration by forestry or other means... to me the nice things about the Allen et al paper are that (1) it find a better-constrained relationship between emissions and peak temperature; (2) which exploits the fact that long timescale processes, which are responsible for much of the uncertainty in ECS, can be ignored if the forcing is short compared to those long timescales; (3) and this results in a nice "exhaustible resource" reframing of the problem which; (4) happens to be pretty tractable for economists.