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Airborne fraction of CO2 stays constant

Prof Wolfcgang Knorr of Bristol has found out that the ability of natural sinks - mainly oceans and forests - to absorb the CO2 emissions hasn't changed by more than 10% or so since 1850. It means that the airborne fraction of CO2 - the fraction of emissions that stays in the air for a very long time - remains constant within the margin of error, near 45%.


Univ. of Bristol press release, The Telegraph, Daily Mail, Bristol 24-7, Bristol Evening Post, Original Bristol 106.5 FM, World Climate Report, Anthony Watts
I have noticed that it was a favorite meme of some alarmists who would say - without having any rational justification - that the airborne fraction was increasing i.e. that an increasing part of the CO2 emissions stays in the air. For example, in 2007, The Reference Frame commented on this very delusion by a guy called David Archer.

His proposition - frequently parroted by Alexander Ač, Joseph Romm, and many others - violates Henry's Law in chemistry that dictates that the concentration of a gas in the ocean remains proportional to its partial pressure in the air, with a constant coefficient.

In principle, the forests could get "saturated" and refuse to absorb greater absolute amounts of CO2 at some point except that there exists absolutely no reason to think that we are approaching this critical regime. In fact, there are good reasons to think that this point of saturation is an order of magnitude away.

The new empirical research shows that just like you would expect, I was right and Archer (of RealClimate.ORG) was wrong.

A detail I find genuinely crazy is that Knorr's own university press release depicts his own research as "controversial" before any controversy about his obviously valid result begins. He also discourages the readers from deducing any conclusions out of his paper. Rather than relying on Nature's will to obey Her own laws, Knorr instructs us in the pink rectangle to search for unnatural explanations why the fraction hasn't changed. ;-)

Well, another inconvenient truth.

I actually find it obvious that the natural sinks will eventually (when the CO2 concentration will be high but the emissions will begin to stagnate) be absorbing a greater part of the CO2 emissions because of an absolute term (absorption of excessive CO2 that is already out there): even if all the industrial CO2 emissions suddenly stopped, the elevated CO2 concentrations would lead Nature to absorb more CO2 than it emits. Within a few centuries, the concentration of 388 ppm would drop very close to 280 ppm - calculable as the equilibrium from the current interglacial temperatures - again.

Some convenient numbers

The total mass of carbon dioxide in the atmosphere is 3 x 10^{15} kilograms while the annual CO2 emissions are 3 x 10^{13} kilograms (30 gigatons), exactly 100 times less. That would mean that the current CO2 concentration in the air, 388 ppm, should annually increase by about 3.9 ppm. But it only increases by 45% of it, around 1.8 ppm. Clearly, the remaining 2.1 ppm are being absorbed by some sinks, mainly oceans and forests.

However, I think it's more sensible to say that this difference (2.1 ppm) between 3.9 ppm (industrial emissions) and 1.8 ppm (observed CO2 increase) arises as a fixed fraction of (388-280) ppm (absorption of 1.95% of the difference between the current concentration and 280 ppm per year) rather than a fixed fraction of 3.9 ppm: it is the extra absorption by which Nature tries to return back to 280 ppm and this absorption would occur even if our CO2 emissions stopped. Nature would keep on "stealing" those 2.1 ppm (or so) of CO2 from the atmosphere every year: why would a forest stop swallowing the CO2 from the atmosphere just because the some power plants stopped emitting elsewhere?

Note that this modified paradigm of mine doesn't change the computations of concentrations since 1850 too much because both the CO2 emissions as well as the difference between the current concentration and 280 ppm have been increasing nearly exponentially with a very similar rate.

If you believe that my general formula is correct and if you adjust the coefficients for the CO2 concentration to currently grow by 1.8 ppm a year, you will find out that if the annual growth of the CO2 emissions is going to be around +1% in average, the CO2 concentration in the air will increase pretty much linearly until 2100 (because the natural absorption will increase and compensate the growing emissions).

Assuming this 1% annual growth of CO2 emissions - that is reasonable to get for a 3% GDP growth (I surely do think that it will be insane to dream about a substantial drop of CO2 emissions in a decade) - we will reach 560 ppm in 2087. If we attribute the whole 20th century warming to the CO2 greenhouse effect and assume the realistic logarithmic dependence of the warming on the concentration, then the 2009-2087 warming would be close to 0.8 °C. The 2009-2109 CO2 warming would be predicted to be nearly uniform (linear) once again, with the total increment equal to 1.05 °C.
To see these calculations, download the Mathematica notebook: PDF preview, NB file
In some sense, this is the upper bound on the CO2 warming because it's extremely likely that a positive part of the 20th century warming had natural causes. Taking this likely fact into account, the prediction for the 21st century warming would drop. For example, if you say that 1/2 of the 20th century warming was caused by the CO2, the predicted warming since 2009 through 2109 will drop to 0.5 °C.

Hat tip: Climate Depot

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