Expert Review Comments on the IPCC WGI AR5 First Order Draft -- Chapter 6 Comment Chapter From From To To No Page Line Page Line Comment Response 6-1 6 0 0 0 0 Overall a very nice job in summaring the Carbon and Biogeochemical Cycles!! [Stefan Gerber, USA] Noted 6-2 6 0 May be it's just the style of writing and/or the word limit but I have found a lot of sentences that are too long Editorial - copyedit to be completed prior to with lots of info being condensed and the result is that these sentences are hard to follow even for a science publication. person like me, let alone the general audience for whom the report is being written. I have tried to pick up on a lot of these sentences in my comments below. [Vivek Arora, Canada] 6-3 6 0 Comments on figures [Vivek Arora, Canada] Noted 6-4 6 0 I believe chapter 6 does a good job at summarizing the carbon cycle as well as cycling of CH4 and N2O. I Taken into account - peat and agricultural particularly like how the chapter was organized to give a perspective on the cycles at different points in history management are mentioned in section 6.3 (glacial-interglacial, Holocene, Last millennium, and since 1750). My biggest concern with chapter six is that deforestation/afforestation seems to be the main focus when considering the effects of landuse change on carbon cycling. There is a wealth of recent literature that documents the depletion of soil organic carbon stores that occur with wetland drainage yet this receives very little attention in the chapter. Where wetlands are discussed the focus is almost entirely on northern peatlands and very little attention is given to temperate freshwater mineral wetlands, which are probably the most threatened wetlands as they are found in the agricultural regions of the world. Furthermore, there is growing evidence that indicates that temperate freshwater mineral soil wetlands are more important carbon sinks than was previously thought. There is also evidence that demonstrate that small agricultural impoundments store tremendous amounts of carbon. Inland freshwater aquatic ecosystems play a huge role in the global carbon cycle. In particular the many millions of small wetlands that remain are of particular importance. Please see Armentano and Menges (1986), Anderson-Teixeira and DeLucia (2011), Battin et al., (2009), Downing et al., (2008), Badiou et al., (2011), Gleason et al., (2006). Chapter 1 in the Biogeochemistry of Submerged Soils by Kirk (2004) gives a particularly good summary of wetland carbon stores both mineral and peat. [Pascal Badiou, Canada] 6-5 6 0 This chapter is very well written, easy to understand and well organised. The main results and messages are Noted. Editorial - copyedit to be completed prior to highlighted. Some typos and mistakes are left in the text. [CATHERINE BELTRAN, France] publication. 6-6 6 0 A good chapter but some rebalancing in length might be necessary (eg, 9 pages on CDR but only 1 page on We have clarified and highlighed the CO2 fertilization permafrost, half a page on methane hydrates, and only a few paragraphs on CO2 fertilisation although clearly text. We did not feel it was appropriate to expand the a very important yet uncertain process). [Olivier Boucher, France] permafrost and hydrate text given the limited literature. As for reducing CDR, we have taken this into account 6-7 6 0 Please have a single spelling for fertilisation/fertilization. [Olivier Boucher, France] Accepted Editorial - changed to fertilization everywhere 6-8 6 0 I would say some of the terminology regarding ocean circulation is frequently if not consistently vague and taken into account - text revised simplistic. The processes by which the deep ocean is ventilated and anthropogenic carbon stored there are usually referred to as “mixing”, which is physically inaccurate and reflects representations used in a much earlier generation of models than those now in use. Go through the text searching on “mix”. I would say most of the occurrences that relate to ocean mixing do not make sense to me (e.g. 3/15, 30/36+37, 39/29+31, 51/36, 70/35; 52/10 and 70/11 are exceptions) On p. 30 we “A more vigorous circulation generally results in more uptake of anthropogenic carbon, compensated by an outgassing of natural carbon”, which at least does not conflate circulation and mixing, but I find this passage quite vague. What is a more vigorous circulation? Does it refer to North Atlantic overturning? Southern Ocean upwelling? Or the shallow of overturning cells of the tropics? And do these all have similar effects on CO2 fluxes (see e.g. Watson 1995 in CP Summerhayes et al eds, “Upwelling in the Ocean: Modern processes and Ancient Records”)? When we move water between two boxes of different tracer concentrations, the tracer concentrations in each box change almost exactly as they would with mixing at the interface between the two boxes. Fickian diffusion is actually a good mathematical representation of this process, whether the mechanism is advection or stirring by mesoscale currents, although in the latter case the exchange coefficient (“eddy diffusion”) is not well Do not Cite, Quote or Distribute Page 1 of 188 Expert Review Comments on the IPCC WGI AR5 First Order Draft -- Chapter 6 Comment Chapter From From To To No Page Line Page Line Comment Response defined. But physically it is NOT a mixing process. A good ref for this is Danabasoglu et al 1994 (Science 264: 1123). It is mainly about stirring by eddies along isopycnals vs z levels (i.e. the Gent-McWilliams parameterization used in most climate models), but I find it a very lucid introduction to the physics of tracer stirring vs mixing. The discussion of mixing and ocean models on p. 40 is unconvincing. It may be true that small-scale mixing is a key process that models must necessarily parameterize, but it is not clear that this is the most important missing process. Some might argue that in the current generation of models mesoscale eddy processes are at least as important a constraint on the models’ realism as small-scale mixing. And it isn’t clear that in the subsequent discussion the authors are not conflating mesoscale stirring and small-scale mixing. I think they are both important, but it is important to be clear when we are talking about which. I think the statement that “physical mixing in the Southern Ocean is thought to have caused most of the 80–100 ppm changes in atmospheric CO2 during glaciations (Sigman et al., 2010)” misrepresents the contents of the Sigman paper. This is a good exercise: go through Sigman et al using your “Search” function and read every passage where “mix” occurs. All of these make physical sense to me, and they do not support the summary/paraphrase quoted above. Yes they do state that there may have been increased stratification (which is certainly a barrier to mixing) in the SO, but when they invoke this as a control on atmospheric CO2 they are talking primarily about advective mechanisms (i.e., ventilation of the deep ocean by high-latitude deep water formation). The only place they invoke mixing as a physical control is when they postulate increased stratification between the abyssal ocean (which in their hypothesis would be primarily filled with water from a SH source in the glacial climate) and the intermediate ocean which they hypothesize to be filled with water of a North Atlantic source that penetrates to the abyss in the interglacial climate). This would have the result of both slowing the loss of DIC from this DIC-rich abyssal water (they are clearly talking about diapycnal mixing here but do not address how important diapycnal mixing is relative to other ventilation mechanisms) and slowing the overturning circulation as per Munk and Wunsch (1998, their ref 62). If the key point here is to point out there are potentially important dynamical processes that models represent poorly, if at all, the fact that diapycnal mixing is actually localized along boundaries to a much larger extent than in models is stated by Munk and Wunsch (see also Wunsch and Ferrari 2004 ARFM 36: 281). Of course, this probably contradicts the hypothesis of Sigman et al who are using a conceptual model closer to Munk and Wunsch 1966 than Munk and Wunsch 1998. It remains a possibility that models are tuned to get about the right amount of deep water formation but that the underlying mechanisms are completely wrong, because the ‘background’ level of diapycnal mixing in the main thermocline is almost certainly much smaller than it is in models. This is also discussed in Chapter 9 (“Evaluation of climate models”). Anderson et al 2009 (Science 323: 1443) also discuss the role of deep ocean ventilation via the SO in glacial- interglacial changes in atmospheric CO2, and explicitly cite advection, not mixing, as the key process. Denton et al 2010 (Science 328: 1652) explicitly identify SO upwelling as a source of the postglacial rise in atmospheric CO2. Another important consideration is that Sigman et al is simply an “ideas” paper: all of the scenarios they sketch are simply hypotheses, and even quite speculative ones. Sigman et al are quite clear about this, and papers of this type should not be cited in a synthesis report as sources of specific facts. [James Christian, Canada] 6-9 6 0 I thought that in WG I the rule is no grey literature citations. Lackner 2010 strikes me as especially problematic Section 6.5. - check Lackner (2010) to be non-grey in this regard.