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Model analysis of ocean carbon storage and transport across climate states
Stockholm University, Faculty of Science, Department of Meteorology .ORCID iD: 0000-0003-4855-7767
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The ocean carbon cycle plays a fundamental role in the Earth’s climate system, on decadal to multi-millennial timescales. Of the carbon held in the ocean, the atmosphere, and the terrestrial biosphere combined, more than 90% resides in the ocean. Carbon enters the surface ocean through air-sea gas exchange and from terrestrial sources. It is transported to the deep ocean with the ocean circulation and through the so-called biological pump, where carbon is taken up in the surface ocean by photosynthetic organisms that fall down and decompose at depth. This thesis contributes to the understanding of the processes involved in ocean carbon storage and transport. It examines how these processes respond to model perturbations, and how this response influences our attempts to simulate glacial-interglacial fluctuations in atmospheric carbon dioxide (CO2).

The thesis investigates the response of the simulated ocean carbon storage, and distribution of the isotopic tracer δ13C, to changes in physical and biological parameters. In the included studies, we use observational as well as proxy records of oceanic properties to evaluate our model simulations. In addition, we use a climate model to interpret proxy evidence of glacial-interglacial changes in ocean δ13C. By using a separation framework, we identify the origin of the carbon in the model ocean, and attribute observed changes to the processes involved.

The results indicate a strong link between ocean carbon storage and the strength of the global ocean overturning circulation. Stronger circulation leads to less carbon storage through a weakening of the biological pump, and through reduced solubility due to an increase in global ocean average temperature.

In simulations of glacial climate, we find that biological adaptability to the surrounding nutrient conditions, through a flexible carbon-to-phosphorus ratio (C/P) in ocean photosynthesis, increases the ocean carbon storage compared to simulations where fixed C/P is applied. The biological flexibility improves the model’s ability to reproduce glacial atmospheric CO2. In line with previous research, we find freshwater input to the North Atlantic to be an important factor for reproducing glacial proxy records. The ensemble of simulations that achieve a good representation of glacial-interglacial δ13C indicates a deglacial whole-ocean change in δ13C of 0.28 ± 0.06‰.

The thesis underlines the importance of the initial state, and the choice of model parameterisations, for the outcome of model ensemble, and intercomparison studies. Finally, it proposes a new method for estimation of ocean carbon transport, and attribution of this transport to different water masses and carbon system processes.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2019. , p. 42
Keywords [en]
Oceanography, Climate, Climate model, Carbon cycle, Paleoclimate
National Category
Climate Research Oceanography, Hydrology and Water Resources Geosciences, Multidisciplinary
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-172894ISBN: 978-91-7797-829-9 (print)ISBN: 978-91-7797-830-5 (electronic)OAI: oai:DiVA.org:su-172894DiVA, id: diva2:1350641
Public defence
2019-10-25, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2019-10-02 Created: 2019-09-11 Last updated: 2019-09-24Bibliographically approved
List of papers
1. The influence of the ocean circulation state on ocean carbon storage and CO2 drawdown potential in an Earth system model
Open this publication in new window or tab >>The influence of the ocean circulation state on ocean carbon storage and CO2 drawdown potential in an Earth system model
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2018 (English)In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 15, no 5, p. 1367-1393Article in journal (Refereed) Published
Abstract [en]

During the four most recent glacial cycles, atmospheric CO2 during glacial maxima has been lowered by about 90-100 ppm with respect to interglacials. There is widespread consensus that most of this carbon was partitioned in the ocean. It is, however, still debated which processes were dominant in achieving this increased carbon storage. In this paper, we use an Earth system model of intermediate complexity to explore the sensitivity of ocean carbon storage to ocean circulation state. We carry out a set of simulations in which we run the model to pre-industrial equilibrium, but in which we achieve different states of ocean circulation by changing forcing parameters such as wind stress, ocean diffusivity and atmospheric heat diffusivity. As a consequence, the ensemble members also have different ocean carbon reservoirs, global ocean average temperatures, biological pump efficiencies and conditions for air-sea CO2 disequilibrium. We analyse changes in total ocean carbon storage and separate it into contributions by the solubility pump, the biological pump and the CO2 disequilibrium component. We also relate these contributions to differences in the strength of the ocean overturning circulation. Depending on which ocean forcing parameter is tuned, the origin of the change in carbon storage is different. When wind stress or ocean diapycnal diffusivity is changed, the response of the biological pump gives the most important effect on ocean carbon storage, whereas when atmospheric heat diffusivity or ocean isopycnal diffusivity is changed, the solubility pump and the disequilibrium component are also important and sometimes dominant. Despite this complexity, we obtain a negative linear relationship between total ocean carbon and the combined strength of the northern and southern overturning cells. This relationship is robust to different reservoirs dominating the response to different forcing mechanisms. Finally, we conduct a drawdown experiment in which we investigate the capacity for increased carbon storage by artificially maximising the efficiency of the biological pump in our ensemble members. We conclude that different initial states for an ocean model result in different capacities for ocean carbon storage due to differences in the ocean circulation state and the origin of the carbon in the initial ocean carbon reservoir. This could explain why it is difficult to achieve comparable responses of the ocean carbon pumps in model intercomparison studies in which the initial states vary between models. We show that this effect of the initial state is quantifiable. The drawdown experiment highlights the importance of the strength of the biological pump in the control state for model studies of increased biological efficiency.

National Category
Biological Sciences Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-154780 (URN)10.5194/bg-15-1367-2018 (DOI)000426909200001 ()
Available from: 2018-04-18 Created: 2018-04-18 Last updated: 2019-09-11Bibliographically approved
2. Variable C/P composition of organic production and its effect on ocean carbon storage in glacial model simulations
Open this publication in new window or tab >>Variable C/P composition of organic production and its effect on ocean carbon storage in glacial model simulations
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2019 (English)In: Biogeosciences Discussions, ISSN 1810-6277, E-ISSN 1810-6285Article in journal (Other academic) Epub ahead of print
Abstract [en]

During the four most recent glacial maxima, atmospheric CO2 has been lowered by about 90--100 ppm with respect to interglacial concentrations. It is likely that most of the atmospheric CO2 deficit was stored in the ocean. Changes of the biological pump, which are related to the efficiency of the biological carbon uptake in the surface ocean and/or of the export of organic carbon to the deep ocean, have been proposed as a key mechanism for the increased glacial oceanic CO2 storage. The biological pump is strongly constrained by the amount of available surface nutrients. In models, it is generally assumed that the ratio between elemental nutrients, e.g. phosphorus, and carbon (C/P ratio) in organic material is fixed according to the classical Redfield ratio. The constant Redfield ratio appears to hold approximately when averaged over basin scales, but observations document highly variable C/P ratios on regional scales and between species. If the C/P ratio decreases when nutrient availability is scarce, as observations suggest, this has the potential to further increase glacial oceanic CO2 storage in response to changes in surface nutrient distributions. In the present study, we perform a sensitivity study to test how a phosphate--concentration dependent C/P ratio influences the oceanic CO2 storage in an Earth system model of intermediate complexity (cGENIE). We carry out simulations of glacial--like changes in albedo, radiative forcing, wind--forced circulation, remineralisation depth of organic matter, and mineral dust deposition. Specifically, we compare model versions with with the classical constant Redfield ratio and an observationally-motivated variable C/P ratio, in which the carbon uptake increases with decreasing phosphate concentration. While a flexible C/P ratio does not impact the model's ability to simulate benthic d13C patterns seen in observational data, our results indicate that, in production of organic matter, flexible C/P can further increase the oceanic storage of CO2 in glacial model simulations. Past and future changes in the C/P ratio thus have implications for correctly projecting changes in oceanic carbon storage in glacial-to-interglacial transitions as well as in the present context of increasing atmospheric CO2 concentrations.

Keywords
Ocean biogeochemistry, Glacial climate
National Category
Climate Research
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-172892 (URN)10.5194/bg-2019-149 (DOI)
Conference
Joint IAPSO-IAMAS-IAGA Assembly 2017, Cape Town, South Africa, Au­gust 27 - Septem­ber 1, 2017; Ocean Sciences Meeting 2018, Portland, OR, USA, February 11 - 16, 2018
Note

Discussion paper under review for publication in Biogeosciences.

Available from: 2019-09-11 Created: 2019-09-11 Last updated: 2019-09-11
3. A revised Earth system model--based analysis of glacial--interglacial changes in ocean δ13C
Open this publication in new window or tab >>A revised Earth system model--based analysis of glacial--interglacial changes in ocean δ13C
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Across the latest deglaciation (from 21 to 0 ka), rearrangements in ocean circulation and carbon reservoirs occurred as climate changed. In the glacial state, atmospheric and terrestrial reservoirs of carbon were smaller, while carbon was stored in the deep ocean. The glacial-interglacial changes in atmospheric CO2 and deep ocean carbon storage are reflected by changes in the intermediate-to-deep ocean vertical δ13C gradient recorded in benthic foraminifera. However, sparse data coverage makes it difficult to infer ocean changes directly from the proxy records. In model studies, such records are often used to assess the validity of model simulations. In this study, we instead use a numerical model to interpolate and extrapolate the available benthic δ13C proxy records of Peterson et al. (2014) for the Holocene (HOL, 0-6 ka) and the Last Glacial Maximum (LGM, 19-23 ka). We apply appropriate boundary conditions for each time slice, and search for the best-possible fit to the proxy records by running ensembles, where we vary the wind stress scaling parameter, the amount of Atlantic-to-Pacific freshwater redistribution, and the fraction of brine relocated from the surface to the deep ocean. For both HOL and LGM, we find that the best fits are acheived when we apply a wind stress scaling of 0.8 and we apply a brine rejection relocation of 20%. However, the best fit for the LGM is found for weak freshwater redistribution, while a stronger redistribution is optimal for HOL. The best-fit simulations reproduce well the shift from a stronger to a weaker surface-to-deep ocean δ13C gradient across the deglaciation, as indicated by the proxy records. The differences in boundary conditions combined with the difference in freshwater redistribution result in a 50% weaker Atlantic Meridional Overturning Circulation (AMOC) at the LGM compared to HOL, while the Pacific oxygen minimum zone is found in the deep (LGM), rather than the intermediate (HOL), ocean. After using model-data data misfit to remove bias from the best-fit simulations, we find that the LGM is more depleted in δ13C compared to HOL, with a deglacial change in whole-ocean δ13C of 0.30‰. For both time slices, good fits to the proxy data are also achieved for other combinations of brine rejection relocation and freshwater redistribution. We therefore compute a bias-corrected ensemble average for the deglacial whole-ocean change in δ13C, to account for uncertainty in the analysis. After weighting the ensemble members by their skill in reproducing the proxy records, we estimate the deglacial whole-ocean change in δ13C (HOL-LGM) to 0.28 ± 0.06. This corresponds to 430 ± 90 Pg C transferred between the terrestrial carbon reservoir and the ocean. This should not be interpreted as an estimate of the overall change in terrestrial carbon storage during the glacial cycle, but as an estimate of the change in carbon with a terrestrial δ13C signature that could be accomodated in the LGM ocean.

Keywords
Model study, Glacial ocean, Glacial climate
National Category
Climate Research
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-172893 (URN)
Note

Manuscript in preparation for Climate of the Past.

Available from: 2019-09-11 Created: 2019-09-11 Last updated: 2019-09-16Bibliographically approved
4. Meridional Ocean Carbon Transport
Open this publication in new window or tab >>Meridional Ocean Carbon Transport
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The ocean's ability to take up and store CO$_{2}$ is a key factor for understanding past and future climate variability. However, qualitative and quantitative understanding of surface-to-interior pathways, and how the ocean circulation affects the CO$_2$ uptake, is limited. Consequently, how changes in ocean circulation may influence carbon uptake and storage and therefore the future climate remains ambiguous.Here we quantify the roles played by ocean circulation and various water masses in the meridional redistribution of carbon.We do so by calculating stream functions defined in Dissolved Inorganic Carbon (DIC) and latitude coordinates, using output from a coupled biogeochemical-physical model. By further separating DIC into components originating from the solubility pump and a residual including the biological pump, air-sea disequilibrium and anthropogenic CO$_2$, we are able to distinguish the dominant pathways of how carbon enters particular water masses.With this new tool, we show that the largest meridional carbon transport occurs in a pole-to-equator transport in the subtropical gyres in the upper ocean. We are able to show that this pole-to-equator DIC transport, and the Atlantic Meridional Overturning Circulation (AMOC) related DIC transport, are mainly driven by the solubility pump. By contrast, the DIC transport associated with deep circulation, including that in Antarctic Bottom Water and Pacific Deep Water, is mostly driven by the biological pump. As these two pumps, as well as ocean circulation, are widely expected to be impacted by anthropogenic changes, these findings have implications for the future role of the ocean as a climate-buffering carbon reservoir.

National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-172840 (URN)
Available from: 2019-09-10 Created: 2019-09-10 Last updated: 2019-09-11Bibliographically approved

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