Tuesday, July 2, 2013

CO2 is a resident of the atmosphere

Sou | 4:50 PM Go to the first of 6 comments. Add a comment

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.


Lars Karlsson said...

Gösta Pettersson's error is not so much about confounding different notions of residence/life time. It is rather about failing to take into account the considerable amount of 14C-free CO2 added by burning fossil fuels. Figure 1 in the WUWT post shows the combined results of (1) CO2 with 14C being absorbed by other reservoirs and (2) the proportion of 14C decreasing as fossil CO2 is added to the atmosphere. If it hadn't been for the latter, the Delta-14C wouldn't have nearly reached 0 at 2005.

Sou said...

Thanks for the clarification, Lars. I admit to not reading the article as carefully as I might have done. My bad.

What I picked up on was this:
"the IPCC states that it takes a few hundreds of years before the first 80% of anthropogenic carbon dioxide emissions are removed from the air. The bombtest curve shows that it takes less than 25 years."

The above suggest he is confusing residence time of a molecule with the carbon cycle as a whole, although 25 years is too long AFAIK.

And this:
"This means that global warming during the 2000th century by necessity has led to a thermal out-gassing of carbon dioxide from the hydrosphere. "

Which suggests he has not allowed for the fact that CO2 in the atmosphere is of such concentration that oceans are still absorbing around half the emissions, and that oceans are acidifying.

And this:
"thermal out-gassing and anthropogenic emissions have provided approximately equal contributions to the increasing carbon dioxide levels over the examined period 1850–2010. During the last two decades, contributions from thermal out-gassing have been almost 40% larger than those from anthropogenic emissions. "

Which suggests again that he is not allowing for the fact that the oceans are absorbing more CO2 than they are 'out-gassing' - as above. It's as if he is double-counting CO2. That is, that he is counting the CO2 being absorbed by the ocean (around 50% emissions) but then saying it's coming out again but in larger quantities than it's going in. Which isn't the case or the oceans would be getting less acidic.

But I see now that I may be wrong about what he's saying. I'm confused by what he means by this:
"the amount of emissions taken up by the oceans has been gravely underestimated by the IPCC due to neglect of thermal out-gassing. "

Now he seems to be saying that the amount being taken up by the oceans is underestimated. So in the end I'm not really sure what he is trying to say. Maybe another case of double-counting. I figure he's trying to redo the carbon budget. But whatever way he cuts it, CO2 wouldn't be rising except for human activity. That's why I figured he was mixing up residence time of a molecule with the overall carbon cycle.

Could you comment? I'd like to get my article right.

Lars Karlsson said...

As for residence times, the bomb curve doesn't show what happens to an individual molecule/atom. After all, some of the 14C atoms absorbed by oceans and plans will come back to the atmosphere after a while. They are just as likely to come back as any other C atoms.

Here is a post in Swedish explaining Gösta's error. There are two figures: Fig 1 is based on measuring 14C as fraction of all C in the atmosphere, and Fig 2 is based on 14C as fraction of dry air. As the amount of air doesn't change much, the latter is a good measure of how 14C changes in absolute terms. As you can see in Fig 2, a considerable amount of atom bomb 14C still remains.

Regaring outgassing from and absorption by oceans, I don't expect Gösta to make sense.

Sou said...

Yes, I think I understand now.

If I've got it correctly, what the test is showing is CO2 going into the ocean or vegetation or whatever, coming out again but when CO2 is absorbed or released it could be a C13 or it could be a C14 isotope. So really it's a mixing time rather than a residence time - would that be a fair interpretation?

Sou said...

Getting my isotopes mixed up. I probably should have written 12C/13C rather than 13C/14C.

Plants have a preference for 12C over 13C, which is one way of determining where the extra atmospheric CO2 is coming from - because the ratio of 12C/13C is rising in the atmosphere as we shoot up more CO2 from fossil fuels (ie long dead plants).

14C is much rarer than 13C and 12C is the most common of all.

Martin Vermeer said...

A good link to what Lars Karlsson describes is
this. See especially Figure 4.

What is happening is the complex interaction between several carbon reservoirs under the influence of a single stratospheric injection of 14C, and ongoing atmospheric injection of 14C-poor C. Watts, or Petterson, shouldn't be allowed within five miles of this, for their own good ;-)