This session invites observational, numerical modeling and analytical studies that enhance the process understanding of ocean storage of carbon and/or heat under various climate scenarios: the contemporary situation of net-positive CO2 emissions and global warming, as well as future scenarios involving the gradual phasing out of CO2 emissions or a warming overshoot followed by net-negative emissions and global cooling. We also seek studies that explore the similarities and differences between ocean storage of carbon and heat, and how ocean uptake —and potential future release— affect climate.
Solicited author: Roland Séférian
The oceans slow the rate of global warming by absorbing each year about 25% of the anthropogenic CO2 emissions and 90% of the additional heat resulting from the Earth energy imbalance induced by the accumulation of greenhouse gases in the atmosphere. The interplay between the ocean heat and carbon uptake, the “Ocean Heat-Carbon Nexus”, links together the responses of the Earth climate and the global carbon cycle to cumulative CO2 emissions and to net zero CO2 emissions. It results from a suite of processes involving the exchange of heat and carbon across the sea-air interface as well as their storage below the mixed-layer and redistribution by the ocean large-scale circulation. The Ocean Heat and Carbon Nexus is assumed to be consistently represented across two modelling platforms used in the latest IPCC assessments: the Earth System Models (ESMs) and the Simple Climate Models (SCMs). However, our research shows significant deficiencies in state-of-the-art SCMs in replicating the ocean heat-carbon nexus of ESMs due to a crude treatment of the ocean thermal and carbon cycle coupling. With one SCM, we show that a more realistic heat-to-carbon uptake ratio exacerbates the projected warming by 0.1°C in low overshoot scenarios and up to 0.2°C in high overshoot scenarios. It is therefore critical to explore how SCMs' physical inconsistencies, such as the representation of the ocean heat-carbon nexus, can affect future warming projections used in climate assessments, not just by SCMs in Working Group 3 but also by ESMs in Working Group 1 via SCM-driven emission-to-concentration translation.
How to cite: Séférian, R., Bossy, T., Gasser, T., Nichols, Z., Dorheim, K., Su, X., Tsutsui, J., and Yeray Santana-Falcón, Y.: Physical inconsistencies in the representation of the ocean heat-carbon nexus in simple climate models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12898, https://doi.org/10.5194/egusphere-egu25-12898, 2025.
The ocean’s capacity to absorb anthropogenic CO2 is predicted to decrease with global warming, contributing to a positive climate-carbon cycle feedback. However, the precise nature of how climate change will impact the ocean’s various carbon pumps and hence atmospheric CO2 remains poorly constrained, especially on multi-centennial time scales. Here, we show that under a high emission scenario, reduced carbon uptake and redistribution of alkalinity leads to ~505 ppm (30%) higher atmospheric CO2 by 2500. Despite compensating changes in biological storage and air-sea disequilibrium, CO2 is still 16% higher due to climate change. These changes are a net response to slowing circulation and increased stratification, which not only reduces carbon uptake but lengthens by hundreds of years the time anthropogenic and biologically-respired CO2 are sequestered in the ocean, with long term implications for climate.
How to cite: Khatiwala, S., Strachan, O., and Schmittner, A.: Complex response of marine carbon pumps to global warming impacts atmospheric CO2 on multi-centennial time scales, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1977, https://doi.org/10.5194/egusphere-egu25-1977, 2025.
The ocean has absorbed approximately 25% of anthropogenic CO2 emissions since the industrial era, playing a critical role in the global carbon cycle. However, the current ocean carbon sink as simulated by the ocean biogeochemistry models of the Global Carbon Budget shows a spread larger than the European Union’s fossil carbon emissions and mismatches with current observation-based estimates. The prime suspect for this deviation is the poorly constrained transfer of carbon between the surface and the interior ocean. This process is called ventilation and is based on the interior ocean carbon gradients that depend on mixing, advective and biological processes.
To address this, we developed a set of metrics based on the new dataset from biogeochemical Argo floats (BGC-Argo) that offer unprecedented observations from the surface to 2000 m, and the GLODAP bottle data. These metrics are a tool to evaluate and optimize ocean ventilation processes and carbon transport between the surface and the interior in ocean models. They target the stratification and mixing (physical variables) as well as the gradients of tracers such as apparent oxygen utilization, dissolved inorganic carbon or dissolved inorganic nitrate. We compute metrics quantifying these depth gradients averaged across large-scale biomes.
With this methodology, we evaluate the ventilation in the model FESOM-REcoM. Our results identify model-observation differences in terms of absolute values and magnitude of gradient in salinity and in the biogeochemical variables in many biomes. Biases in the gradients of biogeochemical properties can partially be explained by biases in the physical stratification of the water column, especially in biomes with high mixing at higher latitudes. In other biomes, biases are attributed to an imperfect representation of biogeochemical processes in the model. We characterize the distribution of biases in FESOM-REcoM, and discuss how to reduce them.
How to cite: Le Chevere, S., Danek, C., Bushinsky, S., and Hauck, J.: Evaluation of the interior ocean ventilation of biogeochemical tracers in a global ocean model using observation-based metrics , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1593, https://doi.org/10.5194/egusphere-egu25-1593, 2025.
The ocean absorbs a quarter of the anthropogenic carbon and 90% of the anthropogenic heat in the Earth system, significantly impacting the climate. On decadal timescales most relevant for climate prediction, the ocean circulation plays a central role in modulating the ocean heat and carbon sinks. It is therefore crucial to understand how these sinks interact with changes in the circulation. We have applied a novel water mass based inverse model, the optimal transformation method (OTM), to study the uptake of heat and carbon by the ocean and its redistribution in the interior by the ocean circulation. The OTM simultaneously calculates budgets of heat, freshwater, and carbon from a combination of observational data products, solving for the air-sea flux and transport and mixing of these tracers in a manner consistent with the available observational data. We apply OTM to a combination of data products: the EN4 objective analysis of temperature and salinity; the ECCO ocean state estimate; our own machine learning reconstruction of ocean interior carbon based on the GLODAP dataset; ERA5 and JRA55 reanalyses of air-sea heat and freshwater fluxes; and air-sea CO2 fluxes from the SeaFlux product. We analyse two decades, estimating global carbon uptake of 2.02 ± 0.22 PgC yr-1 for 1993-2002 and 2.86 ± 0.25 PgC yr-1 for 2003-2012. We find that changes in the carbon uptake between the two decades are dominated by the Southern Ocean (>35°S) and North Pacific (>10°N) basins, and our results also suggest a southwards redistribution of carbon in the Atlantic linked to changes in ocean circulation. Meanwhile, a redistribution of carbon northwards in the Pacific is accompanied by a southwards redistribution of heat.
How to cite: Mackay, N., Ehmen, T., and Watson, A.: Ocean carbon and heat uptake and redistribution diagnosed from observations using a water mass inverse model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10806, https://doi.org/10.5194/egusphere-egu25-10806, 2025.
Over the past 150 years, the ocean has absorbed almost 90% of the excess heat induced by anthropogenic carbon dioxide (CO2) emissions, acting as our planet's main heat reservoir. Multiple mechanisms contribute to ocean heat uptake (OHU) and global heat storage, which redistribute heat from the surface to the deep ocean and across all basins. Nevertheless, a comprehensive picture remains unclear. Within this context, the Atlantic Meridional Overturning Circulation (AMOC) plays a key role in transferring heat to the ocean's deepest layers, with a stronger AMOC related to an increase in global OHU. However, it is difficult to quantify the importance of the AMOC from the analysis of existing simulations from the Coupled Model Intercomparison Project (CMIP), as many processes are simultaneously active.
In this study, we use the climate model EC-Earth3 to investigate how an AMOC weakening induced by a CO2 increase would influence the heat storage inside the ocean. We compare the CMIP abrupt4xCO2 simulation with an idealized experiment with the same forcing but designed to artificially maintain the AMOC strength at preindustrial levels through a positive salinity anomaly in the North Atlantic.
We find that a change in the AMOC strength is associated with a change in heat storage, influencing both the vertical and interbasin redistribution. Due to AMOC weakening, less heat accumulates below 750 m, especially in the Atlantic Ocean, while we observe increased heat storage in intermediate layers and further heat transfer toward the Indo-Pacific Ocean. Overall, we notice a small but significant difference between the two simulations in global heat uptake, increasing in a weaker AMOC state. We hypothesize that a reduced role for AMOC-driven OHU is compensated for by an increase in heat diffusion towards the interior at low latitudes, according to recently developed conceptual models of OHU.
These differences could influence the surface warming pattern and regional sea level rise, with implications for long-term climate changes.
How to cite: Ventrucci, C., Fabiano, F., Davini, P., Mehling, O., and Bellomo, K.: The role of AMOC in controlling ocean heat uptake in idealized abrupt forcing scenarios, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16169, https://doi.org/10.5194/egusphere-egu25-16169, 2025.
The North Atlantic Ocean contributes approximately 30% of the global ocean carbon uptake. This region plays a vital role in anthropogenic carbon uptake and hosts a significant natural carbon cycle driven by physical and biogeochemical processes. This study focuses on understanding the inter-annual variability of air-sea CO2 fluxes, anthropogenic carbon storage, and the role of the Gulf Stream in transporting water masses with low anthropogenic carbon concentrations into the subpolar North Atlantic. We present the development and application of our forward and adjoint ocean carbon cycle and biogeochemistry models within the Estimating the Circulation and Climate of the Ocean (ECCOv4) framework (ECCOv4r2-Dissolved Inorganic Carbon (DIC)). The ECCOv4r2-DIC simulation overall captures the inter-annual variability and decadal trends of ocean carbon uptake in the subpolar North Atlantic. The adjoint model for ocean biogeochemistry is a powerful tool that enables us to investigate the sensitivity of ocean carbon uptake to physical and biogeochemical factors under dynamic ocean conditions. Preliminary results from the adjoint biogeochemistry sensitivity simulations indicate that subpolar North Atlantic carbon storage is highly sensitive to dissolved inorganic carbon (DIC) in the Gulf Stream region on inter-annual timescales (e.g., lag of -4 years). This finding suggests that remote advective carbon transport significantly influences inter-annual carbon variability in the subpolar North Atlantic Ocean.
How to cite: Takano, Y., Jones, D., Williams, R., Forget, G., Lauderdale, J., Munday, D., and Roussenov, V.: North Atlantic Carbon Uptake and Variability: The Gulf Stream's Role in Air-Sea CO2 Flux and Storage, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17516, https://doi.org/10.5194/egusphere-egu25-17516, 2025.
Subtropical cells, which exist in nearly all ocean basins, connect subducting subtropical waters to upwelling sites along the equator. This tight link between the subtropics and the tropics, on a scale of 5-15 years, is well-established in a time-averaged sense by modeling and observations. Recently, evidence has emerged of spice (density-compensated temperature and salinity variations) and potential vorticity anomaly persistence along mean flow pathways on isopycnals. We provide the first global view of subtropical water mass anomaly propagation, using both an observational dataset and the Estimating the Circulation and Climate of the Ocean (ECCO) state estimate Version 4 Release 4. In this global synthesis that complements the existing body of largely regional studies, we find long-lived interannual water mass anomalies that translate along mean advective pathways in all ventilated subtropical gyres. They are detectable over multiple years and several thousand kilometers. Some anomalies are persistent enough to reach both the western boundary and equatorial current systems before being entirely eroded, and thus could form ocean “tunnels” along which heat anomalies could travel to impact remote climate variability. Analysis of ocean tunnel propagation of a passive tracer (spice) and an active tracer (potential vorticity) confirms earlier model results that the active tracer decays more quickly than the passive tracer. Similarities and differences between timing and frequency of the two tracers could provide clues to anomaly formation mechanisms in various subduction regions. The success of ECCO in capturing these phenomena is encouragement to further explore their upstream sources and downstream impacts within this framework.
How to cite: Hersh, C., Wijffels, S., Gebbie, G., and Forget, G.: The long lives of subducted spice and vorticity anomalies in the subtropical oceans, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11039, https://doi.org/10.5194/egusphere-egu25-11039, 2025.
A gravitational sinking of the particulate organic matter (POM) is a key mechanism of the vertical transport of carbon in the deep ocean and its subsequent sequestration. The size spectrum of these particles is formed in the euphotic layer by the primary production and various mechanisms including food web consumption. The mass of particles, as they descend, decreases under bacterial decomposition and the influence of grazing by filter feeders which depends on the water temperature and oxygen concentration, particle sinking velocity, age of the organic particles, ballasting and other factors. In this study, we consider the influence of the size and age of particles, temperature and oxygen concentration on their dynamics and degradation processes. The model takes into account feedback between the degradation rate and sinking velocity of particles. We rely on the known parameterisations, but our Eulerian-Lagrangian approach to analytically and numerically solving the problem differs, allowing the model to be incorporated into biogeochemical global ocean models with relative ease. Two novel analytical solutions of the system of the one-dimensional Eulerian equation for POM concentration and Lagrangian equations for particle mass and position were obtained for constant and age-dependent degradation rates. At a constant rate of particle sinking, they correspond to exponential and power-law profiles of the POM concentration. It was found that feedback between degradation rate and sinking velocity significantly changes POM concentration and POM flux vertical profiles. The calculations are compared with the available POM concentration and flux measurement data for the latitude band of 20-30oN in the Atlantic and Pacific Oceans and 50-60o in the Southern Ocean. The dependence of the degradation rate on temperature significantly affected the profiles of POM concentration enhancing the degradation of sinking particles in the upper layers of the oceans and suppressing it in the deep layers of the oceans. The influence of oxygen concentration in all cases considered was insignificant compared to the temperature distribution with depth.
How to cite: Seo, S., Maderich, V., Brovchenko, I., Kovalets, K., and Kwon, K.: Simple Eulerian-Lagrangian approach to solve equations for sinking particulate organic matter in the ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2675, https://doi.org/10.5194/egusphere-egu25-2675, 2025.
Upper ocean stirring and mixing strongly affect the nutrient flux into the euphotic zone and therefore ocean primary production. Additionally, besides particle sinking, the export of organic and inorganic matter is hugely determined by advective fluxes imposed by physical flows. Since both production and export play a role in oceanic carbon storage, it is important to re-assess its main drivers in models of increased ocean realism. With spatial dimensions below 25 km, sharp fronts, filaments, strong jets and small eddies, submesoscale motions induce large vertical velocities, adding extra transport to the already large lateral stirring induced by the mesoscale (25 km-200 km) field. The impact of resolved submesoscale flows on some aspects of the south Atlantic Ocean carbon cycle is here studied based on a novel global ocean-biogeochemical simulation integrated with the models ICON-O and HAMOCC using a telescoping grid with a resolution refined to approximately 600 m in the south Atlantic. Tracer budgets are used to quantify the relative importance of physical versus biogeochemical processes in the evolution of ocean carbon, including the uptake at the surface and the export to the deep ocean. A comparison between our submesoscale-resolved ocean and biogeochemical simulations with coarser resolutions (10 km and 40 km) sheds some light on the submesoscale role on tracer evolution and highlights expected differences between current climate and mesoscale models and models including the submesoscale. Despite being limited by the short duration of our simulation, this study suggests that submesoscales shape vertical profiles of carbon and nutrients and thereby affect export fluxes and seasonal dynamics.
How to cite: Serra, N. and Ilyina, T.: Impact of submesoscale flows on primary production and export fluxes of carbon in the South Atlantic Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4128, https://doi.org/10.5194/egusphere-egu25-4128, 2025.
The ocean is storing the majority of excess heat in the Earth system resulting from the release of anthropogenic greenhouse gases into the atmosphere. This heat uptake will persist even after cessation of greenhouse gas emissions, and it will continue for centuries in scenarios where global warming is limited to levels set by the Paris Agreement. This continued heat uptake has important implications for regional climate, ecosystems and sea level rise. However, the dynamics of ocean heat uptake and the redistribution of this heat under stabilized global warming remain poorly understood, particularly the time scales involved. Here, we apply the Adaptive Emission Reduction Approach to a fully coupled Earth System Model to simulate different levels of stabilized global warming until the year 3000. We reveal significant differences between the transient phase when surface temperatures first reach the targeted warming level, and the near-stabilized state after close to 1000 years at the warming level. We explore non-linearities in the evolution of the ocean circulation and ventilation over these time scales, as well as the sensitivity of heat uptake and storage to different global warming levels. For example, the stabilization simulations reveal long-term differences across global warming levels in the vertical redistribution of heat, with a relatively warmer upper ocean and colder deep ocean with warmer surface temperatures. We also find a threshold effect between 1.5ºC and 2ºC of global warming, where surpassing this threshold triggers irreversible changes that profoundly impact the redistribution of heat in the ocean. Specifically, during the stabilization phase at 2ºC of global warming and above, the subpolar Southern Ocean shows a recovery of deep convection that leads to an export of colder bottom waters than under pre-industrial conditions, that is not present at 1.5ºC.
How to cite: Silvy, Y., Burger, F. A., and Frölicher, T. L.: Ocean heat uptake and storage during climate stabilization at different global warming levels in GFDL-ESM2M, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8581, https://doi.org/10.5194/egusphere-egu25-8581, 2025.
The biological carbon pump (BCP) plays a key role in the regulation of atmospheric CO2 levels. Export efficiency (Expeff), defined as the proportion of primary production (PP) that is exported as particulate organic carbon (POC) flux below the base of the euphotic zone (EZ), and transfer efficiency (Teff), defined as the ratio of POC flux below the EZ and POC flux attenuated at a given depth in the twilight zone (TZ), are two of the main parameters used as metrics of BCP strength. The objective of this work is to investigate the factors that influence the variability of both parameters at different bloom stages. The APERO cruises aim to investigate the BCP, with emphasis on the TZ (200-1000 m), and were conducted at the PAP site oceanographic observatory during the decline of a spring bloom in June and July 2023. Water and particle profiles (0-1000 m) were collected at five stations and POC fluxes at the base of the EZ were obtained derived from 210Po-210Pb disequilibrium and high depth resolution sediment traps. In addition, data measured from 1989 to 2023 for POC fluxes, at the base of the EZ, derived from 238U-234Th and 210Pb-210Po disequilibrium and at 3000 m depth, derived from moored sediment traps were compiled. Expeff (FluxEz/satellite NPP time-integrated) and Teff (Flux3000m/FluxEz) were quantified, and both values were compared across different years and bloom stages. POC fluxes measured in APERO ranged from 3.1-17 mmol C m⁻² d⁻¹, which agrees well with the value measured in 2021 during the same bloom stage, 13 ± 3 mmol C m⁻² d⁻¹. Expeff presents significant fluctuations and shows a strong intra-annual variability. It changed during the bloom development, from 5-20% in 1989 to 41% in 2012, from 16-42% in 1989 to 14% in 2021 during the bloom peak, and during the decline of the bloom, it decreased from 26 ± 4 % in 2021 to 2.2-11% during the APERO cruise. Finally, Teff exhibits a strong intra-annual variability as the bloom progresses, changing from 6–23% at the beginning of the bloom in 1989, 16% in 2004, and 12% in 2012, to 5–14% at the peak in 1989, and 16% during the postbloom in 2009.
How to cite: López Rodríguez, Á., González González, B., Hurtado Bermúdez, S., Le Moigne, F., Gesson, M., and Villa Alfageme, M.: Analysis of Export and Transfer Efficiency around the PAP-Site Observatory: an update from the APERO project, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13345, https://doi.org/10.5194/egusphere-egu25-13345, 2025.
The efficiency of the ocean to store atmospheric CO2 in the coming century strongly depends on the stability of marine carbon reservoirs. Marine dissolved organic carbon (DOC) contains more carbon than all living biomass on Earth combined (∼660 gigatons C) and is recalcitrant against remineralisation at a decadal to millennial timescale, which offers an additional carbon pump to sequester carbon from active air-sea gas exchange with a millennial-scale stability (microbial carbon pump). However, the fate of this key carbon reservoir in a changing future climate is unknown, because the impact of environmental controls on bacterial remineralisation of DOC to CO2 are not explicitly considered in global Earth System Models.
We developed a dynamical model for dissolved organic matter (DOM) that explicitly depicts the production of DOM through primary production and its degradation by heterotrophic microorganisms, and coupled it interactively to the marine biogeochemistry module of UVic ESCM, an Earth system model of intermediate complexity (EMIC). Being based on present-day simulations with the model, it is revealed that the factor that limits bacterial growth in the model and meta-genomic data indicating bacterial nutrient limitation show a similar pattern in the global ocean. Together with other experimental data, we suggest a strong link between the future developments of DOC and macronutrient cycles.
Our model indicates that an increase in the global DOC pool under global warming ranges from 17 to 42 gigatons C at the end of the 22nd century in a future simulation based on a high-emission scenario (SSP5–8.5). The estimated accumulation rate (2 GtC dec−1) is comparable to the amount of the terrestrial input of DOC to the ocean by rivers, underlining its quantitative relevance for the global DOC budget. Our results suggest that DOM-microbe interactions governed by bacterial nutrient limitation provide negative feedback on the climate state via DOC buildup, reinforcing the growth of DIC sequestration by the conventional biological pump (6 GtC dec−1 for > 1000 m depth) in the same simulation.
How to cite: Kurahashi-Nakamura, T., Dittmar, T., Martiny, A. C., and Lennartz, S. T.: Enhanced storage of carbon in marine dissolved organic matter in scenarios of global warming, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16067, https://doi.org/10.5194/egusphere-egu25-16067, 2025.
Particles sinking on the ocean constitute the vehicles of the Biological Carbon Pump (BCP). As these particles descend, they give form to a complex mixture of biogeochemical materials, each characterised by distinct size, density, porosity, and morphology. Consequently, the velocity of particle sinking (SV) and the flux of particulate organic carbon (POC) exhibit significant variability, influenced by factors such as depth, season, and the characteristics of the ecosystem. The flux of POC and the SV are interconnected parameters; besides, the profile of POC flux attenuation, i.e. the rate at which sinking particles are remineralised and degraded by bacteria and zooplankton, is also strongly dependent on the rate at which the particles sink. Intuitively, faster sinking particles would reach the Twilight Zone in a greater proportion than slow sinking particles; however, this simple correlation is not globally observed in the ocean. Overall, SV is a key variable directly impacting on the strength of the BCP, in spite of that, the methods to estimate particle SV are not standardized and this variable remains poorly measured in the ocean. Therefore, its influence is not properly quantified, nor is how to incorporate this parameter to ocean biogeochemical models.
The utilisation of the disequilibrium between radioactive pairs, 234Th-238U and 210Po-210Pb, allows obtaining both average SV and downward POC flux. In this study, disequilibrium profiles from 15 cruises in the Atlantic and Southern Oceans were examined (including data from COMICS, CUSTARD, APERO and EXPORTS programs), encompassing biogeochemically contrasting sites and various stages of the bloom. This analysis led to a novel compilation of POC flux and SV, coupled with satellite-driven net primary production (NPP) and including export efficiency and transfer efficiency, when available. The objective of this synthesis is to understand the mechanisms associated with the spatial and temporal variation of the SV and to look for patterns in the Biological Carbon Pump efficiency and, ultimately, ocean carbon storage.
How to cite: Villa-Alfageme, M., Melgar, L., López-Rodríguez, Á., Abascal-Ruíz, U., and González-González, B.: Parameterization of sinking velocity rates in the Atlantic Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5016, https://doi.org/10.5194/egusphere-egu25-5016, 2025.
Compelling evidence indicates that ocean circulation is undergoing significant changes due to global warming. These changes include reduced ocean ventilation caused by increased stratification and the weakening of the Atlantic Meridional Overturning Circulation (AMOC). Consequently, this will alter carbon, oxygen, heat and nutrient distribution, and will therefore affect primary production and, by extension, the biological carbon pump. Due to the ocean’s huge capacity for carbon storage, it is imperative that we understand the consequences of these changes.
To examine how ocean ventilation influences the biological carbon pump and overall oceanic carbon storage, two idealised box models of ocean carbon and heat uptake are extended to include biological processes and nutrient cycling. The first model is a one-dimensional box model, with ocean ventilation parameterised by a relaxation timescale that responds to emission-driven warming. The second model is more complex, including a thermocline with a dynamically controlled thickness and meridional overturning circulation, both of which vary with increasing temperatures, determining the extent of ocean ventilation.
These models, previously employed to analyse the ocean’s carbon and thermal response to anthropogenic emissions, are now adapted to explore the effects of changing circulation on the biological carbon pump. A simple nutrient-phytoplankton-zooplankton-detritus (NPZD) biological model is introduced to simulate the role of macronutrient concentrations on phytoplankton and zooplankton growth. Simulations are conducted under scenarios of both constant and changing circulation to investigate the impacts of slower circulation and increased stratification on the biological carbon pump and its contribution to oceanic carbon storage.
How to cite: Baltas, E., Katavouta, A., and Hunt, H.: Exploring the Impact of Changing Ocean Circulation on Carbon Storage due to the Biological Carbon Pump: An Idealised Modelling Approach, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9243, https://doi.org/10.5194/egusphere-egu25-9243, 2025.
The oceans mitigate climate change by absorbing roughly 25% of the anthropogenic carbon that is released. Past reconstructions of air-sea CO2 flux based on surface pCO2 observations have indicated that this carbon sink exhibits decadal variability, appearing to weaken during the 1990s and strengthen in the 2000s. However, the causes of this variability are unclear, and it is poorly represented in climate models and the future climate projections they generate. It also remains uncertain whether the estimated variability is a product of bias due to the limited availability of biogeochemical observations. To address the challenge posed by sparse data, machine learning techniques have been applied to surface pCO2 as well as interior dissolved inorganic carbon (DIC). However, reconstructions of DIC and anthropogenic carbon for the full depth of the global ocean have not yet been achieved.
Our objective is to determine whether the variability in the ocean carbon sink is real and to understand changes in the interior carbon inventory as part of the carbon budget. To this end, we use neural networks to predict the spatiotemporal distributions of full-depth DIC and C* from the 1990s to the 2010s. C* is a quasi-conservative tracer that corrects DIC for biological activity by applying Redfield stoichiometric ratios. ΔC*, the difference in C* between two time points, has been used as a proxy for added anthropogenic carbon.
The neural network is trained on observations from the GLODAPv2.2023 database. We make predictions of DIC and additional C* components - total alkalinity, oxygen, and nitrate - based on the location, depth, temperature, and salinity from the EN4 reanalysis product and atmospheric CO2. Here, we present findings on the spatiotemporal evolution of full-depth interior carbon in the global ocean, providing a quantification of the anthropogenic carbon sink and its variability over time. The interior carbon inventory changes are then compared with current air-sea CO2 flux products. In further work, the results are being combined with a water mass based inverse method to investigate the drivers of variability.
How to cite: Ehmen, T., Mackay, N., and Watson, A.: The spatiotemporal evolution of the global interior ocean’s anthropogenic carbon sink: reconstructed through machine learning, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14150, https://doi.org/10.5194/egusphere-egu25-14150, 2025.
In the event of insufficient mitigation efforts, net-negative CO2 emissions may be required to return to acceptable limits of climate warming as defined by the Paris Agreement. The ocean is an important carbon sink under increasing atmospheric CO2 levels,when physico-chemical carbon-uptake dominates. However, the processes that govern the marine carbon sink under net-negative CO2-emission regimes are unclear. Recent work with an Earth System model of intermediate complexity has shown that under idealized temperature overshoot scenarios CO2 from physical-chemical uptake was partly lost from the ocean at times of net-negative CO2-emissions, while storage associated with the biological carbon pump (DICremin) continued to increase and may even dominate marine excess CO2 storage on multi-centennial time scales (Koeve et al. 2014, Nature Geosciences, https://doi.org/10.1038/s41561-024-01541-y).
Here we extend this work and estimate, for the first time, the cooling potential associated with CO2-storage attributable to the biological carbon pump on centennial time scales, with a focus of conditions of net-negative CO2-emissions. In our approach we use the UVic Earth System model, complemented with explicit model tracers of DICremin and preformed DIC. Changes of these tracers since preindustrial conditions can be traced to either the biological carbon pump or the physical-chemical uptake of anthropogenic CO2, respectively. We quantify the cooling potential of biological pump carbon based on emission pathways perturbed by the change in DICremin since the preindustrial model state. The warming potential of anthropogenic carbon lost from the ocean during times of negative emissions is quantified from emission pathways perturbed by changes of preformed DIC since preindustrial.
How to cite: Koeve, W. and Frenger, I.: The cooling potential of biological pump carbon after temperature overshoot, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21738, https://doi.org/10.5194/egusphere-egu25-21738, 2025.
