Negative emission technologies (NETs) are an integral part of most climate change mitigation scenarios limiting global warming to 1.5 °C above pre-industrial levels. Several different NETs have been proposed, including ocean alkalinization and direct CO2 removal which have been considered as methods with high carbon removal potential. In ocean alkalinization partial pressure of CO2 sea surface is reduced by spreading alkaline material and in direct removal of CO2 it is extracted from sea water and transported to permanent reservoir. To date, most studies on ocean-based NETs with Earth System Models have been based on idealized scenarios where atmospheric carbon is either simply removed by prescribed amount or some NET is deployed at magnitudes that would be extremely challenging to reach if any economic, technical, or political constraints were considered.In this work, we present Earth System Model simulations using a more realistic global deployment scenario for ocean alkalinization with CaO dispersed at ocean surface in the exclusive economic zones of US, Europe, and China. The dispersion scenario is based on current excess capacities in the lime and cement industries in these three regions, and high-end projections on how they could evolve until 2100. We use the high-overshoot SSP5-3.4-OS as the socioeconomic background scenario. We simulate the deployment scenarios with several Earth System Models. We will show results from simulations with alkalinity enhancement deployment initiated in 2030 and 2040. Furthermore, we compare these results with simulations of direct removal of CO2. Here, the direct removal is calculated from the added alkalinity using approximation for CO2 uptake factor using the relation between alkalinity and dissolved inorganic carbon.The results show that the CO2 is being removed from the atmosphere to oceans after the alkalinity deployment. Compared to the control simulation the global CO2 concentration is reduced by about 7 ppm in the deployment scenario starting in 2030 and about 4 ppm in the deployment scenario starting 2040 by end of the century. For real life deployment the efficacy and detectability of the alkalinity enhancement is a major concern. We will show that the temperature change in the earlier deployment scenario (higher removal potential) cannot be distinguished from the annual variability illustrating the problem in detectability. Furthermore, the simulations show the deployment must be constrained in regions with low oceanic transport to inhibit the precipitation of CaCO3 to retain the CO2 removal potential.Using a more realistic scenario for ocean alkalinization we can give a more realistic assessment of its climate effects and explore new research questions such as detectability of local changes in pH or carbon fluxes with slowly increasing deployment rates. In the realistic deployment scenario, ocean alkalinization decreases the CO2 concentration but does not produce a large signal in the temperature. Therefore, this method can be seen as having potential but its role in removing carbon from the atmosphere is limited, according to these scenarios. Furthermore, the wider effects on the Earth system still require more analysis.

The Southern Ocean is a major sink of anthropogenic carbon and excess heat. In this region, the Earth system model projections of these sinks provided by the CMIP5 and CMIP6 scenario experiments show a large model spread. This contributes significantly to the large uncertainties in the overall climate sensitivity and remaining carbon budgets for ambitious climate targets. Hence, a reduction in the uncertainty of the future Southern Ocean carbon and heat sinks is urgently needed. Globally, Bronselaer and Zanna (2020) identified an emergent coupling between anthropogenic carbon and excess heat uptake, highlighting that the passive-tracer behavior of these two quantities is dominant under high-emissions scenarios. This coupling indicates that the use of a single observational constraint might be sufficient to reduce projection uncertainties in both anthropogenic carbon and excess heat uptake. Here, we use this approach for the northern limb of the Southern Ocean (30°S-55°S) where the subduction of intermediate and mode water is known to drive carbon and heat uptake. We found that, in this region, the variations in the models’ contemporary water-column stability over the first 2000 m is highly correlated to both their future anthropogenic carbon uptake and excess heat uptake efficiency. Using observational data of water-column stability, we reduce the uncertainty of future estimates of (1) the cumulative anthropogenic carbon uptake by up to 53% and (2) the excess heat uptake efficiency by 28%. Independent studies have found similar constraints in the Southern Ocean and globally, strengthening our findings (Liu et al., 2023; Newsom et al., 2023; Terhaar et al., 2021, 2022), and pinpointing that a better representation of water-column stratification in Earth system models is essential to improve future anthropogenic climate change projections. Bourgeois, T., Goris, N., Schwinger, J., and Tjiputra, J. F.: Stratification constrains future heat and carbon uptake in the Southern Ocean between 30°S and 55°S, Nat Commun, 13, 340, https://doi.org/10.1038/s41467-022-27979-5, 2022. Bronselaer, B. and Zanna, L.: Heat and carbon coupling reveals ocean warming due to circulation changes, Nature, 584, 227–233, https://doi.org/10.1038/s41586-020-2573-5, 2020. Liu, M., Soden, B. J., Vecchi, G. A., and Wang, C.: The Spread of Ocean Heat Uptake Efficiency Traced to Ocean Salinity, Geophys. Res. Lett., 50, e2022GL100171, https://doi.org/10.1029/2022GL100171, 2023. Newsom, E., Zanna, L., and Gregory, J.: Background Pycnocline Depth Constrains Future Ocean Heat Uptake Efficiency, Geophys. Res. Lett., 50, e2023GL105673, https://doi.org/10.1029/2023GL105673, 2023. Terhaar, J., Frölicher, T. L., and Joos, F.: Southern Ocean anthropogenic carbon sink constrained by sea surface salinity, Sci. Adv., 7, eabd5964, https://doi.org/10.1126/sciadv.abd5964, 2021. Terhaar, J., Frölicher, T. L., and Joos, F.: Observation-constrained estimates of the global ocean carbon sink from Earth system models, Biogeosciences, 19, 4431–4457, https://doi.org/10.5194/bg-19-4431-2022, 2022.

Tipping points are thresholds beyond which large, abrupt and possibly irreversible changes in the climate system or in large scale ecosystems would occur. The crossing of such tipping points under anthropogenic forcing poses a threat to biodiversity, food security, and human societies. However, due to the complexity of the processes involved, it remains notoriously difficult to determine exact thresholds that need to be avoided to stay within a “safe operating space” for humanity. Here, we map, for a variety of mitigation metrics, the crossing of thresholds, which we define to represent a wide range of deviations from the unperturbed state. We assess the crossing of these thresholds in a wide range of plausible future emission pathways: two climate mitigation scenarios (one with a strong overshoot) and one no-mitigation high-emissions scenario. These scenarios are simulated by the latest generation of Earth system models and by two Earth system models of intermediate complexity, for which we created large perturbed-parameter ensembles. Using this comprehensive model database we provide estimates of when and at which warming level 4 mitigation targets (thresholds) for 14 different impact metrics are exceeded along with an assessment of uncertainties. We find that under the high-emissions scenario, even the highest thresholds for many of the impact metrics are exceeded with high confidence, such as the expansion of ocean areas that are undersaturated with respect to aragonite, decreases in plankton biomass, Arctic summer sea ice extent, strength of the Atlantic meridional overturning circulation (AMOC), and subsurface oxygen concentration. The risk of exceeding a given mitigation target decreases under low-emissions and overshoot scenarios. Yet, exceedance of ambitious targets for aragonite undersaturation, Arctic summer sea ice extent, and steric sea level rise (SSLR) are projected to be difficult to avoid (high confidence) even under the low-emissions scenario. The overshoot scenario reduces the risk of exceeding mitigation targets related to Arctic summer sea ice extent, SSLR, AMOC and plankton biomass compared to the high-emissions scenario, particularly in the long-term. Uncertainties in Earth system model projections of net primary production prevent us from concluding on the risk of mitigation target exceedance for this impact metric.

It is now expected carbon emissions will follow an overshoot trajectory. A realistic emission-driven overshoot scenario esm-SSP534-over is available from the CMIP6 archive. We analysed the simulations to examine reversibility of the Arctic sea ice cover. Reversibility here means that at the end of the 21st century the sea ice extent is the same as that at the earlier point in the century with the same atmospheric CO2 concentration. Firstly, in an emission driven simulation the system behaves differently on the upward and downward branches of CO2 concentration. We show it is better to use atmospheric CO2 concentration rather than Arctic surface air temperature, as the relation between the two is not linear. Total Arctic sea ice extent shows consistent behaviour in 3 out of 4 models (CNRM-ESM2, MIROC, UKESM1) with a CO2 concentration threshold above which sea ice becomes irreversible. This can be explained by the continued ocean heat transport into the Arctic even though the Atlantic Meridional Overturing Circulation (AMOC) declines. The NorESM model has very different behaviour, sea ice extent is reversible and even overshoots beyond the present-day extent. We suggest this is caused by the known strong AMOC decline in this model. The analysis indicates Arctic air temperature is a result of the changes in sea ice extent rather than the driving factor, as is often assumed, both ultimately controlled by ocean heat transport. From the available simulations we conclude there is large uncertainty in the future Arctic climate state. This uncertainty extends to the future global air temperatures as different models show different inertia on CO2 concentrations, which only materialises in the downward emission branch. This affects many other climate variables with their own time lag. Climate inertia and time delays in the earth system should be investigated further to improve fidelity of future projection. This necessitates the use of emission-driven scenarios instead of concentration-driven ones which do not allow for the full inclusion of internal earth system feedbacks. We acknowledge funding from the projects COMFORT (grant agreement no. 820989) and OceanNETs (grant agreement no. 869357) under the European Union’s Horizon 2020 research and innovation programme, and from the EC Horizon Europe project OptimESM “Optimal High Resolution Earth System Models for Exploring Future Climate Changes”, grant 101081193 and UKRI grant 10039429, from the project EPOC, EU grant 101059547 and UKRI grant 10038003. For the EU projects the work reflects only the authors’ view; the European Commission and their executive agency are not responsible for any use that may be made of the information the work contains.

AbstractThe physiological tolerances of marine species toward ambient temperature and oxygen can jointly be evaluated in a single metric: the metabolic index. Changes therein characterize a changing aerobic habitat tailored to species‐specific thermal and hypoxia sensitivity traits. If the geographical limits of marine species as indicated by critical thresholds of the metabolic index shift abruptly in response to ocean warming and deoxygenation, aerobic habitat could potentially be lost abruptly. Here, we assess the spatio‐temporal detectability of abrupt shifts in potential habitats for selected marine species within the Shared Socioeconomic Pathway 5–8.5 (SSP5‐8.5) scenario run with the fully coupled Norwegian Earth System Model version 2 (NorESM2‐LM). We use an environmental time series changepoint detection routine and analyze the number and timing of these abrupt changes over the past, present and future. We construct nine ecophysiotypes with low, medium, and high resting vulnerability to hypoxia and sensitivity of hypoxia vulnerability to temperature, respectively, with six different thresholds for minimal oxygen demand. For all ecophysiotypes with positive temperature sensitivity to hypoxia, the volume of non‐viable habitat in the upper ocean expands between 1850 and 2100. Changepoints in the metabolic index are detected in 49.0 ± 9.2% of the volume that eventually becomes non‐viable for all ecophysiotypes over the course of the 21st century. More than 75% of these abrupt shifts occur in response to warming close to the surface, while at depth, the abrupt shifts driven by changes in oxygen partial pressure become more important, with potentially severe consequences for marine species, populations, and ecosystems.

Ocean alkalinity enhancement (OAE) deliberately modifies the chemistry of the surface ocean to enhance the uptake of atmospheric CO2. The chemical efficiency of OAE (the amount of CO2 sequestered per unit of alkalinity added) depends, among other factors, on the background state of the surface ocean, which will significantly change until the end of this century and beyond. Here, we investigate the consequences of such changes for the long-term efficiency of OAE. We show, using idealized and scenario simulations with an Earth system model, that under doubling (quadrupling) of pre-industrial atmospheric CO2 concentrations, the simulated mean efficiency of OAE increases by about 18% (29%) from 0.76 to 0.90 (0.98). We find that only half of this effect can be explained by changes in the sensitivity of CO2 sequestration to alkalinity addition itself. The remainder is due to the larger portion of anthropogenic emissions taken up by a high-alkalinity ocean. Importantly, both effects are reversed if atmospheric CO2 concentrations were to decline due to large-scale deployment of land-based (or alternative ocean-based) carbon dioxide removal (CDR) methods. By considering an overshoot pathway that relies on large amounts of land-based CDR, we demonstrate that OAE efficiency indeed shows a strong decline after atmospheric CO2 concentrations have peaked. Our results suggest that the assumption of a constant, present-day chemical efficiency of OAE in integrated assessment modeling and carbon credit assignments could lead to economically inefficient OAE implementation pathways.

Ocean Alkalinization deliberately modifies the chemistry of the surface ocean to enhance the uptake of atmospheric CO2. Here, we quantify, using idealized and scenario Earth system model (ESM) simulations, changes in carbon cycle feedbacks and in the seasonal cycle of the surface ocean carbonate system due to ocean alkalinization. We find that both, the sensitivity to changes in atmospheric CO2 concentration (carbon-concentration feedback) as well as the sensitivity to temperature changes (carbon-climate feedback) are enhanced. While the temperature effect, which decreases ocean carbon uptake, remains small in our model, the carbon-concentration feedback enhances the uptake of carbon due to alkalinization by more than 20% compared to the carbon sequestration that alkalinity addition would facilitate at constant CO2 levels. This effect depends on the trajectory of atmospheric CO2 concentration, and leads to an increased loss of carbon from the ocean if net emissions become negative. The seasonal cycle of air-sea CO2 fluxes is enhanced due to an increased buffer capacity in an alkalinized ocean. The seasonal cycle of H+-ion concentration is also enhanced, although it remains smaller than under preindustrial conditions. This, together with an increased seasonal cycle of the aragonite saturation state in some regions, has the potential to adversely affect ecosystem health.

Understanding and limiting the spread of ocean carbon sink projections are crucial to effectively guide the development of climate mitigation policies, determine accurate future carbon budget and subsequently climate change. The North Atlantic and the Southern Ocean are two of the most intense sink regions for anthropogenic CO2 emissions, while the tropical Pacific is the largest outgassing system. Nevertheless, CMIP6 models simulate growing inter-model spread in future carbon fluxes in these regions. In this study, we apply an emergent constraint approach to reduce the projections uncertainties under the high-CO2 future scenario. The efficiency of surface-to-deep transport of anthropogenic carbon is commonly identified as the key mechanisms driving the systematic inter-model spread in high latitude regions. For the North Atlantic region, we further use a genetic algorithm to optimize our identified emergent constraint relationship by isolating the region where contemporary model bias strongly correlates with the projection spread. The interior biogeochemical state is key for constraining the future tropical Pacific CO2 flux. Our study consolidates the importance of improving representations of anthropogenic carbon ventilation mechanisms in models and sustaining carbon and watermass monitoring network in these regions to improve the fidelity of future model projections.

The Arctic Ocean is generally undersaturated in CO2 and acts as a net sink of atmospheric CO2. This oceanic uptake is strongly modulated by sea ice, which can prevent air-sea gas exchange and has major impacts on stratification and primary production. Moreover, carbon is stored in sea ice with a ratio of alkalinity to dissolved inorganic carbon that is larger than in seawater. It has been suggested that this storage amplifies the seasonal cycle of seawater pCO(2) and leads to an increase in oceanic carbon uptake in seasonally ice-covered regions compared to those that are ice-free. Given the rapidly changing ice scape in the Arctic Ocean, a better understanding of the link between the seasonal cycle of sea ice and oceanic uptake of CO2 is needed. Here, we investigate how the storage of carbon in sea ice affects the air-sea CO2 flux and quantify its dependence on the ratio of alkalinity to inorganic carbon in ice. To this end, we present two independent approaches: a theoretical framework that provides an analytical expression of the amplification of carbon uptake in seasonally ice-covered oceans and a simple parameterization of carbon storage in sea ice implemented in a 1D physical-biogeochemical ocean model. Sensitivity simulations show a linear relation between ice melt and the amplification of seasonal carbon uptake. A 30 % increase in carbon uptake in the Arctic Ocean is estimated compared to ice melt without amplification. Applying this relationship to different future scenarios from an earth system model that does not account for the effect of carbon storage in sea ice suggests that Arctic Ocean carbon uptake is underestimated by 5 % to 15 % in these simulations.

Abstract. Abrupt changes in ocean biogeochemical variables occur as a result of human-induced climate forcing as well as those which are more gradual and occur over longer timescales. These abrupt changes have not yet been identified and quantified to the same extent as the more gradual ones. We review and synthesise abrupt changes in ocean biogeochemistry under human-induced climatic forcing. We specifically address the ocean carbon and oxygen cycles because the related processes of acidification and deoxygenation provide important ecosystem hazards. Since biogeochemical cycles depend also on the physical environment, we also describe the relevant changes in warming, circulation, and sea ice. We include an overview of the reversibility or irreversibility of abrupt marine biogeochemical changes. Important implications of abrupt biogeochemical changes for ecosystems are also discussed. We conclude that there is evidence for increasing occurrence and extent of abrupt changes in ocean biogeochemistry as a consequence of rising greenhouse gas emissions.

Negative emission technologies (NETs) are an integral part of most climate change mitigation scenarios limiting global warming to 1.5 °C above preindustrial levels. Several different NETs have been proposed, including ocean alkalinization that has been considered as one method with high carbon removal potential. To date, most studies on NETs with Earth System Models have been based on idealized scenarios where atmospheric carbon is either simply removed by prescribed amount or some NET is deployed at magnitudes that would be extremely challenging to reach if any economic, technical, or political constraints were considered. Here, we present a more realistic global deployment scenario for ocean alkalinization with Ca(OH)2 dispersed at ocean surface in the exclusive economic zones of US, EU, and China, based on their respective production capacities. The dispersion scenario is based on current excess capacities in the lime and cement industries, and high-end projections on how they could evolve until 2100. We use the high-overshoot SSP5-3.4-OS as the socioeconomic background scenario. We simulate the deployment scenario with two different Earth System Models: EC-Earth and NorESM2-LM. In addition to this sophisticated scenario, we carry out an idealized scenario with a uniform addition of 0.5 Gt Ca(OH)2 per year in the same coastal areas. The preliminary results show that the idealized 0.5 Gt Ca(OH)2 flux decreased the atmospheric CO2 concentration by 7 ppm in the first 15 years. The effects on ocean carbon uptake and surface ocean pH were strongly localized near the dispersion regions. The early version of the dispersion zone also included the Baltic Sea and the Mediterranean Sea, which led to significant increase in the alkalinity in these sea regions as the water exchange with the wider oceans are limited there. By providing a more realistic scenario for ocean alkalinization, we can give also more realistic assessment of climate effects and explore new research questions such as detectability of local changes in pH or carbon fluxes with slowly increasing deployment rates.

With ongoing climate change, multiple stressors including ocean warming, deoxygenation, ocean acidification and limited nutrient availability are expected to lead to considerable regime shifts within marine ecosystems [1]. However, distinguishing such abrupt shifts from long-term trends in physical and biogeochemical ocean variables may not only be obscured by the natural variability of the system, but also the complexity of the ecosystem itself. Moreover, species-dependent physiological tolerances are likely going to limit the detectability of crossing of thresholds or tipping points of the whole ecosystem. The metabolic index describes temperature-dependent hypoxic tolerances with respect to the oxygen supply [2]. Critical values of the metabolic index indicate the geographical limits of marine species, therefore it is a useful metric to describe the extent of a potential habitat. Here, we assess the spatio-temporal detectability of abrupt changes in such a potential habitat for selected marine species using an environmental time series changepoint detection routine developed by [3]. We compare the number and timing of these abrupt changes in different Shared Socioeconomic Pathways (SSPs) run with the fully coupled Norwegian Earth System Model version 2 (NorESM2), i.e., analysing the SSP1-26, SSP-5-34-OS, and SSP5-85 scenarios. Preliminary results reveal global, regional and local abrupt changes of lost metabolically viable potential habitat in relation to environmental stressors under different evolving climates. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 820989 (project COMFORT). The work reflects only the authors’ view; the European Commission and their executive agency are not responsible for any use that may be made of the information the work contains. [1] Heinze et al., 2020, The quiet crossing of tipping points, PNAS, 118(9) [2] Deutsch et al., 2020, Metabolic trait diversity shapes marine biogeography, Nature, 585, 557-562 [3] Beaulieu and Killick, 2018, Distinguishing trends and shifts from memory in climate data, Journal of Climate, 31(23), 9519-9543

The Southern Ocean between 30°S and 55°S is a major sink of excess heat and anthropogenic carbon, but model projections of these sinks remain highly uncertain. Reducing such uncertainties is required to effectively guide the development of climate mitigation policies for meeting the ambitious climate targets of the Paris Agreement. Here, we show that the large spread in the projections of future excess heat uptake efficiency and cumulative anthropogenic carbon uptake in this region are strongly linked to the models' contemporary stratification. This relationship is robust across two generations of Earth system models and is used to reduce the uncertainty of future estimates of the cumulative anthropogenic carbon uptake by up to 53% and the excess heat uptake efficiency by 28%. Our results highlight that, for this region, an improved representation of stratification in Earth system models is key to constrain future carbon budgets and climate change projections.

How does storage of carbon in sea ice impacts the partial pressure of CO 2 ?To answer this question, one can consider the upper layer of the ocean, partially covered by sea ice forming and melting.The atmospheric pCO 2 and wind speed at 10m height are known, as are the ice concentration and meltingfreezing rate and the ocean surface temperature and salinity.The surface ocean pCO 2 (in µatm) would vary along the seasonal cycle due to changes in temperature, salinity, air-sea gas exchange, dilution and concentration related to surface freshwater fluxes (either from precipitation-evaporation, or from ice melt and formation).Biology would also impact carbonate properties due to production and respiration.Finally, advection and mixing could also modify surface properties.Two scenarios emerge, with for sole difference the storage and release of alkalinity and dissolved inorganic carbon (DIC) by sea ice.

The Southern Ocean is a major sink of anthropogenic carbon and excess heat. The Earth system model projections of these sinks provided by the CMIP5 and CMIP6 scenario experiments show a large model spread contributing to the large uncertainties in climate sensitivity and remaining carbon budgets for ambitious climate targets. A recent study identified an emergent coupling between anthropogenic carbon and excess heat uptake, highlighting that the passive-tracer behavior of these two quantities is dominant under high-emission scenarios. This coupling indicates that the use of a single observational constraint might be sufficient to reduce projection uncertainties in both anthropogenic carbon and excess heat uptake. In the northern limb of the Southern Ocean (30°S-55°S) where the subduction of intermediate and mode water is known to drive carbon and heat uptake, we find that the variations in model´s contemporary water-column stability over the first 2000 m is highly correlated to both its future anthropogenic carbon uptake and excess heat uptake efficiency. Using observational data, we reduce the uncertainty of future estimates of (1) the cumulative anthropogenic carbon uptake by up to 53% and (2) the excess heat uptake efficiency by 28%. Our results show that improving the representation of water-column stratification in Earth system models should be prioritized to improve future anthropogenic climate change projections.

Dataset of one-dimensional runs for investigation on the sea ice carbon pump. Associated with Sect. 3.1 and 4.1 of manuscript "Underestimation of oceanic carbon uptake in the Arctic Ocean: Ice melt as predictor of the sea ice carbon pump".

The North Atlantic and Southern Oceans are major sinks of anthropogenic carbon and excess heat. The Earth system model projections of these sinks provided by the CMIP5 and CMIP6 scenario experiments remain highly uncertain, hindering an effective development of climate mitigation policies for meeting the ambitious climate targets laid down in the Paris agreement. A recent study identified an emergent coupling between anthropogenic carbon and excess heat uptake, highlighting the dominant passive-tracer behavior of these two quantities under high-emission scenarios. This coupling potentially allows for the use of a single observational constraint to reduce these projection uncertainties. As a first step, we investigate the causes of these uncertainties in the Southern Ocean (30°S-55°S) by looking regionally at different contemporary physical and biogeochemical quantities. We find that the variations in model´s contemporary water-column stability over the first 2000 m is highly correlated to both its future anthropogenic carbon uptake and excess heat uptake efficiency. Using an observation-based estimate of contemporary water-column stability, this allows us to reduce the uncertainty of future estimates of (1) the cumulative anthropogenic carbon uptake by up to 50% and (2) the excess heat uptake efficiency by 23%. Our results show that improving representation of water-column stratification in Earth system models should be prioritized to constrain future carbon budget and climate change projections.

The uptake of anthropogenic carbon (Cant) by the ocean leads to ocean acidification, causing the reduction of pH and the saturation states of aragonite (Ωarag) and calcite (Ωcalc). The Arctic Ocean is particularly vulnerable to ocean acidification due to its naturally low pH and saturation states and due to ongoing freshening and the concurrent reduction in total alkalinity in this region. Here, we analyse ocean acidification in the Arctic Ocean over the 21st century across 14 Earth system models (ESMs) from the latest Coupled Model Intercomparison Project Phase 6 (CMIP6). Compared to the previous model generation (CMIP5), models generally better simulate maximum sea surface densities in the Arctic Ocean and consequently the transport of Cant into the Arctic Ocean interior, with simulated historical increases in Cant in improved agreement with observational products. Moreover, in CMIP6 the inter-model uncertainty of projected changes over the 21st century in Arctic Ocean Ωarag and Ωcalc averaged over the upper 1000 m is reduced by 44–64 %. The strong reduction in projection uncertainties of Ωarag and Ωcalc can be attributed to compensation between Cant uptake and total alkalinity reduction in the latest models. Specifically, ESMs with a large increase in Arctic Ocean Cant over the 21st century tend to simulate a relatively weak concurrent freshening and alkalinity reduction, while ESMs with a small increase in Cant simulate a relatively strong freshening and concurrent total alkalinity reduction. Although both mechanisms contribute to Arctic Ocean acidification over the 21st century, the increase in Cant remains the dominant driver. Even under the low-emissions Shared Socioeconomic Pathway 1-2.6 (SSP1-2.6), basin-wide averaged Ωarag undersaturation in the upper 1000 m occurs before the end of the century. While under the high-emissions pathway SSP5-8.5, the Arctic Ocean mesopelagic is projected to even become undersaturated with respect to calcite. An emergent constraint identified in CMIP5 which relates present-day maximum sea surface densities in the Arctic Ocean to the projected end-of-century Arctic Ocean Cant inventory is found to generally hold in CMIP6. However, a coincident constraint on Arctic declines in Ωarag and Ωcalc is not apparent in the new generation of models. This is due to both the reduction in Ωarag and Ωcalc projection uncertainty and the weaker direct relationship between projected changes in Arctic Ocean Cant and changes in Ωarag and Ωcalc.

A quantification of carbon fluxes in the coastal ocean and across its boundaries with the atmosphere, land, and the open ocean is important for assessing the current state and projecting future trends in ocean carbon uptake and coastal ocean acidification, but this is currently a missing component of global carbon budgeting. This synthesis reviews recent progress in characterizing these carbon fluxes for the North American coastal ocean. Several observing networks and high-resolution regional models are now available. Recent efforts have focused primarily on quantifying the net air–sea exchange of carbon dioxide (CO2). Some studies have estimated other key fluxes, such as the exchange of organic and inorganic carbon between shelves and the open ocean. Available estimates of air–sea CO2 flux, informed by more than a decade of observations, indicate that the North American Exclusive Economic Zone (EEZ) acts as a sink of 160±80 Tg C yr−1, although this flux is not well constrained. The Arctic and sub-Arctic, mid-latitude Atlantic, and mid-latitude Pacific portions of the EEZ account for 104, 62, and −3.7 Tg C yr−1, respectively, while making up 51 %, 25 %, and 24 % of the total area, respectively. Combining the net uptake of 160±80 Tg C yr−1 with an estimated carbon input from land of 106±30 Tg C yr−1 minus an estimated burial of 65±55 Tg C yr−1 and an estimated accumulation of dissolved carbon in EEZ waters of 50±25 Tg C yr−1 implies a carbon export of 151±105 Tg C yr−1 to the open ocean. The increasing concentration of inorganic carbon in coastal and open-ocean waters leads to ocean acidification. As a result, conditions favoring the dissolution of calcium carbonate occur regularly in subsurface coastal waters in the Arctic, which are naturally prone to low pH, and the North Pacific, where upwelling of deep, carbon-rich waters has intensified. Expanded monitoring and extension of existing model capabilities are required to provide more reliable coastal carbon budgets, projections of future states of the coastal ocean, and quantification of anthropogenic carbon contributions.