CO2 is a resident of the atmosphere

Sometimes it's as if WUWT is going methodically through the list of SkepticalScience myths, trying valiantly to revive them.  This time someone called Gösta Pettersson is muddling the residence time of an individual CO2 molecule with the time taken for all the CO2 in the atmosphere to go through a complete carbon cycle or the time taken to get to a new equilibrium state in the earth system.

Edit: Lars Karlsson has made a couple of comments pointing out that the issues in Gösta Pettersson's article are not as simple as I made them out to be.  (See comments below).

Any individual molecule of CO2 will get used up in photosynthesis or absorbed by the ocean (at present the ocean is still a net absorber of CO2 rather than a net emitter, because of the partial pressure of atmospheric CO2). But no sooner does that happen than another molecule pops into the air to replace it - maybe in exchange with the ocean or via plant respiration.  And while all this is going on we keep adding more CO2 to the air and the oceans, by burning fossil fuels and other activities like land clearing.

That ongoing cycle between vegetation, oceans, soil and atmosphere determines the residence time of a single molecule.  The length of time taken before atmospheric CO2 reaches a new equilibrium is much longer - of the order of centuries to millenia.

I rather like this diagram from realclimate.org to get a sense of the time frames involved.  It's designed to illustrate climate sensitivity.  Click the diagram to see a larger version.

Source: RealClimate.Org

There are any number of diagrams of the carbon cycle. For example this one from the NASA/Globe program. (Click to enlarge):

Credit: NASA/Globe Program

In the comments a number of people point out the mistake Gösta Pettersson made. Some deniers refuse to accept it, but that's par for the course at WUWT.  The question remains - why does Anthony Watts promote these false myths on his blog?  It can't be for any good reason.