Hope everyone had a good, and safe, festive season. Towards the end of last year I wrote a couple of posts about the ocean carbonare cycle. I then ended up in a debate about what it would take to stabilise concentrations (hint: stabilising emissions will not stabilise concentrations).
One issue that people still seem unsure about is why we expect some fraction of our emissions to remain in the atmosphere for thousands of years. I thought I would try to illustrate this using the basic ocean carbonate chemistry that I described here.
Basically, in equilibrium, the amount of dissolved inorganic carbon (DIC) in the ocean determines the partial pressure of CO2 and, hence, the atmospheric CO2 concentration via Henry’s Law. The top panel of the figure on the right shows this. A DIC of mol/kg produces an atmospheric CO2 concentration of 280ppm (pre-industrial). As the DIC increases, so does the equilibrium atmospheric CO2 concentration.
We also know that the ocean holds about 38000 GtC (giga-tonnes of inorganic carbon). Therefore, we can associate a change in DIC with a change in the total amount of carbon in the ocean (i.e., the increase in inorganic carbon in the ocean is approximately ). Similarly, you can get the increase in atmospheric CO2 using . The sum of the increase in inorganic carbon in the ocean and the increase in atmospheric CO2 would be our total emission; the fraction of that in the atmosphere would be the residual airborne fraction. This is shown in the bottom panel of the figure on the right. The residual airborne fraction increases from about 15% for emissions of 100s of GtC (we’ve already emitted 600 GtC) to almost 30% if we were to emit as much as 5000 GtC.
The above is all approximate, but I think the basic idea is about right (happy to be corrected if it’s not). It also looks similar to what is obtained in Archer (2005), from which I’ve taken the table below. The “yes” refers to the analysis including temperature feedbacks (as does mine), while the “no” refers to it not including CaCO3 and silicate weathering (mine doesn’t either). The 4th column is total emissions (in GtC) and the 3 final columns are after times of 1kyr, 10kyr, and 100kyr and the reason the values are constant is because weathering isn’t included.
Essentially, in the absence of weathering (which occurs on kyr timescales) the oceans cannot dissolve all our emissions, and the residual amount in the atmosphere increases from about 15% (for total emissions of around 1000GtC) to around 30% (for total emissions of around 5000 GtC).
If anyone would like to download the code I used to produce these figures, it is here. You may need to uncomment some of the lines to get both figures.