Because the climate system responds to perturbations on a range of different time-scales, Earth would keep warming up even if we immediately stopped emitting CO₂. In the absence of human intervention, it will also take a long time for atmospheric CO₂ levels to return to pre-industrial values through natural processes. These are arguments for reducing emissions as quickly as possible, but a related question is: how many emitted carbon are we locked in for?

What I mean by this is that we will have to emit CO₂ to transition to a carbon-neutral society. This could be called society's “carbon inertia”. For instance, two of the “wedges” proposed by Socolow and Pacala to stabilize carbon emissions are (1) increasing wind electricity capacity by a factor of ten relative to today, and (2) doubling the average fuel efficiency of cars from 30mpg to 60mpg. These are both worthwhile goals, but how much carbon would be emitted building all those wind turbines and high-efficiency cars?

In the worst case, we would have to burn all the available fossil fuels to transform to a carbon neutral society. In a better scenario it would be possible to go through with this transformation while leaving a significant amount of carbon in the ground, but this would require the process to be highly optimized. In the best case, of course, we would be able to overhaul our energy infrastructure while emitting a negligible amount of CO₂.

Calculating society's carbon inertia is hard. It would be relatively easy if we had perfect integrated assessment models and could explore different possibilities. But to give an order of magnitude estimate, let’s first take the "increasing wind capacity by a factor of ten" wedge. The mean lifecycle emissions of wind-turbines is about 12 g CO₂-eq / kWh and we need to go from a capacity of roughly 60 TWh to roughly 6000 TWh. The total emissions for this increase in capacity is then $$ 5940 \times 10^{12} \times 12 \mathbin{/} 10^3 \mathbin{/} 10^6 = 71.3 Mtons CO_2-eq. $$ Given that annual global CO₂ emissions are about 10 Gtons, this is encouraging.

Now let's take the "doubling the average fuel efficiency of cars" wedge. Suppose instead we replaced every non-electric car with an electric car that effectively had zero emissions, assuming that all electricity was generated by renewables. There are about 1 billion cars in the world and it takes about 20 tons CO₂-eq to build a standard car (in fact building electric cars often requires slightly more carbon). So replacing every car on Earth would take 20 Gtons CO₂-eq; roughly the same order of magnitude as two years' worth of global emissions.

So we have estimates of about 1% of annual global CO₂ emissions and 200% annual global CO₂ emissions for the two wedges. Socolow and Pacala's eight wedges only keep emissions constant at today's levels; 16 wedges are required to eliminate all carbon emissions and so, if we assume that the two wedges discussed here bracket the range of emissions required for all the wedges, 10-20 years is a reasonable guess for society's current carbon inertia. This suggests that we could successfully transition to a carbon-neutral society while leaving a significant amount of carbon in the ground, though the level of international cooperation required to do this may be prohibitive.