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1. Magnitude, Trends, and Variability of the Global Ocean Carbon Sink From 1985 to 2018

   https://doi.org/10.1029/2023GB007780  

   DeVries, Tim; Yamamoto, Kana; Wanninkhof, Rik; Gruber, Nicolas; Hauck, Judith; Müller, Jens Daniel; Bopp, Laurent; Carroll, Dustin; Carter, Brendan; Chau, Thi‐Tuyet‐Trang; et al (October 2023, Global Biogeochemical Cycles)

   Abstract This contribution to the RECCAP2 (REgional Carbon Cycle Assessment and Processes) assessment analyzes the processes that determine the global ocean carbon sink, and its trends and variability over the period 1985–2018, using a combination of models and observation‐based products. The mean sea‐air CO₂flux from 1985 to 2018 is −1.6 ± 0.2 PgC yr⁻¹based on an ensemble of reconstructions of the history of sea surface pCO₂(pCO₂products). Models indicate that the dominant component of this flux is the net oceanic uptake of anthropogenic CO₂, which is estimated at −2.1 ± 0.3 PgC yr⁻¹by an ensemble of ocean biogeochemical models, and −2.4 ± 0.1 PgC yr⁻¹by two ocean circulation inverse models. The ocean also degasses about 0.65 ± 0.3 PgC yr⁻¹of terrestrially derived CO₂, but this process is not fully resolved by any of the models used here. From 2001 to 2018, the pCO₂products reconstruct a trend in the ocean carbon sink of −0.61 ± 0.12 PgC yr⁻¹ decade⁻¹, while biogeochemical models and inverse models diagnose an anthropogenic CO₂‐driven trend of −0.34 ± 0.06 and −0.41 ± 0.03 PgC yr⁻¹ decade⁻¹, respectively. This implies a climate‐forced acceleration of the ocean carbon sink in recent decades, but there are still large uncertainties on the magnitude and cause of this trend. The interannual to decadal variability of the global carbon sink is mainly driven by climate variability, with the climate‐driven variability exceeding the CO₂‐forced variability by 2–3 times. These results suggest that anthropogenic CO₂dominates the ocean CO₂sink, while climate‐driven variability is potentially large but highly uncertain and not consistently captured across different methods. 

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2. Improved atmospheric constraints on Southern Ocean CO ₂ exchange

   https://doi.org/10.1073/pnas.2309333121  

   Jin, Yuming; Keeling, Ralph F; Stephens, Britton B; Long, Matthew C; Patra, Prabir K; Rödenbeck, Christian; Morgan, Eric J; Kort, Eric A; Sweeney, Colm (February 2024, Proceedings of the National Academy of Sciences)

   We present improved estimates of air–sea CO₂exchange over three latitude bands of the Southern Ocean using atmospheric CO₂measurements from global airborne campaigns and an atmospheric 4-box inverse model based on a mass-indexed isentropic coordinate (M_θe). These flux estimates show two features not clearly resolved in previous estimates based on inverting surface CO₂measurements: a weak winter-time outgassing in the polar region and a sharp phase transition of the seasonal flux cycles between polar/subpolar and subtropical regions. The estimates suggest much stronger summer-time uptake in the polar/subpolar regions than estimates derived through neural-network interpolation of pCO₂data obtained with profiling floats but somewhat weaker uptake than a recent study by Long et al. [Science374, 1275–1280 (2021)], who used the same airborne data and multiple atmospheric transport models (ATMs) to constrain surface fluxes. Our study also uses moist static energy (MSE) budgets from reanalyses to show that most ATMs tend to have excessive diabatic mixing (transport across moist isentrope, θ_e, or M_θesurfaces) at high southern latitudes in the austral summer, which leads to biases in estimates of air–sea CO₂exchange. Furthermore, we show that the MSE-based constraint is consistent with an independent constraint on atmospheric mixing based on combining airborne and surface CO₂observations. 

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3. Atmospheric pCO ₂ Response to Stimulated Organic Carbon Export: Sensitivity Patterns and Timescales

   https://doi.org/10.1029/2024GL108462  

   Holzer, Mark; DeVries, Tim; Pasquier, Benoît (June 2024, Geophysical Research Letters)

   Abstract The ocean's organic carbon export is a key control on atmospheric pCO₂and stimulating this export could potentially mitigate climate change. We use a data‐constrained model to calculate the sensitivity of atmospheric pCO₂to local changes in export using an adjoint approach. A perpetual enhancement of the biological pump's export by 0.1 PgC/yr could achieve a roughly 1% reduction in pCO₂at average sensitivity. The sensitivity varies roughly 5‐fold across different ocean regions and is proportional to the difference between the mean sequestration timeτ_seqof regenerated carbon and the response timeτ_preof performed carbon, which is the reduction in the preformed carbon inventory per unit increase in local export production. Air‐sea CO₂disequilibrium modulates the geographic pattern ofτ_pre, causing particularly high sensitivities (2–3 times the global mean) in the Antarctic Divergence region of the Southern Ocean. 

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4. Scale‐Dependent Drivers of Air‐Sea CO ₂ Flux Variability

   https://doi.org/10.1029/2024GL111911  

   Fay, Amanda R; Carroll, Dustin; McKinley, Galen A; Menemenlis, Dimitris; Zhang, Hong (October 2024, Geophysical Research Letters)

   Abstract In climate studies, it is crucial to distinguish between changes caused by natural variability and those resulting from external forcing. Here we use a suite of numerical experiments based on the ECCO‐Darwin ocean biogeochemistry model to separate the impact of the atmospheric carbon dioxide (CO₂) growth rate and climate on the ocean carbon sink — with a goal of disentangling the space‐time variability of the dominant drivers. When globally integrated, the variable atmospheric growth rate and climate exhibit similar magnitude impacts on ocean carbon uptake. At local scales, interannual variability in air‐sea CO₂flux is dominated by climate. The implications of our study for real‐world ocean observing systems are clear: in order to detect future changes in the ocean sink due to slowing atmospheric CO₂growth rates, better observing systems and constraints on climate‐driven ocean variability are required. 

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5. Seasonal variability of surface ocean carbon uptake and chlorophyll‐a concentration in the West Antarctic Peninsula over two decades

   https://doi.org/10.1029/2024GL112446  

   Turner, Jessica S; Munro, David R; Fay, Amanda; Stammerjohn, Sharon; Kim, Heather H; Schofield, Oscar; Dierssen, Heidi (February 2025, Geophysical Research Letters)

   The Southern Ocean plays a vital role in global CO₂uptake, but the magnitude and even the sign of the flux remain uncertain, and the influence of phytoplankton phenology is underexplored. This study focuses on the West Antarctic Peninsula, a region experiencing rapid climate change, to examine shifts in seasonal carbon uptake. Using 20 years of in situ air‐sea CO₂flux and satellite‐derived Chlorophyll‐a, we observe that the seasonal cycles of both air‐sea CO₂flux and Chlorophyll‐a intensify poleward. The amplitude of the seasonal cycle of the non‐thermal component of surface ocean pCO₂increases with increasing latitude, while the amplitude of the thermal component remains relatively stable. Pronounced biological uptake occurs over the shelf in austral summer despite reduced CO₂solubility in warmer waters, which typically limits carbon uptake through physical processes. These findings underscore the prominence of biological mechanisms in regulating carbon fluxes in this rapidly changing region. 

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