Airborne fraction

Given the recent news that the rise in CO2 has greened the planet, I can see a new “skeptic” meme appearing. Essentially, I can see them arguing that both climate sensitivity is too high, and that nature will take up more of our emissions than we currently think. Consequently, they will argue that we will not warm much because climate sensitivity is low and because projections of atmospheric CO2 rises are too high. For example Judith Curry ended her post by saying

As Nic Lewis points out, this paper alters the RCP scenarios in terms of resulting atmospheric CO2 content; i.e. the RCP scenarios are significantly too high.

I actually don’t understand this particular issue all that well, so thought I’d write this in the hope that some of my commenters might know more than I do (not hard, I hear you say). I should start, however, by pointing out that I don’t think that this greening of the planet is such a surprise and I think it has been included in climate studies for quite some time already. The rise in atmospheric CO2 is about 46% of what we’ve emitted in total; this is called the airborne fraction. About one-quarter of our emissions have been absorbed by the biosphere, and greening of the planet is probably an expected outcome of this.

Credit: Pierre Friedlingstein

Credit: Pierre Friedlingstein

The key issue is really how the airborne fraction will change as we continue to emit CO2. One of the authors of the paper that Nic Lewis mentions above is Pierre Friedlingstein. I found this presentation of his, which includes the figure on the right. It looks rather uncertain, but it seems that the airborne fraction only really starts to increase if we follow an emission pathway above RCP4.5.

So, maybe some of the higher emission pathways will lead to slower increases in atmospheric CO2 than we currently think, but then again, maybe not. If it is only likely to impact the higher emission pathways, this would be good, but not necessarily a reason to pay less attention to the potential risks associated with continuing to emit CO2. Of course, the uncertainty means that we can’t even be confident that this will indeed be the case, and there are other also factors, like ocean acidification, to take into account.

Okay, this post has got somewhat convoluted and isn’t really going anywhere, which is partly because I’m a bit tired, and partly because this is a topic about which I’m not that familiar. I’ll make one point as to why this may be less relevant than it might at first seem. At the moment, the airborne fraction is about 46%. If we were to halt emissions, we’d expect about 20-30% of our emissions to remain in the atmosphere for thousands of years – technically, the long-term enhancement in atmospheric CO2 will be equivalent to 20-30% of our emissions.

One interesting result from climate models is that if we halt emissions, atmospheric CO2 will reduce in such a way that global temperatures will – on average – stabilise (i.e., they will essentially stop rising). If we’re over-estimating the airborne fraction along some of the higher emission pathways, but not the long-term airborne fraction, this would seem to suggest that if we were to halt emissions, global temperatures would – rather than stabilising – continue to rise to the same equilibrium. In other words, if our estimates of the long-term airborne fraction is about right, then even if we’re over-estimating the airborne fraction along some of the higher emission pathways, our long-term warming projections are – I think – unchanged. At least, this would seem to be the case.

So, if this whole greening issue has long-term significance, then it would seem to require that we’re both over-estimating the airborne faction along some of the emission pathways, and over-estimating how much CO2 will remain in the atmosphere once we stop emitting. I’m not even sure if the former is a reasonable inference, but the latter seems a bit of a stretch; at the moment, at least. Anyway, I’ll stop there. If anyone has a better understanding of this than I do, or know where to find out more, comments would be appreciated.

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37 Responses to Airborne fraction

  1. Magma says:

    Some history for you, and to save other readers the time of looking it up. The first IPCC report in 1990 summarized 770 studies on the effect of CO2 on C3 crop growth. So no, this is not new, although I suspect the shrieks of “CO2 is plant food!” will be louder than usual from the deniosphere.

    “In the overwhelming majority of cases the effect of CO2 on productivity has been shown to be beneficial – if all other factors are maintained constant – and a 10-50% increase in dry matter accumulation can occur in most species with a doubling of ambient CO2 concentration. In a summary of 770 studies the mean mature yield increase for C3 crops (see below) was 26% but the extremes ranged from a decrease of 20% to an increase of 200%. For temperate cereals the mean increase observed in grain yield was 36%. The large range in effect of CO2 can, to some extent, be attributed to species differences, however few realistic studies under comparable conditions have been made.” IPCC First Assessment Report Working Group II, Section 2-4 (1990)

    As for how much of the CO2 uptake will stay in plant material, that’s dependent of the interplay of insect pests, rot and fire competing with new growth. Hopefully without generalizing too much (it’s a long way from my field of expertise), century to millennial-scale preservation of plant material seems to be confined to wet bogs, cold regions not subject to widespread summer forest fires, and wet tropical and subtropical forests that are, again, not subject to widespread dry season forest fires.

    In a warmer world with more intense dry seasons and wildfires, additional CO2 sequestration into plant material may be too short-lived to be of much use.

  2. Magma says:

    I just saw Curry’s self-serving comments quoted in the BBC article:
    It is inappropriate to dismiss the arguments of the so-called contrarians, since their disagreement with the consensus reflects conflicts of values and a preference for the empirical (i.e. what has been observed) versus the hypothetical (i.e. what is projected from climate models). These disagreements are at the heart of the public debate on climate change, and these issues should be debated, not dismissed.

    Once again, Curry is playing dishonest rhetorical games. Note that her clear implication is that contrarians are empirical and observation-based, whereas the AGW consensus is hypothetical and model-based.

    As I recall, and I may be wrong, little of Curry, Lindzen, Spencer & Christy, Pielke Sr. or other high profile contrarian’s research ever involved boots on the ground (or flotation suits on the water) field work or the collection and analysis of primary data. They are largely modelers and reanalyzers of other researchers data. Curry’s comment is a nod (or dog-whistle) to the standard denier trope that anthropogenic climate change is purely theoretical and based on a few flawed computer models and heavily-modified temperature data.

  3. Magma,
    Thanks, very useful.

    On another note, here’s a video from Ed Wiebe, circa 2007, showing greening and browning in Canada under an A2 SRES scenario.

    http://climate.uvic.ca/model/movies.old/2100_640x480.mov

  4. Tom Curtis says:

    Anders, the notes from Friedlingstein do not specifically reference this paper, and do specifically reference other earlier papers for panels other than that which you have shown. It is not clear, therefore, that it is an explanation of the paper, or of the state of knowledge with that paper taken into account. Indeed, from the page url, it would appear to represent the state of knowledge circa 2012.

    More importantly, you have misunderstood the panel you show. Specifically, you state, “maybe some of the higher emission pathways will lead to slower increases in atmospheric CO2”. Contrary to that claim, however the graph shows a higher airborne fraction for higher emission pathways, with an increasing airborne fraction for rcp 6 and 8.5 relative to current values.

  5. Tom Curtis says:

    Comment currently in moderation.

  6. Michael Hauber says:

    Nic Lewis is quoted at BBC as saying this means the IPCC projections are too high. In actual fact the projections are for a fixed Co2 pathway, and so this study changes nothing about whether they are too high or not. What it does change is the likelihood of following the higher pathways decreases and the likelihood of a lower pathway decreases. Projections based on earth system modelling which allows Co2 to vary according to natural feedbacks are still in an early stage, and are discussed in the IPCC report as well. From what I’ve seen such models have been overestimating the current level of Co2, so it is not surprise that Co2 uptake by vegetation may be better than previously assumed. We can hope this continues in the future, but I see no guarantees that this will be so. Also there are other reasons to suspect that sensitivity may be higher, especially when looking at how models are working with clouds and it seems to be a recurring theme that models that better simulate clouds tend to show a higher sensitivity.

    To me it is no surprise that as scientists study climate change they should be finding different reasons why sensitivity may be higher or lower than previously thought. Some find only the reasons for higher sensitivity and claim we are headed for disaster, and others find only the reasons for lower sensitivity and claim there is nothing to worry about.

  7. Probably quite relevant older paper (one of the authors is Pierre Friedlingstein).

    Chris Jones et al. 2013, “Twenty-First-Century Compatible CO2 Emissions and Airborne Fraction Simulated by CMIP5 Earth System Models under Four Representative Concentration Pathways”

    http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-12-00554.1

  8. Michael Hauber says:

    And reading the paper, it looks like there is no basis for a claim that greening is more than included in models:

    We compare satellite-based LAI anomalies with LAI anomalies simulated by ten global ecosystem models driven by eCO2 (+46 ppm over the study period), climate, nitrogen deposition and LCC (Supplementary Section 7). Multi-Model Ensemble Mean (MMEM) LAI anomalies, with all these drivers considered, generally agree with averaged satellite observations at the global scale (r = 0.85, p < 0.01; Fig. 2a).

  9. Ethan Allen says:

    airborne fraction (AF) ~ % of CO2 emissions that stay in the atmosphere?

    Someone would have to look at the land component of the carbon budget to see if the greening makes a significant net negative contribution to that component (all other components being equal (don’t change) which might not hold). Atmospheric CO2 has not slowed down to date.

    Here are two papers I recently found …
    A probabilistic analysis of cumulative carbon emissions and long-term planetary warming
    http://iopscience.iop.org/article/10.1088/1748-9326/10/11/115007?fromSearchPage=true
    (Published 16 November 2015)

    The link between a global 2 °C warming threshold and emissions in years 2020, 2050 and beyond
    http://iopscience.iop.org/article/10.1088/1748-9326/7/1/014039?fromSearchPage=true
    )Published 26 March 2012)

    IMHO, the “greening” issue is welcome but is a bait and switch from the central issue, substantively reducing CO2 emissions this century. Greening will save our sorry souls is just more ‘head in the sand’ thinking IMHO.

    BAU -> greening -> more food -> more people -> more (or the same) per capita emissions -> more total emissions

    Per capita emissions currently sit on a high point of ~1.4 tonnes C per capita globally. I’m currently going through these numbers per XOM (2040), BP (2035) and Shell (2050) carbon intensive outlooks. Also need to throw EDGAR 1970-2014 into that cata set (to do rather shortly).

  10. Tom Curtis says:

    Michael Hauber, it is not clear how the paper could disagree with prior estimates of CO2 uptake by plants. The observed data in the paper is relative greeness of observed surface areas. That is only indirectly connected to CO2 uptake and cannot be turned into an estimate of CO2 uptake without a string of assumptions. In constrast, using the decline in atmospheric O2, a very direct check on biosphere uptake of CO2 can be made, with results that have been known since 1996.

  11. anoilman says:

    Anders… great video… Government scientists have confirmed that BC’s forests are indeed greening. They are sinking more carbon that they used to, and growth is now 3-4% higher.
    http://communications.uvic.ca/releases/release.php?display=release&id=1526

    We’ve had major droughts in the prairies for the last 2 years or so..

  12. For several years now, in agriculture—along with the fertilisation effect—numerous articles have referred to the increase in pests, pathogens and weeds as a result of increased CO2. Here is one typical example of the articles I’ve seen: http://glisa.umich.edu/climate/agriculture.

    I suspect in due course it will be shown that the positive benefits of greening only occur up to a certain point, after which negative effects outweigh any positive.

  13. anoilman says:

    johnrussell40: It varies regionally. So areas will do better. Other areas will not.

  14. Willard says:

    A ClimateBall ™ episode, featuring Judy’s misrepresentation of a single but relevant sentence:

    https://judithcurry.com/2016/04/26/rise-in-co2-has-greened-planet-earth/#comment-781053

  15. Brigitte says:

    On the greening stuff here is some quite sensible BBC reporting, I think – about 40 mins in http://www.bbc.co.uk/programmes/b0783lf1

  16. Tom,
    Yes, I realise that the slides are a few years old, but I didn’t make that clear. I did say I was tired and confused 🙂

    Specifically, you state, “maybe some of the higher emission pathways will lead to slower increases in atmospheric CO2”. Contrary to that claim, however the graph shows a higher airborne fraction for higher emission pathways, with an increasing airborne fraction for rcp 6 and 8.5 relative to current values.

    Sorry, I explained that very badly. I meant relative to what we expect for those pathways, rather than relative to what it is today today. However, looking at it again, the ranges are relatively small, so, yes, they do both suggest an increase in airborne fraction relative to today.

    So, maybe you’re right that the talk slides aren’t that relevant, but I was really just looking for something that indicated how the airborne fraction would change for the various emission pathways.

  17. Tapani,
    Thanks, that papers seems to be where some of the figures in Pierre Friedlingstein’s talk came from.

    Brigitte,
    I’ll have a listen, thanks.

  18. Willard,
    Yes, I saw that exchange. Ignoring that it’s not so much that Matt Ridley has highlight greening, but that he has used it

    to argue against cuts in carbon emissions to mitigate climate change

  19. BBD says:

    As I understand it, CO2 fertillisation is likely transient and will be limited / overprinted by other environmental factors (water availability; nutrient availability; plant thermal envelope; insect herbivory). Add in wildfires and soil temperature increase for completeness’ sake and the longer-term impact on sequestration rates is moot.

    It’s about as comfort-giving as the (non) evidence for low S.

    * * *

    A side note for those whingeing about supposed BBC bias against contrarians: the BBC report I read (Harrabin) featured quotes from both Lewis and Curry.

  20. Dikran Marsupial says:

    IIRC the terrestrial sink is not very well characterized, so it is often inferred in carbon budgets as the residual implied by the mass balance equation when anthropogenic and oceanic fluxes (which are well comparatively understood) have been accounted for. This implies that climatologists are very well aware of (i) our uncertainty in estimating terrestrial fluxes and (ii) that the carbon cycle models nevertheless do a reasonable job of representing atmospheric increases. As neither Prof. Curry (“wow”) nor Nic Lewis are experts on the carbon cycle (neither am I), perhaps they should be a bit more reserved in drawing conclusions from one newly published paper. Getting a paper in a journal is the first step towards acceptance by the research community, not the last, and it isn’t very rational to latch onto a new study as if it overturns a whole field just because you like the conclusions. Perhaps it would be better if the media had an 18 month embargo on all science stories to give the community a chance to have a good look at it first? ;o)

  21. Dikran Marsupial says:

    From the abstract of another paper co-authored by Prof. Friedlingstein

    “Variations within this range depend on the probability of staying below 2 °C and on end-of-century non-CO2 warming. Current CO2 emissions are about 40 GtCO2 yr−1, and global CO2 emissions thus have to be reduced urgently to keep within a 2 °C-compatible budget.”

    which suggests that greening is not going to keep us out of trouble.

    Incidentally, one of the more plausible explanations for the “missing sink” (i.e. that inferred from mass balance and what we know of the terrestrial fluxes) is an increase in boreal forests, which implies some greening is expected there, although it may be due to increasing temperatures, rather than directly from increasing CO2 levels.

  22. Dikran,
    Yes, I think had a glance at that paper. Seems that despite the possibility of continued greening, the estimates for the carbon budget in that paper are roughly what one would expected: 590 – 1240 GtCO2 from 2015 onwards, or 160 – 340 GtC. We’re currently emitting about 10GtC/year, so something like another 16 – 34 years from now at the current rate.

  23. verytallguy says:

    Some observations and questions, no answers:

    1. It’s a remarkable demonstration of how profound changes can be driven by greenhouse gases. I’d guess we don’t have a great understanding of how this will affect ecosystems, even if it will be net beneficial (whatever that really means in this context). Though I’d guess there would likely be few strong negatives for food production at least?
    2. Is the degree of greening estimated by this study more, or less than previous predictions? It’s not clear to me.
    3. What do other measurement methodologies show?
    4. *if* this is showing more greening than expected, then presumably the land sink for CO2 is currently higher than previous estimates. As you’d expect this to drop off (as other nutrients become limiting), then I’d guess this study could result in future estimates of airborne fraction being revised *up* not down?

  24. Vtg,
    I agree about the profound changes. If anything this is the main issues. If we continue as we are, there will be changes that could be quite significant and we will have to deal with them, even if some of them could (will be, probably) positive.

    Is the degree of greening estimated by this study more, or less than previous predictions? It’s not clear to me.

    From reading – briefly – some of Friedlingstein’s other work, the suggestion seems to be that the observations of tropical uptake (I think) are consistent with the models, but at the low end (i.e., the airborne fraction will increase, but will be on the lower side of the range). That’s my understanding. For example, for RCP8.5 it might increase to 0.6, rather than 0.7. However, there may well be nuances to this that I don’t understand and, I think, the models still do not consider other carbon cycle feedbacks like permafrost release.

  25. verytallguy says:

    On other matters, in reference to a previous thread, this is yet another example of how climate “sceptics” have their right to free speech abused.

    Only two scientists independent of the authors are quoted in the BBC article. Just two!

    Only both of these are “sceptics”, and neither has any expertise in the carbon cycle. Why only two more layman “sceptics” than the zero expert scientists ramming the manufactured “consensus” down our throats?

    What about all the other “sceptics” who are ignorant of the area being reported? Their free speech is under attack, clearly.

    The Stalinist BBC promoting Lysenkoist “science” needs to be cut down to size.

  26. verytallguy says:

    Further showing my ignorance, I’d guess that the new(ish) OCO-2 satellite data should greatly inform our understanding of the carbon cycle:

    Also, just an excuse to post this rather beautiful video simulation of atmospheric CO2 concentrations. I cheerfully confess to knowing nothing of the model behind it, perhaps others can inform on that:

  27. paulski0 says:

    vtg,

    4. *if* this is showing more greening than expected, then presumably the land sink for CO2 is currently higher than previous estimates.

    Figure 2 a) and c) I think show that there is no discrepancy in the long term change compared to expected, where expected is defined by the carbon cycle models used. The linear regression trend across the average of three satellite datasets is slightly larger than the trend for the multi-model ensemble mean, but it looks to me that just accounting for uncertainty in the linear trend estimation would reveal no statistically significant difference. Without even getting into structural uncertainties in the observations.

    3. What do other measurement methodologies show?

    Not sure either, though study periods need to be taken into account when comparing. For example the start date for this study (1982) is clearly in a local minimum for land carbon uptake, and therefore we should expect a slightly enhanced trend compared to the “true” long-term change.

  28. Lars Karlsson says:

    What silly silly games Curry is playing:

    “It is inappropriate to dismiss the arguments of the so-called contrarians, since their disagreement with the consensus reflects conflicts of values and a preference for the empirical (i.e. what has been observed) versus the hypothetical (i.e. what is projected from climate models).”

    I’d rather say that contrarians are like surgeons that don’t want to operate unless they have observed that the patient has died. They are the pathologists of climate science.

  29. Lars,
    It’s a bizarre thing to say. I’m sure there are some scientists who tend to focus on empirical things, and others who focus on models. However, good scientists are aware of the value of both. That Judith seems to think it good that she, and those she supports, focus on empirical things and largely ignore models, suggests she doesn’t really understand how best to evaluate scientific evidence.

  30. Lars Karlsson says:

    ATTP, I suspect that Curry wrote that for those readers who don’t know how science works.

  31. Willard says:

    There’s nothing silly there, Lars. Judy’s signalling the virtues of empiricism and associates them with contrarians. RichardM did a similar thing in his Quora response:

    Lukewarmists. They, too, stick to the science. They recognize there is a danger but feel it is uncertain. We should do something, but it can be measured. We have time.

    http://www.huffingtonpost.com/quora/the-classifications-of-cl_b_9729598.html

    The ambiguity of “measured” provides a nice touch.

    How RichardM divided everything on top and lumped everything under the lukewarm church is a rare thing of beauty.

  32. Dikran Marsupial says:

    “focus on empirical things”

    shame the “empirical things” of the future are not available at the present time.

  33. MMM says:

    If I had an hour, I’d track down:
    RCP projected emissions and CO2 concentrations (http://tntcat.iiasa.ac.at/RcpDb/dsd?Action=htmlpage&page=compare)
    Observed CO2 concentrations (ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_annmean_gl.txt)
    and estimated emissions (cait.wri.org)
    and see which RCP we’re closest to in terms of emissions, and whether concentration is lower or higher than that RCP. My rough eyeball suggests that in fact, concentrations are a bit low in comparison to emissions, but I didn’t do the leg-work to calculate the land-use emissions in WRI so I could add it to the CO2-only numbers and then convert from CO2-equivalent to PgC…

    Not that the “greening” paper contributes directly to this discussion as far as I can tell, despite Nic Lewis’ opinions.

    -MMM

  34. Marco says:

    “That Judith seems to think it good that she, and those she supports, focus on empirical things and largely ignore models, suggests she doesn’t really understand how best to evaluate scientific evidence.”

    Some people might have noted that a model was used to infer that CO2 fertilization was the most important factor. Other people, Curry in particular…apparently not so much.

  35. BBD says:

    Or to paraphrase:

    “You are a very clever young man but you can’t fool me. It’s models all the way down!”

  36. pendantry says:

    My tuppence:

    1. It’s my understanding that even if we were to halt all emissions today, inertia in the system guarantees continued warming for about another four decades. I know I haven’t understood all that’s been said in this article and comment thread, but I haven’t recognised any acknowledgement of this salient factor. (Perhaps it’s simply that my understanding is wrong?)

    2. How does the ‘improved greening’ compare as a proportion with the rate at which humanity is bulldozing forests per year? That is to say; surely it’s meaningless unless we accept we need to stop chopping down the trees that some seem to think are going to save our behinds.

  37. It’s my understanding that even if we were to halt all emissions today, inertia in the system guarantees continued warming for about another four decades.

    Technically, this isn’t true. If we completely halt all emissions, global temperatures will actually – on average – stabilise. This is because halting all emissions will lead to a reduction in atmospheric CO2 that would roughly cancel the warming that would have occured had atmospheric concentrations remained fixed. I think that there will be some hemispherical variation in that the NH will actually cool slightly since it has more land and will have warmed faster, while the SH will continue to warm a little. It’s discussed in RealClimate post.

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