This was an ask stoat question, and probably a fairly easy one, so I'll have a go.
First of all, what is it? AF (ie, Airbo(u)rne Fraction, is the proportion of human emitted CO2 that stays in the atmosphere, the rest being sunk in land or ocean. Now it is important not to confuse the "proportion that stays in the atmosphere" with "the concentration in the atmosphere" otherwise you get silly little skeptics running around thinking that "airbourne fraction is constant" means that CO2 has stopped increasing. Sigh. However, I see that last time I looked at this I was having to slap down the other side who seemed to think that AF was rapidly rising to 100%. Sigh #2.
Having poked around a bit after the recent Knorr paper, I find that skeptical science seems to have done a pretty good job on the story in general, so I don't think there is any need to explain that side much more (or, you can have mt and Eli). But since I've started this post I'd better find something to say. And that is... two things:
First off, there does seem to be some "pressure" to find increasing AF. That it should increase doesn't seem to be a strong scientific prediction, but it wouldn't be very surprising given increasing human emissions and possible degredation of sinks. And if it was increasing, that make future CO2 predictions more exciting. Which brings me on to the second point:
As someone commented, AF itself isn't a very physical variable. The real physical variables are the sources and the sinks. What we can measure fairly easily are sources (fossil fuel use well; deforestation harder) and the concentration. From those you can compute the sinks and the AF. Or you can try to estimate the sinks directly but that is imprecise. But now suppose you need to predict *future* CO2 levels (and if you want to project future climate change, you do need to). You can run your economic models forwards and deduce emissions, but modelling the sinks is hard (you can stuff them all into a coupled GCM with carbon model, and the Hadley folk at least can do that, but I'm not sure how accurate it is thought to be). So it is an awful lot easier just to use the constant scaling factor of AF to deduce future CO2 levels. And so you have this "tuning knob" and what value should it be? Since AF ~ 0.5, I think most people use a half, which seems about fair - you don't know its future value accurately, but then you're guessing at the emissions too, so it all washes out togther. But of course, if you knew that AF was going up, you could get to higher CO2 levels earlier. That would be bad, wouldn't it?
[Update: per L in the comment "Some folks talk about GHGs running away a bit, what with all the forest fires and permafrost melting -- this would also show up as an increased AF, wouldn't it". More easily, it would show up as yet another thing you can measure, the year-by-year change in CO2 levels. Which was what the earlier post, and its graph, was about. Which shows that nothing too wildly exciting is occurring.
ps: we're up to comment ~9,800. Nearly at the majic 10k, for which there will be a Prize! -W]
I think your two points were worth reading. There's something else that hasn't seemed to receive much discussion, either. Some folks talk about GHGs running away a bit, what with all the forest fires and permafrost melting -- this would also show up as an increased AF, wouldn't it (at least in the way it's currently measured)?
I wanna win the 10k comment prize!!!!
I got bogged down at WUWT when this first came out, quite some time ago. It became quickly clear to me that most of the confusion was due to the wording of the press release, as well as people not reading the paper, or even looking at the main figure. I think an examination of how a poor press release distorts the discussion is probably more interesting than the topic of AF itself.
I don't want to win the 10k prize.
Probably hafta pay shipping costs myself.
Looks like there's a new study suggesting only about 8 ppm of extra CO2 will get into the atmosphere with more warming (earlier estimates were around 40 ppm).
Might be easier to look at a shift in the isotope ratios.
Re #5: From what I can gather about their approach (extrapolating from recent-past natural changes), it's neither here nor there on the issue of sink saturation.
I don't think you can really talk about future airborne fraction in the absence of a specific emissions scenario.
For example, I posit that the total carbon sink in 2011 will be fairly independent of the actual emissions: the sink is mostly a function of atmospheric concentration, and the atmospheric concentration changes very little from year to year. So, if we drop CO2 emissions to, say, one third their current value in 2011, I'd bet that the airborne fraction would drop to about zero.
I do think it is fairly likely that AF will go up with increasing temperature due to stratification of the surface ocean, and therefore less ocean uptake. Also, I think there is reason to believe that AF will go up as CO2 concentration increases over some minimum threshold because at some point, ecosystems will no longer be carbon limited and will be nitrogen or water or something-else limited instead. (Though there is some ability of plants to shift their C:N ratios, there is disagreement in the ecosystem community about how much adaptation potential there is)
On the other hand, maybe increased precip and growing seasons will increase ecosystem uptake.
As I understand it, it's more complex (and even less sensible) than "the proportion of human emitted CO2 that stays in the atmosphere". As I understand it, airborne fraction is the rise in atmospheric CO2 in a year, divided by the human CO2 emissions in that year.
[Good to have that subtlety pointed out. I think that is what I said, or meant to say. But yes, you can measure it year-by-year, or decade-by-decade, or over all time. But people usually mean year-by-year -W]
In fact, the back of my envelope says that about a third of atmospheric carbon is exchanged with the land and ocean every year, and about one part in 300 is absorbed, net. Both of these numbers are functions of total atmospheric CO2 (which change very slowly), and they apply equally to our emissions and to any other CO2 molecule in the atmosphere.
[Ah, "Both of these numbers are functions of total atmospheric CO2" may be misleading. I think that for the ocean absoption, physically, yes it probably is a function of conc. For the land sink, it may well be more like a constant number -W]
So 0.3 of the CO2 from my central heating today will be in the ocean this time next year, and 0.003 will stay there. Ditto the CO2 from my next breath.
It is right to be concerned about the net absorption number, and to wonder whether sinks are filling up. I used to worry about that a great deal, because sea-surface temperature and acidity trends will reduce the ocean sink. But then I found out, I think, that the surface waters are mostly acting as a buffer for the real sink, the deep ocean, and that will take forever to fill up.
The problem with airborne fraction is that it connects this net absorption number to a completely unrelated number: human emissions. If net human emissions are E, and net land/ocean absorption is A, then the airborne fraction is F = (E - A)/E. As it happens, at current atmospheric CO2 levels, A is about half E, so F is 0.5. But this is a coincidence. If E doubled next year, F would be be 0.75. If it halved, F would be 0.
So for modelling, we should absolutely not be using F as a knob (i.e. rearranging that equation as A = E(1-F), or rather as dC = EF). We should be modelling A as a function of C (total atmospheric carbon), so we get dC = E - A(C). Probably in the domain we actually care about, the function A(C) is very close to linear, so we can write dC = E - aC, for some value a around 0.003 according to the back of my envelope.
i'm not sure that this is what you meant, but I don't think that "standard" projections of future CO2 concentrations are drawn from emissions scenarios using a constant 0.5 AF... emissions are converted into concentrations using the Bern-CC (climate-carbon) model forced by a "prescribed" climate change (?).
"It should be noted that the standard SRES A2 concentration value of 836 ppm was calculated in the TAR with the Bern carbon cycle-climate model (BERN-CC; Joos et al., 2001) that accounted for the climate-carbon cycle feedback."
>Marcus "I do think it is fairly likely that AF will go up with increasing temperature due to stratification of the surface ocean, and therefore less ocean uptake."
>Nick Barnes "But then I found out, I think, that the surface waters are mostly acting as a buffer for the real sink, the deep ocean, and that will take forever to fill up."
What proportion of ocean water is 'surface water'? Also what proportion of this years surface water was surface water last year?
(I have seen circa 1000 years for ocean circulation but ... not above sort of details.)
Re surface water vs deep ocean, turnover times, see this. 10% of surface CO2 goes into the deep ocean each year.