|Review of a revised version of "Ocean Carbon Uptake Under Aggressive Emission Mitigation" by Sean Ridge and Galen McKinley|
I have reviewed a first version of this manuscript, and I am happy to see that the revised manuscript has
improved a lot. I have a few remaining issues (listed below), and I would recommend the manuscript
for publication in Biogeosciences after the authors have addressed these.
The analogue of vertical diffusion used by the authors is misleading as far as the impulse response function is
concerned. In lines 160-168 it is stated: "The convolution integral (Equation 7) is derived from the model’s surface
anthropogenic carbon tendency equation", which is not correct. The impulse response function (IRF) doesn't know the
gradient of anthropogenic carbon, and I don't see how equation 9 and 7 are related. Of course, it is possible to reproduce
results from a box-diffusion model sufficiently well by fitting an IRF (as in Joos et al. 1996). But an IRF is not based
in any way on the assumption that the vertical mean ocean state can be represented by a 1d-diffusion approach as in
equation 8. The IRF is an empirical fit to model results, representing all processes that the fitted model included.
Later we find statements like
*lines 385-388: "Our one-dimensional ocean carbon cycle model represents multiple physical
processes that remove carbon to depth as a single diffusive process that is constant in time (Equation 8) using an effective
vertical diffusivity, K_z,eff . The value for this term in the one-dimensional model has been set (Section 2.3) so as to
mimic advective, eddy-diffusive and watermass transformation processes occurring in CESM.", which wrongly suggests that
the IRF would use a vertical diffusivity as a parameter.
*lines 406-408: "The remainder of the climate-carbon feedback is related is due to changing physical transport, which
in the one-dimensional model is due only to the vertical carbon gradient and ocean circulation is constant." Again, the IRF
does not know the vertical gradient of carbon.
I would strongly suggest that the authors revise the above mentioned parts of the manuscript. I see that the authors need to
introduce the vertical C-gradient somehow, to be able to define the "gradient effect", but please do this in a way that avoids
the impression that this is parametrized in the IRF.
Also, it would be good to mention/discuss that the "gradient effect" as used by the authors is found by residual (difference
between the "historical scaling" and the ccc-simulation) so you don't actually need a model that models a vertical gradient
to derive it.
The concept of a constant sink rate, which allows for defining the "historical scaling" is a central point, but
it is introduced too late in the text (lines 63-70). It would make the text easier to understand if these lines
could be moved up to somewhere after the definition of k_S (lines 34-44). I see that lines 63-70 deal with the
ocean sink only while lines 34-44 more generally deal with the land and ocean sink, but then please generalize
lines 63-70 (you write about "...the theoretical prediction of constant sink efficiency..." already in line 40,
and for a reader not familiar with the concept this is unclear).
line 50-51: Please consider replacing "climate-carbon feedbacks" by "carbon cycle feedbacks" (here both,
carbon-climate and carbon-concentration feedbacks are meant).
line 74: RCP8.5 is not a "business-as-usual" scenario, it should be called a "high emission" scenario.
line 100-105: Please double check whether this could be more concise. It seems to me some sentences are more
or less duplicate.
Section 2.2: Please spell out which version of CESM you are using and add a reference for the whole model. Please
also add a suitable reference for the RCPs (e.g. 10.1007/s10584-011-0148-z). It remains unclear to me why the athors
are reluctant to properly acknowledge CESM scientists and engineers, as I commented in my first review.
lines 177-178: "...we perform two sensitivity experiments...". It is only one sensitivity experiment that is
performed, isn't it? The "historical scaling" is not an experiment with the 1d-model. Also, please describe
the sensitivity experiment briefly in the main text (not only in Table 1). E.g. "...we perform a sensitivity
experiment ... where the buffer factor is kept at pre-industrial level" or similar.
line 188: From Figure 6 in Randerson et al. (2015) I roughly read that AMOC is reduced by 10 Sv in 2080. This
is quite a significant reduction. So the point is not that CESM hasn't significant circulation changes but rather
that the carbon cycle in CESM seems to be relatively insensitive to these changes (on a global scale). Please
consider revising this sentence.
line 199-200: "The pCO2ocn closely follows pCO2atm , with the same sign." I don't understand what the authors
mean with "with the same sign". Please clarify.
line 217: "Projected Spatial Redistribution of the Anthropogenic Carbon Air-Sea Flux". I think "redistribution"
is not a good wording here. Maybe better "Projected Spatial Patterns of Anthropogenic Air-Sea Carbon Flux"?
lines 290-291: "...the ocean would absorb 158 Pg C_ant from 2020 to 2080". You mean would absorb 158 Pg C_ant
in addition, right? Please consider making this clearer.
line 304-305: Please check the logic of this sentence (it is not clear what "because" refers to).
line 392: "differences" is a bit unclear. I think the authors mean "changes".
lines 400-402: "We use our one-dimensional model to estimate climate-carbon feedbacks for CESM." If this is really the
case a bit more explanation would be appropriate, but it seems the authors use the feedback values from Arora et
al. (2013). Please add either more explanation as to how you estimate the feedback with the 1d-model, or delete
this sentence. Also CESM's gamma_o is not only weaker than the CMIP5 mean, but it has the weakest ocean carbon-climate
feedback of all models.
lines 403-404: "For CESM, decline in ocean carbon uptake due to climate-carbon feedbacks in high emission scenarios is
an order of magnitude smaller than due to change in ocean chemistry (Randerson et al., 2015)." I cannot see that the
Randerson paper supports this statement. They cannot separate climate-carbon feedbacks from feedbacks due ocean chemistry
with their experiments. Please clarify/justify this statement.
line 431: Please specify after which year carbon in the upper 100m starts to decrease.
line 24: "is controlled" -> "is further controlled"
line 27: "...dominates regional patterns anthropogenic..." -> "...dominate regional patterns of anthropogenic..."
line 48: "that the future" -> "that in the future"
line 59: This sentence duplicate the previous one, please consider merging the two sentences.
line 82: Please consider writing "... referenced to the year 1990, and expressed as a percentage".
line 145: "based in impulse response functions" -> "based on a impulse response function"
line 147: "...to the RCP4.5 and RCP8.5 CO2 concentration pathways" -> "...to the RCP CO2 concentration pathways"
(applies to all RCPs, not oply 4.5 and 8.5)
Section 2.5: There is a couple of instances where the superscript "ML" is missing in the notation for "C_ant^ML".
Please check this throughout this section.
line 198: There is a minus sign missing in the equation, please check this.
line 233: "Globally-mean" -> "Global-mean"
line 239: "CESM-simulated air-sea anthropogenic carbon uptake" -> "CESM-simulated anthropogenic carbon uptake"
line 233: "(Figure 2b)" -> "(Figure 2c)"
line 249: "at that location" -> "at that depth"
line 259: "fro" -> "from"
line 276: "by the the" -> "by which the"
line 318: delete "acts"
line 324: "aroudn" -> "around"
line 326: "to a less net uptake" -> "to less net uptake"
line 362: "...are renewed connection..." Please check grammar of this sentence.