Carbon Inertia

28 Jun 2016

It is often said that we’re locked in to a certain amount of warming: the planet would keep warming up even if we immediately stopped emitting CO2, for a little while at least. This is partly because the climate system responds to perturbations on a range of different time-scales, some of them quite long. In the absence of human intervention, it will also take a long time for atmospheric CO2 levels to return to pre-industrial values through geological processes. These are arguments for reducing emissions as quickly as possible, but a related question is: how many emissions are we locked in for?

What I mean by this is that, assuming we don’t want to tank the global economy and revert to being hunter-gatherers, we will have to emit CO2 to transition to a carbon-neutral society. This could be called the “carbon inertia” of our society, and doesn't seem to be discussed very often. For instance, two of the “wedges” proposed by Socolow and Pacala 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 as far as I can tell the authors don’t take into account 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 available fossil fuels just to transform to a carbon neutral society. In a slightly better case it would be possible to go through with this transformation while leaving a significant amount of carbon in the ground, but doing this would require the process to be highly optimized. It’s hard to imagine countries successfully co-ordinating and co-operating to the extent that this would require. In the best case, of course, we would be able to overhaul our energy infrastructure without emitting much CO2 at all.

Calculating our 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. This estimate gives the mean lifecycle emissions of wind-turbines as about 12 g CO2-eq / kWh. We need to go from a capacity of about 60 TWh to about 6000 TWh. The total emissions for this increase in capacity is then roughly $$ 5940 \times 10^{12} \times 12 \mathbin{/} 10^3 \mathbin{/} 10^6 = 71.3 Mtons CO_2-eq. $$ 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. Presumably, this means replacing every inefficient car with a more efficient one. 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. Let's also assume that manufacturing an electric car produces the same emissions as manufacturing a regular car $($in fact building electric cars often requires slightly more carbon$)$. There are about 1 billion cars in the world and this article gives an estimate of 20 tons CO2-eq to build a standard-sized car. 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. If we assume that these roughly bracket the range of the required wedges, 5-10 years seems like a reasonable first guess for our current carbon inertia, on top of our regular emissions $($which, to be fair, would be decreasing over the course of the transition$)$. This is better than I expected, though it would be good to look at other possible wedges and also think about other limiting factors, like the finite supply of rare-Earth metals. But it does suggest that we could successfully transition to a carbon-neutral society while leaving some carbon in the ground.