Airbourne fraction

Posted by William M. Connolley on October 23, 2007
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AF (ie, Airbo(u)rne Fraction, ie the proportion of emitted CO2 that stays in the atmos, the rest being sunk in land or ocean) is in the news; I wrote up part of it recently (and detected some nonsense about it a year ago). There is a PNAS paper, Canadell et al; Eli has already done it.

When talking about the N Atlantic results, I was unwise enough to say “But airbourne fraction is still about 55%, so this can’t be happening globally.” This was in the context of the North Atlantic halving its uptake, which I instinctively thought couldn’t happen globally or it would be obvious in the CO2 data. But thinking about it more…

..and somewhat prompted by Eli, I wonder if this is true. Suppose we assume the AF is 50%, and of the remaining 25% goes into the ocean and 25% into land sinks, which is reasonable. And suppose we wonder if a decline by half of the ocean sink would be obvious. Well the “before” state would be an AF of (1-1/4-1/4) = 50%, which is the assumption I started with. And the “after” state would be AF of (1-1/4-1/8) = 62.5%. Could you hide that in the interannual variations? Probably. Fig 2 of Canadell et al shows a tiny trend from 1960-2006; but from 1992 to 2006 there is a (by eye) trend from about 20% to 40%, with big swings. So yes, there could be a global change in ocean uptake by 50%, and it wouldn’t be too obvious from the atmospheric data. So I was wrong, hurrah. Was that clear?

In fact the ocean trend from Canadell et al. is about 5% over 40 years (ie, a decline from 30% to 25%), the land stays roughly constant (note that the atmos is measured, the ocean comes from an ocean GCM, and the land is the remainder), so the AF goes up by 5% (errrm, and since the AF is about 50%, thats a relative increase of 10%) over the same period.

i-a37538bd60b90526822b69b147f15464-co2-rise.png But… in all of this one shouldn’t forget the bottom line, which is CO2 in the atmosphere. Which to me looks fairly linear (top pic) and the rate of increase has been below 3 ppm since 2000 – since the 1998 El Nino, in fact (bottom pic). Thats less than a 1% per year increase, which is what the SRES a1b assumed for the “future” (ie past 2000, ie now).

This points up the difference between the atmospheric concentration and the emissions growth rate, which as Canadell et al say is now 3.3%/yr, but you can’t really see that in the CO2 data. Of course if we keep growing at 3.3%/yr you soon will be able to see it.

Lastly, the atmos conc is also affected by the AF, and C et al claim to detect an increasing trend in AF. Weeeeeellll… C et al only get P=0.89, which is well short of the 95% sig you would normally want on a trend (and thats for the *best* of 3 methods they use). So I’m not too convinced by their trend. It only accounts for 18% of the atmos growth anyway (the remainder being mostly economic growth and some increased carbonisation, probably because of switching from oil to coal, and Chinese use of coal).

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  1. #1 Adam
    2007/10/26

    Looking at that upper graph, it looks like prior to 1997 the trend matches the blue trend line, then 98/99 rise above it before 2000-2003 drops below it again, and since then it’s risen above it again (all years approximate). Is there any way to plot the variation in trends over say a three year period and to see if it has increased these last three years? Or am I misreading the graph?

    I realise that <5 years is probably not a good timescale for trends, but it could be something to keep an eye on (which you are obviously doing otherwise you wouldn’t have these posts).

    [I think you're right, but reading off the upper graphs the deviations are all too small to be useful -W]

  2. #2 Adam
    2007/10/26

    Hey! You scrubbed my caveat. ;)

    Okay, thanks for the reply.

  3. #3 Eli Rabett
    2007/10/26

    I thought the most interesting thing was the HUGE variations in the fraction taken up by the land. Is there any good analysis of this (e.g. warm years vs. cold lots of rain vs. none) on a global scale?

    [Dunno. Land is the residual of atmos and ocean, so it could be errors in the ocean component. But it seems more likely that land would be more variable - droughts, temperature swings, etc -W]

  4. #4 Hank Roberts
    2007/10/26

    Always worth searching by author, e.g., some info here:
    http://scholar.google.com/scholar?q=%22author%3AJ.%20G.+author%3ACanadell%22

  5. #5 Eli Rabett
    2007/10/29

    True on the land being more variable (OTOH its raining in Africa, there drought in Iran. . . ) but the land goes to carbon neutral in some years, (and I suspect even becomes a net source in cases)

  6. #6 Dano
    2007/10/29

    If I’m briefing a decision-maker with this info., I’m going to state we should choose this metric and irrigated land under ag. Why irrigated? That’s going to change and it’s higher productivity. It will be a lower negative at green-up and increased heat will lower productivity, thus lowering sinks. Monitoring these numbers will be essential for directing policy toward further lower C or allowing some emissions for x time.

    Aside, but relevant: as we offshore our C emissions, will it become necessary to limit our offshore emissions as well and make Kyote-type things more relevant after all?

    Best,

    D

  7. #7 Nick Barnes
    2010/01/02

    Can you write something about AF, relating Canadell to Knorr? My view is that AF is really a bogus number, and that the figure of merit is the net ocean uptake, which is some sort of function of atmospheric CO2 concentration. But what sort of function? Etc.