François Primeau

Ocean iron fertilization (OIF) is being discussed as a potential carbon dioxide removal (CDR) strategy¹. Yet the trade-offs between climate benefits and ecological costs remain poorly understood^2,3,4. Here we use a process-rich ocean biogeochemical model with validated representations of marine biodiversity and biogeochemistry⁵ to quantify fertilization potential and ecosystem impact across ten ocean biomes over 60 years. We find regional disparities in climate ecosystem trade-offs: biomes with comparable CDR efficiency have contrasting ecological outcomes. The Southern Ocean and the equatorial Pacific achieve the highest efficiencies through distinct pathways: the former through non-local downstream carbon export, the latter through local export with downstream loss, as fertilized blooms deplete macronutrients in source water, suppressing non-local productivity. Consequently, equatorial fertilization reduces energy flows to higher trophic levels, expands oxygen-minimum zones (OMZs) and reduces macrozooplankton biomass. These ecosystem impacts are potentially avoidable by shifting deployment to higher latitudes. Post-fertilization recovery experiments reveal ecosystem resilience in the Southern Ocean but persistent perturbations in the equatorial Pacific owing to regional iron retention. Balancing climate benefits and ecological risks yields three tiers: the Southern Ocean (higher efficiency/lower risk); the equatorial Pacific and global fertilization (higher efficiency/higher risk); and the subtropics (lower efficiency/moderate risk). Sixty years of fertilization yield net CDR of 1.1–5.3 ppm, with more than half re-emitted within decades post-fertilization. The disconnect between local interventions and global outcomes complicates robust carbon crediting and raises equity and governance concerns.

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The ocean’s biological carbon pump exports atmospheric CO ₂ to the deep ocean, where it can remain sequestered for decades to centuries, and attempts to artificially enhance this natural carbon sink by fertilizing portions of the open ocean could help mitigate the impacts of excessive anthropogenic CO ₂ emissions. However, differences in the cycling rates of carbon and other nutrients may impact the long-term response to ocean fertilization. In this study, we use a steady-state global biogeochemical inverse model, optimized to match hydrographic observations, to examine how differential production, remineralization, and circulation-driven re-exposure timescales of organic carbon and phosphorus affect long-term carbon sequestration. We partition global organic matter production based on the time required for regenerated carbon and phosphorus to return to the ocean surface. We find that less than 15% of total organic carbon and 31% of total organic phosphorus production remains sequestered in the ocean interior for ≥ 1 y, with only 3.3% (1.8 Pg C y ⁻¹ ) and 8.3% (0.046 Pg P y ⁻¹ ), respectively, remaining for a century or longer. The C:P ratio of the sequestration flux declines with increasing residence time, from 255:1 for total production to 98:1 for material sequestered for 100+ years, indicating that carbon is recycled to the surface more rapidly than phosphorus. This decoupling between carbon and phosphorus sequestration timescales could result in a “productivity hangover,” where the slow recovery of surface phosphate leads to a long-term suppression of global productivity, reducing the net removal of atmospheric CO ₂ .

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 ∼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 ∼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.

The biological carbon pump (BCP) transfers CO ₂ from the surface ocean to depth, helping regulate atmospheric CO ₂ and accounting for roughly one‐third of glacial–interglacial CO ₂ changes. Its strength is widely estimated from satellite‐based relationships between net primary production (NPP) and export efficiency, but these approaches cannot distinguish organic matter rapidly recycled near the surface from that exported to depth. This ambiguity inflates export estimates and obscures BCP variability. Here, using a tracer‐constrained inverse biogeochemical model, we explicitly quantify the “rapid recycling” fraction of NPP that does not contribute to deep‐ocean sequestration. We find that ∼48%–60% of satellite‐measured NPP is respired within the euphotic zone within hours to days, leaving a climatological mean carbon export of 13.72–15.55 Pg C yr ⁻¹ —consistent across multiple satellite NPP products. Export fluxes derived from our inverse model agree more closely with sediment trap and ²³⁴ Th observations than those from empirical NPP–export relationships or Earth system models. Of course, the inverse model's steady‐state assumption and dependence on a low‐resolution climatological circulation field prevent future predictions and limit its accuracy in coastal and polar regions. These results still show that deep‐ocean tracer constraints yield robust BCP estimates despite large surface productivity uncertainties, and they provide the first global‐scale quantification of rapid surface recycling as a major limitation of satellite‐based export assessments.

The abyssal ocean may provide adequate sequestration for Carbon Capture and Storage and Carbon Dioxide Removal. The transit of carbon from seafloor release to ocean surface can take millennia, as it occurs through many water pathways characterized by long‐tailed transit‐time distributions (TTDs). However, multi‐millennial simulations of TTDs are typically prohibitively expensive. Here, we introduce an idealized but computationally efficient methodological framework for computing TTDs offline from climate‐model archives. We applied this framework to one Earth System Model to estimate the deep ocean sequestration efficiency for the 2030s and 2090s ocean circulations. We found that sequestration is most efficient on abyssal plains isolated from deep branches of the conveyor belt, such as the North Pacific Basins, where less than 10% of the water reaches the surface after 1,000 years. The climate‐warming‐induced slowdown of the 2090s deep‐ocean circulation extends return times by about 30%, which exceeds internal climate variability (%).

Ocean sediments may provide adequate long-term storage for carbon dioxide removal (CDR), with the abyssal ocean providing extra sequestration. The transit of carbon from seafloor release to ocean surface can take up to millennia, as it occurs through many pathways characterized by long-tailed transit-time distributions (TTDs). Here, we introduce an idealized methodological framework for efficiently computing these TTDs from climate-model archives. We apply this framework to one Earth System Model and estimate the deep ocean sequestration efficiency for the 2030s and 2090s ocean circulations. We find the highest sequestration efficiencies on abyssal plains isolated from the deep branches of the conveyor belt, such as the North Pacific Basins, where less than 10% of water makes contact with the surface after 1000 years. The climate-warming induced slowdown of the 2090s deep ocean circulation extends return times by about 30%, which exceeds internal variability (~20%) estimated from the ensemble of simulations.

The editorial team of AGU Advances is grateful for the excellent contributions of our peer reviewers. We rely on their expertise to ensure that the manuscripts submitted to the journal undergo a rigorous, fair, and timely review. Remarkably, during 2024, the journal benefitted from the dedication from 273 reviewers, contributing a total of 338 reviews. These reviewers represented 24 countries. These reviewers provided insights of tremendous and generous value, and they assisted our authors in strengthening the rigor, quality, and presentation of their scholarship. Peer reviewing provides a natural way to engage in continuous learning and professional development. The majority of our reviewers are geoscientists, although we also have interdisciplinary contributions as the scope of Advances covers the extended domain of geosciences, intersecting with economics, communication and computational science, and the social sciences at large. Authors benefit greatly from reviewers' comments and suggestions: already more than 10 years ago, a study reported that most authors (90%) believe that peer review improved the last paper they published (Mulligan et al., 2013, https://doi.org/10.1002/asi.22798). Although the research and publishing arena is rapidly changing, peer review is considered the optimal standard for evaluating and selecting quality scientific manuscripts for publication, and therefore is highly deserving of our appreciation. We thank all of our peer reviewers for their selfless service and dedication to the scientific community. Your continuing support to the authors and editorial team of AGU Advances is deeply appreciated.

The biological carbon pump (BCP) transfers CO₂ from the surface ocean to deep waters and has historically regulated atmospheric CO₂ levels, accounting for around one-third of glacial-interglacial CO₂ fluctuations. Current satellite-based methods for estimating BCP strength rely on empirical relationships between net primary production (NPP) and carbon export ratio. However, these methods contain a critical flaw: they cannot distinguish between organic matter that is quickly recycled near the surface and organic matter that reaches the deep ocean. Using an inverse biogeochemical model, we demonstrate that ~60% of satellite-measured NPP is rapidly recycled in the euphotic zone and never influences deep ocean tracer distributions. This finding has two important implications. First, the true strength of the BCP is better constrained by deep ocean tracer distributions than by satellite measurements - our model shows consistent export rates regardless of which satellite NPP product is used. Second, while deep tracers robustly constrain the mean state of the BCP, they are less useful for capturing temporal variability. We show that combining satellite observations with inverse modeling could help detect interannual variations in BCP strength, though longer observational records of both satellite and hydrographic data will be needed to make these temporal signals statistically significant.

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 CO ₂ drawdown and air‐sea CO ₂ 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.