The Southern Ocean plays a disproportionately large role in the global carbon cycle, absorbing around 40% of anthropogenic CO₂ taken up by the global ocean (Kathiwala et al., 2009; Frölicher et al., 2015; Williams et al., 2023). This capacity is tightly linked to its overturning oceanic circulation, in which deep, carbon-rich water wells up to the surface, and dense masses are formed that sink back to deeper parts of the ocean. Throughout this dynamic process, seawater carbon content is altered by high biological activity and strong air–sea gas exchange. Through tight coupling between the Southern Ocean’s deeper water masses and the atmosphere, it regulates atmospheric CO₂ concentrations and thus exerts a critical influence on Earth’s climate. However, the Southern Ocean is undergoing rapid physical and biogeochemical change, with changing vertical circulation, surface ocean properties and a marked decline in sea ice extent over the past decades. While anthropogenic carbon uptake is predominantly a physical process, contemporary carbon fluxes (natural plus anthropogenic) have a large imprint by biological production and decomposition of organic matter, even more so when it comes to regional and seasonal variability.
Despite its importance, the processes governing the Southern Ocean carbon sink remain poorly constrained. Satellite observations are mostly restricted to surface processes and cannot observe beneath the extensive winter sea ice. Shipboard observations are heavily biased toward summer, when biological activity dominates. In the winter, physical drivers of carbon cycling, such as mixing, dense water- and sea ice formation, prevail over biogeochemical drivers. However, wintertime processes are much less constrained than summertime due to lack of multidisciplinary in situ observations. Rare year-round time series have shown that biogeochemical and physical observations in autumn, winter, and spring are necessary to understand the seasonal and interannual variability of carbon uptake in the high latitude, sea ice covered ocean (Venables et al., 2013; Droste et al., 2025). Other studies have shown that targeting heavily under sampled regions and seasons has the potential to significantly improve data products for the partial pressure of CO₂ (pCO₂) (Dong et al., 2024; Heimdal et al., 2024), which feed into global ocean CO₂ flux estimates and the Global Carbon Budget (Friedlingstein et al., 2025). The biogeochemical (BGC) Argo float array has led to tremendous improvements in addressing the seasonal data gap (Sarmiento et al., 2023). However, autonomous float measurements are limited to off-shelf regions and endangered by sea ice cover, and the biogeochemical data are constrained to sensor availability and certain technical limitations. To advance our ability to understand variability, to detect trends, and to accurately assess model outputs, we need to address the seasonal and spatial data gaps in the Southern Ocean.
Ongoing changes in ocean circulation, stratification, and sea ice dynamics are expected to profoundly reshape Southern Ocean CO₂ exchange and storage. Stratification, as a result of ocean freshening, has maintained the Southern Ocean CO₂ sink over the last decades, but this may change if CO₂-rich subsurface waters reach the thinning surface layer (Olivier & Haumann, 2025). Maintaining and expanding the decade-long BGC Argo array is therefore crucial (Roemmich et al., 2019; Owens et al., 2022). Autonomous measurements need to be integrated with high-accuracy biogeochemical observations made on ships, covering a full seasonal cycle, to obtain the highest quality BGC sensor data output. Targeted regional campaigns using multiple platforms need to be coordinated and synthesized with modelling and satellite observations, data assimilation, and machine learning.
The Southern Ocean is changing and understanding its carbon cycle and the role it plays globally has become urgent. The Southern Ocean carbon cycle is the result of a tightly- coupled system that can only be fully understood from a multi-disciplinary and multi-platform approach. Under InSync, this working group aims to coordinate and guide this approach for carbon-essential variables on an international level.
To achieve a robust, year-round assessment of the Southern Ocean carbon cycle, our working group proposes a coordinated multi-platform observing strategy that leverages the full diversity of existing and emerging observational capabilities. We envision combining measurements from large national research vessels, non-research ships operating in Antarctic waters, and small platforms such as sailing vessels, to maximise spatial and seasonal coverage of key carbon variables across all Antarctic sectors. Where possible, large research vessels could provide high-quality hydrographic and biogeochemical sections, including repeat sections that have been maintained by various programs for decades. Sailing vessels would complement these operations by sampling coastal or rarely visited regions and providing additional temporal resolution. Both large research vessels and sailing vessels should be equipped, wherever possible, with an extensive underway system comprising pCO₂.
To leverage high resolution and truly year-round data coverage from existing BGC-Argo floats (the SOCCOM program will deploy BGC-Argo floats until 2027), ship-based multi-disciplinary observations at the surface and throughout the water column will be designed to provide crucial calibration opportunities for float BGC sensors, as well as opportunities to better constrain ocean processes. The deployment of BGC-Argo floats with carbonate system (e.g., pH) and other BGC sensors (e.g., nitrate, dissolved oxygen, fluorescence, backscatter) needs to be expanded to continue this pan-Southern Ocean time series, extending it throughout the InSync period and beyond to the 5th International Polar Year (2032-2033). Other uncrewed surface vehicles, such as gliders and saildrones, with BGC sensors will be deployed to target specific regions with features of interest and timescales that are challenging to be captured by floats. To capture critical vertical fluxes in the ocean interior and deep ocean seasonal dynamics, mooring arrays will be deployed in key water mass transformation zones and shelf regions. Where feasible, moorings would incorporate biogeochemical sensors, sediment traps, and automated samplers to quantify carbon export and upper-ocean ecosystem processes. Sea ice-covered regions remain challenging for data-collection, even for autonomous robotic systems. This will be addressed by enhancing the use of marine mammal- borne instruments that could provide valuable environmental and biogeochemical measurements in ice-covered regions. In parallel, we aim to improve integration with land- based oceanographic and atmospheric time series (e.g., Rothera Oceanographic Time Series, IDEAL time series at Maxwell Bay and South Bay).
Satellite missions, including upcoming bio-optical programs such as PACE, will be important partners; harmonised protocols for linking floats, gliders, ships, and remote sensing will be key. Our methodological plan also includes developing advanced frameworks for synthesising 4-D observations, through data assimilation, machine learning, regional synthesis efforts, and close collaboration with modelling teams. These modelling groups, already operating high-resolution biogeochemical and Earth system models, will help identify observational gaps, test sampling strategies, and quantify the added value of different observational compo ents.
Across all platforms, we intend to harmonise sampling protocols, ensure consistent use of reference materials, and align with international networks to guarantee data interoperability. Overall, our approach aims to design an observing system capable of capturing the processes driving the Southern Ocean carbon cycle, from surface fluxes to deep carbon export, while recognising that priorities will evolve through continued discussion between observational scientists and modellers.
| Timeframe | Task |
|---|---|
| 2025 - Establishment of WG and contribute to white papers |
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| 2026 - Carbon Cycling White Paper and funding applications |
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| 2027-2029 - Implementation of science plans |
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| 2030-2032 - Data synthesis and publication |
The Carbon Cycling Working Group will build on (and contribute to) a broad network of existing international initiatives. Strong partnerships with these programs are essential to avoid duplication, maximise impact, and ensure that our activities integrate seamlessly into the global ocean carbon observing system.
We will coordinate closely with major data-oriented networks such as SOCONET and SOCAT, which provide high-quality surface ocean CO₂ observations, and GLODAP, which curates global ship-based biogeochemical data. Collaboration with GO-BGC and SOCCOM will support the integration and interpretation of biogeochemical float observations, including data assimilation and links with Earth System Models. The working group will also engage with capability groups within SOOS, such as SOFLUX (air-sea fluxes) and the SO-OA Hub (ocean acidification), to ensure alignment with community best practices and shared scientific priorities.
Partnerships will extend to specialised scientific communities, including BEPSII for sea ice biogeochemistry, SCAR-ICEPRO for linking modern and paleo perspectives on Southern Ocean change, and SCAR-INSTANT for interdisciplinary studies of ice-ocean-atmosphere interactions. Links to satellite remote-sensing groups, in particular through the IOCCG, will support integration of in situ carbon-cycle observations with emerging space-based capabilities.
Finally, coordination with other InSync working groups, such as the planned initiative on seasonal cycles of trace metals and microbial life, will help ensure coherence across disciplinary boundaries. Through these partnerships, the Carbon Cycling WG aims to connect diverse observing, modelling, and synthesis communities, facilitating a unified international effort to understand and monitor the Southern Ocean carbon cycle.
