Marine ecosystem role in setting up preindustrial and future climate

Domain: earth_science
Published: 2025-03-05
Editorial outcome: minor_revision

Abstract
The ocean ecosystem is a vital component of the global carbon cycle, storing enough carbon to keep atmospheric CO2 considerably lower than it would otherwise be. However, this conception is based on simple models, neglecting the coupled land-ocean feedback. Using an interactive Earth system model, we show that the role ocean biology plays in controlling atmospheric CO2 is more complex than previously thought. Atmospheric CO2 in a new equilibrium state after the biological pump is shutdown increases by more than 50% (163 ppm), lower than expected as approximately half the carbon lost from the ocean is adsorbed by the land. The abiotic ocean is less capable of taking up anthropogenic carbon due to the warmer climate, an absent biological surface pCO2 deficit and a higher Revelle factor. Prioritizing research on and preserving marine ecosystem functioning would be crucial to mitigate climate change and the risks associated with it.

Introduction
As a key climate regulator, the ocean has slowed anthropogenic climate change by absorbing 91% of the heat trapped in the Earth system between 1971 and 20181 and 26% of anthropogenic carbon emitted since the preindustrial period2. Subsequently, it will play a crucial role in determining future climate. Air-sea fluxes of carbon dioxide primarily arise from the air-sea disequilibrium of the partial pressure of CO2, driven by two main mechanisms: (i) the solubility pump, associated with CO2 gas solubility and circulation, and (ii) the biological pump, the photosynthetic conversion of near-surface dissolved inorganic carbon (DIC) into organic matter that is exported into the ocean interior via gravitational sinking and circulation3. Through mechanism (ii), the biological carbon pump (BCP) effectively reduces surface-preformed DIC while increasing remineralized DIC at depth. This vertical redistribution and variations in preformed to remineralized DIC determines the partitioning of ocean-atmosphere carbon stocks4. Using an atmosphere-ocean equilibrium relationship, it is estimated that when the remineralized carbon stock is removed, atmospheric CO2 levels would be approximately 150-240 ppm higher in the new equilibrium state5.

Currently the BCP is often considered to operate at steady state, with the physically-driven solubility pump considered to have dominated the projected increase in current and future carbon sink and storage rates6,7, due to the limited observational evidence and knowledge on the non-linear interactions and feedbacks between BCP and other Earth system components8. As a result, there is currently no consensus on the projected changes in BCP efficiency.

Repeated attempts to quantify the importance of BCP for future carbon storage have been conducted using ocean models of varying complexities. These include altering the chemical stoichiometry of organic matter production9, changing the remineralization depth10, enhancing micronutrient fertilization11, and simulating future climate change with an abiotic ocean (i.e., by complete removal of all marine organisms)12, among others. All these studies were performed with stand-alone ocean models, neglecting the carbon cycle feedback processes that occur between the ocean, land, and atmosphere. Such approaches can either underestimate or overestimate the impact of changing BCP on the long-term ocean carbon uptake8. In the face of rapidly intensifying climate change and anthropogenic pressures on marine ecosystems, better quantification of BCP’s role in mitigating future climate change is of fundamental importance for climate science and policy developments.

Here, we assess the role of the BCP in setting up the preindustrial climate and quantify the impacts of its removal on future carbon sequestration and climate change. We apply the state-of-the-art Norwegian Earth System Model (NorESM2)13,14 in its fully interactive configuration to simulate quasi-equilibrium preindustrial climate and subsequently project historical and future climate scenarios with and without marine organisms (see “Methods”). The impacts across three future scenarios, ranging from high CO2 emissions SSP5-8.5 (Shared Socioeconomic Pathways) to a strong mitigation SSP1-2.615, are investigated. Our simulations allow for prognostic atmospheric CO2 concentration, taking into account carbon fluxes from land and ocean and the associated feedback to the climate system. The preindustrial reorganization of the carbon pools across the Earth system in the absence of BCP leads to more than 50% (163 ppm) higher atmospheric CO2 and a 1.6°C warmer surface temperature. Consequently, the Earth system’s capacity to buffer anthropogenically-induced climate change is hampered. Despite having lower DIC content, an abiotic ocean counterintuitively has a higher surface DIC and pCO2 concentration, leading to a higher Revelle factor and a reduced anthropogenic carbon uptake. Future land and ocean carbon sinks are projected to reduce considerably, accelerating and amplifying anthropogenic climate change.

Results
Preindustrial states
Two distinct quasi-equilibrium preindustrial climate states are simulated, with and without ocean biology (hereafter referred to as REF and Abiotic, respectively; see Methods and Supplementary Table 1). Key climate metrics and carbon budget estimates for these two preindustrial states are summarized in Table 1. The Abiotic ocean releases roughly 730 Pg C to the atmosphere during the 2000-year spin-up period (Supplementary Fig. 1). Of this, 345 Pg C is adsorbed by land, primarily through CO2 fertilization-induced vegetation growth16. The remainder stays in the atmosphere, leading to a 163 ppm higher CO2 concentration (445 ppm) compared to REF (282 ppm). This increases global mean surface temperature by 1.64°C, while sea surface temperature (SST) increases by 1.15°C. This warming lead to considerable impacts on the Earth system, such as reducing Atlantic Meridional Overturning Circulation (AMOC) strength (9%) and sea-ice area in the Arctic and Antarctic (23 and 24%, respectively; Table 1). Changes in sea-ice extent are seasonally non-uniform, with September Arctic sea-ice reducing the most, by more than half. Without ocean biology, global ocean temperature increases from 3.56 to 4.66°C, and as the surface ocean equilibrates with the higher atmospheric CO2 concentration, its Revelle factor increases by 16%.

The spatial patterns of surface temperature in the two pre-industrial climate states are similar but warmer everywhere in the Abiotic (Supplementary Fig. 2a, b). The Abiotic ocean exhibits stronger CO2 outgassing in the tropical oceans and ingassing in the mid-to-high latitudes (Supplementary Fig. 2d, e). The zonal outgassing band along the Southern Ocean circumpolar fronts seen in REF, associated with the upwelling of remineralized carbon-rich water masses17, switches into a net sink in Abiotic. In the subtropical oligotrophic, the rates of CO2 flux are relatively similar in REF and Abiotic. The altered land carbon budget includes an increased vegetation carbon pool, with the largest increase simulated in the tropics region, followed by high-latitude ecosystems in the northern hemisphere (Supplementary Fig. 2g, h).

The stronger and expanded tropical outgassing in the Abiotic is consistent with (i) an absence of biological consumption of surface DIC and (ii) warming-induced lower CO2 solubility, both of which increase surface pCO2. In the high latitudes, stronger uptakes can be explained by the amplification of the seasonal cycle. Supplementary Fig. 3 illustrates that the removal of ocean biology fundamentally alters the seasonal cycle of the surface carbonate system. Firstly, the thermal-driven pCO2 variability is amplified due to higher background pCO218 and higher SST seasonal variation. Secondly, the lack of summer productivity and winter upwelling of remineralized carbon-rich deep water amplify the thermally driven pCO2 variability. Thirdly, the indirect effect of land biosphere changes (stronger summer productivity and winter respiration) leads to an amplification of the atmospheric CO2 seasonal cycle (Supplementary Fig. 3a, d). These three effects combine to increase the air-sea pCO2 gradient, with a stronger effect during winter, and manifest in a stronger high-latitude carbon sink in the Abiotic ocean (with up to a three-fold increase; Supplementary Fig. 3c, f). We note that in our quasi-equilibrium pre-industrial Abiotic ocean, the sedimentary carbon content is reduced by 391 Pg C (Table 1), due to the absence of organic matter accumulations and dissolution of organic materials. This additional DIC to the water column could contribute to the overall oceanic carbon release. We also note that our experiment represents an extreme hypothetical case with a complete absence of marine productivity. When we consider a 10% reduction in primary production, a range projected by models19, Supplementary Fig. 4 shows that the preindustrial atmospheric CO2 would approximately be 10 ppm higher, while the land carbon budget increases by 9 Pg C, implying ocean outgassing of approximately 30 Pg C.

Historical and future projections
Projections of historical and future climate (1850–2100) show accelerated climate change in Abiotic relative to REF (Fig. 1 and Supplementary Fig. 5). Atmospheric CO2 in REF grows from 281 to 415 (SSP1-2.6), 569 (SSP2-4.5), and 1061 (SSP5-8.5) ppm by 2100 (Fig. 1a and Supplementary Fig. 5a), consistent with the CO2 pathways defined in the CMIP6 (Coupled Model Intercomparison Project phase 6) protocol15. In the Abiotic, CO2 increases from 445 to 685 (SSP1-2.6), 883 (SSP2-4.5), and 1433 (SSP5-8.5) ppm. The stronger CO2 increase in the Abiotic (by 106, 150, and 208 ppm in SSP1-2.6, SSP2-4.5, and SSP5-8.5, respectively) is caused by the land and ocean carbon sinks diminishing by as much as 91% (i.e., land sink in SSP2-4.5). The weaker ocean carbon sink, despite higher atmospheric CO2, is attributed to the higher surface ocean pCO2 and lower air-sea pCO2 disequilibrium (Fig. 1f).

Figure 2 summarizes that without ocean biology, 68 to 83% of the fossil fuel emissions would remain in the atmosphere by 2100, as compared to only 37 to 65% in simulations that include ocean biology.

These larger anthropogenic CO2 residuals being stored in the atmosphere translate to a stronger climate sensitivity by accelerating climate change in Abiotic, with broad implications for various components of the Earth system (Fig. 1 and Supplementary Fig. 5). Under SSP5-8.5, the global mean surface temperature in Abiotic increases by nearly 5°C by 2100, a 30% stronger warming rate than in REF (Table 1). Regionally, the largest warming occurs at high latitudes, with approximately 2 °C additional warming in the Arctic due to polar amplification20 (Supplementary Fig. 6). Similarly, the AMOC strength, the sea-ice area, and the ocean mixed layer depth, are projected to decline faster in Abiotic (Fig. 1c, g–i). A complete removal of ocean biology leads to the system crossing the 2°C warming threshold more than ten years earlier, with the disappearance of summer Arctic sea ice occurring more than 50 years earlier (Fig. 1b, g). This accelerated climate change is also evident in the lower emissions scenarios of SSP1-2.6 and SSP2-4.5 (Supplementary Table 2). The transient climate response to CO2 emissions (TCRE) for the REF and Abiotic are 1.69 and 2.13°C Eg C−1, respectively (Supplementary Fig. 7), implying a 20% reduction of the allowable CO2 emissions for a specified global warming target when ocean biology is removed.

The reduced terrestrial carbon sinks in the Abiotic reflects the saturation of land carbon uptake in a warmer, higher CO2 world due to the higher preindustrial vegetation carbon pool21,22 (Fig. 1e and Table 1), hence weakening the negative terrestrial carbon cycle feedback associated with CO2 fertilization. In the ocean, warmer and higher surface pCO2 reduces seawater CO2 solubility and buffering capacity23, leading to a lower air-sea pCO2 disequilibrium and less CO2 uptake for a given increase in atmospheric CO2 concentration (Fig. 1d, f and Table 1). The lack of ocean production-induced pCO2 deficit, which has significant impacts at higher atmospheric CO2 levels, also plays an important role24.

Next, we analyze how the spatio-temporal dynamics of the anthropogenic carbon (Cant) sink and storage in the ocean are altered by the absence of ocean biology. The cumulative ocean uptake of Cant (i.e., the difference between the transient historical + future and the preindustrial control simulations) over the 1850–2100 period is spatially heterogeneous, with deep water formation regions of the North Atlantic and the Southern Ocean emerging as the most intense sink regions (Fig. 3). These key gateways for Cant sinks allow for efficient transport of Cant-rich surface water to the interior for long-term storage25,26. Stronger uptake is simulated in the high-emission SSP5-8.5 than SSP2-4.5 and SSP1-2.6, with regions of net Cant outgassing (more prominently in Abiotic) are simulated in the subtropics, where surface Cant converges27. The Abiotic Ocean absorbs considerably less Cant than REF in the subpolar North Atlantic and parts of the Southern Ocean (Fig. 3c, f, i).

We investigate the driving mechanisms of the lower Abiotic Cant uptake, focusing in the subpolar North Atlantic (dashed green outlines in Fig. 3c, f, i). Since the seasonal cycle of CO2 fluxes is fundamentally altered in Abiotic, we assess the projection of different drivers in each season (Supplementary Fig. 8). The air-sea gradient of pCO2 determines to first order the direction of CO2 flux. In REF, the oceanic pCO2 stays below the atmospheric values, and the subpolar North Atlantic is a net CO2 sink all year, consistent with observations and models28. This is also seen in the Abiotic at the beginning of the historical period but is progressively altered by the faster oceanic pCO2 growth rate, resulting in a remarkable decline in the CO2 sink. More importantly, under SSP5-8.5 (and other scenarios) oceanic pCO2 grows faster than atmospheric pCO2 during spring and summer, therefore switching from uptake to outgassing. This is primarily driven by the combination of stronger warming and the higher initial surface Revelle factor, which reduce the Abiotic ocean capacity to offset the growing atmospheric pCO2 (Supplementary Fig. 9), as depicted by the weaker surface DIC growth rate (Supplementary Fig. 8).

Methods

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors have fully adressed my concerns and I find the revised paper presents important findings suitable for publication in Nature Communications. In particular, the paper highlights and quantifies the response of the land sink to changes in the ocean, and the role of ecosystems in setting up mean pre-conditions that also have an influence on anthropogenic uptake of CO2 by the oceans.\n\nThe authors have revised all the sentences which I identified as not being fully justified by the manuscript results (bar one, see below, which I had not originally seen). They have also introduced a more realistic simulation with a reduction of 10% in productivity which puts the full abiotic results into context.\n\nI have a few comments remaining:\n\np. 6, lines 237-241. The comment on the role of ocean sediments appears in the discussion only and is not backed by analysis in the results section, other than a line that is not commented in Table 1. Given the strong comment made on the possible implications for deep-sea mining and trawling, the sentence on lines 240-241 needs to be backed by analysis in the results section. Here also, please be conscious that the abiotic results are extreme scenarios and should not be presented in a way that can be interpreted as plausible effects of mining/trawling, which would be considerably smaller and potentially negligible. The sentence rightly says this needs further investigation.\n\np. 7, lines 277-278. The study cited in reference 36 and the Gardian newspaper article cited in the letter to the editor grossly overinterpret the effect of recent climate variability in 2023 (linked to El Nino) as suggestion that it might indicate a collapse of the land sink. Study 36 simply adds Land sink + land-use change emissions (both affected by El Nino) to highlight a very low land sink. However, the land sink did not behave unusually compared to other El Nino periods. Reference 36 is a pre-print that has yet to be challenged by peer-reviewed. Whereas the comment in the current paper is valid (that the longevity of the natural carbon sinks is not guaranteed), the reference is not, at least not by itself. For example, Friedlingstein et al. 2024 (also a preprint) makes the case that the land sink will recover in 2024. Please expand references here to back the statement made with peer-reviewed publications (such as the IPCC).\nhttps://essd.copernicus.org/preprints/essd-2024-519/\n\nFigure 1e. The land panel has discontinuity around 2020, both with the broken lines and with the following unusual and strong decrease in the land sink (around 2020-2040), which are not explained in the paper and appear wrong to me. These need to be verified and explained, at the minimum in the methods or Supplementary material.\n\nOn a side note to the authors, a similar analysis of an abiotic land would be interesting and complementary to the current paper.", "reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_3": "Manuscript Review for Nature Communications\nSubmitted: 12/23/2025\nTitle: Marine ecosystem role in setting up preindustrial and future climate\nThe manuscript examines the biological carbon pump’s (BCP) total role in the global ocean carbon sink and its importance in regulating historic and future atmospheric CO2 and global temperature. The conclusions challenge the notion that the BCP plays a secondary role to the solubility and circulation components of the ocean sink in absorbing excess atmospheric CO2, demonstrating how its absence increases the Revelle factor and surface pCO2 under preindustrial conditions. The paper explores the carbon cycle’s sensitivity to biological processes by examining its extreme boundaries, providing clear, conceptual comparisons. While the finding that disabling the BCP could cause climatologically significant atmospheric CO2 and global temperature increases underscores its role in climate regulation, a total biological shut-down is not very plausible and represents an extreme case. In response to reviewer feedback, the authors incorporated a partial productivity reduction experiment for more realistic insights. By including the moderate case of (90%) productivity, consistent with the 10% decline projected by CMIP5 and CMIP6 models, the study expands its relevance.\n\nThe authors should consider providing the total inventories of carbon in each reservoir (land, ocean, atmosphere, wherever carbon is stored) at the beginning of their model runs. While it is true that similar studies often omit this information, it would be greatly beneficial for other modeling groups that want to compare the behavior of different Earth System models. Because ESMs are typically spun up with a prescribed atmospheric pCO2, the inventory of carbon will be different in every model. As a result, the relative redistribution of carbon between the different reservoirs as a result of a change in the biological pump will be different for different models. If the reference model had a very weak BCP, then shutting it off would have a weaker impact. This applies to the land biosphere as well– the strength of the CO2 fertilization effect, for example, would differ based on how the model is tuned.\n\nThe study makes several unique contributions that are significant to the field and related fields.\n\nOne strength of the study is its integration of the coupled land-ocean-atmosphere system, a relatively uncommon approach in BCP research. In the Main section, another study is mentioned that simulates future climate change with an abiotic ocean– however, that study used a stand-alone ocean model. Their interconnected framework adds important nuance to the field by including carbon cycle feedback processes that occur between land, ocean, and atmosphere.\n\nIn the discussion, the highlighted seasonality-driven ocean-atmosphere CO2 dynamics, particularly in the North Atlantic and Southern Ocean, offer valuable insights into underexplored climate feedbacks related to seasonality. These feedbacks are particularly uncertain because of the lack of year-round temporal coverage of observed BCP variables. The amplification of seasonal pCO2 outgassing in key ocean sink regions and its impact on long-term carbon sequestration is particularly noteworthy.\n\nThe study highlights noteworthy spatial differences in the storage of anthropogenic carbon between biotic and abiotic simulations, particularly within specific ocean regions such as the subpolar Atlantic. The authors also identify the mechanisms driving these differences with a well-supported explanation, demonstrating how factors like the faster oceanic pCO2 growth rate, higher initial surface Revelle factor, and seasonal variations in air-sea pCO2 gradients contribute to the reduced carbon uptake in abiotic simulations.\n\nA few concerns could be addressed and do not prohibit publication.\n\nThe decision to include only high- and low- end emissions scenarios raises questions about the exclusion of moderate scenarios, which could yield unique feedback patterns beyond averaging the extremes. Including a brief discussion of this choice would provide helpful context to the design of the study.\n\nKey results of the paper rely on a direct link between increased/decreased primary productivity and increased/decreased carbon sequestration by the BCP. Increased primary productivity does not always lead to increases in effective carbon storage or even carbon export past shallow ocean layers. An acknowledgement of the nuanced relationship between primary productivity, carbon export, and sequestration in the discussion would be valuable. Variations in productivity by region or organism type may have differing impacts on BCP efficiency, complicating direct links between productivity reductions and CO2 sequestration losses. Addressing this complexity would help the study in guiding future climate modeling efforts, carbon cycle research, and policy development.\n\nSeveral points in the main text could be clarified to strengthen the narrative.\n\nLines 38-40: The explanation of how the BCP influences the ratio of preformed to remineralized carbon, and its direct link to total ocean carbon storage, could be clearer. Consider stating directly that the BCP reduces surface preformed carbon by converting it into export production carbon, thus increasing remineralized carbon at depth. A clarification would help avoid ambiguity, emphasizing that the BCP primarily redistributes carbon within the ocean rather than storing it in an isolated reservoir. Consider rephrasing to clarify and emphasize the specific role of the BCP in partitioning ocean-atmosphere carbon stocks.\n\nLines 40-42: The reference to “removing remineralized carbon” was initially confusing without clearer context. Since remineralized carbon results directly from BCP activity, specifying whether the term refers to modeled carbon stocks or the conceptual absence of the BCP would reduce potential misunderstandings. This could be interpreted as the authors taking out remineralized carbon, so that the deep ocean has less carbon, and then re-equilibrating the model. Alternatively, this could mean that the BCP was “shut off”, and then they got a readjustment in a different way with a different amount of carbon.\n\nLines 43-45: Consider clarifying in particular what aspects of the carbon sink are considered to be dominated by the solubility pump rather than BCP– for example, its current magnitude and variability, or projected changes under climate scenarios?\n\nLines 45-48: Consider mentioning the critical role of limited observational evidence and lack of consensus on current changes in BCP efficiency when discussing gaps in the understanding of the current and future BCP.\n\nLines 50-53: While the examples of modeling approaches used to study the BCP here are relevant, the list appears selective. Consider rephrasing to signal the listed examples are representative rather than exhaustive.\n\nLines 53-56: The assertion that neglecting feedback processes leads to overestimation of the BCP’s impact could benefit from a short explanation. Why does the omission consistently result in overestimation, rather than introducing more general uncertainty?\n\nLines 60-61: The term “marine ecosystem” might be too broad. If the study specifically shuts down the BCP, consider using that term to align more closely with the experiment design."}, "author_response": "Response letter to manuscript\nChanges in response to Reviewers’ comments are marked with ‘green’ color in the\nrevised manuscript with track changes. Line numbers refer to the revised manuscript\nwith track changes.\nReviewer #1 (Remarks to the Author):\nThe authors have fully addressed my concerns and I find the revised paper presents\nimportant findings suitable for publication in Nature Communications. In particular, the\npaper highlights and quantifies the response of the land sink to changes in the ocean,\nand the role of ecosystems in setting up mean pre-conditions that also have an\ninfluence on anthropogenic uptake of CO2 by the oceans.\nWe thank reviewer#1 for his/her positive feedback and additional constructive\ncomments. We have considered all the new comments in our latest revision.\nThe authors have revised all the sentences which I identified as not being fully justified\nby the manuscript results (bar one, see below, which I had not originally seen). They\nhave also introduced a more realistic simulation with a reduction of 10% in productivity\nwhich puts the full abiotic results into context.\nResponses to additional comments are provided below.\nI have a few comments remaining:\np. 6, lines 237-241. The comment on the role of ocean sediments appears in the\ndiscussion only and is not backed by analysis in the results section, other than a line\nthat is not commented in Table 1. Given the strong comment made on the possible\nimplications for deep-sea mining and trawling, the sentence on lines 240-241 needs to\nbe backed by analysis in the results section. Here also, please be conscious that the\nabiotic results are extreme scenarios and should not be presented in a way that can be\ninterpreted as plausible effects of mining/trawling, which would be considerably\nsmaller and potentially negligible. The sentence rightly says this needs further\ninvestigation.\nIn the revision (end of Sect. ‘Preindustrial states’), we have commented on the role of\nsedimentary carbon loss (in Abiotic ocean) in increasing water column DIC and\npotentially the ocean outgassing:\nL150-153: “We note that in our quasi-equilibrium preindustrial Abiotic ocean, the\nsedimentary carbon content is reduced by 391 Pg C (Table 1), due to the absence of\norganic matter accumulations and dissolution of organic materials. This additional DIC\nto the water column could contribute to the overall oceanic carbon release.”\nWe agree that our extreme scenarios should not be interpreted as plausible effects of\nmining/trawling, and have rephrased the aformentioned text in the discussions from\n“Our simulations also suggest that the dissolution of organic material in the ocean\nsediment considerably alters the water column DIC budget and subsequently the air-\nsea CO fluxes. This implies that the impact of human-induced disruption of marine\n2\nsediment, e.g., through deep sea mining or fish trawling, on the ocean carbon cycle\nshould be thoroughly investigated.”\nto:\nL294-300: “Our simulations, though extreme, suggest that the dissolution of\nsedimentary organic material can alters the water column DIC budget and\nsubsequently the air-sea CO fluxes. This has obvious parallels to human induced\n2\ndisturbances of the seafloor by trawling or dredging; however, these will be much\nsmaller in size and hence future studies applying a more realistic scenario would be\nnecessary to investigate the impact of human-induced disruption of marine\nsediments.”\np. 7, lines 277-278. The study cited in reference 36 and the Gardian newspaper article\ncited in the letter to the editor grossly overinterpret the effect of recent climate\nvariability in 2023 (linked to El Nino) as suggestion that it might indicate a collapse of the\nland sink. Study 36 simply adds Land sink + land-use change emissions (both affected\nby El Nino) to highlight a very low land sink. However, the land sink did not behave\nunusually compared to other El Nino periods. Reference 36 is a pre-print that has yet to\nbe challenged by peer-reviewed. Whereas the comment in the current paper is valid\n(that the longevity of the natural carbon sinks is not guaranteed), the reference is not,\nat least not by itself. For example, Friedlingstein et al. 2024 (also a preprint) makes the\ncase that the land sink will recover in 2024. Please expand references here to back the\nstatement made with peer-reviewed publications (such as the IPCC).\nhttps://essd.copernicus.org/preprints/essd-2024-519/\nWe agree that reference#36 is not ideal when discussing to the longevity of natural\ncarbon sinks. We have now rephrased the statement below and reference Ch. 5 of the\nIPCC-AR6 report (Canadell et al., 2021), which states the slowing down of natural CO\n2\nsink rate under future high-warming scenarios (i.e. box FAQ5.1):\n“The projected lower land and ocean carbon sinks in the future provides a glimpse of\npotential accelerated climate change, in light of a recent study raising concern on the\nlongevity of natural carbon sink [36].”\nTo (now moved to Discussion section):\nL258-260: “This accelerated climate change also illustrates the potential consequence\nif the longevity of natural carbon sink is reduced faster or earlier than initially expected\n(Canadell et al., 2021).”\nFigure 1e. The land panel has discontinuity around 2020, both with the broken lines and\nwith the following unusual and strong decrease in the land sink (around 2020-2040),\nwhich are not explained in the paper and appear wrong to me. These need to be verified\nand explained, at the minimum in the methods or Supplementary material.\nThank you for noticing this. The discontinuity is due to the separation between historical\n(1850-2014) and future scenario (2015-2100) 10-yr averaging applied to land carbon\nuptake. We have revised Fig. 1 in the revision. The strong decrease in ‘net’ land sink\naround 2020-2040 seen in SSP5-8.5 is associated with the carbon lost due to land-use\nchange, fire and harvested carbon. Figure R1 below illustrates that the strong sink\nreduction disappears when the land carbon sink is calculated only based on net\nprimary production minus respiration (NEP, panel c).\nFigure R1. Time-series of 10-yr averaged of (a) net land carbon sink, (b) net ecosystem exchange (exclude\nland-use change), and (c) net ecosystem production (exclude land-use change, fires, and harvest pool\nfluxes) under the (dark-blue) historical and SSP5-85 scenario, and (light-blue) preindustrial control.\nWe have added the following at the end of Methods section:\nL411-413: “We note that the noticeable decline in net land CO uptake in the early 21st\n2\ncentury under the SSP5-8.5 scenario (Fig. 1e) is associated with the loss of carbon due\nto the prescribed land use change and fires.”\nOn a side note to the authors, a similar analysis of an abiotic land would be interesting\nand complementary to the current paper.\nWe agree, though since land uptakes are primarily biotic-driven, this would consider\nonly land use change.\nReviewer #2 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nWe thank reviewer#2 for supporting the co-review process of our manuscript.\nReviewer #3 (Remarks to the Author):\nThe manuscript examines the biological carbon pump’s (BCP) total role in the global\nocean carbon sink and its importance in regulating historic and future atmospheric CO2\nand global temperature. The conclusions challenge the notion that the BCP plays a\nsecondary role to the solubility and circulation components of the ocean sink in\nabsorbing excess atmospheric CO2, demonstrating how its absence increases the\nRevelle factor and surface pCO2 under preindustrial conditions. The paper explores the\ncarbon cycle’s sensitivity to biological processes by examining its extreme boundaries,\nproviding clear, conceptual comparisons. While the finding that disabling the BCP\ncould cause climatologically significant atmospheric CO2 and global temperature\nincreases underscores its role in climate regulation, a total biological shut-down is not\nvery plausible and represents an extreme case. In response to reviewer feedback, the\nauthors incorporated a partial productivity reduction experiment for more realistic\ninsights. By including the moderate case of (90%) productivity, consistent with the 10%\ndecline projected by CMIP5 and CMIP6 models, the study expands its relevance.\nWe thank reviewer #3 for providing thorough critical and constructive feedback on our\nmanuscript. We have addressed all of the raised comments below.\nThe authors should consider providing the total inventories of carbon in each reservoir\n(land, ocean, atmosphere, wherever carbon is stored) at the beginning of their model\nruns. While it is true that similar studies often omit this information, it would be greatly\nbeneficial for other modeling groups that want to compare the behavior of different\nEarth System models. Because ESMs are typically spun up with a prescribed\natmospheric pCO2, the inventory of carbon will be different in every model. As a result,\nthe relative redistribution of carbon between the different reservoirs as a result of a\nchange in the biological pump will be different for different models. If the reference\nmodel had a very weak BCP, then shutting it off would have a weaker impact. This\napplies to the land biosphere as well– the strength of the CO2 fertilization effect, for\nexample, would differ based on how the model is tuned.\nAs requested, we have added the following at the end of the ‘Methods section’:\nL408-411: “At the start of the transient historical simulation (1850), the total carbon\ninventories for the atmosphere, land, and ocean (water column+sediment) reservoirs\nfor REF (Abiotic) are 599 (945), 3115 (3458), and 39547 (38829) Pg C, respectively.”\nThe study makes several unique contributions that are significant to the field and\nrelated fields. One strength of the study is its integration of the coupled land-ocean-\natmosphere system, a relatively uncommon approach in BCP research. In the Main\nsection, another study is mentioned that simulates future climate change with an\nabiotic ocean– however, that study used a stand-alone ocean model. Their\ninterconnected framework adds important nuance to the field by including carbon cycle\nfeedback processes that occur between land, ocean, and atmosphere.\nIn the discussion, the highlighted seasonality-driven ocean-atmosphere CO2 dynamics,\nparticularly in the North Atlantic and Southern Ocean, offer valuable insights into\nunderexplored climate feedbacks related to seasonality. These feedbacks are\nparticularly uncertain because of the lack of year-round temporal coverage of observed\nBCP variables. The amplification of seasonal pCO2 outgassing in key ocean sink regions\nand its impact on long-term carbon sequestration is particularly noteworthy.\nThe study highlights noteworthy spatial differences in the storage of anthropogenic\ncarbon between biotic and abiotic simulations, particularly within specific ocean\nregions such as the subpolar Atlantic. The authors also identify the mechanisms driving\nthese differences with a well-supported explanation, demonstrating how factors like the\nfaster oceanic pCO2 growth rate, higher initial surface Revelle factor, and seasonal\nvariations in air-sea pCO2 gradients contribute to the reduced carbon uptake in abiotic\nsimulations.\nWe are grateful to reviewer #3 for this insightful comment and in acknowledging the\nuniqueness of our approach and importance contributions to the ocean carbon cycle\ncommunity.\nA few concerns could be addressed and do not prohibit publication.\nThe decision to include only high- and low- end emissions scenarios raises questions\nabout the exclusion of moderate scenarios, which could yield unique feedback patterns\nbeyond averaging the extremes. Including a brief discussion of this choice would\nprovide helpful context to the design of the study.\nThe two extreme scenarios (SSP1-2.6 and SSP5-8.5) were selected to illustrate the\nrobustness of our conclusions that the absence of biological carbon pump would lead\nto reduced future land and ocean capacity to absorb excess anthropogenic CO from\n2\nthe atmosphere across these emission pathways. In order to increase the\ncomprehensiveness of our findings, we have decided to performed the additional SSP2-\n4.5 (moderate CO emissions scenario) and have added both simulations with and\n2\nwithout biology. The result is fully consistent and does not change our main\nconclusions. We have denoted this in the revised manuscript by including relevant\ninformation (on SSP2-4.5 results) throughout the text.\nKey results of the paper rely on a direct link between increased/decreased primary\nproductivity and increased/decreased carbon sequestration by the BCP. Increased\nprimary productivity does not always lead to increases in effective carbon storage or\neven carbon export past shallow ocean layers. An acknowledgement of the nuanced\nrelationship between primary productivity, carbon export, and sequestration in the\ndiscussion would be valuable. Variations in productivity by region or organism type may\nhave differing impacts on BCP efficiency, complicating direct links between productivity\nreductions and CO2 sequestration losses. Addressing this complexity would help the\nstudy in guiding future climate modeling efforts, carbon cycle research, and policy\ndevelopment.\nWe have included the following statement in the discussions section to acknowledge\nthe described nuanced relationship and cite a reference:\nL279-286: “In Earth system models, the BCP is generally positively associated with\nsurface primary productivity, nevertheless there is a nuanced relationship between\nprimary production, carbon export, and carbon sequestration. Spatial and temporal\nvariations in surface productivity, and ecosystem structure, may have non-linear\nimpacts on BCP efficiency, complicating the direct links between reductions in\nproductivity and CO sequestration (Frenger et al., 2024). Addressing this complexity\n2\nwould be valuable to guide future climate modeling efforts, carbon cycle research, and\npolicy development.”\nSeveral points in the main text could be clarified to strengthen the narrative.\nLines 38-40: The explanation of how the BCP influences the ratio of preformed to\nremineralized carbon, and its direct link to total ocean carbon storage, could be clearer.\nConsider stating directly that the BCP reduces surface preformed carbon by converting\nit into export production carbon, thus increasing remineralized carbon at depth. A\nclarification would help avoid ambiguity, emphasizing that the BCP primarily\nredistributes carbon within the ocean rather than storing it in an isolated reservoir.\nConsider rephrasing to clarify and emphasize the specific role of the BCP in partitioning\nocean-atmosphere carbon stocks.\nWe have revised the sentence:\n“The biological carbon pump (BCP) regulates the ratio of preformed to remineralised\ncarbon dioxide dissolved in the ocean interior, and hence how much carbon is stored in\nthe ocean.”\nto:\nL51-54: “Through mechanism (ii), the biological carbon pump (BCP) effectively reduces\nsurface preformed DIC while increasing remineralised DIC at depth. This vertical\nredistribution and variations in preformed to remineralised DIC determines the\npartitioning of ocean-atmosphere carbon stocks.”\nLines 40-42: The reference to “removing remineralized carbon” was initially confusing\nwithout clearer context. Since remineralized carbon results directly from BCP activity,\nspecifying whether the term refers to modeled carbon stocks or the conceptual\nabsence of the BCP would reduce potential misunderstandings. This could be\ninterpreted as the authors taking out remineralized carbon, so that the deep ocean has\nless carbon, and then re-equilibrating the model. Alternatively, this could mean that the\nBCP was “shut off”, and then they got a readjustment in a different way with a different\namount of carbon.\nThe analytical relationships of Goodwin et al. (2008) consider removal of remineralized\ncarbon from the ocean reservoir. We have revised the sentence:\n“Using an atmosphere-ocean equilibrium relationship, it is estimated that the\natmosphere CO2 levels would be 150-240 ppm higher when the remineralised carbon is\nremoved [5].”\nto:\nL57-59: “Using an atmosphere-ocean equilibrium relationship, it is estimated that\nwhen the remineralised carbon stock is removed, atmospheric CO2 levels would be\napproximately 150-240 ppm higher in the new equilibrium state [5].”\nLines 43-45: Consider clarifying in particular what aspects of the carbon sink are\nconsidered to be dominated by the solubility pump rather than BCP– for example, its\ncurrent magnitude and variability, or projected changes under climate scenarios?\nWe meant the projected long-term changes, i.e. increase, in carbon sinks. We revised\nthe sentence:\n“Currently the BCP are often considered to operate at steady state, with the physically-\ndriven solubility pump considered to have dominated the current and future carbon\nsink and storage rates [6, 7], …”\nTo:\nL62-64: “Currently the BCP is often considered to operate at steady state, with the\nphysically-driven solubility pump considered to have dominated the projected increase\nin current and future carbon sink and storage rates [6, 7], …”\nLines 45-48: Consider mentioning the critical role of limited observational evidence and\nlack of consensus on current changes in BCP efficiency when discussing gaps in the\nunderstanding of the current and future BCP.\nWe revised the following:\n“… partly because Earth system models project large uncertainties in future primary\nproduction [8] and partly because of a lack of knowledge on the non-linear interactions\nand feedbacks between BCP and other Earth system components.”\nto:\nL64-67: “… due to the limited observational evidence and knowledge on the non-linear\ninteractions and feedbacks between BCP and other Earth system components. As a\nresult, there is currently no consensus on the projected changes in BCP efficiency. ”\nLines 50-53: While the examples of modeling approaches used to study the BCP here\nare relevant, the list appears selective. Consider rephrasing to signal the listed\nexamples are representative rather than exhaustive.\nWe have added “, among others.” at the end of the original sentence (L77-78).\nLines 53-56: The assertion that neglecting feedback processes leads to overestimation\nof the BCP’s impact could benefit from a short explanation. Why does the omission\nconsistently result in overestimation, rather than introducing more general uncertainty?\nWe agree that the net effect does not necessarily have to be an overestimation. We\nhave rephrased the following:\n“Such approaches can considerably overestimate the impact of changing BCP on the\nlong-term ocean carbon uptake [9].”\nto:\nL80-81: “Such approaches can either underestimate or overestimate the impact of\nchanging BCP on the long-term ocean carbon uptake [8].”\nLines 60-61: The term “marine ecosystem” might be too broad. If the study specifically\nshuts down the BCP, consider using that term to align more closely with the experiment\ndesign.\nWe replaced “marine ecosystem” with “BCP” (L85)."}, {"round": 2, "reviewer_comments": {"reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_3": "We believe that the authors have addressed all the issues from the reviews."}, "author_response": ""}]

Decision letter