Tuesday, December 27, 2011
In Friday's post, I noted that continued drying of tropical forests raises the potential that the global carbon sink might weaken in the future. This is one of a variety of such possibilities that one sees news stories about every so often (for example here and here). The background is that, as humanity has been emitting fossil fuel carbon dioxide over the last 150 years or so, not all of it stays in the atmosphere: the elevated concentrations in the air mean that the atmosphere and the ocean and land biosphere are out of equilibrium and some of the CO2 leaves the atmosphere and goes into the ocean and biosphere each year, partially offsetting our emissions. Once out of the atmosphere it no longer affects the climate (though it may have other effects such as ocean acidification and causing ecological changes).
It's pretty easy to assess this oneself with no more than a basic memory of high school chemistry, the Mauna Loa CO2 data, and the BP data on carbon dioxide emissions. From the Mauna Loa data we know how the concentration of CO2 in the air changes each year and we want to convert that into a number of gigatonnes of extra carbon floating above us. To do that we need two things: to know the total mass of the atmosphere - 5.1480 x 106 GT according to the Wiki - and a way to convert the volume mixing ratio of CO2 into a fraction of that total weight that is carbon (for consistency I will just work in carbon terms - to get carbon dioxide masses just multiply by 44/12). To do this we can use the fact that the carbon atom in a CO2 molecule has an atomic mass of 12, while nitrogen (N2 - molecular weight 28) is 78.084% of the atmosphere by volume, oxygen (O2 - molecular weight 32) is 20.946% by volume and Argon (atomic weight 40) is 0.9340%. We can safely ignore the remaining trace components.
So we want to multiply the change in CO2 volume concentration by 5.1480 x 106 *12/(0.78084x28 + 0.20946x32 + 0.00934*40) to get a change in the mass of carbon in the atmosphere (the denominator is basically the molecular weight of the average molecule in the atmosphere and the numerator has the atomic weight of carbon - 12 - and this is how we convert from volume terms to weights).
Alternatively, if you are willing to just trust me to get it right, here's how the change in carbon in the atmosphere stacks up against human emissions:
As you can see, the annual increase in carbon in the atmosphere is always less than what we emit and the difference is the natural sink. This is quite noisy depending on fluctuations in how well the world's plants and water bodies did in fixing carbon versus the activities of decomposing bacteria, fires, etc in releasing it back to the atmosphere again. In this graph, the symptom of the global sink starting to fail would be that the red line would start to approach or even - heaven forbid - cross the blue line. You'll see that if anything there is a bit of a tendency to the other direction in the last decade or so. To look at this more carefully we can take the sink as a fraction of emissions each year:
Here we see that the fraction of emissions that get reabsorbed by the planet each year seems to be increasing not decreasing. However, unlike some scientists I could name - I will disclose that this uptrend is not statistically significant - simple linear regression gives a trend of 2.1%/decade ± 1.9%/decade, and the result (with p=0.237) is entirely consistent with being really flat and the apparent uptrend just a fluke of the noise. But at any rate, there is certainly no evidence of the sink getting weaker.
So while we might worry about increasing drought causing runaway feedbacks in the climate in the future, there is no evidence of a weakening global carbon sink at present.
Of course, what still does have a statistically significant uptrend is the rate at which carbon increases in the atmosphere:
A regression on that red line gives an increase of 0.56 GT/decade and the p-level is miniscule (t statistic of 6) - as I mentioned the other day the concentration of CO2 in the atmosphere is increasing quadratically (because our rate of emissions is increasing at a roughly linear rate).