Watt about the CO2 residence time?

Watts Up With That (WUWT) has a recent post by Christopher Monckton called On CO₂ residence times – the chicken or the egg. Ths post discusses the bombtest curve shown in the figure below. This curve shows the decay of atmospheric ¹⁴C after the end of nuclear testing. It shows a half life of around 10 years and that most is gone within 50 years.

The conclusion that is drawn (based on some earlier work by Gosta Petterson) is that this implies that the enhanced atmospheric CO₂ concentration (resulting from anthropogenic emissions) should disappear within a few decades rather than over hundreds of years, as claimed by the IPCC. I’ve already discussed this in an earlier post, so won’t go into too much detail here. The basic point is, though, that what Gosta is confusing is the residence time of an individual molecule (which is indeed only a few years) with the time it would take for an enhancement to decay [Amendment : Not only is my interpretation here a bit simplistic, but I believe that Gosta Petterson has also acknowledged an error in his first calculation. See the comments by Lars Karlsson for details]. An individual molecule will only stay in the atmosphere for a few years before being absorbed by the biosphere or the oceans. It is, however, typically replaced by another molecule from the biosphere and oceans and so, on short timescales, this carbon cycle doesn’t change the atmospheric concentration.

Christopher Monckton points out that

Mr. Born, in comments on my last posting, says the residence time of CO2 has no bearing on its atmospheric concentration: “It’s not an issue of which carbon isotopes we’re talking about. The issue is the difference between CO2 concentration and residence time in the atmosphere of a typical CO2 molecule, of whatever isotope. The bomb tests, which tagged some CO2 molecules, showed us the latter, and I have no reason to believe that the residence time of any other isotope would be much different.”

So, I think Mr Born is pointing out that the bombtest curve is telling us something about the residence time of individual molecules, not the timescale over which a concentration enhancement would decay.

Christopher then goes through various things in great detail that I won’t dissect here as I don’t have the time or the energy. The conclusion of his post begins with

It is because the consequences of this research are so potentially important that I have set out an account of the issue here at some length. It is not for a fumblesome layman such as me to say whether Professor Pettersson and Professor Salby (the latter supported by Professor Lindzen) are right.

It’s quite good to show some circumspection. However, it is a little odd that someone who is so certain about much of the science associated with global warming/climate change is now suddenly uncertain about this, especially as it’s not a particularly tricky aspect of this topic. Christopher could try reading this, or this, or this, or this, or this.Christopher then finishes his post by saying

Or is Mr. Born right?

If I’ve understood what Mr Born is saying, then the answer to this question is Yes.