Dominance of the Southern Ocean in Anthropogenic Carbon and Heat Uptake in CMIP5 Models

Anthropogenic climate change will drive extensive mass loss across both the Antarctic (AIS) and Greenland Ice Sheets (GrIS), with the potential for global climate system feedbacks, especially in polar regions. Historically, the high-latitude North Atlantic and Southern Ocean have been critical regions for anthropogenic heat and carbon uptake, but our understanding of how this uptake will be altered by future freshwater discharge is incomplete. We assess each ice sheet's impact on global ocean anthropogenic heat and carbon storage for a high-emission scenario over the -century using a coupled Earth system model. We explore the impact of contemporaneous mass loss from both ice sheets on anthropogenic heat and carbon storage and quantify their linear and nonlinear contributions. Notably, added freshwater reduces ocean heat and carbon storage by 2,100, and the sum of individual freshwater effects differ from those induced by simultaneous freshwater discharge from both ice sheets. Combined AIS and GrIS freshwater engenders distinct anthropogenic storage anomalies-particularly in the high-latitude Southern Ocean and North Atlantic. From 2080 to 2100, GrIS freshwater exerts primary control on the temporal evolution of global ocean heat storage, while global ocean carbon storage is modulated by the linear AIS and GrIS freshwater impacts. Nonlinear impacts of simultaneous ice sheet discharge have a non-negligible contribution to the evolution of global ocean heat storage. Further, anthropogenic heat changes are realized more quickly in response to ice sheet discharge than anthropogenic carbon. Our results highlight the need to incorporate both ice sheets actively in climate models to accurately project future global climate.

The Southern Ocean is a major sink of anthropogenic carbon and excess heat. In this region, the Earth system model projections of these sinks provided by the CMIP5 and CMIP6 scenario experiments show a large model spread. This contributes significantly to the large uncertainties in the overall climate sensitivity and remaining carbon budgets for ambitious climate targets. Hence, a reduction in the uncertainty of the future Southern Ocean carbon and heat sinks is urgently needed.Globally, Bronselaer and Zanna (2020) identified an emergent coupling between anthropogenic carbon and excess heat uptake, highlighting that the passive-tracer behavior of these two quantities is dominant under high-emissions scenarios. This coupling indicates that the use of a single observational constraint might be sufficient to reduce projection uncertainties in both anthropogenic carbon and excess heat uptake. Here, we use this approach for the northern limb of the Southern Ocean (30°S-55°S) where the subduction of intermediate and mode water is known to drive carbon and heat uptake. We found that, in this region, the variations in the models’ contemporary water-column stability over the first 2000 m is highly correlated to both their future anthropogenic carbon uptake and excess heat uptake efficiency. Using observational data of water-column stability, we reduce the uncertainty of future estimates of (1) the cumulative anthropogenic carbon uptake by up to 53% and (2) the excess heat uptake efficiency by 28%. Independent studies have found similar constraints in the Southern Ocean and globally, strengthening our findings (Liu et al., 2023; Newsom et al., 2023; Terhaar et al., 2021, 2022), and pinpointing that a better representation of water-column stratification in Earth system models is essential to improve future anthropogenic climate change projections.Bourgeois, T., Goris, N., Schwinger, J., and Tjiputra, J. F.: Stratification constrains future heat and carbon uptake in the Southern Ocean between 30°S and 55°S, Nat Commun, 13, 340, https://doi.org/10.1038/s41467-022-27979-5, 2022.Bronselaer, B. and Zanna, L.: Heat and carbon coupling reveals ocean warming due to circulation changes, Nature, 584, 227–233, https://doi.org/10.1038/s41586-020-2573-5, 2020.Liu, M., Soden, B. J., Vecchi, G. A., and Wang, C.: The Spread of Ocean Heat Uptake Efficiency Traced to Ocean Salinity, Geophys. Res. Lett., 50, e2022GL100171, https://doi.org/10.1029/2022GL100171, 2023.Newsom, E., Zanna, L., and Gregory, J.: Background Pycnocline Depth Constrains Future Ocean Heat Uptake Efficiency, Geophys. Res. Lett., 50, e2023GL105673, https://doi.org/10.1029/2023GL105673, 2023.Terhaar, J., Frölicher, T. L., and Joos, F.: Southern Ocean anthropogenic carbon sink constrained by sea surface salinity, Sci. Adv., 7, eabd5964, https://doi.org/10.1126/sciadv.abd5964, 2021.Terhaar, J., Frölicher, T. L., and Joos, F.: Observation-constrained estimates of the global ocean carbon sink from Earth system models, Biogeosciences, 19, 4431–4457, https://doi.org/10.5194/bg-19-4431-2022, 2022.

<p>As the climate warms due to greenhouse gas emissions, the ocean absorbs excess heat and carbon. The patterns of ocean excess heat and carbon storage appear tightly linked when the large-scale circulation is fixed. This unique link is not shared with any other ocean tracer, such as <span>Chlorofluorocarbons</span> (CFCs). At the same time, ocean excess carbon storage patterns are mostly unchanged whether the large-scale circulation is free to evolve, or fixed to the pre-industrial circulation pattern, as the climate warms. Here, we interpret the reason for this behavior by breaking ocean carbon storage into two parts: uptake of atmospheric anomalies by the surface ocean, and subsequent internal storage by the ocean’s circulation. We show that the patterns of surface ocean carbon anomalies are dictated by mean state biogeochemical properties and therefore mostly unchanged by circulation changes. Furthermore, surface biogeochemical properties are strongly shaped by the ocean temperature, providing a link between ocean heat and carbon uptake. CFCs on the hand, lack chemical buffering and therefore the patterns of CFC storage do not correlate with heat as much as carbon patterns do. The patterns of surface anomalies ultimately explain most of the differences in how temperature, carbon and CFCs are stored by the ocean, while changes in internal pathways are of secondary importance. Furthermore, the ratio of total ocean carbon and heat storage is roughly constant across warming scenarios and climate models, which might have further implications for relating ocean carbon storage to important climate metrics, such as the transient response to cumulative emissions.</p>

Abstract. The ocean response to carbon emissions involves the combined effect of an increase in atmospheric CO2, acting to enhance the ocean carbon storage, and climate change, acting to decrease the ocean carbon storage. This ocean response can be characterised in terms of a carbon–concentration feedback and a carbon–climate feedback. The contribution from different ocean basins to these feedbacks on centennial timescales is explored using diagnostics of ocean carbonate chemistry, physical ventilation and biological processes in 11 CMIP6 Earth system models. To gain mechanistic insight, the dependence of these feedbacks on the Atlantic Meridional Overturning Circulation (AMOC) is also investigated in an idealised climate model and the CMIP6 models. For the carbon–concentration feedback, the Atlantic, Pacific and Southern oceans provide comparable contributions when estimated in terms of the volume-integrated carbon storage. This large contribution from the Atlantic Ocean relative to its size is due to strong local physical ventilation and an influx of carbon transported from the Southern Ocean. The Southern Ocean has large anthropogenic carbon uptake from the atmosphere, but its contribution to the carbon storage is relatively small due to large carbon transport to the other basins. For the carbon–climate feedback estimated in terms of carbon storage, the Atlantic and Arctic oceans provide the largest contributions relative to their size. In the Atlantic, this large contribution is primarily due to climate change acting to reduce the physical ventilation. In the Arctic, this large contribution is associated with a large warming per unit volume. The Southern Ocean provides a relatively small contribution to the carbon–climate feedback, due to competition between the climate effects of a decrease in solubility and physical ventilation and an increase in accumulation of regenerated carbon. The more poorly ventilated Indo-Pacific Ocean provides a small contribution to the carbon cycle feedbacks relative to its size. In the Atlantic Ocean, the carbon cycle feedbacks strongly depend on the AMOC strength and its weakening with warming. In the Arctic, there is a moderate correlation between the AMOC weakening and the carbon–climate feedback that is related to changes in carbonate chemistry. In the Pacific, Indian and Southern oceans, there is no clear correlation between the AMOC and the carbon cycle feedbacks, suggesting that other processes control the ocean ventilation and carbon storage there.

<p>The Southern Ocean south of 30°S, occupying about a third of global surface ocean area, accounts for approximately 40% of the past anthropogenic carbon uptake and about 75% of excess heat uptake by the ocean. However, Earth system models have large difficulties in reproducing the Southern Ocean circulation, and therefore its historical and future anthropogenic carbon and excess heat uptake. In the first part of the talk, we show that there exists a tight relation across two Earth system model ensembles (CMIP5 and CMIP6) between present-day sea surface salinity in the subtropical-polar frontal zone, the formation region of mode and intermediate waters, and the past and future anthropogenic carbon uptake in the Southern Ocean. By using observations and Earth system model results, we constrain the projected cumulative Southern Ocean anthropogenic carbon uptake over 1850-2100 by the CMIP6 model ensemble to 158 ± 6 Pg C under the low-emissions scenario SSP1-2.6 and to 279 ± 14 Pg C under the high emissions scenario SSP5-8.5. Our results suggest that the Southern Ocean anthropogenic carbon sink is 14-18% larger and 46-54% less uncertain than estimated by the unconstrained CMIP6 Earth system model results. The identified constraint demonstrated the importance of the freshwater cycle for the Southern Ocean circulation and carbon cycle. In the second part of the talk, potential emergent constraints for the Southern Ocean excess heat uptake will be discussed.</p>

Clouds are a major source of uncertainty in climate model projections over the Southern Ocean and Antarctica1. The inaccurate representation of clouds in climate models results in biases in the net radiative balance which has knock-on effects on the ability of models to represent sea surface temperatures, ocean heat uptake, sea ice cover, and ultimately large-scale circulation in the Southern Hemisphere2,3,4,5,6. Evidence suggests that this is due to the poor representation of mixed phase clouds in models—the dominant cloud type in this region. As part of the Southern Ocean Clouds project, we have conducted two flying campaigns out of Rothera research station which is located on the Antarctic peninsula, in order to investigate the composition of clouds over the Southern Ocean. Over the course of two field seasons (one in the 2022-23, and one in the 2024-25 Antarctic season) we performed more then 40 flights consisting of over 140 flying hours. During these flights we measured ice crystal, water droplet and aerosol number concentrations and sizes. We also collected Ice Nucleating Particles on filters in addition to performing measurements of meteorological parameters, turbulence, and radiative balance.  Here we will present an overview of the flying campaign of the 2024-25 season, and compare the observations conducted this year to those which were made during our previous campaign in the 2022-23 season. Although we saw higher droplet number concentrations for the 2024-25 campaign than during the 2022-23 campaign, both revealed the presence of higher droplet number concentrations at higher altitudes (> 2000 m asl) indicating a potential long range source for these.  1 Bodas-Salcedo, A., et al., 2014: Origins of the Solar Radiation Biases over the Southern Ocean in CFMIP2 Models. J. Climate, 27, 41–56, https://doi.org/10.1175/JCLI-D-13-00169.1.  2 Lauer, A., et al., 2018: Process-level improvements in CMIP5 models and their impact on tropical variability, the Southern Ocean and monsoons. Earth Syst. Dynam., 9, 33–67, https://doi.org/10.5194/esd-9-33-2018.  3 Frölicher, T. L., et al., 2015: Dominance of the Southern Ocean in Anthropogenic Carbon and Heat Uptake in CMIP5 Models. J. Climate, 28, 862–886, https://doi.org/10.1175/JCLI-D-14-00117.1.  4 Ferrari and Ferreira 2011: what processes drive the ocean heat transport? Ocean. Model., 38, 171-186, https://doi.org/10.1016/j.ocemod.2011.02.013.  5: Ceppi P., et al., 2012: Southern Hemisphere Jet latitude biases in CMIP5 models linked to shortwave cloud forcing. Geophys. Res. Lett, 39, 19: https://doi.org/10.1029/2012GL053115.  6 Y. Hwang, D.M.W. Frierson, 2013: Link between the double-Intertropical Convergence Zone problem and cloud biases over the Southern Ocean, Proc. Natl. Acad. Sci. U.S.A., 110 (13) 4935-4940, https://doi.org/10.1073/pnas.1213302110 

<p>The Southern Ocean is a major sink of anthropogenic carbon and excess heat. The Earth system model projections of these sinks provided by the CMIP5 and CMIP6 scenario experiments show a large model spread contributing to the large uncertainties in climate sensitivity and remaining carbon budgets for ambitious climate targets. A recent study identified an emergent coupling between anthropogenic carbon and excess heat uptake, highlighting that the passive-tracer behavior of these two quantities is dominant under high-emission scenarios. This coupling indicates that the use of a single observational constraint might be sufficient to reduce projection uncertainties in both anthropogenic carbon and excess heat uptake. In the northern limb of the Southern Ocean (30°S-55°S) where the subduction of intermediate and mode water is known to drive carbon and heat uptake, we find that the variations in model´s contemporary water-column stability over the first 2000 m is highly correlated to both its future anthropogenic carbon uptake and excess heat uptake efficiency. Using observational data, we reduce the uncertainty of future estimates of (1) the cumulative anthropogenic carbon uptake by up to 53% and (2) the excess heat uptake efficiency by 28%. Our results show that improving the representation of water-column stratification in Earth system models should be prioritized to improve future anthropogenic climate change projections.</p>

The global ocean has taken up an estimated 89% of excess heat and 26% of annual CO2 emissions resulting from anthropogenic activity in recent decades. Despite the crucial role of the ocean in the climate system, there remains significant uncertainty around ocean heat and carbon uptake. Processes of ocean carbon and heat uptake and redistribution are among the leading processes limiting scientists’ ability to predict the rate and magnitude of global climate change. Furthermore, despite being responsible for as much as 40–50% of global annual oceanic CO2 uptake, the Southern Ocean remains the most controversial ocean basin, with large differences in both the magnitude and variability of CO2 fluxes across various estimation methods. The disagreement regarding the magnitude and variability of Southern Ocean CO2 fluxes largely arises due to the sparsity and uneven distribution of in-situ observations of carbon in the ocean, a problem which also constrains estimates of ocean heat uptake. However, machine learning has been extensively demonstrated in recent years to be an effective tool for overcoming such data limitations through gap-filling methods.We leverage existing expertise in machine learning methods from generating the SOM-FFN and MOBO-DIC ocean carbon data products and apply this expertise to the development of a novel machine learning generated ocean heat uptake data product for the Southern Ocean. We integrate new Earth Observation data from Copernicus satellites with in-situ observation data from the Southern Ocean as input for our new data products. Here, we present a beta-version of our new machine-learning based products of carbon and heat uptake in the Southern Ocean and a first analysis of the variability and trends of the carbon and heat uptake of these data products. This work presents our first steps towards data products which will allow us to identify transport pathways of carbon and heat within the ocean interior. The novel machine learning-based products will also be used to support the advancement of Earth System Models and climate change predictions.

The effect of the Southern Ocean on global climate change is assessed using Earth system model projections following an idealized 1% annual rise in atmospheric CO2. For this scenario, the Southern Ocean plays a significant role in sequestering heat and anthropogenic carbon, accounting for 40% ± 5% of heat uptake and 44% ± 2% of anthropogenic carbon uptake over the global ocean (with the Southern Ocean defined as south of 36°S). This Southern Ocean fraction of global heat uptake is however less than in historical scenarios with marked hemispheric contrasts in radiative forcing. For this idealized scenario, inter-model differences in global and Southern Ocean heat uptake are strongly affected by physical feedbacks, especially cloud feedbacks over the globe and surface albedo feedbacks from sea-ice loss in high latitudes, through the top-of-the-atmosphere energy balance. The ocean carbon response is similar in most models with carbon storage increasing from rising atmospheric CO2, but weakly decreasing from climate change with competing ventilation and biological contributions over the Southern Ocean. The Southern Ocean affects a global climate metric, the transient climate response to emissions, accounting for 28% of its thermal contribution through its physical climate feedbacks and heat uptake, and so affects inter-model differences in meeting warming targets. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Projected changes in ocean heat and carbon storage are assessed in terms of the added and redistributed tracer using a transport-based framework, which is applied to an idealized climate model and a suite of six CMIP5 Earth system models following an annual 1% rise in atmospheric CO2. Heat and carbon budgets for the added and redistributed tracer are used to explain opposing regional patterns in the storage of ocean heat and carbon anomalies, such as in the tropics and subpolar North Atlantic, and the relatively reduced storage within the Southern Ocean. Here the added tracer takes account of the net tracer source and the advection of the added tracer by the circulation, while the redistributed tracer takes account of the time-varying circulation advecting the preindustrial tracer distribution. The added heat and carbon often have a similar sign to each other with the net source usually acting to supply the tracer. In contrast, the redistributed heat and carbon consistently have an opposing sign to each other due to the opposing gradients in the preindustrial temperature and carbon. These different signs in heat and carbon redistribution can lead to regional asymmetries in the climate-driven changes in ocean heat and carbon storage. For a weakening in the Atlantic overturning and strengthening in the Southern Ocean residual circulation, the high latitudes are expected to have heat anomalies of variable sign and carbon anomalies of a consistently positive sign, since added and redistributed tracers are opposing in sign for heat and the same sign for carbon there.

<p>The Southern Ocean is one of today's largest sink of carbon, having absorbed about 10\% of the anthropogenic carbon emissions. Southern Ocean's dynamics are principally modulated by the strength of the Southern Hemispheric westerlies,  which are projected to increase over the coming century. Here, using a high-resolution ocean-sea-ice-carbon cycle model, we explore the impact of idealized changes in Southern Hemispheric westerlies on the ocean carbon storage . We find that a 20\% strengthening of the Southern Hemispheric westerlies leads to a $\sim$25 Gt loss of natural carbon, while an additional 13 Gt of anthropogenic carbon is absorbed compared to the control run, thus resulting in a net loss of $\sim$12 GtC from the ocean over a period of 42 years. This tendency is enhanced if the westerlies are also shifted polewards, with a total natural carbon loss of almost 37 GtC, and an additional anthropogenic carbon uptake of 18 GtC. While both experiments display a large natural carbon loss south of 10$^\circ$S, the amplitude is three times greater in the poleward strengthening case, which is  not fully compensated by the increase in anthropogenic carbon content. However, the poleward wind shift leads to significant differences in the pattern of DIC change due to a weakening of the upper overturning cell,  which leads to an increase in natural and total carbon north of 35$^\circ$S in the upper 2000 m.</p>

The uptake and storage of anthropogenic carbon in the North Atlantic is investigated using different configurations of ocean general circulation/carbon cycle models. We investigate how different representations of the ocean physics in the models, which represent the range of models currently in use, affect the evolution of CO2 uptake in the North Atlantic. The buffer effect of the ocean carbon system would be expected to reduce ocean CO2 uptake as the ocean absorbs increasing amounts of CO2. We find that the strength of the buffer effect is very dependent on the model ocean state, as it affects both the magnitude and timing of the changes in uptake. The timescale over which uptake of CO2 in the North Atlantic drops to below preindustrial levels is particularly sensitive to the ocean state which sets the degree of buffering; it is less sensitive to the choice of atmospheric CO2 forcing scenario. Neglecting physical climate change effects, North Atlantic CO2 uptake drops below preindustrial levels between 50 and 300 years after stabilisation of atmospheric CO2 in different model configurations. Storage of anthropogenic carbon in the North Atlantic varies much less among the different model configurations, as differences in ocean transport of dissolved inorganic carbon and uptake of CO2 compensate each other. This supports the idea that measured inventories of anthropogenic carbon in the real ocean cannot be used to constrain the surface uptake. Including physical climate change effects reduces anthropogenic CO2 uptake and storage in the North Atlantic further, due to the combined effects of surface warming, increased freshwater input, and a slowdown of the meridional overturning circulation. The timescale over which North Atlantic CO2 uptake drops to below preindustrial levels is reduced by about one-third, leading to an estimate of this timescale for the real world of about 50 years after the stabilisation of atmospheric CO2. In the climate change experiment, a shallowing of the mixed layer depths in the North Atlantic results in a significant reduction in primary production, reducing the potential role for biology in drawing down anthropogenic CO2.

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.

The Southern Ocean plays a vital role in mitigating global warming through its uptake of anthropogenic carbon ( C ant ). Because of its dynamic nature, the Southern Ocean circulation is challenging to accurately simulate with today’s Earth system models (ESMs), which often have insufficient resolution to resolve small but important mesoscale processes. Here, we assess how the modeled Southern Ocean C ant uptake and storage are affected by the explicit simulation of mesoscale eddies. Specifically, we compare a global ocean biogeochemistry simulation having eddy-rich resolution (0.1°) in the Southern Ocean with a 0.5° simulation where eddy effects are parameterized. We find that explicitly simulating Southern Ocean mesoscale eddies enhances Southern Ocean C ant uptake by 10% and the global C ant interior storage by 8%. Steeper—and more realistic—density slopes in the eddy-rich model facilitate the upward transport of deep waters, yielding enhanced C ant uptake via various processes: lower surface C ant concentrations, elevated surface salinity, increased vertical mixing, and higher chemical carbon uptake capacity. Explicitly simulating eddies enhances C ant inventories in both bottom and mode/intermediate waters but with a 30% shift toward bottom waters, which could potentially affect the long-term sequestration of C ant . These findings, consistent across an additional model family, help reconcile discrepancies between observations and ESMs, which often underestimate the Southern Ocean C ant uptake. This study emphasizes the need for adequate model resolution, or improved eddy parameterizations, to accurately simulate the global carbon cycle and to reduce uncertainties in future climate projections informing climate policy. Significance Statement The Southern Ocean plays a vital role in mitigating global warming through its uptake of anthropogenic carbon. However, this process remains poorly constrained in Earth system models, which often have a resolution insufficient to simulate small but important ocean features like mesoscale eddies. This study explores whether high-resolution ocean models, which capture these eddies, differ in their absorption of anthropogenic carbon in the Southern Ocean. We find that higher-resolution models take up and store around 10% more anthropogenic carbon in the Southern Ocean with respect to lower-resolution ones, owing to a circulation better matched to observations. These findings help reconcile discrepancies between models and observations and suggest that many low-resolution Earth system models may underestimate Southern Ocean anthropogenic carbon storage, potentially biasing projections toward higher levels of global warming.

The ocean removes man-made (anthropogenic) carbon from the atmosphere and thereby mitigates climate change. Observations from global hydrographic surveys reveal the spatial and temporal evolution of the ocean inventory of anthropogenic carbon and suggest substantial decadal variability in historical storage rates. Here, we use a 100-member ensemble of an Earth system model to investigate the influence of external forcing and internal climate variability on historical changes in ocean anthropogenic carbon storage over 1994 to 2014. Our findings reveal that the externally forced, decadal changes in storage are largest in the Atlantic (2–4 mmol m−3 decade−1) and positive nearly everywhere. Internal climate variability modulates regional ocean anthropogenic carbon storage trends by up to 10 mmol m−3 decade−1. The influence of internal climate variability on decadal storage changes is most prominent at depths of ∼300 m and at the edges of the subtropical gyres. Internal variability in anthropogenic carbon in the extratropics has high spectral power on decadal to multi-decadal timescales, indicating that the approximately decadal repetitions of hydrographic surveys may produce storage change estimates that are heavily influenced by internal climate variability.