the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Oct 2023
Reviews and syntheses: Abrupt ocean biogeochemical change under human-made climatic forcing – warming, acidification, and deoxygenation
Abstract. Abrupt changes in ocean biogeochemical variables occur as a result of human-induced climate forcing as well as those which are more gradual and occur over longer timescales. These abrupt changes have not yet been identified and quantified to the same extent as the more gradual ones. We review and synthesise abrupt changes in ocean biogeochemistry under human-induced climatic forcing. We specifically address the ocean carbon and oxygen cycles because the related processes of acidification and deoxygenation provide important ecosystem hazards. Since biogeochemical cycles depend also on the physical environment, we also describe the relevant changes in warming, circulation, and sea ice. We include an overview of the reversibility or irreversibility of abrupt marine biogeochemical changes. Important implications of abrupt biogeochemical changes for ecosystems are also discussed. We conclude that there is evidence for increasing occurrence and extent of abrupt changes in ocean biogeochemistry as a consequence of rising greenhouse gas emissions.
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RC1: 'Comment on bg-2023-182', Anonymous Referee #1, 16 Dec 2023
The manuscript prepared by Heinze et al. provides a synthesis of climate change-related effects in the oceans, building on multiple model studies conducted at different time and spatial scales and available data to analyze the impacts of changing temperature, acidification and deoxygenation on carbon and nutrient cycles as well as ecosystems in different regions. The figure (23) and table summarizing the findings are helpful to set future research priorities in different areas.
This document is extremely relevant, well written and well researched, and will provide a great contribution to the field. Suggested changes (detailed below) are aimed at clarifying concepts and improving figures to aid with interpretation. Since this manuscript is likely to be read and cited by a broad audience, some concepts that are familiar for climate scientists and oceanographers should still be defined in more detail. The only more general suggestion I would make is to add some discussion of the lack of representation of wetlands in models, since they play a large role in transforming and exporting organic and inorganic carbon (including potentially large alkalinity fluxes in some regions) to the open ocean.
L73-75: this sentence is a bit confusing, could be rephrased
L160-161: add a reference
L166-167: should say where HABs happen and provide a definition of what HABs are
L178-180: Could rephrase this sentence, a non-careful (or not familiar with the topic) reader might interpret alkalinity to be H+ excess
L253: Distribution of species?
L291: should define runaway effects
Figure 2: Should add reference period for anomaly, and would be helpful to add a horizontal line at zero on panel A
L334-340: Should add some references for hysteresis
Figure 5: Would be useful to add a panel with a map showing the location of the data
Figure 6: On panel A, colobar needs labeling, axes need coordinates, and arrows are hard to see.
L670: Should provide a definition of ECS in this paragraph
L684: The beginning of the paragraph felt like a sudden change of subject (happens in other parts of the text as well). Not a problem with the content and mostly an aesthetic choice, but adding a sentence summarizing the previous paragraph or introducing the new one might help.
L728: What are the critical levels?
L790: Wouldn’t it make sense to switch sections 4.1.3 and 4.1.4?
L803-804: The Siberian coastline… I was a bit confused by this sentence
L808: net export of phosphorus… needs a reference
L850-851: Paragraph ends implying regions of the SO will be discussed, but next section is on Arctic
Figure 10: Numbers are very hard to read
L1004-1005: Should add some coordinates for intermediate water resurfacing
L1040: Under which scenario?
L1045-1047: It is not very clear to me, from the description, when/how decrease in temperature and nutrient would impact phytoplankton. Should it say “lower peak” in the previous sentence? If there are changes in phytoplankton community composition it would be good to add this information here.
L1131-1133: This sentence is a bit confusing
L1165: Detects instead of detectable?
L1256L Define Nr
Section 5.1 – Consider adding some examples of areas where trends are higher (e.g. Gulf of Maine, which has good length of observations)
Figure 15: Add coordinates to maps and titles to panels d-f
L1485-1486: Which CDR is being considered here, and why would increased climate sensitivity delay increase as well? I think this last sentence could be expanded
Figures 17,18: Colorbars need to be labeled and numbers are too small
L1579-1580: Wouldn’t the percentage left in the atmosphere depend on the total amount of CO2 added? Are the different scenarios close enough to justify the 10% range?
Figure 19: Colorbar should indicate it is for O2
Figure 21: Numbers are very small
L1770: “The as such defined”?
Citation: https://doi.org/10.5194/bg-2023-182-RC1 -
AC1: 'Reply on RC1', Christoph Heinze, 12 Mar 2024
Author response to Review #1
The referee’s remarks are kept in italics.
The referee’s main points:
The manuscript prepared by Heinze et al. provides a synthesis of climate change-related effects in the oceans, building on multiple model studies conducted at different time and spatial scales and available data to analyze the impacts of changing temperature, acidification and deoxygenation on carbon and nutrient cycles as well as ecosystems in different regions. The figure (23) and table summarizing the findings are helpful to set future research priorities in different areas.
This document is extremely relevant, well written and well researched, and will provide a great contribution to the field. Suggested changes (detailed below) are aimed at clarifying concepts and improving figures to aid with interpretation. Since this manuscript is likely to be read and cited by a broad audience, some concepts that are familiar for climate scientists and oceanographers should still be defined in more detail. The only more general suggestion I would make is to add some discussion of the lack of representation of wetlands in models, since they play a large role in transforming and exporting organic and inorganic carbon (including potentially large alkalinity fluxes in some regions) to the open ocean.
Our response to the main points:
The authors would like to thank the reviewer for the positive and constructive review and the detailed comments and the work put into this review. It is correct that the manuscript is thought for a broad audience, especially also in view that the manuscript is expected to be registered (in addition to be the Biogeosciences publication) in the Encyclopedia of Geosciences https://encyclopedia-of-geosciences.net/
We are currently discussing in the author team whether we will include more details about the underlying concepts in separate “boxes” (as, e.g., done in past IPCC reports) accompanying the main text or in dedicated appendices after the main text body at the end of the manuscript. (At the same time, this could also satisfy the recommendation by Referee #2; see our respective response to this referee.)
At this stage, we plan to add discussions on the importance of wetlands as dynamical biogeochemical areas for influencing matter delivery to the ocean – including their lacking representation in most models – under the discussion of abrupt regional changes in the respective sections on warming, carbon, and oxygen cycling. The following recent paper and references therein will be used for this: Regnier, P., Ward, N.D., Izquierdo, A., Baldwin, A.H., Day, D., McElhinny, J., Patel, K., Vargas, R., Zheng, J., Exchange Consortium, and Myers-Pigg, A.: Coastal inundation regime moderates the short-term effects of sediment and soil additions on seawater oxygen and greenhouse gas dynamics: a microcosm experiment. Front. Mar. Sci. 10:1308590, 2023, doi: 10.3389/fmars.2023.1308590
L73-75: this sentence is a bit confusing, could be rephrased
We plan to split this sentence up into two sentences and rephrase: “However, it is not as yet conclusively quantified, what ecosystem impact these abrupt ocean environmental changes (often of local or regional extent) have versus smoothly monotonically changing conditions. Nevertheless there are some studies about abrupt state variable changes that have resulted in
negative consequences for ecosystems (Bond et al., 2015; Hoegh-Guldberg et al., 2007; Wernberg et al., 2016, 2021; Chan et al., 2008).”
L160-161: add a reference
We intend to cite the following two papers and will add the references to the reference list:
Chen-Tung, A.C., Liu K.-K., and Macdonald, R.: Continental Margin Exchanges, in Ocean Biogeochemistry - the Role of the Ocean Carbon Cycle in Global Change, edited by Michael J. R. Fasham, pp. 53-97, Springer, Berlin et al., 2003, ISBN 978-3-642-62691-3, DOI 10.1007/978-3-642-55844-3
Terhaar, J., Lauerwald, R., Regnier, P., Gruber, N., and Bopp, L.: Around one third of current Arctic Ocean primary production sustained by rivers and coastal erosion. Nature Communications, 12(1), 169, 2021, https://doi.org/10.1038/s41467-020-20470-z
L166-167: should say where HABs happen and provide a definition of what HABs are
We will include an item on harmful algae blooms in the glossary and refer to it in the main text.
L178-180: Could rephrase this sentence, a non-careful (or not familiar with the topic) reader might interpret alkalinity to be H+ excess
We suggest to rephrase the sentence to: “The foundation for this reactivity of CO2 in seawater is the balance of acids and bases of different strengths present, which is quantified through alkalinity (Rakestraw, 1949; Middelburg et al., 2020; Wolf-Gladrow et al., 2007; Dickson, 1981, 1992).”
L253: Distribution of species?
Yes, it should be “distribution of species”. Thank you.
L291: should define runaway effects
Good point. We plan to replace “runaway effects” by “positive feedback processes”.
Figure 2: Should add reference period for anomaly, and would be helpful to add a horizontal line at zero on panel A
Figure 2 will be amended for these suggestions.
L334-340: Should add some references for hysteresis
We plan to add as references:
Scheffer, M., Carpenter, S., Foley, J.A., Folke, C., and Walker, B.: Catastrophic shifts in ecosystems, Nature, 413(6856), 591-596, 2001, doi: Doi 10.1038/35098000.
Litzow, M. A., and Hunsicker, M.E.: Early warning signals, nonlinearity, and signs of hysteresis in real ecosystems. Ecosphere 7(12):e01614, 2016, doi: 10.1002/ecs2.1614
Figure 5: Would be useful to add a panel with a map showing the location of the data
A respective location map will be added.
Figure 6: On panel A, colorbar needs labeling, axes need coordinates, and arrows are hard to see.
Respective amendments for this figure will be made.
L670: Should provide a definition of ECS in this paragraph
There is already a respective glossary item in place. We will add “see glossary” in the main text here.
L684: The beginning of the paragraph felt like a sudden change of subject (happens in other parts of the text as well). Not a problem with the content and mostly an aesthetic choice, but adding a sentence summarizing the previous paragraph or introducing the new one might help.
We will change this paragraph and also other locations in the text accordingly to improve readability. This will be part of an overall streamlining of the text.
L728: What are the critical levels?
This depends on whether one considers the disappearance of only summer ice cover or also winter ice cover. It depends further on the greenhouse gas emission scenario and the corresponding climate sensitivity of the Earth system model providing the projection under this scenario. Examples are given in Hezel et al. (2014), such as for scenario RCP8.5 and CMIP5 model output: “September sea ice disappears with a mean temperature increase of 2.4 ◦C (range 0.4–6.2 ◦C), and March sea ice disappears at a mean temperature increase of 8.2 ◦C (range 7.1–10.3 ◦C).” As the reviewers ask for a reduction in length of the manuscript we feel that we cannot really go into details here and giving just some numbers out of context would also not be sound. Because we cite respective papers for the interested reader we suggest that we make no change here.
L790: Wouldn’t it make sense to switch sections 4.1.3 and 4.1.4?
Yes, we will switch the sequence of the sections.
L803-804: The Siberian coastline… I was a bit confused by this sentence
We suggest to change the sentence to: “The Siberian coastline is transitioning from a lower to a higher coastal erosion regime that can happen locally over short time due to regional sea-ice decline and respective increase in wave action.”
L808: net export of phosphorus… needs a reference
We will add es reference: Torres-Valdés, S., Tsubouchi, T., Bacon, S., Naveira-Garabato, A.C., Sanders, R., McLaughlin, F.A., Petrie, B., Kattner, G., Azetsu-Scott, K., and Whitledge, T.E.: Export of nutrients from the Arctic Ocean, J. Geophys. Res. Oceans, 118, 1625–1644, 2013, doi:10.1002/jgrc.20063.
L850-851: Paragraph ends implying regions of the SO will be discussed, but next section is on Arctic
Section 4.2 (to which lines 850-851) belong is a general introduction section after which all regional hot pots will be considered subsequently. We can add a text in line 851 saying: “We will now consider the different regions in more detail.” In general, we will streamline the text and try to achieve a smooth and logical transition from one section to the next during this process.
Figure 10: Numbers are very hard to read
We will try to increase the numbers or the figure as such to make the numbers better readable.
L1004-1005: Should add some coordinates for intermediate water resurfacing
We will add the latitude band 49°N-65°N according to Ridge and McKinley, 2020).
L1040: Under which scenario?
Abrupt subpolar gyre cooling in the North Atlantic happen under various scenarios according to Swingedouw et al. (2021) and, therefore, no single scenario can be named here.
L1045-1047: It is not very clear to me, from the description, when/how decrease in temperature and nutrient would impact phytoplankton. Should it say “lower peak” in the previous sentence? If there are changes in phytoplankton community composition it would be good to add this information here.
We do not know what the reviewer aims at with his question concerning a “’lower peak’ in the previous sentence” as this sentence does not discuss bloom peaks specifically. We will modify the sentence in lines 1045-1047 to: “In a warming climate, analysis of CMIP6 model output indicates a tendency towards earlier phytoplankton bloom initiation and peak time by about one month due to faster mixed layer shallowing in spring (Yamaguchi et al., 2022).” A firm statement on phytoplankton community information cannot be given at this stage based on the model results by the authors.
L1131-1133: This sentence is a bit confusing
We suggest to change the sentence into two sentences: “Warming and acidification have antagonistic effects on the Fe(II): While warming promotes the oxidation of Fe(II) in the course of enhanced trace metal interactions, a pH decrease leads to a slowing down of the Fe(II) oxidation and in parallel with increased Fe(III) solubility promotes Fe(II) persistence.”
L1165: Detects instead of detectable?
We suggest changing the respective sentence to: “The latest ensemble of Earth system models from CMIP6 projects deoxygenation to occur throughout most of the upper 2000 m prior to the end of the 21st century (Tjiputra et al., 2023).”
L1256 Define Nr
Nr was already introduced in section 1.2.3.. However, we suggest to add a glossary item: “Nr: Forms of compounds that include reactive nitrogen atoms in contrast to the molecular nitrogen N2 in the atmosphere and other Earth system reservoirs that is very stable due to its triple-bond between the two nitrogen molecules. The inventory of Nr in the Earth system has increased dramatically due to the production of artificial fertiliser.”
Section 5.1 – Consider adding some examples of areas where trends are higher (e.g. Gulf of Maine, which has good length of observations)
In view of the length of the manuscript, we suggest not to go into detail, but refer the reader to the paper by Oliver et al. (2021), when it comes to monitoring of marine heat waves. We suggest to include the following sentence in line 1329: “A detailed overview concerning the monitoring of marine heatwaves and their trends including example is given in Oliver et al. (2021)”.
Figure 15: Add coordinates to maps and titles to panels d-f
We plan to add titles to panels d-f. We are not sure whether coordinates to the small maps in the different panels will enhance them, as either the numbers would be very small are only very few numbers could be added that would not add further essential information. In the IPCC assessment reports no coordinates have been used in cases of such small maps.
L1485-1486: Which CDR is being considered here, and why would increased climate sensitivity delay increase as well? I think this last sentence could be expanded
We plan to change the sentence in lines 1485-1486 to: “The recovery of the upper ocean from excess ocean heat content will be strongly delayed after CDR (from the atmosphere) and this delay increases for higher climate sensitivities because larger amours of excess heat from the large marine reservoir have to be equilibrated with the atmosphere (Jeltsch-Thömmes et al., 2020).”
Figures 17,18: Colorbars need to be labeled and numbers are too small
We plan to amend Figures 17 and 18 accordingly.
L1579-1580: Wouldn’t the percentage left in the atmosphere depend on the total amount of CO2 added? Are the different scenarios close enough to justify the 10% range?
We talk here explicitly of approximate numbers over long timescales where the ocean is flushed through many times and also the dissolution of calcium carbonate as well as terrestrial weathering contribute to the buffering. We plan to add the following sentence in line 1582: “For a more detailed analysis of the atmospheric retention of anthropogenic CO2 taking into account the non-linearity of the marine carbonate buffer system, please, see Archer (2005).”
Figure 19: Colorbar should indicate it is for O2
The colour bar description will be amended.
Figure 21: Numbers are very small
We plan to improve the readability of the numbers.
L1770: “The as such defined”?
We suggest to change the sentence to: “The Arctic Ocean pelagic habitat distribution as established for the 1950s will shift to a "New Arctic" after a transitional period between 2030 and 2060, suggesting a potential tipping point in the Arctic Ocean marine habitat after 2070.”Citation: https://doi.org/10.5194/bg-2023-182-AC1
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AC1: 'Reply on RC1', Christoph Heinze, 12 Mar 2024
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RC2: 'Comment on bg-2023-182', Anonymous Referee #2, 12 Feb 2024
The manuscript by Heinze and coauthors provides an exhaustive review of abrupt changes in ocean biogeochemistry, with a focus on carbon (C) and oxygen (O2) cycles, under the influence of anthropogenic drivers. As the ocean adjusts and evolves under the effects of human perturbations to the climate systems, many biogeochemical changes have been documented. Of particular interest for their potential ecological effects are abrupt changes that occur over fast timescales relative to natural variations. These abrupt changes — which may also include permanent “tipping points” are much less well characterized than general, progressive changes. Thus the review by Heinze et al. is timely and relevant for the ocean biogeochemistry community.
This is a very extensive review, that is well organized in subtopics ranging from global to regional changes, including persistent changes, extreme events, reversibility, and ecosystem consequences, all covered with a fair amount of detail. The manuscript also includes extensive introductory material. Most of the manuscript is informed by prior work, with a notable preponderance of fairly recent references — perhaps because of the relatively new topic. The Authors also include results from ongoing research, which add depths and insights to specific questions. Some of these sections are particularly stimulating, e.g., Sections 5 (Fig. 15) and 6 (Figs. 16-19). Overall, I thinks this will be a useful reference for a broad community.
There are some aspects of the review that are not completely convincing. It is exhaustive, but at times it also feels unnecessarily detailed; it is probably not a paper to read from bottom to end in one sitting, but rather a collection of different sub-topics that may be of individual interest for different readers. Much of the background information, and very exhaustive discussion of non-abrupt change can be found in other publications, e.g., the ocean and biogeochemistry chapters of the last two IPCC Reports, and related papers. As a reviewer, I found myself tempted multiple times to skip entire background paragraphs that appeared often more detailed than necessary. A significant portion of this background material could be condensed or edited for conciseness, making for a much more streamlined read.
On a first read I often struggled to understand exactly what counts as “abrupt” change rather than just progressive change; by “zooming out” to timescales long enough, any gradual change would look abrupt; much of the practice of defining abrupt changes depend on the baseline variability one considers, and this includes some degree of arbitrariness. These points are addressed in Section 2, which attempts to lay out the methodological criteria for the review; these are in practice mostly qualitative criteria. But several examples of changes that are characterized as abrupt could easily be characterized as manifestations of internal climatic variability (e.g., Fig. 2b); it is not always clear how irreversible many of these changes are in practice; any claim of an anthropogenic cause would have to be supported by a (painstaking) process of attribution that is generally not applied in the paper or the studies cited. These limitations make at times for a frustrating read. That said, with appropriate caveats, the review does provide a valuable overview of a variety of examples of potentially important biogeochemical changes, many of which may deserve additional scrutiny and investigation. I just wish it was more focused.
Specific comments:
This is definitely a paper that would benefit from a table of contents — I hope the BGS format allows to include one. A table of contents would help the more casual readers to better orient themselves in the large body of text, and more easily and quickly find topics more relevant to their interests.
The title could be somewhat revised; something like: “Biogeochemical change under human-made climate forcing … with a focus on abrupt and irreversible change” would be somewhat more accurate because the distinction between progressive change and abrupt or reversible change can be somewhat blurred.
I feel that Fig. 23 and Table 1, which provide excellent summaries of the changes discussed in the text, would be more useful early on in the paper, again, as a roadmap for the reader. While they currently function as a summary at the end, positioning them early on could enhance reader engagement, preventing them from being overlooked by readers who might lose focus towards the end.
Section 1.1 is unnecessarily detailed. Much of this information can be taken from publications such as the IPCC reports. It does not add essential information to understand abrupt/irreversible changes. The authors could confidently assume that the BGS readers know what man-made climate change is and what it is caused by.
Section 1.2.2 is unnecessarily detailed. No need to explain the details of aqueous carbon chemistry, etc.
Lines 178-180: “The foundation for this reactivity of CO2 is the presence …” I see what the authors mean but this could be rephrased, because CO2 would react equally well with pure water where there’s no excess of cations. (In which case the alkalinity would be essentially zero.)
Line 210 “In the case of iron, a decrease in pH will affect …” Here you could be more specific and say whether the pH drop would increase or decrease Fe chelation, rather than just saying it changes it.
Line 245: my impression is that O2 between 20-60 mmol/m3 is too hight to enable a switch to anaerobic reactions; while there’s no generally accepted threshold, O2<5 mmol/m3 is a more likely starting point, e.g. for water column denitrification and other reductions.
Lines 278-280: Perhaps discuss that, with a progressive trend, e.g. warming, an “extreme” relative to a fixed baseline may become persistent at some point.
Line 291: I’m confused by the term “runaway”, as it implies a persistent positive feedback, and in this case I am not sure there are practical examples of that; you could either provide an explanation or use some different term.
Section 2.2: this category seems particularly hard to define univocally. E.g. is a particularly strong shift in the phase of the PDO, as observed in the Pacific in recent decades, a tipping point? Probably not if it is part of natural variability; unfortunately the observational record is often too short to characterize this type of events. E.g., for the study of Chan et al., 2008 it is not clear it signals a tipping point rather than a natural phase shift in upwelling and bottom oxygenation.
Section 3.1: the introduction of this section seems too broad and not necessarily needed for this type of paper. The paleo-section also feels unnecessarily detailed.
Section 3.2: here too there are parts that don’t feel as relevant, e.g. the paragraph at Lines 434-448 on future emission scenarios.
Section 3.3: the entire first paragraph reads as background on oceanic C-system response to CO2 emissions; is it necessary in a paper about abrupt change? Likewise, multiple other paragraphs (e.g., Lines 483-490 on negative emissions) seems out of topic.
Lines 501-502, “do not offer consistent results…” but see the recent work by Busecke et al., 2022, AGU advances, which provides a reconciled picture of O2 changes in oxygen minimum zones under global warming.
Lines 540-569: this is accurate and interesting (and probably too detailed), but it is unclear to me what makes these changes “abrupt” since they depend on a progressive and fairly slow deoxygenation trend.
Section 4, general: for many of the following examples, the “abrupt” character of the changes discussed is unclear; some do not seem necessarily driven by man0made warming but they could be part of natural (e.g. multidecadal) variability); some are a consequence of the slow secular change.
Lines 604-622: this reads as a too detailed excursus. Also I am not sure this counts as a tipping point; the authors are rather describing many interactions between elements of the current system that may change under a variety of drivers.
Lines 640-645 and Fig. 5: these are biogeochemical changes that may be part of multi-decal variability, couched within a section of warming and physical change; they feel detouring and not completely relevant. Again, is decadal variability a central point of discussion in a paper on man-made abrupt change?
Lines 650-657: too much detail; this could be summarized in a short sentence.
Figure 7: this is a very detailed figure on a very specific example: is this level of detail needed? Also, again, it is not clear how abrupt the change discussed is.
Lines 746-775: even after reading this massive paragraph a couple times, I remain unsure what counts as abrupt change here and what the specific focus is on.
Lines 798-812: lots of detail on coastal erosion that feel quite removed from the central topic of abrupt BGC change.
Lines 814-824, and Fig. 8: how does this relate to abrupt change? It is a model sensitivity (river vs. no-river).
Section 4.2: This is again a review on CO2 uptake about the ocean; the focus should be rather be on abrupt change.
Section 4.2.1: again, many instances of progressive changes are discussed, rather than focusing on abrupt change.
Fig. 9: These show gradual changes, not abrupt — unless a millennial timescale is considered.
Fig. 10: you could explain what the white colors mean — my take is that they show thick sea ice. Also, clarify in the caption what we are looking at; e.g. what region.
Line 914: Fransson et al., in Review: paper in review should be avoided unless there’s a preprint; but here it may be the paper in press in the bibliography?
979-1001: this does not seem directly related to abrupt change, but rather a review of future climate change in the Southern Ocean.
Line 996: “Together, these trigger the shift from …” I struggled with this sentence, it could be clarified.
Section 4.2.6: “the second most important oceanic region for anthropogenic CO2 uptake” and “with avoided outgassing serving as a net sink”: I see what the Authors mean, but it could be better explained; I imagine the equatorial pacific will remain a region of CO2 outgassing, so there will not be actual uptake there, but the effect on the atmospheric CO2 budget would be analogous to net uptake.
Lines 1115-1120: this entire sentence is confusing and may need rewriting.
Lines 1125-1130: this feels like a potentially important section, but it is not very clearly written, so I suggest a rephrasing.
Lines 1149-1153: this section on ligand-induced feedbacks could be better explained, e.g. by pacing the reader through the steps of the (positive?) feedback.
Section 4.3.2: Again, the relation to abrupt change is not obvious; the first paragraph seem too basic/introductory and unnecessary.
Lines 1204-1208: Fine, but for context, the figure show an O2 decline of ~1% over a nearly centennial period, this does not deem either significant or particularly abrupt, especially since O2 remains way above typical hypoxic thresholds.
Section 5.2: again there’s not much of a discussion of abrupt changes, but rather a review of extremes.
Lines 1382-1386: this could be clarified.
Line 1405: “which generally do not support marine life” this is completely inaccurate, as plenty of animal species can live at O2<60 mmol/m3; life in general can live in the absence of O2.
Section 5.3: This entire section could benefit from a reframing in light of the work of Deutsch et al., e.g. Deutsch et al., 2015, Science; 2020 Nature.
Lines 1451-1460: this is an interesting paragraph; there are no citations so I assume it’s new research. I suggest maybe explaining what controls the different ECS in the specific versions of the model.
Section 7.1, last paragraph. Cite the work by Dutkiewicz et al., 2015.
Lines 1676-1680: maybe add a clarification on the extent to which phytoplankton shifts are driven by shifts in water mass boundaries vs. ecological interactions.
Fig. 21: you could add more detail in the caption, e.g. explaining what the colors are in the bottom rows, etc.; the colorbars are very hard to read.
Section 7.5: this is very focused on the Iberian Peninsula upwelling, but upwelling systems are much more varied, so it’s a missed opportunity. There’s a rich literature on upwelling changes, from tests of Bakun’s hypothesis to recent analysis with CMIP models (Wang et al. 2015, Nature; Rykaczewski et al., 2015, GRL; Sydeman et al., 2014).
Section 7.6: this does not feel like a very careful or thorough review of a vast topic; it’s very focused on European seas. See work by Free et al. 2019, Science, Fredston et al. 2023, Nature, Lotze et al. 2019, PNAS, Tittensor et al., 2021, Nature Climate Change, etc.
Citation: https://doi.org/10.5194/bg-2023-182-RC2 -
AC2: 'Reply on RC2', Christoph Heinze, 12 Mar 2024
Author response to Review #2
The referee’s remarks are kept in italics.
The referee’s main points:
The manuscript by Heinze and coauthors provides an exhaustive review of abrupt changes in ocean biogeochemistry, with a focus on carbon (C) and oxygen (O2) cycles, under the influence of anthropogenic drivers. As the ocean adjusts and evolves under the effects of human perturbations to the climate systems, many biogeochemical changes have been documented. Of particular interest for their potential ecological effects are abrupt changes that occur over fast timescales relative to natural variations. These abrupt changes — which may also include permanent “tipping points” are much less well characterized than general, progressive changes. Thus the review by Heinze et al. is timely and relevant for the ocean biogeochemistry community.
This is a very extensive review, that is well organized in subtopics ranging from global to regional changes, including persistent changes, extreme events, reversibility, and ecosystem consequences, all covered with a fair amount of detail. The manuscript also includes extensive introductory material. Most of the manuscript is informed by prior work, with a notable preponderance of fairly recent references — perhaps because of the relatively new topic. The Authors also include results from ongoing research, which add depths and insights to specific questions. Some of these sections are particularly stimulating, e.g., Sections 5 (Fig. 15) and 6 (Figs. 16-19). Overall, I thinks this will be a useful reference for a broad community.
There are some aspects of the review that are not completely convincing. It is exhaustive, but at times it also feels unnecessarily detailed; it is probably not a paper to read from bottom to end in one sitting, but rather a collection of different sub-topics that may be of individual interest for different readers. Much of the background information, and very exhaustive discussion of non-abrupt change can be found in other publications, e.g., the ocean and biogeochemistry chapters of the last two IPCC Reports, and related papers. As a reviewer, I found myself tempted multiple times to skip entire background paragraphs that appeared often more detailed than necessary. A significant portion of this background material could be condensed or edited for conciseness, making for a much more streamlined read.
On a first read I often struggled to understand exactly what counts as “abrupt” change rather than just progressive change; by “zooming out” to timescales long enough, any gradual change would look abrupt; much of the practice of defining abrupt changes depend on the baseline variability one considers, and this includes some degree of arbitrariness. These points are addressed in Section 2, which attempts to lay out the methodological criteria for the review; these are in practice mostly qualitative criteria. But several examples of changes that are characterized as abrupt could easily be characterized as manifestations of internal climatic variability (e.g., Fig. 2b); it is not always clear how irreversible many of these changes are in practice; any claim of an anthropogenic cause would have to be supported by a (painstaking) process of attribution that is generally not applied in the paper or the studies cited. These limitations make at times for a frustrating read. That said, with appropriate caveats, the review does provide a valuable overview of a variety of examples of potentially important biogeochemical changes, many of which may deserve additional scrutiny and investigation. I just wish it was more focused.
Our response to the main points:
We are very grateful to the referee for the constructive and comprehensive review and the work invested for this review.
Referee #2 asks for a streamlining and shortening of the background material sections including descriptions of non-abrupt changes that may be shortened as readers may already be familiar with these items or can read them in, e.g., the recent IPCC assessment report chapters. We note that the article has been developed for a broad audience including readers that are not familiar with some of the concepts, especially in view of getting the manuscript registered as a an item in the Encyclopedia of Geosciences https://encyclopedia-of-geosciences.net/ In order to satisfy both the streamlining of some known concepts to the biogeochemical experts (Referee #2) and on the other hand not to lose readers from outside of the marine biogeochemistry discipline (Referee #1) we suggest as an option to include more details about the underlying concepts in separate “boxes” (as, e.g., done in the past IPCC reports) accompanying the main text or in dedicated appendices after the main text body at the end of the manuscript. This would make it easier for biogeochemical experts to skip text parts about concepts that they are already familiar with. We will also try to shorten the text, where possible, to enable easier reading.
We appreciate the comment of the referee on the definition or delimitation of abrupt versus non-abrupt changes. However, we think that our approach is useful because we include not only different reference time frames for the definition of abrupt changes, but also respectively differing regional extents of abrupt changes. Abrupt changes are certainly also caused by internal climate variability. However, the statistics of internal climate variability are expected to change under human-made climate forcing. To this end we cite in addition now the recent paper of Heinze et al. (Heinze, C., Michel, C., Torsvik, T., Schwinger, J., and Tjiputra, J. F.: More frequent abrupt marine environmental changes expected. Geophysical Research Letters, 51, e2023GL106192, 2024, https://doi.org/10.1029/2023GL106192), which has not yet been available at the time of the submission of the review for Biogeosciences. We agree that attributing specific abrupt changes to anthropogenic climate forcing is not always strictly possible, however, for some issues such as global ocean warming, acidification, and deoxygenation, and some other cases it is possible at least with a certain degree of probability. This has been discussed already throughout the manuscript.
Specific comments:
This is definitely a paper that would benefit from a table of contents — I hope the BGS format allows to include one. A table of contents would help the more casual readers to better orient themselves in the large body of text, and more easily and quickly find topics more relevant to their interests.
We browsed through a suite of Reviews and Syntheses papers in Biogeosciences and could not yet identify a paper, where a table of contents was used. However, we are open for such an inclusion. It would need to be clarified with the editor and the editorial staff. Regardless of the table of contents, we think that the reader benefits from reading the paper from start to end. It then makes most sense. Through the design of the paper, we experimented with versions of different structures, with the present one being the most useful in our experience.
The title could be somewhat revised; something like: “Biogeochemical change under human-made climate forcing … with a focus on abrupt and irreversible change” would be somewhat more accurate because the distinction between progressive change and abrupt or reversible change can be somewhat blurred.
We consider to change the title to one in the line of the following options:
"Reviews and syntheses: Human-made climatic forcing causes ocean biogeochemical change – how abrupt and irreversible is it?”
“Reviews and syntheses: Human-induced ocean warming, acidification, and deoxygenation – what abrupt biogeochemical changes can happen?”
“Reviews and syntheses: Human-induced ocean warming, acidification, and deoxygenation – abrupt versus gradual biogeochemical changes”
“Reviews and syntheses: Human-induced ocean warming, acidification, and deoxygenation – gradual and abrupt biogeochemical changes”
I feel that Fig. 23 and Table 1, which provide excellent summaries of the changes discussed in the text, would be more useful early on in the paper, again, as a roadmap for the reader. While they currently function as a summary at the end, positioning them early on could enhance reader engagement, preventing them from being overlooked by readers who might lose focus towards the end.
We had this option discussed internally intensely. We will pick up this discussion again. The disadvantage of bringing Figure 23 and Table 1 at the beginning would be, that these materials would be out of context and can hardly be discussed. But we will explore an option where we use Figure 23 and Table 1 in the introduction and then get back to them in the concluding remarks. The corresponding decision will depend on the streamlining process of the entire paper.
Section 1.1 is unnecessarily detailed. Much of this information can be taken from publications such as the IPCC reports. It does not add essential information to understand abrupt/irreversible changes. The authors could confidently assume that the BGS readers know what man-made climate change is and what it is caused by.
The article shall be accessible to a broad readership, who not always have read the IPCC reports. A concise summary of anthropogenic forcing is needed to understand the rest of the paper. Often presentations of human-caused forcings are imprecise and forget about biogeochemical climate forcing in addition to physical climate forcing. The IPCC texts on human and natural climate forcings/drivers are extremely elaborate. We think that our introduction here is appropriate.
Section 1.2.2 is unnecessarily detailed. No need to explain the details of aqueous carbon chemistry, etc.
We suggest as an option (in order to satisfy reviewers #2 and also #1) to transfer the basic information to either “boxes” or appendices to make the knowledge accessible for the non-experts and to provide an easier read for the experts.
Lines 178-180: “The foundation for this reactivity of CO2 is the presence …” I see what the authors mean but this could be rephrased, because CO2 would react equally well with pure water where there’s no excess of cations. (In which case the alkalinity would be essentially zero.)
We suggest to rephrase the sentence to: “The foundation for this reactivity of CO2 in seawater is the balance of acids and bases of different strengths present, which is quantified through alkalinity (Rakestraw, 1949; Middelburg et al., 2020; Wolf-Gladrow et al., 2007; Dickson, 1981, 1992).”
Line 210 “In the case of iron, a decrease in pH will affect …” Here you could be more specific and say whether the pH drop would increase or decrease Fe chelation, rather than just saying it changes it.
We will add the following text: “For example, a decrease in pH will likely increase the organic chelation of iron by increasing the conditional stability constants of iron-ligand complexes (Avendaño et al., 2016) and hence the solubility of iron in seawater.” The reference will be added to the reference list (Avendaño, L., Gledhill, M., Achterberg, E.P., Rérolle, V.M.C, and Schlosser, C.: Influence of Ocean Acidification on the Organic Complexation of Iron and Copper in Northwest European Shelf Seas; a Combined Observational and Model Study. Front. Mar. Sci. 3:58, 2016, doi: 10.3389/fmars.2016.00058).
Line 245: my impression is that O2 between 20-60 mmol/m3 is too hight to enable a switch to anaerobic reactions; while there’s no generally accepted threshold, O2<5 mmol/m3 is a more likely starting point, e.g. for water column denitrification and other reductions.
Yes, correct, thank you for identifying this glitch. We will amend the text accordingly.
Lines 278-280: Perhaps discuss that, with a progressive trend, e.g. warming, an “extreme” relative to a fixed baseline may become persistent at some point.
We plan to change the sentence in lines 277-279 to: “Regional changes in the frequency of occurrence and extent of extreme events (where normally the value would approach the average level after an extreme event, but also changes in extreme event statistics can be important here including the baseline for identifying extreme events).”
Line 291: I’m confused by the term “runaway”, as it implies a persistent positive feedback, and in this case I am not sure there are practical examples of that; you could either provide an explanation or use some different term.
Good point. We plan to replace “runaway effects” by “positive feedback processes”.
Section 2.2: this category seems particularly hard to define univocally. E.g. is a particularly strong shift in the phase of the PDO, as observed in the Pacific in recent decades, a tipping point? Probably not if it is part of natural variability; unfortunately the observational record is often too short to characterize this type of events. E.g., for the study of Chan et al., 2008 it is not clear it signals a tipping point rather than a natural phase shift in upwelling and bottom oxygenation.
The referee used the term ‘tipping point’ in her/his comment. However, we would say that in these cases the term ‘regime shift’ because a tipping point would be a threshold in forcing or system properties that induces a regime shift. The referee re-iterates more or less what we describe in our paragraph, which addresses both natural and human-caused changes. We also address the problem of lacking observations. In order to better specify this type of changes in section 2.2 we suggest to add the following text at the end of the section: “In order to be more quantitative, we refer to this category of changes if the shift occurs over a much shorter time than corresponding stable reference periods. Analysis of projections with an Earth system model indicates increased abrupt changes occurrence due to anthropogenic climate change that will last several centuries for warming, deoxygenation, and acidification (Heinze et al., 2023).” The additional reference (Heinze, C., Michel, C., Torsvik, T., Schwinger, J., & Tjiputra, J. F.: More frequent abrupt marine environmental changes expected. Geophysical Research Letters, 51, e2023GL106192, 2024, https://doi.org/10.1029/2023GL106192) will be added to the reference list.
Section 3.1: the introduction of this section seems too broad and not necessarily needed for this type of paper. The paleo-section also feels unnecessarily detailed.
We think that the anthropogenic climate forcing and the associated climate change need to be put into perspective with major global changes in the past. These past abrupt changes in the global dimension have been slower than the anthropogenic event. Thus its uniqueness can only be understood in describing the overall context of past Earth system change. We plan to streamline and shorten this section, however, to improve readability.
Section 3.2: here too there are parts that don’t feel as relevant, e.g. the paragraph at Lines 434-448 on future emission scenarios.
The model intercomparison experiments ZECMIP and CDRMIP are absolutely essential in providing insight into the reversibility of global anthropogenic climate change. Thus, they need to be described here because other sections of the paper build on them. This is particularly needed for a sound analysis of AMOC changes.
Section 3.3: the entire first paragraph reads as background on oceanic C-system response to CO2 emissions; is it necessary in a paper about abrupt change? Likewise, multiple other paragraphs (e.g., Lines 483-490 on negative emissions) seems out of topic.
We suggest as an option to export the background information to “boxes” accompanying the main text or appendices at the end of the manuscript so that the information will be available to the non-experts and to enhance readability for the experts.
Lines 501-502, “do not offer consistent results…” but see the recent work by Busecke et al., 2022, AGU advances, which provides a reconciled picture of O2 changes in oxygen minimum zones under global warming.
We plan to add the following text in line 503: ”For the Pacific OMZ a more detailed picture could be achieved, indicating an expansion of the overall broad OMZ but at the same time a shrinking of the very low oxygen core (Busecke et al., 2022).” The additional reference (Busecke, J. J. M., Resplandy, L., Ditkovsky, S. J., & John, J. G.: Diverging fates of the Pacific Ocean oxygen minimum zone and its core in a warming world. AGU Advances, 3, e2021AV000470, 2024, https://doi. org/10.1029/2021AV000470) will be added to the reference list.
Lines 540-569: this is accurate and interesting (and probably too detailed), but it is unclear to me what makes these changes “abrupt” since they depend on a progressive and fairly slow deoxygenation trend.
This paragraph is still in the global changes section and considering the natural background (e.g. stable Holocene conditions) we think that inclusion is justified. We plan to shorten and streamline the paragraph to enhance readability.
Section 4, general: for many of the following examples, the “abrupt” character of the changes discussed is unclear; some do not seem necessarily driven by man0made warming but they could be part of natural (e.g. multidecadal) variability); some are a consequence of the slow secular change.
We try to identify abrupt changes as they occur and on what observational or model-induced basis is available. That there is a grey zone cannot be entirely avoided. However, in a world and at a time, where quite a number of papers on potential AMOC shut-down and changes in the Atlantic
current system are provided, a view on available evidence is necessary. Attribution of the various changes to anthropogenic climate change is difficult due to a lack of baseline observations.
Lines 604-622: this reads as a too detailed excursus. Also I am not sure this counts as a tipping point; the authors are rather describing many interactions between elements of the current system that may change under a variety of drivers.
We disagree because this change in the North Atlantic current system is a recently detected and well documented change over a short period. We will try, however, to shorten the text.
Lines 640-645 and Fig. 5: these are biogeochemical changes that may be part of multi-decal variability, couched within a section of warming and physical change; they feel detouring and not completely relevant. Again, is decadal variability a central point of discussion in a paper on man-made abrupt change?
Due to a lack of open ocean time series, we only have limited observations available to detect abrupt changes in the ocean state variables. The Irminger Sea time series is a rare exception and needs to be mentioned here. It is premature to term this time series as decadal variability or not.
Lines 650-657: too much detail; this could be summarized in a short sentence.
We will try to shorten and streamline the text here.
Figure 7: this is a very detailed figure on a very specific example: is this level of detail needed? Also, again, it is not clear how abrupt the change discussed is.
The reorganisation of the water flow through the Arctic is an important feature of the changing general circulation pattern. We are not sure, why the referee wants to downplay it as ‘very specific’. It needs to be seen in the context of fast changes in sea ice cover and hydrography in the Arctic Ocean.
Lines 746-775: even after reading this massive paragraph a couple times, I remain unsure what counts as abrupt change here and what the specific focus is on.
We tried to summarise the ongoing change in the Southern Ocean in a – given the complexity of the system – fairly concise paragraph including abrupt sea ice changes. We highlighted where we think that changes are persistent. We will try to shorten and streamline the text.
Lines 798-812: lots of detail on coastal erosion that feel quite removed from the central topic of abrupt BGC change.
Coastal erosion in connection with (abrupt) sea ice retreat has a strong impact on ocean biogeochemistry. In order to support this we cite now the two following papers and add these to the references list:
Chen-Tung, A.C., Liu K.-K., and Macdonald, R.: Continental Margin Exchanges, in Ocean Biogeochemistry - the Role of the Ocean Carbon Cycle in Global Change, edited by Michael J. R. Fasham, pp. 53-97, Springer, Berlin et al., 2003, ISBN 978-3-642-62691-3, DOI 10.1007/978-3-642-55844-3
Terhaar, J., Lauerwald, R., Regnier, P., Gruber, N., & Bopp, L.: Around one third of current Arctic Ocean primary production sustained by rivers and coastal erosion. Nature Communications, 12(1), 169, 2021, https://doi.org/10.1038/s41467-020-20470-z
Lines 814-824, and Fig. 8: how does this relate to abrupt change? It is a model sensitivity (river vs. no-river).
We suggest to replace the text in lines 817-819 by: “The total flux of riverine nitrates to the Arctic is currently decreasing, however different watersheds have different trends and rates (ArcticGro database, https://arcticgreatrivers.org/, last accessed 06.10.2023; Tank et al. (2023)), which suggests a differential change in Arctic shelf ecosystem provinces in the future.” The reference of Zolkos et al. (2022) will be removed from the references list. The new reference (Tank, S.E., McClelland, J.W., Spencer, R.G.M., Shiklomanov, A.I., Suslova, A., Moatar, F., Amon, R.M.W. Cooper, L.W., Elias, G., Gordeev, V.V., Guay, C., Gurtovaya, T.Y., Kosmenko, L.S., Mutter, E.A., Peterson, B.J., Peucker-Ehrenbrink, B., Raymond, P.A., Schuster, P.F., Scott, L., Staples, R., Striegl, R.G., Tretiakov, M., Zhulidov, A.V., Zimov, N., Zimov, S., and Holmes, R.M.: Recent trends in the chemistry of major northern rivers signal widespread Arctic change. Nat. Geosci. 16, 789–796, 2023, https://doi.org/10.1038/s41561-023-01247-7) will be added to the references list.
We plan to replace the caption for Figure 8 with the following text: “Figure 8: Arctic shelf productivity is controlled by nutrient flux of rivers, shown in NEMO MEDUSA 1° historical integrations with and without the riverine inputs of nutrients (“River” and “No-River” respectively) compared to observations (satellite-derived). Observed changes in nutrient flux from different river watersheds can impact shelf primary production a nd produce regime shifts.”
Section 4.2: This is again a review on CO2 uptake about the ocean; the focus should be rather be on abrupt change.
We will shorten and streamline this introductory text. At the same time we think an introduction with some background information is important as context especially for the non-experts.
Section 4.2.1: again, many instances of progressive changes are discussed, rather than focusing on abrupt change.
The referee is correct in the sense that the pH change is not happening as a step function. However, the acidification in cold high latitude waters is indeed faster progressing than elsewhere. The transition from calcite or aragonite over- to undersaturation happens regionally in only few years and makes a large change to ecosystem behaviour and biogeochemistry, especially CaCO3 preservation or dissolution that can be seen as a similar change as physical phase changes, i.e. liquid water or ice.
Fig. 9: These show gradual changes, not abrupt — unless a millennial timescale is considered.
As described in our previous comment above, the essential element is the Ω=1 line separating over- and undersaturated waters, an example of gradual changes reaching a tipping point.
Fig. 10: you could explain what the white colors mean — my take is that they show thick sea ice. Also, clarify in the caption what we are looking at; e.g. what region.
We will improve the figure caption.
Line 914: Fransson et al., in Review: paper in review should be avoided unless there’s a preprint; but here it may be the paper in press in the bibliography?
Yes, sorry for the confusion, the reference in the main text (under review) had not been updated to “in press” at the time of submission of our manuscript. The paper is published meanwhile and citation plus reference in the reference list will be updated accordingly. The reference is:
Fransson, A., Chierici, M., Granskog, M.A., Dodd, P.A. and Stedmon, C.A.: Impacts of glacial and sea-ice meltwater, primary production, and ocean CO2 uptake on ocean acidification state of waters by the 79 North Glacier and northeast Greenland shelf. Front. Mar. Sci. 10:1155126, 2023, doi: 10.3389/fmars.2023.1155126
979-1001: this does not seem directly related to abrupt change, but rather a review of future climate change in the Southern Ocean.
We will shorten this paragraph and focus on the increase of the Southern Ocean carbon sink in relation to sea ice loss and the changes in solubility-driven/mixing-driven uptake modes.
Line 996: “Together, these trigger the shift from …” I struggled with this sentence, it could be clarified.
This sentence will be reformulated during the streamlining process (see comment above).
Section 4.2.6: “the second most important oceanic region for anthropogenic CO2 uptake” and “with avoided outgassing serving as a net sink”: I see what the Authors mean, but it could be better explained; I imagine the equatorial pacific will remain a region of CO2 outgassing, so there will not be actual uptake there, but the effect on the atmospheric CO2 budget would be analogous to net uptake.
Yes, correct, a decrease in outgassing is similar to an increase in uptake considering the overall net fluxes. We will also mention that the rising atmospheric CO2 partial pressure due to progressing anthropogenic CO2 emissions will partly work as a ‘lid’ on the ocean to reduce outgassing (by reducing the air-sea CO2 gradient locally).
Lines 1115-1120: this entire sentence is confusing and may need rewriting.
We suggest to reformulate this passage to: “Within the water column, there is usually a transition from CaCO3 over- to undersaturation happening characterised by the CaCO3 lysocline or the calcium carbonate compensation depth CCD. The lysocline is the depth interval over which a sudden corrosiveness for CaCO3 shell material is encountered. The CCD is the depth level at which CaCO3 dissolution and CaCO3 supply from particle fluxes out of shallower layers compensate; below the CCD thus no CaCO3 sediments would be found. The shoaling of both the CaCO3 lysocline and the CCD due to ocean acidification has a strong impact on deep-sea ecosystems including cold water corals (Fransner et al., 2022; Gehlen et al., 2014; Guinotte and Fabry, 2008).”
Lines 1125-1130: this feels like a potentially important section, but it is not very clearly written, so I suggest a rephrasing.
We suggest to revise this section to: “The persistence of Fe in the ocean is a function of pH, T, as well as S, and the amount/quality of organic matter present (Santana-Casiano et al., 2022), because these parameters affect its speciation, i.e. its distribution over different chemical forms: Iron is present in seawater both as Fe(II) and Fe(III). Fe(II) tends to oxidise to Fe(III), but can still be found in oxic conditions in the ocean, due to complexation with organic ligands and involvement in different redox reactions. Still, typically, Fe tends to be dominated by Fe(III). Over 99% of the soluble fraction of Fe(III) is complexed with organic ligands. The different forms of Fe have different solubilities (inorganic Fe(III) is almost insoluble at oceanic pH) and bioavailability, affecting how fast Fe is removed by precipitation or biological uptake.”
Lines 1149-1153: this section on ligand-induced feedbacks could be better explained, e.g. by pacing the reader through the steps of the (positive?) feedback.
We suggest to replace this section of text by: “The fact that iron-binding ligands themselves are a product of biological activity creates the potential for feedbacks, which can act to amplify or mediate climate-related changes in iron availability. Ligands that are a by-product of biological activity (such a marine humics) would tend to become more abundant by some climate-driven reduction of iron-limitation in the ocean, increasing Fe solubility, and slowing the loss of iron from the ocean, hence a positive feedback. Ligands that are produced in reaction to iron limitation, such as bacterial siderophores, would do the opposite, and constitute a negative feedback. These feedbacks have been studied in a global box model of the oceanic iron cycle (Völker and Ye, 2022). In this study it was shown that including a description of the cycling of organic ligands similar to the one in Völker and Tagliabue (2015) into the model improved its fit of the phosphorus and iron cycle to data. Although not all parameters describing the ligand cycling in the model could be constrained, it was shown that the overall feedback caused by the biological production of ligands is likely positive, albeit not very strongly so. This means that changes in the availability of iron caused by external, climate-related factors could be somewhat stronger than extrapolated based on constant ligands.”
Section 4.3.2: Again, the relation to abrupt change is not obvious; the first paragraph seem too basic/introductory and unnecessary.
We will shorten and streamline the text but retain some part of it as background information for the non-experts.
Lines 1204-1208: Fine, but for context, the figure show an O2 decline of ~1% over a nearly centennial period, this does not deem either significant or particularly abrupt, especially since O2 remains way above typical hypoxic thresholds.
The interesting part is here that the O2 decline starts here not synchronous with the warming. However, we tend to agree with the reviewer here, and will remove the figure (plus related text) also in order to shorten the manuscript.
Section 5.2: again there’s not much of a discussion of abrupt changes, but rather a review of extremes.
Extreme events qualify as abrupt changes themselves and further can trigger ecosystem regime shifts. Therefore, we see no issue here. Extremes as one category of abrupt changes have been in general introduced in section 2.3.
Lines 1382-1386: this could be clarified.
We suggest to add the following text in line 1387: “With a baseline following the mean trend of a specific state variable, only extremes with respect to a certain shorter reference period are identified and counted, while for a fixed baseline (e.g. year 1850 or 1950 for model results or observations), extremes are of larger extent and also duration.”
Line 1405: “which generally do not support marine life” this is completely inaccurate, as plenty of animal species can live at O2<60 mmol/m3; life in general can live in the absence of O2.
We will change the text to: “that is important for many higher organisms including economically important fishes (Keeling et al., 2010)”.
Section 5.3: This entire section could benefit from a reframing in light of the work of Deutsch et al., e.g. Deutsch et al., 2015, Science; 2020 Nature.
Thank you for this comment. In the revised section 5.3 we will highlight the importance of studying low-oxygen extreme events due to the strong correlation between species-specific tolerance for hypoxia and regional distribution of marine species as described by Deutsch et al., 2015 and Deutsch et al., 2020. However, only a few studies looking at low-oxygen extreme event characteristics and projected changes therein exist to date, and even fewer that include species-specific oxygen requirements. We would like to add that within section 1.2.3. the impacts of low-oxygen on marine organisms are introduced in more detail, including the work by Deutsch et al., 2015 and Deutsch et al., 2020. (The references had already been included in the references list.)
Lines 1451-1460: this is an interesting paragraph; there are no citations so I assume it’s new research. I suggest maybe explaining what controls the different ECS in the specific versions of the model.
Yes, this is new research (building on the work by Jeltsch-Thömmes et al., 2020). In EMICs (Earth system models of intermediate complexity) such as the one used, different ECSs are simulated simply by introducing an arbitrary feedback to air temperature changes without specifying the physical reason for such a feedback.
Section 7.1, last paragraph. Cite the work by Dutkiewicz et al., 2015.
We plan to add the following text in line 1644 of section 7.1: “An overview of acidification impacts on phytoplankton communities can be found in Dutkiewicz et al., 2015.” The additional reference (Dutkiewicz, S., Morris, J.J., Follows, M.J., Scott, J., Levitan, O., Dyhrman, S.T., and Berman-Frank, I.: Impact of ocean acidification on the structure of future phytoplankton communities, Nature Climate Change, 1002-1006, 2015, DOI: 10.1038/NCLIMATE2722) will be added to the reference list.
Lines 1676-1680: maybe add a clarification on the extent to which phytoplankton shifts are driven by shifts in water mass boundaries vs. ecological interactions.
We plan to add the following text in line 1679: “In an Eulerian framework, some cases of an apparent shift in ecosystem structure can be caused by lateral or vertical movements of water masses (e.g. through shifting water mass boundaries or fronts) instead for ecological reasons.”
Fig. 21: you could add more detail in the caption, e.g. explaining what the colors are in the bottom rows, etc.; the colorbars are very hard to read.
We plan to increase the colour bar information and will amend the caption for more details concerning the bottom row.
Section 7.5: this is very focused on the Iberian Peninsula upwelling, but upwelling systems are much more varied, so it’s a missed opportunity. There’s a rich literature on upwelling changes, from tests of Bakun’s hypothesis to recent analysis with CMIP models (Wang et al. 2015, Nature; Rykaczewski et al., 2015, GRL; Sydeman et al., 2014).
We feel a little bit in a grid-lock situation here, because we should shorten the paper, but extend it here for more material that does not directly address ecosystem changes. As a compromise we will plan to add the Californian Upwelling changes and the CMIP studies in line1741: “Another example of an upwelling change of direct ecological relevance is the upwelling of corrosive water onto the shelf off California (Feely et al., 2008). A general trend of an intensification of coastal upwelling systems and also changes in geographical distribution have been identified mostly on the basis of Earth system model analysis (Sydeman et al, 2014; Wang et al., 2015; Rykaczewski et al., 2015).” The additional references will be added to the references list (Sydeman, W. J., García-Reyes, M., Schoeman, D.S., Rykaczewski, R.R., Thompson, S.A., Black, B.A., and Bograd, S.: Climate change and wind intensification in coastal upwelling ecosystems, Science, 345, issue 6192, 77-80, 2014, doi: 10.1126/science.1251635. Wang, D., Gouhier, T.C., Menge, B.A., and Ganguly, A.R.: Intensification and spatial homogenization of coastal upwelling under climate change, Nature, 2015, doi:10.1038/nature14235. Rykaczewski, R. R., Dunne, J.P., Sydeman, W.J., García-Reyes, M., Black, B.A., and Bograd, S.J.: Poleward displacement of coastal upwelling-favorable winds in the ocean’s eastern boundary currents through the 21st century, Geophys. Res. Lett., 42, 6424–6431, 2015, doi:10.1002/2015GL064694.).
Section 7.6: this does not feel like a very careful or thorough review of a vast topic; it’s very focused on European seas. See work by Free et al. 2019, Science, Fredston et al. 2023, Nature, Lotze et al. 2019, PNAS, Tittensor et al., 2021, Nature Climate Change, etc.
As for the previous item, we think that we cannot provide an in-depth review of this vast topic. Otherwise the paper would grow in extent even further. In order to account for other studies, we plan to modify the start of section 7.6 through adding the following text: “Climatic warming has implications for fish stocks including a general reduction in maximum sustainable yield (Free et al., 2019) and biomass (Lotze et al., 2019; Tittensor et al., 2021).”
The reference of Fredston et al. (2023) will be added to the sentence ending in line 1746.
The additional references will be added to the references list (Fredston, A. L., Cheung, W.W.L., Frölicher, T.L., Kitchel, Z.J., Maureaud, A.A., Thorson, J.T., Auber, A., Mérigot, B., Palacios-Abrantes, J., Lourdes, M., Palomares, D., Pecuchet, L., Shackell, N.L., and Pinsky, M.L.: Marine heatwaves are not a dominant driver of change in demersal fishes, Nature volume 621, pages 324–329, 2023, https://doi.org/10.1038/s41586-023-06449-y. Free, C. M., Thorson, J.T. Pinsky, M.L., Oken, K.L., Wiedenmann, J., and Jensen, O.P.: Impacts of historical warming on marine fisheries production, Science 363, 979–983, 2019, doi: 10.1126/science.aau1758. Lotze, H. K., Tittensor, D.P., Bryndum-Buchholz, A., Eddy, T.D., Cheung, W.W.L., Galbraith, E.D., Barange, M., Barrier, N., Bianchi, D., Blanchard, J.L., Bopp, L., Büchner, M., Bulman, C.M., Carozza, D.A., Christensen, V., Coll, M., Dunne, J.P., Fulton, E.A., Jennings, S., Jones, M.C., Mackinson, S., Maury, O., Niiranen, S., Oliveros-Ramosx, R., Roy, T., Fernandes, J.A., Schewel, J., Shin, Y.-J., Silva, T.A.M., Steenbeek, J., Stock, C.A., Verley, P., Volkholz, J., Walker, N.D., and Worm, B.: Global ensemble projections reveal trophic amplification of ocean biomass declines with climate change, PNAS, 116, 12907–12912, 2019, doi: 10.1073/pnas.1900194116. Tittensor, D. P., Novaglio, C., Harrison, C.S., Heneghan, R.F., Barrier, N., Bianchi, D., Bryndum-Buchholz, L.A., Britten, G.L., Büchner, M., Cheung, W.W.L., Christensen, V., Coll, M., Dunne, J.P., Eddy, T.D., Everett, J.D., Fernandes-Salvador, J.A., Fulton, E.A., Galbraith, E.D., Gascuel, D., Guiet, J., John, J.G., Link, J.S., Lotze, H.K., Maury, O., Ortega-Cisneros, K., Palacios-Abrantes, J., Petrik, C.M., du Pontavice, H., Rault, J., Richardson, A.J., Shannon, L., Shin, Y.-J., Steenbeek, J., Stock, C.A., and Blanchard, J.L.: Next-generation ensemble projections reveal higher climate risks for marine ecosystems, Nature Climate Change, 11, 973–981, 2021, https://doi.org/10.1038/s41558-021-01173-9.).Citation: https://doi.org/10.5194/bg-2023-182-AC2
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AC2: 'Reply on RC2', Christoph Heinze, 12 Mar 2024
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RC3: 'Comment on bg-2023-182', Anonymous Referee #3, 21 Feb 2024
This paper provides an extensive review of a wide range of topics in marine biogeochemistry and climate change that loosely fall under the heading of "abrupt change". I can imagine that the authors' goal is to advance a new research theme on abrupt change, building on other themes that have been embraced by the community in recent years on topics such as "tipping points" and "extreme events".
While the breadth of scholarship and bibliography are impressive, the paper has one large problem, which is that it is very long, mainly because the bounds on what topics are within and without the scope of the paper are too vaguely drawn. I expect few if any readers will undertake reading it from start to end. I also expect the paper will prove difficult to use as reference on subtopics because the outline seems haphazard.
I appreciate the challenge the authors have undertaken. The word "abrupt" doesn't define a mechanistic category, and only vaguely defines a phenomenological category. The paper provides very few observational examples in the modern era to clarify what is meant by abrupt. Most of the examples are from models. One of the better observational examples, involving the Australian marine heat wave leading to ecosystem shifts, can possibly be understood as an extreme warming event triggering an ecological tipping point. In this and other examples, it's not clear how adding the label "abrupt" has led to sharper thinking or research focus.
A characteristic of the current manuscript is that, at the section and paragraph level, the lead sentences rarely establish a clear link to the theme of abrupt change. Rather, they more typically launch the reader off on a tangent. Also, the writing could be tighter in places. Some sentences don’t make a lot of sense if you read them carefully, in part because of loose usage of terms.
The paper embraces three broad categories of abruptness (lines 271-280). Although I struggle a bit to understand these categories, it seems the first of these is geologic, e.g. PETM, glacial-interglacial, and the second involves the full Anthropocene (which of course is abrupt when seen from a geologic perspective), and the third involves recent changes that are more abrupt than the forcings that have driven the Anthropocene. With these choices, the authors have categorized essentially all forms of environmental changes as being abrupt, with very few exceptions, e.g. the ~50 million year Cenozoic cooling trend. No wonder the paper is so long.
The first fix I recommend is to discard the first two categories of abruptness as out of scope. In other words, I'd cut all content about paleo and the full Anthropocene from the paper except insofar as this content helps understand the third category. I note that even the second category alone is too broad. By categorizing all human-induced climate change as "abrupt", the authors have obligated themselves to carry out a lengthy review that is largely redundant to IPCC reports and other texts. In my view, this is not the place for a broad review of modern climate change. Nor is it the place for a scattered review of paleo changes. This first fix is also consistent with the title of the paper, which suggests a focus on abrupt change within the recent era. This fix would eliminate most of Section 2 and all of Section 3. I also think Section 1 can be cut because this paper is not the right place for tutorial on the greenhouse effect or on how carbon chemistry is altered by ocean CO2 uptake.
Even after adopting these changes, there is still a question of how to shape the remaining content. I have several suggestions:
(1) I think the introductory content of a paper of reduced scope needs to summarize the main mechanistic reasons why oceanic biogeochemical responses are expected to be slowly varying (there are many such reasons) and then picking them apart to identify exceptions. Without this context, a young scientist reading this paper as a foundational document might come away with a skewed perspective.
(2) The main content of a paper of reduced scope needs revisions to avoid the problem I mention above about tangential content. For example, the topics of extreme events (Section 5) and irreversibility or tipping points (Section 6) already have their following in the community. While I appreciate that a focus on abrupt change will connect with these topics, this paper should be defining those connections rather than carrying out stand-alone reviews of those topics. Addressing this point will necessitate other changes to the outline.
(3) The main content of the paper of reduced scope needs to shift slightly away from models towards observed results. And similarly, I suggest drawing a clearer demarcation between abrupt changes that are documented in the modern era, versus abrupt changes that may occur in the future.
Taking a step back, I also suggest the authors take seriously another possibility, which is to set aside the current draft and start by writing a succinct document (< 2000 words) for a higher profile journal. Such a document would of necessity have a tighter logic and flow. A shorter document would force a correction to the problem I note above about tangential content.
Another unrelated point is that despite its length, the document seemingly overlooks two possible categories of recent abrupt changes: One type is associated with shifting ocean or meteorological fronts, which can lead to abrupt changes at a location that a front has moved across. Another type is associated with the superposition of natural decadal variability and long-term secular change. The superposition of these two types of changes can lead to a staircase-like pattern (hiatus, jump, hiatus, jump, etc).
I now turn to more detailed comments. These include comments within sections that I have suggested be cut entirely. Some of these comments are thus irrelevant if my previous advice is followed.
73-77. This is an example of a sentence that doesn't really make sense when read carefully. What kind of abrupt ocean environmental change is under consideration? Are these events already happening, or just hypothesized to happen in the future? Further, I think none of the cited references describe abrupt environmental changes outside of the ecosystem response. For example, Wernberg et al 2016 describes an abrupt ecological response to an extreme heat wave. Was the extreme heat wave abrupt? Maybe. But that is not how Wernberg et al chose to describe it. A key characteristic of this event was that temperatures exceeded previous bounds. Whether it started or ended abruptly is thus not particularly relevant. The ecological response might well have been "abrupt" in an objective sense. But this is not relevant to the point being made in this sentence (lines 73-77).
148. Note unclosed parentheses.
166. This veers dangerously towards treating a model results as established facts. (I could point to other examples of this).
200. What abrupt changes are we talking about?
376. What is meant by "such changes". Specifically, what aspect of glacial-interglacial changes have no analogues except under previous catastrophic events. Also, as we lack ice core records to resolve CO2 changes prior to about 800 kyr BP, our knowledge of earlier changes (and their degree of abruptness) is quite poor.
423. An example of tangential content. This entire paragraph has unclear relevance to the topic of abrupt change. One strong link between the TCRE framework and the topic of abruptness is strangely overlooked by authors, i.e. that if warming is tied to cumulative emissions, then warming will not be abrupt in relation to human activities. Abrupt global warming requires a breakdown of the linear TCRE relationship.
432-426. The deviations from linearity arise not just from quantification methods and NETs.
437. An example of vague language. What does it mean to attenuate something that is abrupt? Low past filtering? To the extent that anthropogenic warming has been abrupt (relative to geologic changes) it cannot be attenuated because it's already in the past.
445-447. I found this sentence hard to parse.
450. Section 3.3. An example of the general bias that I pointed out in my preamble. A key missing point is that the main mechanism by which the ocean absorbs (or buffers) atmospheric CO2 involves steady processes that will not impose abrupt changes in air-sea flux, even in response to abrupt changes in human emissions or non-oceanic CDR. Also, as far as I can tell, nothing in this section relates to abrupt changes outside of the overall anthropogenic perturbation.
458. The 92% figure depends on how much CO2 is ultimate emitted (due to non-linear carbon chemistry and carbon ion limitation). I think a lower number is appropriate for many future scenarios.
491. Section 3.4. Another example of tangential content. This is a nice review of controls on deoxygenation. But none of this concerns abrupt changes other than in the sense that the anthropogenic perturbation is abrupt on geologic timescales.
516-518. Could be worded better to clarify that this concerns future changes.
Figure 4. Depth of contour map is not identified clearly. I also can't clearly visualize what is being verbalized. The numbers are identifying what features exactly?
Figure 5. What longitude, latitude are shown? What season?
Figure 6. The map does not clearly indicate the gyres and currents. Where is the flow that exports water via the Norwegian Trench?
684-701. A very notable characteristic of the Mediterranean outflow is its high salinity. Why no mention of salinity in this section?
958. Section 4.2.3 I saw little in this section that relates to "abrupt" change.
1322. I'd replace "have been described" to "have been observed"
1580. Same concern here as above for line 458.
1901-1903. An example of a sentence that didn't make much sense to me.Citation: https://doi.org/10.5194/bg-2023-182-RC3 -
AC3: 'Reply on RC3', Christoph Heinze, 12 Mar 2024
Author response to Review #3
The referee’s remarks are kept in italics.
The referee’s main points:
This paper provides an extensive review of a wide range of topics in marine biogeochemistry and climate change that loosely fall under the heading of "abrupt change". I can imagine that the authors' goal is to advance a new research theme on abrupt change, building on other themes that have been embraced by the community in recent years on topics such as "tipping points" and "extreme events".
While the breadth of scholarship and bibliography are impressive, the paper has one large problem, which is that it is very long, mainly because the bounds on what topics are within and without the scope of the paper are too vaguely drawn. I expect few if any readers will undertake reading it from start to end. I also expect the paper will prove difficult to use as reference on subtopics because the outline seems haphazard.
Our response to this main point:
We thank the referee for the time and effort this person has invested to review this manuscript. We think that the paper needs to be read as a whole. The outline, outcomes, and conclusion will become clear then. We do not know whether the referee has indeed read the paper in one sweep or not. The paper without references is 77 pages long with 23 Figures and one large table. One page in discussion format of BG does correspond to less than one respective page in print. So, whether one considers this paper as “very long” or not is – at least partially – subjective. Yes, there are shorter reviews around, and yes, there are longer reviews around. We have developed the outline from a zero-order draft, over a first-order draft, to a second-order draft that provided the basis for the submitted version of the discussion paper. The referee states that “the outline seems haphazard”. We do not think that this is true, also based on the feedback by other readers and other referees.
I appreciate the challenge the authors have undertaken. The word "abrupt" doesn't define a mechanistic category, and only vaguely defines a phenomenological category. The paper provides very few observational examples in the modern era to clarify what is meant by abrupt. Most of the examples are from models. One of the better observational examples, involving the Australian marine heat wave leading to ecosystem shifts, can possibly be understood as an extreme warming event triggering an ecological tipping point. In this and other examples, it's not clear how adding the label "abrupt" has led to sharper thinking or research focus.
Our response to this main point:
We have clearly and carefully defined in section 2 what we mean by abrupt change. We do not know, what the referee means by relating an adjective (“abrupt”) to a noun (“mechanistic category”, “phenomenological category“). We describe in our paper the difficulty of observing abrupt biogeochemical changes because only few time series measurements exist. Many changes that would fall into the second category (2.2) may simply not be resolved by existing monitoring networks. Analysis of output from well-tested Earth system models is a necessity here to fill the corresponding gap, and the only way to sound out future changes under different possible forcing scenarios. Similar to our response to referee #2: In order to better specify this type of changes in section 2.2 we suggest adding the following text at the end of the section: “In order to be more quantitative, we refer to this category of changes if the shift occurs over a much shorter time than corresponding stable reference periods. Analysis of projections with an Earth system model indicates increased abrupt changes occurrence due to anthropogenic climate change that will last several centuries for warming, deoxygenation, and acidification (Heinze et al., 2023).” The additional reference (Heinze, C., Michel, C., Torsvik, T., Schwinger, J., & Tjiputra, J. F.: More frequent abrupt marine environmental changes expected. Geophysical Research Letters, 51, e2023GL106192, 2024, https://doi.org/10.1029/2023GL106192) will be added to the reference list.
A characteristic of the current manuscript is that, at the section and paragraph level, the lead sentences rarely establish a clear link to the theme of abrupt change. Rather, they more typically launch the reader off on a tangent. Also, the writing could be tighter in places. Some sentences don’t make a lot of sense if you read them carefully, in part because of loose usage of terms.
Our response to this main point:
We aim to shorten and streamline the paper and improve its readability.
The paper embraces three broad categories of abruptness (lines 271-280). Although I struggle a bit to understand these categories, it seems the first of these is geologic, e.g. PETM, glacial-interglacial, and the second involves the full Anthropocene (which of course is abrupt when seen from a geologic perspective), and the third involves recent changes that are more abrupt than the forcings that have driven the Anthropocene. With these choices, the authors have categorized essentially all forms of environmental changes as being abrupt, with very few exceptions, e.g. the ~50 million year Cenozoic cooling trend. No wonder the paper is so long.
Our response to this main point:
We will update the description of category 2.2 as suggested in the response to reviewer #2. The changes that refer to the full Anthropocene refer to category 2.1 and not 2.2. We do by no means categorise all environmental changes as abrupt. We relate the change in focus always to a temporal and spatial baseline: Global changes on a millennial stable baseline, reginal to basin-wide changes on a stable decadal-to-centennial baseline, and regional extreme events on a stable baseline of several years or few decades.
The first fix I recommend is to discard the first two categories of abruptness as out of scope. In other words, I'd cut all content about paleo and the full Anthropocene from the paper except insofar as this content helps understand the third category. I note that even the second category alone is too broad. By categorizing all human-induced climate change as "abrupt", the authors have obligated themselves to carry out a lengthy review that is largely redundant to IPCC reports and other texts. In my view, this is not the place for a broad review of modern climate change. Nor is it the place for a scattered review of paleo changes. This first fix is also consistent with the title of the paper, which suggests a focus on abrupt change within the recent era. This fix would eliminate most of Section 2 and all of Section 3. I also think Section 1 can be cut because this paper is not the right place for tutorial on the greenhouse effect or on how carbon chemistry is altered by ocean CO2 uptake.
Our response to this main point:
The referee wants to retain only the category of extreme events, which would be a too-limited view. Next to extreme events, there are many other ongoing marine environmental changes.
Even after adopting these changes, there is still a question of how to shape the remaining content. I have several suggestions:
(1) I think the introductory content of a paper of reduced scope needs to summarize the main mechanistic reasons why oceanic biogeochemical responses are expected to be slowly varying (there are many such reasons) and then picking them apart to identify exceptions. Without this context, a young scientist reading this paper as a foundational document might come away with a skewed perspective.
(2) The main content of a paper of reduced scope needs revisions to avoid the problem I mention above about tangential content. For example, the topics of extreme events (Section 5) and irreversibility or tipping points (Section 6) already have their following in the community. While I appreciate that a focus on abrupt change will connect with these topics, this paper should be defining those connections rather than carrying out stand-alone reviews of those topics. Addressing this point will necessitate other changes to the outline.
(3) The main content of the paper of reduced scope needs to shift slightly away from models towards observed results. And similarly, I suggest drawing a clearer demarcation between abrupt changes that are documented in the modern era, versus abrupt changes that may occur in the future.
Our response to this main point:
Thank you for these suggestions. To 1: We have provided such context throughout the paper but agree that we can make this point clearer, i.e. why some biogeochemical changes occur slowly. We will take this into account when streamlining the manuscript. However, there is a whole spectrum of biogeochemical marine changes, and some occur on shorter timescales, e.g., manifested in many seasonal variations. To 2: Earlier on, the referee wanted us to retain only extreme events (category 2.3), and now the referee asks us to only touch on them by connecting to these. We do not know how to respond to these contradictory recommendations. To 3: Due to only few open ocean time series measurements we have to use also model output to a large extent and - naturally - for future projections.
Taking a step back, I also suggest the authors take seriously another possibility, which is to set aside the current draft and start by writing a succinct document (< 2000 words) for a higher profile journal. Such a document would of necessity have a tighter logic and flow. A shorter document would force a correction to the problem I note above about tangential content.
We already had published a corresponding perspective paper (Heinze, C., Blenckner, T., Martins, H., Rusiecka, D., Doscher, R., Gehlen, M., Gruber, N., Holland, E., Hov, O., Joos, F., Matthews, J. B. R., Rodven, R., and Wilson, S.: The quiet crossing of ocean tipping points, Proceedings of the National Academy of Sciences of the United States of America, 118, 2021, 10.1073/pnas.2008478118).
Another unrelated point is that despite its length, the document seemingly overlooks two possible categories of recent abrupt changes: One type is associated with shifting ocean or meteorological fronts, which can lead to abrupt changes at a location that a front has moved across. Another type is associated with the superposition of natural decadal variability and long-term secular change. The superposition of these two types of changes can lead to a staircase-like pattern (hiatus, jump, hiatus, jump, etc).
Our response to this main point:
We plan to add these forms of abrupt change to make the description in section 2.2 clearer. The aspect concerning shifts of frontal systems will also be addressed in section 7 (see our response to reviewer #2 for lines 1676-1680).
Specific comments:
I (=referee) now turn to more detailed comments. These include comments within sections that I have suggested be cut entirely. Some of these comments are thus irrelevant if my previous advice is followed.
73-77. This is an example of a sentence that doesn't really make sense when read carefully. What kind of abrupt ocean environmental change is under consideration? Are these events already happening, or just hypothesized to happen in the future? Further, I think none of the cited references describe abrupt environmental changes outside of the ecosystem response. For example, Wernberg et al 2016 describes an abrupt ecological response to an extreme heat wave. Was the extreme heat wave abrupt? Maybe. But that is not how Wernberg et al chose to describe it. A key characteristic of this event was that temperatures exceeded previous bounds. Whether it started or ended abruptly is thus not particularly relevant. The ecological response might well have been "abrupt" in an objective sense. But this is not relevant to the point being made in this sentence (lines 73-77).
In this passage we intentionally focus on the ecosystem response to abrupt environmental change. In order to improve readability, we plan to revise the text by splitting up the sentence: “It is not as yet conclusively quantified, how ecosystems respond to abrupt ocean environmental changes in comparison to smoothly and monotonically changing conditions. There are at least some studies, in which abrupt biogeochemical state variable changes have been recorded together with negative consequences for ecosystems (Bond et al., 2015; Hoegh-Guldberg et al., 2007; Wernberg et al., 2016, 2021; Chan et al., 2008).”
148. Note unclosed parentheses.
Thank you. The parentheses opened in line 148 will be closed in line 151.
166. This veers dangerously towards treating a model results as established facts. (I could point to other examples of this).
We will change “experience” to “may experience” in line 166.
200. What abrupt changes are we talking about?
All categories of abrupt change.
376. What is meant by "such changes". Specifically, what aspect of glacial-interglacial changes have no analogues except under previous catastrophic events. Also, as we lack ice core records to resolve CO2 changes prior to about 800 kyr BP, our knowledge of earlier changes (and their degree of abruptness) is quite poor.
As we discuss paleo-analogues (or the lack of them) for the anthropogenic climate change, “such changes” refers to those, where the heat and/or carbon inventory of the ocean is changed through external forcing and not through a redistribution in the Earth system as described in the previous sentence. Paleo-reconstructions often have limitations but this does not make them useless.
423. An example of tangential content. This entire paragraph has unclear relevance to the topic of abrupt change. One strong link between the TCRE framework and the topic of abruptness is strangely overlooked by authors, i.e. that if warming is tied to cumulative emissions, then warming will not be abrupt in relation to human activities. Abrupt global warming requires a breakdown of the linear TCRE relationship.
TCRE refers to the endpoint of warming for cumulative emissions on the global average, not to trajectories of parts of the Earth system between start and end. Non-linear effects may come into play, especially under CDR, as discussed from line 425 on.
432-426. The deviations from linearity arise not just from quantification methods and NETs.
What we have written is based on respective references concerning this point. It would have been good to receive a concrete suggestion on what specifically could be added here.
437. An example of vague language. What does it mean to attenuate something that is abrupt? Low past filtering? To the extent that anthropogenic warming has been abrupt (relative to geologic changes) it cannot be attenuated because it's already in the past.
We will amend the text in line 438 from “attenuated” to “reduced from the high level”.
445-447. I found this sentence hard to parse.
We plan to amend the text in line 446 from “was caused” to “was caused originally”.
450. Section 3.3. An example of the general bias that I pointed out in my preamble. A key missing point is that the main mechanism by which the ocean absorbs (or buffers) atmospheric CO2 involves steady processes that will not impose abrupt changes in air-sea flux, even in response to abrupt changes in human emissions or non-oceanic CDR. Also, as far as I can tell, nothing in this section relates to abrupt changes outside of the overall anthropogenic perturbation.
Yes, but this section is exactly about the global anthropogenic CO2 perturbation (as the section heading or 3.3 says), which is abrupt on the background of the long Holocene stable period.
458. The 92% figure depends on how much CO2 is ultimate emitted (due to non-linear carbon chemistry and carbon ion limitation). I think a lower number is appropriate for many future scenarios.
We suggest changing the text here to: “If no negative emissions are applied and no other effective long-term CO2 sinks contribute, the ocean will absorb significant amounts of CO2 from the atmosphere over several tens of thousands of years until it has buffered ca. 92% of the atmospheric CO2 perturbation (Bolin and Eriksson, 1959; Egleston et al., 2010; Revelle and Suess, 1957). Because the buffer factor depends critically on the CO2 partial pressure of the surface waters, the percentage buffered will be somewhat lower the higher the total anthropogenic CO2 emission rates will be.”
491. Section 3.4. Another example of tangential content. This is a nice review of controls on deoxygenation. But none of this concerns abrupt changes other than in the sense that the anthropogenic perturbation is abrupt on geologic timescales.
This section is about global deoxygenation on the background of a long reference period as the section heading says.
516-518. Could be worded better to clarify that this concerns future changes.
This sentence will be revised (see response to referees #1 and #2). This does not only concern future changes, it also concerns past mass extinction events that may have been caused by this mechanism.
Figure 4. Depth of contour map is not identified clearly. I also can't clearly visualize what is being verbalized. The numbers are identifying what features exactly?
We plan to include depth information in this map and also add “latitude” to the y-axis. We think the numbers are described in the caption and should be understandable together with text of section 4.1.1. We aim at adding an introductory sentence to the figure caption in order to make the figure easier to understand.
Figure 5. What longitude, latitude are shown? What season?
Figure 5 will be amended accordingly.
Figure 6. The map does not clearly indicate the gyres and currents. Where is the flow that exports water via the Norwegian Trench?
We will improve the map concerning the size of the arrows and will improve the indication of dynamic features.
684-701. A very notable characteristic of the Mediterranean outflow is its high salinity. Why no mention of salinity in this section?
We agree that the Mediterranean outflow is important for the salinity of the Atlantic Ocean, but adding this information does not contribute to the focus of the manuscript here.
958. Section 4.2.3 I saw little in this section that relates to "abrupt" change.
We tried to summarise the ongoing change in the Southern Ocean including abrupt changes in sea ice. We highlighted where we think that changes are persistent. We will try to shorten and streamline the text.
1322. I'd replace "have been described" to "have been observed"
As we refer also to Earth system models in this sentence, we think that “observed” is not the right term here. We suggest to use “have been determined”.
1580. Same concern here as above for line 458.
Here the principle is outlined and the number is given as approximate.
1901-1903. An example of a sentence that didn't make much sense to me.
The sentence will be changed to: “When one considers the planetary boundaries and the processes as well forcings that determine them in coupling, the planetary boundaries are in general approached earlier than if seen separately in isolation, which suggests even more stringent mitigation targets for staying within the safe operating space (Lade et al., 2020; Steinacher et al., 2013; Willcock et al., 2023).”Citation: https://doi.org/10.5194/bg-2023-182-AC3
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AC3: 'Reply on RC3', Christoph Heinze, 12 Mar 2024
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