Paul S -
I'm aware of the logarithmic nature of the climate response - that is why I specified a particular starting level - 400ppmv. 500 might be a better choice bit it doesn't matter much, as a rough estimate is all I was looking for.
The point about emissions reductions is crucial. Reducing emissions has absolutel no effect <b>unless</b> the unburnt fossil fuels remain in the ground I.e. they are not simply burnet at a later date. The extra Co2 in the atmosphere is cumulative, so the only question that is relevant is
'<i>what proportion of the finite quantity of fossil fuel will eventually be used'</i>
It doesn't matter over how many years this quantity takes to be reached, so slowing down the rate it is approached is an irrelevance. The only important change will be to have economically recoverable fuels left in the ground <i>deliberately</i> and <i>permanently</i>. Otherwise the end result is the same. changing emissions rates merely changes the speed (very slightly) with which the end is approached.
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