Abstract Large‐scale farming and purposeful sinking of seaweed has been suggested as a marine Carbon Dioxide Removal (mCDR) strategy. Farmed seaweed uptakes dissolved inorganic carbon (DIC) from the mixed layer, resulting in a CO2 deficit that causes an influx of atmospheric CO2 into the surface ocean. The carbon‐rich seaweed is then harvested and conveyed to depth where it is either eventually remineralized back to DIC or incorporated into the sediment. To explore abiotic drivers of seaweed mCDR efficiency and durability, we simulate the advection and mixing of a DIC perturbation through the ocean with a steady‐state global ocean circulation model that includes an interacting atmosphere with realistic air‐sea gas exchange. We find that initially many locations in the global ocean can be nearly as efficient as direct air capture (DAC) and are temporarily durable (sequestered for >100 years) if biomass is conveyed rapidly (≥1,000 m d−1) to a depth (≥2,000 m) before it is eventually ventilated back to the atmosphere. Durability timescales and efficiencies are significantly reduced for slower vertical conveyance (<100 m d−1) and shallower depths (<500 m). Seaweed CDR efficiency and durability metrics are determined by local rates of vertical mixing, global circulation pathways, CO2 air‐sea gas exchange timescales, and the rate that the seaweed is conveyed to the seafloor. Understanding the abiotic controls on seaweed mCDR efficiency and durability as well as its biotic impacts on surface and benthic ecosystems are critical for assessing the efficacy of large‐scale seaweed cultivation and purposeful sequestration as a mCDR strategy.

The ocean's biological carbon pump transports organic carbon from the surface to depth via three main pathways: the gravitational sinking of particles, active transport by vertically migrating zooplankton, and mixing and advection of suspended and dissolved organic carbon. Here, we use a global data-assimilated ocean biogeochemical model to diagnose the seasonal variability of carbon export and sequestration by these gravitational, migrant, and mixing pumps. The total carbon export and sequestration are 10.2 +/- 0.8 PgC yr-1 and 1,339 +/- 17 PgC, respectively, similar to previous estimates that did not consider seasonality. However, the seasonality of the export and sequestration pathways is highly variable, especially in the high latitudes. In subpolar regions, the seasonal amplitude of the pumps is similar to 40%-60% of the annual mean: export and sequestration by the gravitational and migrant pumps peak in the summer, while the mixing pump strongly opposes this seasonality, reaching a maximum during the winter. The sequestration time of exported carbon is generally higher during winter than summer in the subpolar regions, helping to augment carbon sequestration during the less productive winter months. The gravitational "e-ratio," or ratio of gravitational carbon export to net primary production, has a seasonal variability of similar to 0.1 at high latitudes, with higher values in the summer compared to winter. Resolving seasonality reduces the inferred geographic variability of the e-ratio compared with annual-mean models, demonstrating the importance of seasonal observations and models to understand and quantify the processes regulating carbon export and sequestration.

Proxy records of seawater radiocarbon (14C/C) provide strong constraints on how changes in ocean ventilation contributed to the increase in atmospheric CO2 during the termination of the last ice age (approximate to 18,000-to-12,000 years ago). One outstanding problem, however, is the existence of anomalously low deglacial benthic foraminiferal 14C/C in the intermediate-depth Eastern Tropical North Pacific (ETNP) near the Gulf of California (GoC). This deglacial ETNP 14C/C anomaly is hypothesized to reflect either (a) an artifact of the proxy record, (b) the advection of low 14C/C seawater, or (c) the input of 14C/C-depleted geologic carbon related to local seafloor volcanism. To test these hypotheses, we first use new sediment-trap and seaweed 14C/C to establish a new baseline understanding of ETNP seawater 14C/C, which suggest that anomalously low 14C/C is upwelled in the modern GoC. We then apply new geochemical experiments to test and ultimately validate the utility of the benthic foraminiferal 14C/C as a proxy for seawater 14C/C. Finally, we present a compilation of published and new glacial-interglacial benthic foraminiferal 14C/C records, specifically developed to map the spatial and temporal variability of the intermediate-depth water mass containing the deglacial ETNP 14C/C anomaly. These results clearly show that the ETNP deglacial 14C/C anomaly develops near the GoC mouth, concomitant with local hydrothermal systems. Considering these results and those of our companion paper (Green et al., 2026, https://doi.org/10.1029/2025pa005217), we argue that the input of pH-neutral geologic carbon from hydrothermal vents near and within the GoC could explain the anomalous intermediate-depth 14C/C values both during the deglaciation and today.

Ocean ventilation, or the transfer of tracers from the surface boundary layer into the ocean interior, is a critical process in biogeochemical cycles and the climate system. Here, we assess steady‐state ventilation patterns and timescales in three models of ocean transport: a 1 global configuration of the Nucleus for European Modeling of the Ocean (NEMO), a recent 2 solution of the Ocean Circulation Inverse Model (OCIM), and a 2 solution of the Total Matrix Intercomparison (TMI). We release artificial dyes in six surface regions of each model and compare equilibrium dye distributions as well as ideal age distributions. We find good qualitative agreement in large‐scale dye distributions across the three models. However, the distributions indicate that TMI and OCIM are more diffusive than NEMO. A shallow bias of North Atlantic ventilation in NEMO contributes to a stronger presence of the North Atlantic dye in the mid‐depth Southern Ocean and Pacific. This isopycnal communication between the North Atlantic surface and the mid‐depth Pacific is very slow, however, and NEMO simulates a maximum age in the North Pacific (NP) about 900 years higher than the data‐constrained models. Overly slow NP ventilation persists across NEMO sensitivity experiments encompassing our current best knowledge of diapycnal and isopycnal mixing, pointing to biases in subarctic Pacific dynamics. This study provides a synoptic picture of deep ocean ventilation and a framework for assessing its representation in general circulation models.

The ocean provides both socioeconomic and climate benefits, but these benefits can potentially conflict when fishing disrupts the carbon sequestration potential of marine macrofauna and disturbs carbon-rich sediments. A lack of understanding about the spatial overlap between fishing activity and key carbon sequestration areas hinders efforts to incorporate climate considerations into fisheries management. Here, we identify potentially conflicting oceanic carbon areas with pelagic (COCA-P) and bottom-trawling fisheries (COCA-BT) separately. We show that COCA-P cover 11.2% of the ocean, with 60% occurring in the high seas, while COCA-BT cover 3.7% of the ocean, mainly in coastal areas where 56.4% of global trawling catches occur. Only 1% of COCAs overlap both fishery types, suggesting that targeted adjustments in fishing practices could mitigate conflicts. These findings provide critical insights for fisheries management, informing policies that balance food security, economic interests, and carbon sequestration in the ocean.

Net sea-air CO2 flux can be calculated from observations of seawater and atmosphere partial pressure of CO2 (pCO2) and estimates of the gas transfer velocity. Typically, these quantities are calculated at a monthly resolution, which misses potentially important high-frequency temporal variability. Here, we calculated sea-air CO2 flux at a 3-hourly resolution using a 10-year mooring data set (2011-2020) from the central California coastal upwelling region. We identified a significant flux of CO2 from the ocean to the atmosphere due to a positive correlation between seawater pCO2 and wind speed at timescales of hours to days, particularly during the late spring and early summer upwelling season. Accounting for this variability changes the region from a net sink to a net source of CO2 to the atmosphere. These findings imply that CO2 fluxes computed from monthly-resolution data may miss important shorter-term variability that contributes to a net outgassing of CO2 from the ocean.

Although the role of marine macrofauna in the ocean carbon cycle is increasingly understood, the cumulative impacts of fisheries and climate change on this pathway remain overlooked. Here, using a marine ecosystem model, we estimate that each degree of warming reduces macrofauna biomass and carbon export by 4.2% and 2.46%, respectively. Under a high emission scenario (SSP 5-8.5), this translates to a 13.5% ± 6.6% decline in export by 2100, relative to the 1990s. Fishing further amplifies this reduction by up to 56.7% ± 16.3%, creating a sequestration deficit of 14.6 ± 10.3 GtC by 2100. On average, a 1% biomass loss from fishing results in a 0.8% decline in carbon export. However, sequestration durability (~600 years) remains unaffected. While measures restoring commercial macrofaunal biomass could yield carbon benefits comparable to mangrove restoration, multiple uncertainties limit their inclusion in the Nature-based Climate Solution portfolio, highlighting the need for further research.

Marine carbon dioxide removal (mCDR) is gaining interest as a tool to meet global climate goals. Because the response of the ocean-atmosphere system to mCDR takes years to centuries, modeling is required to assess the impact of mCDR on atmospheric CO2 reduction. Here, we use a coupled ocean-atmosphere model to quantify the atmospheric CO2 reduction in response to a CDR perturbation. We define two metrics to characterize the atmospheric CO2 response to both instantaneous ocean alkalinity enhancement (OAE) and direct air capture (DAC): the cumulative additionality (alpha) measures the reduction in atmospheric CO2 relative to the magnitude of the CDR perturbation, while the relative efficiency () quantifies the cumulative additionality of mCDR relative to that of DAC. For DAC, alpha is 100% immediately following CDR deployment, but declines to roughly 50% by 100 years post-deployment as the ocean degasses CO2 in response to the removal of carbon from the atmosphere. For instantaneous OAE, alpha is zero initially and reaches a maximum of 40%-90% several years to decades later, depending on regional CO2 equilibration rates and ocean circulation processes. The global mean approaches 100% after 40 years, showing that instantaneous OAE is nearly as effective as DAC after several decades. However, there are significant geographic variations, with approaching 100% most rapidly in the low latitudes while stays well under 100% for decades to centuries near deep and intermediate water formation sites. These metrics provide a quantitative framework for evaluating sequestration timescales and carbon market valuation that can be applied to any mCDR strategy.

Abstract The ocean is a major carbon sink and takes up 25%–30% of the anthropogenically emitted CO2. A state‐of‐the‐art method to quantify this sink are global ocean biogeochemistry models (GOBMs), but their simulated CO2 uptake differs between models and is systematically lower than estimates based on statistical methods using surface ocean pCO2 and interior ocean measurements. Here, we provide an in‐depth evaluation of ocean carbon sink estimates from 1980 to 2018 from a GOBM ensemble. As sources of inter‐model differences and ensemble‐mean biases our study identifies (a) the model setup, such as the length of the spin‐up, the starting date of the simulation, and carbon fluxes from rivers and into sediments, (b) the simulated ocean circulation, such as Atlantic Meridional Overturning Circulation and Southern Ocean mode and intermediate water formation, and (c) the simulated oceanic buffer capacity. Our analysis suggests that a late starting date and biases in the ocean circulation cause a too low anthropogenic CO2 uptake across the GOBM ensemble. Surface ocean biogeochemistry biases might also cause simulated anthropogenic fluxes to be too low, but the current setup prevents a robust assessment. For simulations of the ocean carbon sink, we recommend in the short‐term to (a) start simulations at a common date before the industrialization and the associated atmospheric CO2 increase, (b) conduct a sufficiently long spin‐up such that the GOBMs reach steady‐state, and (c) provide key metrics for circulation, biogeochemistry, and the land‐ocean interface. In the long‐term, we recommend improving the representation of these metrics in the GOBMs.

This study characterized ocean biological carbon pump metrics in the second iteration of the REgional Carbon Cycle Assessment and Processes (RECCAP2) project. The analysis here focused on comparisons of global and biome‐scale regional patterns in particulate organic carbon (POC) production and sinking flux from the RECCAP2 ocean biogeochemical model ensemble against observational products derived from satellite remote sensing, sediment traps, and geochemical methods. There was generally good model‐data agreement in mean large‐scale spatial patterns, but with substantial spread across the model ensemble and observational products. The global‐integrated, model ensemble‐mean export production, taken as the sinking POC flux at 100 m (6.08 ± 1.17 Pg C yr−1), and export ratio defined as sinking flux divided by net primary production (0.154 ± 0.026) both fell at the lower end of observational estimates. Comparison with observational constraints also suggested that the model ensemble may have underestimated regional biological CO2 drawdown and air‐sea CO2 flux in high productivity regions. Reasonable model‐data agreement was found for global‐integrated, ensemble‐mean sinking POC flux into the deep ocean at 1,000 m (0.65 ± 0.24 Pg C yr−1) and the transfer efficiency defined as flux at 1,000 m divided by flux at 100 m (0.122 ± 0.041), with both variables exhibiting considerable regional variability. The RECCAP2 analysis presents standard ocean biological carbon pump metrics for assessing biogeochemical model skill, metrics that are crucial for further modeling efforts to resolve remaining uncertainties involving system‐level interactions between ocean physics and biogeochemistry.

Abstract The ocean's organic carbon export is a key control on atmospheric pCO2 and stimulating this export could potentially mitigate climate change. We use a data‐constrained model to calculate the sensitivity of atmospheric pCO2 to 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 pCO2 at average sensitivity. The sensitivity varies roughly 5‐fold across different ocean regions and is proportional to the difference between the mean sequestration time τseq of regenerated carbon and the response time τpre of performed carbon, which is the reduction in the preformed carbon inventory per unit increase in local export production. Air‐sea CO2 disequilibrium modulates the geographic pattern of τpre, causing particularly high sensitivities (2–3 times the global mean) in the Antarctic Divergence region of the Southern Ocean.

Trawling the seafloor can disturb carbon that took millennia to accumulate, but the fate of that carbon and its impact on climate and ecosystems remains unknown. Using satellite-inferred fishing events and carbon cycle models, we find that 55-60% of trawling-induced aqueous CO2 is released to the atmosphere over 7-9 years. Using recent estimates of bottom trawling’s impact on sedimentary carbon, we found that between 1996-2020 trawling could have released, at the global scale, up to 0.34-0.37 Pg CO2 yr-1 to the atmosphere, and locally altered water pH in some semi-enclosed and heavy trawled seas. Our results suggest that the management of bottom-trawling efforts could be an important climate solution.

The ocean's biological carbon pump (BCP) affects the Earth's climate by sequestering CO2 away from the atmosphere for decades to millennia. One primary control on the amount of carbon sequestered by the biological pump is air-sea CO2 disequilibrium, which is controlled by the rate of air-sea CO2 exchange and the residence time of CO2 in surface waters. Here, we use a data-assimilated model of the soft tissue BCP to quantify carbon sequestration inventories and time scales from remineralization in the water column to equilibration with the atmosphere. We find that air-sea CO2 disequilibrium enhances the global biogenic carbon inventory by similar to 35% and its sequestration time by similar to 70 years compared to identical calculations made assuming instantaneous air-sea CO2 exchange. Locally, the greatest enhancement occurs in the subpolar Southern Ocean, where air-sea disequilibrium increases sequestration times by up to 600 years and the biogenic dissolved inorganic carbon inventory by >100% in the upper ocean. Contrastingly, in deep-water formation regions of the North Atlantic and Antarctic margins, where biological production creates undersaturated surface waters which are subducted before fully equilibrating with the atmosphere, air-sea CO2 disequilibrium decreases the depth-integrated sequestration inventory by up to similar to 150%. The global enhancement of carbon sequestration by air-sea disequilibrium is particularly important for carbon respired in deep waters that upwell in the Southern Ocean. These results highlight the importance of accounting for air-sea CO2 disequilibrium when evaluating carbon sequestration by the biological pump and for assessing the efficacy of ocean-based CO2 removal methods.

The daily vertical migrations of fish and other metazoans actively transport organic carbon from the ocean surface to depth, contributing to the biological carbon pump. We use an oxygen-constrained, game-theoretic food-web model to simulate diel vertical migrations and estimate global carbon fluxes and sequestration by fish and zooplankton due to respiration, fecal pellets, and deadfalls. Our model provides estimates of the carbon export and sequestration potential for a range of pelagic functional groups, despite uncertain biomass estimates of some functional groups. While the export production of metazoans and fish is modest (~20% of global total), we estimate that their contribution to carbon sequestered by the biological pump (~ 800 PgC) is conservatively more than 50% of the estimated global total (~1300 PgC) and have a significantly longer sequestration time scale (~250 years) than previously reported for other components of the biological pump. Fish and multicellular zooplankton contribute about equally to this sequestered carbon pool. This essential ecosystem service could be at risk from both unregulated fishing on the high seas and ocean deoxygenation due to climate change.

The ocean contains about 40 times more carbon than the atmosphere, storing 38,000 Pg C as dissolved inorganic carbon (DIC) versus 900 Pg C as carbon dioxide (CO 2 ) in the present atmosphere. The biological carbon pump contributes to ocean carbon storage by moving organic carbon out of the surface ocean into deeper waters in sinking particles, vertically migrating organisms and physical circulation. Century-scale (≥100 years) storage of the resulting biogenic DIC is commonly assumed to occur exclusively in the deep ocean, typically below 1,000 m. However, recent work has shown that carbon can be sequestered at century scales above 1,000 m in many ocean regions, in what we call ‘continuous vertical sequestration’. Here we calculate the century-scale carbon sequestration flux driven by the biological pump throughout the water column by combining previously published estimates of organic carbon flux and modelled values of water-mass sequestration time distributions. We estimate that the flux of organic carbon that is sequestered for ≥100 years in the contemporary ocean by the combined action of various biological pump pathways is 0.9–2.6 Pg C yr −1 , which is up to six times larger than previous estimates based on the organic carbon flux reaching the deep ocean.

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 CO2 flux from 1985 to 2018 is -1.6 +/- 0.2 PgC yr(-1) based on an ensemble of reconstructions of the history of sea surface pCO(2) (pCO(2) products). 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(-1) by an ensemble of ocean biogeochemical models, and -2.4 +/- 0.1 PgC yr(-1) by two ocean circulation inverse models. The ocean also degasses about 0.65 +/- 0.3 PgC yr(-1) of terrestrially derived CO2, but this process is not fully resolved by any of the models used here. From 2001 to 2018, the pCO(2) products 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 CO2 dominates the ocean CO2 sink, while climate-driven variability is potentially large but highly uncertain and not consistently captured across different methods.

The RECCAP2 global ocean project provides an assessment of the mean, trends, and variability of the global ocean carbon sink for the period 1985-2018. The analysis is based on a comprehensive assessment of models and observation-based products, including global ocean biogeochemical models (GOBMs), pCO2 observation-based air-sea CO2 flux products, ocean data assimilation models, and DIC-observation based products. We find that the mean ocean CO2 sink from 1985-2018 is -1.7±0.3 PgC yr-1 as diagnosed by pCO2-observation based air-sea CO2 flux products. The dominant component of the global air-sea CO2 flux is the oceanic uptake of anthropogenic CO2, which is estimated at between -2.0 to -2.6 PgC yr-1 using a range of GOBMs, assimilation models and DIC-based products. The second largest component of the global air-sea CO2 flux is the outgassing of terrestrially-derived CO2, which is estimated at 0.65±0.3 PgC yr-1 but is not yet fully resolved by RECCAP2 models. The trend in the global air-sea CO2 flux from 1985-2018 ranges from -0.26 PgC yr-1 decade-1 in the GOBMs to -0.39 PgC yr-1 decade-1 in the pCO2 products. Over the 2001-2018 period, when the pCO2-based estimates benefit from improved data coverage, they predict a strengthening trend in the ocean carbon sink of -0.63 PgC yr-1 decade-1. This is driven primarily by the trend in anthropogenic carbon uptake of -0.41 PgC yr-1 decade-1, and secondarily by a climate-forced trend of -0.28 PgC yr-1 decade-1. This climate-forced strengthening of the ocean carbon sink since 2001 is not diagnosed in the GOBMs, and the reasons for this trend remain unclear. We find that 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. GOBMs suggest that the climate-driven variability is about 4-8% of the global mean carbon sink, while the climate-driven variability is about 9-14% of the global mean flux in the observation-based pCO2 products. In all, the RECCAP2 analysis provides a state-of-the-art summary of our current knowledge of the ocean carbon sink, and the mechanisms driving its magnitude, trends, and variability over time.

Abstract Barystatic sea level rise (SLR) caused by the addition of freshwater to the ocean from melting ice can in principle be recorded by a reduction in seawater salinity, but detection of this signal has been hindered by sparse data coverage and the small trends compared to natural variability. Here, we develop an autoregressive machine learning method to estimate salinity changes in the global ocean from 2001 to 2019 that reduces uncertainties in ocean freshening trends by a factor of four compared to previous estimates. We find that the ocean mass rose by 13,000 ± 3,000 Gt from 2001 to 2019, implying a barystatic SLR of 2.0 ± 0.5 mm/yr. Combined with SLR of 1.3 ± 0.1 mm/yr due to ocean thermal expansion, these results suggest that global mean sea level rose by 3.4 ± 0.6 mm/yr from 2001 to 2019. These results provide an important validation of remote‐sensing measurements of ocean mass changes, global SLR, and global ice budgets.