Subtleties in comparing fossil fuel reserves to CO2 concentrations
As reported in Climate Progress, climatologist James Hansen has done comparisons of fossil fuel reserves from two sources to concentrations of carbon dioxide in the atmosphere, using a conversion factor of 2.12 GtC per ppm of CO2. For example, see Figure 1 from Hansen’s more detailed writeup:
Figure 1. CO2 emissions by fossil fuels (1 ppm CO2 ~ 2.12 GtC, where ppm is parts per million of CO2 in air and GtC is gigatons of carbon). Alternative estimates of reserves and potentially recoverable resources are from EIA (2011) and GAC (2011).
Such comparisons are fraught with pitfalls. In this case, Hansen appears to be comparing the carbon content in fuels with CO2 concentrations in the atmosphere assuming that all of the carbon from burning the fuel would go into the atmosphere. This is fine as long as it is clear what is being assumed, but this detail is not included on the graph, in the caption, or in the paper I link to above. (There may be a more detailed paper of which I’m not aware, so if anyone knows of it, please send me a link. I’ve also tried to find the EIA (2011) and GAC (2011) references without success.)
In reality, some fraction of emitted carbon (called the “airborne fraction”) will stay in the atmosphere and the rest will be absorbed by the oceans and biota on land. The exact value of the airborne fraction varies. Right now it’s roughly 50% but if we continue on our current path, it will rise. For example, in the MIT no-policy case, the airborne fraction measured cumulatively in the 21st century would be about 65%, which demonstrates the declining ability of the oceans to take up carbon in a high emissions world.
It would be better when making graphs like Figure 1 to assume an airborne fraction (say 65% for a world in which we exploit many fossil fuel reserves) and to note that the conversion to ppm assumes that factor. Otherwise this way of presenting the data, which has many good reasons to recommend it, may be misleading to those unfamiliar with the subtleties.
In my post on the illusion of fossil fuel abundance, I use the lower-bound reserve and resource estimates from the IIASA Global Energy Assessment to make different but related points, but don’t make the link to concentrations explicitly. I’m working now to add ppm estimates to give the context in the graphs for that analysis.
