Resolving ecological feedbacks on the ocean  carbon sink in Earth system models

Armstrong McKay, David I.; Cornell, Sarah E.; Richardson, Katherine; Rockström, Johan

doi:https://doi.org/10.5194/esd-12-797-2021

Articles | Volume 12, issue 3

https://doi.org/10.5194/esd-12-797-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/esd-12-797-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 12, issue 3

Research article

|

14 Jul 2021

Research article |

| 14 Jul 2021

Resolving ecological feedbacks on the ocean carbon sink in Earth system models

David I. Armstrong McKay, Sarah E. Cornell, Katherine Richardson, and Johan Rockström

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Final revised paper (published on 14 Jul 2021)
Supplement to the final revised paper
Preprint (discussion started on 15 Jul 2020)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review of McKay et al.', Anonymous Referee #1, 10 Aug 2020
- AC1: 'Author’s Initial Response to Anonymous Referee #1', David Armstrong McKay, 08 Oct 2020
RC2: 'Review of "Resolving ecological feedbacks on the ocean carbon sink in Earth system models" by Armstrong McKay et al.,', Jamie Wilson, 10 Sep 2020
- AC2: 'Author’s Initial Response to Referee #2 (Jamie Wilson)', David Armstrong McKay, 08 Oct 2020
SC1: 'Interactive Comment on McKay et al., 2020 ESDD ``Resolving ecological feedbacks on the ocean carbon sink in Earth system models''', Karin Kvale, 20 Sep 2020
- AC3: 'Author’s Response to Short Comment #1 (Karin Kvale et al.)', David Armstrong McKay, 08 Oct 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (20 Oct 2020) by Michel Crucifix

AR by David Armstrong McKay on behalf of the Authors (08 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (09 Feb 2021) by Michel Crucifix

RR by Jamie Wilson (12 Feb 2021)

RR by Anonymous Referee #1 (01 Mar 2021)

Suggestions for revision or reasons for rejection

I’d like to thank the authors for addressing the majority of the points identified. I have a few follow-up points on some of the responses, and a couple of suggestions the authors might like to consider that may help improve the manuscript.

Follow-up:

- A significant outstanding omission remains the contextualising with CMIP5 (I accept the authors' stated issues with CMIP6). The authors assert that there is little information on this, but omit Fu et al. (2016)’s quite comprehensive study on this specific subject. This should provide helpful relevant context that the manuscript is currently weak on.

Fu, W., Randerson, J. T., and Moore, J. K.: Climate change impacts on net primary production (NPP) and export production (EP) regulated by increasing stratification and phytoplankton community structure in the CMIP5 models, Biogeosciences, 13, 5151–5170, https://doi.org/10.5194/bg-13-5151-2016, 2016.

- While the manuscript is now clearer on how CaCO3 production is parameterised in the model, I was a little confused about how it dissolves down the water column. Line 218-219 states …

“BIOGEM by default uses a fixed remineralisation profile similar to the Martin curve for the sinking labile fractions of both POC and PIC”

… which tends to suggest that PIC dissolution follows the Martin curve. More typically it is assumed that it has a relationship with the CCD, beneath which carbonate is undersaturated and dissolution occurs.

- I previously asked about mixing in the study’s model …

“Ln. 169: does the model resolve seasonal mixing of different ecological regimes?

No, but insolation and light attenuation is seasonal, and ecoGEnIE does reproduce seasonal variation in primary production. (pg.8 ln.249 – pg.9 ln.263)”

This suggests to me that there is no seasonal mixing. However, I’m a little confused because the new manuscript revision explicitly mentions (ln. 229-230) …

“photosynthesis … subject to light limitation, photoacclimation, and seasonal light attenuation within a variable mixed layer depth”

Figure S48 also tends to suggest that there is a representation of the mixed layer, so there’s perhaps just a mix-up in the description that needs clearing up.

Additional specific points:

- As an aside, it would probably increase reader interest in this manuscript if the headline changes in carbon cycle processes were quantified in the abstract. This block of text could really benefit from having numbers (e.g. from Table 2) added where suggested:

“We find that including TDR strengthens the biological pump relative to default runs due to increased nutrient recycling (+X%), while ECOGEM weakens the biological pump by enabling a shift to smaller plankton classes (-Y%). However, interactions with carbonate chemistry cause opposite sign responses for the carbon sink in both cases: TDR leads to a smaller sink relative to default runs (-P%) whereas ECOGEM leads to a larger sink (+Q%). Combining TDR and ECOGEM results in a net strengthening of the biological pump (+A%)and a small net reduction in carbon sink relative to default (-B%).”

- Supplementary figures that show OBS and then comparative model deltas could be better arranged to allow readers to directly compare the plots. At present, readers must flip back and forth to compare plots making something that should be easy needlessly difficult. For example, Figures S1, S3 and S5 (and S2, S4 and S6) should be rearranged onto the same page so that readers can easily compare patterns of bias. This should be repeated throughout the supplementary material. Also, Figure S13 shows the model-obs difference, but without showing the observations, so the reader cannot tell how large the biases really are.

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ED: Reconsider after major revisions (05 Mar 2021) by Michel Crucifix

AR by David Armstrong McKay on behalf of the Authors (30 Apr 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (11 May 2021) by Michel Crucifix

RR by Jamie Wilson (14 May 2021)

ED: Publish as is (17 May 2021) by Michel Crucifix

AR by David Armstrong McKay on behalf of the Authors (19 May 2021)

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Short summary

We use an Earth system model with two new ocean ecosystem features (plankton size traits and temperature-sensitive nutrient recycling) to revaluate the effect of climate change on sinking organic carbon (the biological pump) and the ocean carbon sink. These features lead to contrary pump responses to warming, with a combined effect of a smaller sink despite a more resilient pump. These results show the importance of including ecological dynamics in models for understanding climate feedbacks.