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Curran Papers on CO2 uptake

Curran Papers on CO2 uptake: suggesting "Peak CO2 uptake was 2006"

Very recently these Papers became more widely known / published by - among others - the Guardian, Skeptical Science and a discussion in the FB "Climate Change MOOC" group.

I am not a scientist, but over the last 24 hours have formed a pretty strong opinion on this. I could simply state it here, but perhaps it is fitting to open the floor as it were to serious discussion?

All in favor: say "aye" :)


  • I've long held the view that this would happen sooner or later. It's a shame it is sooner, if that's what is already happening. I wonder about the oceans, too.
  • I agree in general; my biggest fear has been Methane Feedback. This, in the first 20 years after release - which to all intents and purposes is the horizon we must focus on anyway - is pound for pound some 100 times worse than CO2. Methane releases in both Arctic tundra and (Arctic) Ocean(s) have recently been detected and made public.

    But on the Papers: not being a scientist I can only comment on this in general. The whole Paper seems to be based on the premise that the seasonal fluctuations in CO2 levels measured all over the world are linked to photosynthesis. Yes, there is that link, but is there any peer-reviewed paper quantifying exactly how much! Anyone?

    We do know that CO2 uptake and emissions are not attributable to one single source, there are other factors as well. Agriculture for one: soil respiration might fluctuate with seasonal (traditional) soil tillage. Oceans have an uptake and respiration cycle as well, which possibly could be seasonal (temperature) driven?

    AFAIK we can not yet quantify the effect of photosynthesis as a percentage of the whole uptake and if that is correct, the Papers are indeed based on an assumption, not on scientific "gospel".

    I'm sure there are more scientific arguments "out there" and I'm equally sure they will be learning experiences!

  • There are another two papers that are not unrelated. They don't find that sinks are reducing. What they do say is that when CO2 decreases, the sinks will also decrease. There are links to the articles in ERL and Nature Climate Change in the article at the Met Office. (Both open access, with free registration in the case of NCC.)
    ...How will carbon sinks change as we remove CO2 from the atmosphere?
    A paper published in Environmental Research Letters shows how natural carbon sinks may weaken in response to efforts to remove CO2 from the atmosphere. Understanding this effect is vital to devise pathways consistent with the UN's ambitious 1.5 °C and 2 °C climate targets. "We expect natural ecosystems to continue to absorb CO2 but as society reduces emissions, or removes CO2 from the atmosphere, this ecosystem service will itself weaken," explained Dr Jones....
  • I was "trying to absorb" - pardon the pun - this...   and it seems o "not support" the Curran Papers.

  • Thanks for these links Francis.    The Curran paper makes exceptional that should make headlines.... I mean it would be really I am looking for that convincing evidence ...and i am afraid (or should I say pleased) i dont see it in the papers. Indeed the data that is shown for example in the Tamino article doesn't support this claim. Curran shows a graph that supposedly fits the data but we don't see the data wouldn't fit the quadrilateral graph exactly and El Nino or La Nina years should show up. This is not discussed in the paper at all. We don't see the extent of these departures. The best fit graph that Curran supposes would for example be heavily influenced by the last 18 months of CO2 levels influenced by the recent El Nino. The yearly increase from last August for global monthly averages is a staggering 3.6 ppm ( ). Ok this suggest that we may be entering a phase whereby the biosphere is starting to have difficulty maintaining the previous sinking of CO2 but this would be best seen as speculative at this stage. If Curran had believed that the Mauno Loa data showed peak carbon had occurred in 2006 one would also expect to see the Barrow data being analysed where greater seasonal swings occur. So in short I have my doubts.
  • @ aTTP: Yes indeed I had noticed the - albeit short - exchange on Twitter. One Reason I took it here is that Twitter, limited as it is to 140 chars. is *totally* inadequate to facilitate serious discussions over Papers! :( James Curran himself repeatedly asked to be proven wrong (he'd LOVE it, his words) and I hope we can give him just that!

    @ PatH: Thanks very much for that contribution! Glad to have suggested a useful link :)
  • Thanks again PatH and also for directing me to the later Tamino article in the Climate Change MOOC group! I think it is worth sharing here as well!
  • edited October 2016
    Well, there *is* at least one Paper that has quantified datasets on CO2 emissions by all major components after all! 

    Accurate assessment of anthropogenic carbon dioxide (CO2 ) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere is important to better understand the global carbon cycle, support the climate policy process, and project 5 future climate change. Present-day analysis requires the combination of a range of data, algorithms, statistics and model estimates and their interpretation by a broad scientific community. Here we describe datasets and a methodology developed by the global carbon cycle science community to quantify all major components of the global carbon budget, including their uncertainties

    A comment detailed this as follows: >>  Taken the year 2011: human emissions were 10.4±1.0 PgC. Growth in the atmosphere was 3.6±0.2 PgC (only 35% from the emissions) Oceans got 2.7±0.5 PgC (26%) and land 4,1±0.9 PgC (39%).  << 

    Is this Paper, these datasets in any way reflected in the Curran Papers under discussion?

  • IIRC, that paper is mainly about a single year (2011) and also determines the terrestrial sink by simply closing the budget (i.e., emissions - atmos - oceans).  I don't think it's really related to the Curran paper which is suggesting a slowdown in the terrestrial sink.  I haven't had a chance to really look at the Curran paper, though.
  • edited October 2016
    @ aTTP: (CURRAN Papers; attached for all!). My point - that I should have clarified better - is that terrestrial uptake, being just a portion - which in itself consists of several components - yet in the paper is seemingly "blamed" for all of the seasonal fluctuations.. is that not a stretch? Why not assume that Ocean uptake exhibits a certain seasonal fluctuation f.i.?
  • The yearly cycle showing seasonal cycles are consistent with mainly terrestrial uptake because we see the seasonal cycle much stronger in the NH where there is more land. I also recall that isotope variations confirm this. The fluctuations from year to year (promoted by ENSO changes) also indicate a mainly biological seen from similar isotope variations. The oceans absorbing more or less CO2 would not show the isotope variation taht we see. (Plants preferably absorb and therefore emit the lighter C12 isotope and we see the corresponding change in the C13/C12 ratio.)

  • The first Tamino article mentioned, from 1 June 2012, is really interesting. But note that it considers the amplitude of the annual cycle in the Mauna Loa record. I looked specifically at the bit of the annual cycle that is the drop. You can easily imagine a situation in which the drop begins to decrease (as is suggested in my analysis) while the immediately subsequent rise increases. Then the amplitude could remain pretty much the same, or potentially even increase. You could postulate, I think, that the rising limb of the annual cycle, during the N. Hemisphere winter, could certainly increase if warmer temperatures promoted more active biodegradation of leaf litter - just one example.
  • edited October 2016
    @PatH Came upon this, and figure Ozone (near the surface) is probably mostly concentrated to NH...
    (Perhaps the issue is with the presentation of the data in the Curran Papers, less than with the premise??)

    >> Trees also reduce their photosynthesis due to ozone. That makes it a double whammy for climate change — as a greenhouse gas, ozone directly causes global warming; but it also does so indirectly, by reducing the amount of CO2 that trees take up. <<
  • @James Curran  Do you have a graph that shows how the drops fit your theoretical best fit quadratic equation? i would expect to see variations due to the ENSO cycles.

  • I'm not sure it's possible to say anything definitive about the longer term carbon take-up of the entire N Hemisphere by studying the seasonal cycle sampled at just one location. For example, this poster observes the same change in amplitude and attributes (at least in part) the change to the different timing of circulation of airmasses from Eurasia and N America that are eventually sampled at Mauna Loa.

    Take a look at the Global Carbon Project, a detailed annual study done by many specialist scholars. They bring together inventories, observations and models. They conclude that the land uptake, although very variable, shows no sign of any recent decrease. Here are a couple of slides from that presentation.

    I would have to regard this study as much more reliable--using as it does many different lines of evidence--than the Curran & Curran work.

    I do think that the land sink needs to be watched closely and, indeed, I have written about how carbon-cycle feedbacks may hold some nasty surprises for us over decades to come. I just do not think that major changes have been observed yet.
  • edited October 2016
    In reply to PatH of 18 October:  Here is the plot of residuals (not published) from the quadratic fit line in Fig 3 of the first of the two published papers (April 2016).  I don't see any pattern and you can see the autocorrelation doesn't suggest much going on.  Except, as I mention in the paper, there is a statistically significant increase in the absolute value of the residuals over time - which could fit with increasing variability of weather patterns.
  • Thanks James. However with these residuals typically of 10% or greater I would not have confidence in saying peak carbon has occurred.  The downward trend lately may well be ENSO related. Taking this forward in time over the next few years may well see other better fitting quadratics. Also have you tried your analysis on the Barrow data where there are larger seasonal cycles. 
  • I chose Mauna Loa for the obvious reason that it's generally understood to be the most representative single station for what's happening to planetary CO2.  However - interesting suggestion - and I've had a very quick look at Barrow.  Using monthly figures (I used weekly in the published papers) and the necessary de-trending etc, there appears to be a steady increase over the whole period in the size of the intra-annual drop of CO2.  This plot is visually a very different shape from the equivalent for Mauna Loa.  It suggests that the circum-polar regions are continuing to green up and be more sequestration-active.  I guess that's what you'd expect as previously semi-frozen land areas begin to unfreeze... 
  • James Curran, How did you define the drop, was it from certain month to certain month, or from peak-to-peak?
  •  Ari_Iokimaki   A bit more sophisticated than either of those. If you go to the website with the abstract and click on my name, then you can email me.  I can easily provide you deatails then.  Thanks.
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