Ana C. Franco

In the Northeast Pacific, the marine carbonate system's variability across timescales is not well constrained. Here, we quantify observed seasonal and non‐seasonal variability in Dissolved Inorganic Carbon (DIC), partial pressure of carbon dioxide () and aragonite saturation state and discuss potential drivers. We used three decades of observations from four Line P time series stations, the longest marine carbonate system time series in the Northeast Pacific (1990–2019). To gauge the spatial extent of the variability patterns, we used output from a global ocean model representing the observed period. In the Northeast Pacific, seasonal and non‐seasonal variability at 10 m was minimal, mostly damped by the opposing influence of DIC and temperature changes at both seasonal and interannual timescales. For DIC and , the seasonal cycle dominated total variability in the top 60–70 m, with mean‐transect 10 m seasonal amplitudes of 35 3 μmol and 0.31 0.04, respectively. In the upper 60–70 m, the magnitude of non‐seasonal variability was at least half that of the seasonal variability for most variables. From five climate indices examined, we focused on the basin‐scale Pacific Decadal Oscillation index (PDO) to investigate potential drivers of non‐seasonal variability, with 20%–40% of the non‐seasonal variability in DIC and associated to this index. In the Northeast Pacific, positive PDO periods were linked to a mean reduction in 10 m DIC of 5 μmol and an increase in 10 m of 0.04 for each PDO unit increase, which could potentially reduce the occurrence and severity of ocean acidification events. The opposite could be expected during negative PDO periods.

Marine heatwaves and biogeochemical extremes such as deoxygenation and acidification are intensifying with climate change, becoming more frequent, persistent, and spatially extensive. Of particular concern are compound events, simultaneous extremes in multiple stressors, which can interact nonlinearly and trigger severe ecosystem disruptions. While their occurrence under long-term warming is increasingly documented, much less is known about their evolution in overshooting scenarios, where global temperatures temporarily exceed the Paris Agreement’s 1.5 °C target before declining through large-scale deployment of carbon dioxide removal (CDR). Such pathways raise critical questions about whether and when marine stress conditions can return to earlier states. Here we use simulations from the Horizon Europe project RESCUE (Response of the Earth System to overshoot, climate neutrality and negative emissions), which develops pairs (overshoot vs straight-stabilization) of novel socio-economic scenarios incorporating a broad portfolio of CDR strategies and arriving at the same cumulative carbon budget by the end of the century. We assess differences in the spatial patterns, frequency, intensity, and duration of compound events between an overshoot and its respective straight-stabilization trajectory. In addition, we evaluate ecosystem exposure to cumulative stress using indices for heat, hypoxia, and acidification, defined as exposure time below ecologically dangerous thresholds for marine organisms. Our analysis focuses on the persistence of these new extreme regimes and on when and if they can be reversed.

Substantial natural variability can obscure the detection of anthropogenic long-term trends in the marine carbonate system (e.g., ocean acidification). Yet the magnitude of the trends and variability remains poorly constrained due to limited marine carbonate system observations. Here, we use a Bayesian machine-learning approach based on Gaussian Process Regression (GPR) to decompose total variability of ocean acidification-related variables into seasonal, interannual and long-term components. The method is first applied to three decades of observations from the Line P carbon program, the longest marine carbonate system timeseries in the Northeast Pacific (1990-2019), typically taking samples three times per year. We found that over the period from 1990 to 2019, the local oceanic uptake of anthropogenic carbon dioxide from the atmosphere was the main driver of long-term changes in the marine carbonate system, including acidification. The seasonal cycle of dissolved inorganic carbon and the aragonite saturation state (both indicators of ocean acidification) was the dominant contributor to total variability in the top 60-70 m of the water column, with a mean surface seasonal amplitude of 35 ± 3 µmol kg−1 and 0.31 ± 0.04, respectively. In this depth range, the magnitude of the interannual variability was at least half of the seasonal variability for most variables. We then apply GPR to output from a global ocean biogeochemical model subsampled as per availability of observations, to assess the observational effort required to detect future ocean carbon trends, with a particular focus on detecting signals related to potential marine carbon dioxide removal interventions.

As the largest active carbon reservoir on Earth, the ocean is a cornerstone of the global carbon cycle, playing a pivotal role in modulating ocean health and the Earth's climate system. Understanding these crucial roles requires access to a broad array of data products documenting the changing chemistry of the global ocean as a vast and interconnected system. This review article provides an overview of 68 existing ocean carbonate chemistry data products and data product sets, encompassing compilations of cruise datasets, derived gap-filled data products, model simulations, and compilations thereof. It is intended to help researchers identify and access data products that best align with their research objectives, thereby advancing our understanding of the ocean's evolving carbonate chemistry. The list will be updated periodically to incorporate new data products. The most up-to-date list is available at https://oceanco2.github.io/co2-products/ (Gregor and Jiang, 2026).

The Northeast Pacific is an important net sink of atmospheric CO2. However, substantial natural variability modulates the long-term increase of seawater partial pressure of CO2 (pCO2), potentially influencing the magnitude of the CO2 sink. In addition to the seasonal cycle, the Pacific Decadal Oscillation (PDO) is known to play a role in driving a large fraction of the non-seasonal variability in the region. Yet, the magnitude of this natural variability, especially periods of high surface dissolved inorganic carbon (DIC), are not well constrained. Here we quantify the seasonal and non-seasonal variability in DIC and pCO2 using observations from the Line P program, the longest marine carbonate system time-series transect in the NE Pacific (1990-2019), as well as an ensemble of historical simulations with an Earth system model (EC-Earth-CC). Preliminary results show that the mean amplitude of the DIC seasonal cycle is similar across our NE Pacific transect (23-30 µmol kg-1) and decreases with depth to less than 5 µmol kg-1 at 60 to 70 m. In contrast, the non-seasonal variability remains approximately constant with depth, ranging between 10 – 20 µmol kg-1. We quantify the role of the PDO in driving this residual non-seasonal variability, and analyse the contrasting impact of temperature and DIC changes in controlling surface pCO2 during opposite phases of the PDO.

Carbon Dioxide Removal (CDR) technologies are imperative for achieving net zero emissions, a crucial feat to meet the 2º target set in the Paris Agreement. Ocean Alkalinity Enhancement (OAE) is a marine CDR technology that consists of increasing the Total Alkalinity (TA) of the ocean by depositing alkaline minerals to ocean surface waters. The increase in TA reduces the sea surface partial pressure of CO2 (pCO2), thereby enhancing oceanic CO2 uptake or reducing oceanic CO2 outgassing. Despite the potential of OAE to reduce atmospheric CO2 concentrations, the realistic implementation of OAE faces substantial impediments, including logistical feasibility and the lack of international ocean governance for its deployment in open waters. To address these obstacles and incentivize the development of a policy framework for OAE, we set forward optimal conditions that maximize the efficiency of OAE in the North Pacific Ocean, leveraging natural climatic variability induced by the Pacific Decadal Oscillation (PDO). The addition of TA at high Dissolved Inorganic Carbon (DIC) concentrations has the potential to induce a stronger decrease in pCO2 than at lower DIC concentrations. Therefore, natural temporal increases in surface DIC concentrations could potentially predispose the system for enhanced OAE efficiency. The PDO induces multi-decadal variations in the carbonate system, with the potential to influence the spatiotemporal variability in OAE efficiency. PDO phases have been shown to be predictable up to a decade ahead, thereby providing a practical indication for logistical planning of OAE deployment. We analyze the influence of the PDO on OAE efficiency in the North Pacific Ocean through four Earth System Model simulations under a high emission scenario (RCP8.5) spanning from 2020 to 2100. Using theoretical CO2 uptake efficiencies, as defined by Tyka et al. (2022) and Renforth and Henderson (2017), we describe how PDO states modulate variability in uptake efficiency via their control on DIC and TA concentrations. Subsequently, we analyze the realized uptake efficiency by contrasting oceanic air-water CO2 fluxes (FCO2) in simulations with continuous and homogenous global OAE deployment against simulations without CDR intervention per unit of added TA. Early results show regional differences in OAE efficiency rates during different PDO phases. During positive PDO phases, theoretical CO2 uptake efficiencies decrease in the Northeast Pacific while increasing in the central Western Pacific, corresponding to respectively lower and higher DIC concentrations. The inverse responses are observed during negative PDO phases. We discern differences between theoretical and realized CO2 uptake efficiencies, indicating the role of additional influential variables. Our study provides new insights into the impact of the PDO on OAE efficiency and the potential to optimize CDR strategies by aligning them with natural climatic variations.

Oxygen is crucial for the survival of marine species. Yet, the ocean has experienced a loss of approximately 2% of its oxygen inventory since the last century, resulting in adverse impacts on marine life and ecosystems. In particular, changes in the gap between the supply and demand for dissolved oxygen lead to physiological and ecological variations, which cause alterations in habitats and food webs for fish and ecosystem services. These changes vary over time and by region, and the heterogeneous characteristics of marine species bring about non-linear consequences to human society. Despite this, identifying the potential ripple effects of deoxygenation on human society is challenging due to the integrated impacts of other stressors, such as global warming and ocean acidification, and their varying changes depending on environmental conditions and regions, such as upwelling and eutrophication. Therefore, we conducted a literature review on ocean deoxygenation and its effects on fish dynamics and the ecosystem, with a focus on the environmental and societal impact, to present crucial considerations and pathways for future research on ocean deoxygenation. We found that quantitative approaches are necessary to assess the dynamic changes under deoxygenation, and the consequent effects on marine ecosystems should be verified to exploit the natural resources from the ocean. One of the most reliable approaches to quantifying the ripple impacts of deoxygenation is to model spatial and temporal changes with other climate stressors, forming a global network encompassing socio-economic and regional effects of this global change to facilitate and improve capabilities to address the impacts of ocean deoxygenation.

Bottom waters of the northeast Pacific continental shelf naturally experience localized hypoxic conditions, with significant influences on food webs and biogeochemical cycling. In August 2021, extreme hypoxia was detected from several measurement platforms along the southern British Columbia continental shelf, with oxygen concentration −1, and a difference from the seasonal climatology of more than 2 standard deviations. Early and intense remote upwelling and local density shifts were associated with an anomalously strong spring phytoplankton bloom, which likely stimulated localized respiration of subsurface organic matter. This event was concurrent with unsuitable habitat for Pacific halibut and calcite and aragonite undersaturation throughout most of the water column. The drivers of this extreme low oxygen event could be enhanced under future climate change, with potentially significant impacts on marine ecology and biogeochemistry.

Improving our understanding of how the ocean absorbs carbon dioxide is critical to climate change mitigation efforts. We, a group of early career ocean professionals working in Canada, summarize current research and identify steps forward to improve our understanding of the marine carbon sink in Canadian national and offshore waters. We have compiled an extensive collection of reported surface ocean air–sea carbon dioxide exchange values within each of Canada's three adjacent ocean basins. We review the current understanding of air–sea carbon fluxes and identify major challenges limiting our understanding in the Pacific, the Arctic, and the Atlantic Ocean. We focus on ways of reducing uncertainty to inform Canada's carbon stocktake, establish baselines for marine carbon dioxide removal projects, and support efforts to mitigate and adapt to ocean acidification. Future directions recommended by this group include investing in maturing and building capacity in the use of marine carbon sensors, improving ocean biogeochemical models fit-for-purpose in regional and ocean carbon dioxide removal applications, creating transparent and robust monitoring, verification, and reporting protocols for marine carbon dioxide removal, tailoring community-specific approaches to co-generate knowledge with First Nations, and advancing training opportunities for early career ocean professionals in marine carbon science and technology.

Ocean warming and deoxygenation are already modifying the habitats of many aerobic organisms. Benthic habitat in the Northeast Pacific is sensitive to deoxygenation, as low oxygen concentrations occur naturally in continental shelf bottom waters. Here, we examine the potential impacts of deoxygenation and ocean warming on the habitat distribution of Pacific halibut (Hippoglossus stenolepis), one of the most commercially important groundfish in North America. We combine fisheries‐independent Pacific halibut survey data (1998–2020) with oceanographic measurements and a regional ocean circulation model to investigate current and future (end of 21st century) influences of deoxygenation and warming on optimal Pacific halibut habitat. We use the observations and model output to derive a metabolic index of Pacific halibut‐specific suitable habitat. Our results show high Pacific halibut counts in regions where the metabolic index is greatest and demonstrate that interannual variability in Pacific halibut abundance is coherent with the Pacific Decadal Oscillation. Working with model projections, we examine potential future changes in suitable Pacific halibut habitat by the end of the century under a high carbon dioxide emissions scenario. These projections indicate that suitable Pacific halibut habitat may largely disappear off the coast of Washington state, retreating approximately 5° latitude northward. In bottom waters along coastal British Columbia and Alaska continental shelf, Pacific halibut habitat is projected to decrease by about 50%. Such habitat changes may potentially drive a northward shift in Pacific halibut, with significant implications for commercial fisheries.