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Anthropogenic climate change is projected to lead to ocean warming, acidification, deoxygenation, reductions in near-surface nutrients, and changes to primary production, all of which are expected to affect marine ecosystems. Here we assess projections of these drivers of environmental change over the twenty-first century from Earth system models (ESMs) participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) that were forced under the CMIP6 Shared Socioeconomic Pathways (SSPs). Projections are compared to those from the previous generation (CMIP5) forced under the Representative Concentration Pathways (RCPs). A total of 10 CMIP5 and 13 CMIP6 models are used in the two multi-model ensembles. Under the high-emission scenario SSP5-8.5, the multi-model global mean change (2080–2099 mean values relative to 1870–1899) ± the inter-model SD in sea surface temperature, surface pH, subsurface (100–600 m) oxygen concentration, euphotic (0–100 m) nitrate concentration, and depth-integrated primary production is +3.47±0.78 ∘C, -0.44±0.005, -13.27±5.28, -1.06±0.45 mmol m−3 and -2.99±9.11 %, respectively. Under the low-emission, high-mitigation scenario SSP1-2.6, the corresponding global changes are +1.42±0.32 ∘C, -0.16±0.002, -6.36±2.92, -0.52±0.23 mmol m−3, and -0.56±4.12 %. Projected exposure of the marine ecosystem to these drivers of ocean change depends largely on the extent of future emissions, consistent with previous studies. The ESMs in CMIP6 generally project greater warming, acidification, deoxygenation, and nitrate reductions but lesser primary production declines than those from CMIP5 under comparable radiative forcing. The increased projected ocean warming results from a general increase in the climate sensitivity of CMIP6 models relative to those of CMIP5. This enhanced warming increases upper-ocean stratification in CMIP6 projections, which contributes to greater reductions in upper-ocean nitrate and subsurface oxygen ventilation. The greater surface acidification in CMIP6 is primarily a consequence of the SSPs having higher associated atmospheric CO2 concentrations than their RCP analogues for the same radiative forcing. We find no consistent reduction in inter-model uncertainties, and even an increase in net primary production inter-model uncertainties in CMIP6, as compared to CMIP5.

Measurements show large decadal variability in the rate of CO₂ accumulation in the atmosphere that is not driven by CO₂ emissions. The decade of the 1990s experienced enhanced carbon accumulation in the atmosphere relative to emissions, while in the 2000s, the atmospheric growth rate slowed, even though emissions grew rapidly. These variations are driven by natural sources and sinks of CO₂ due to the ocean and the terrestrial biosphere. In this study, we compare three independent methods for estimating oceanic CO₂ uptake and find that the ocean carbon sink could be responsible for up to 40% of the observed decadal variability in atmospheric CO₂ accumulation. Data-based estimates of the ocean carbon sink from pCO₂ mapping methods and decadal ocean inverse models generally agree on the magnitude and sign of decadal variability in the ocean CO₂ sink at both global and regional scales. Simulations with ocean biogeochemical models confirm that climate variability drove the observed decadal trends in ocean CO₂ uptake, but also demonstrate that the sensitivity of ocean CO₂ uptake to climate variability may be too weak in models. Furthermore, all estimates point toward coherent decadal variability in the oceanic and terrestrial CO₂ sinks, and this variability is not well-matched by current global vegetation models. Reconciling these differences will help to constrain the sensitivity of oceanic and terrestrial CO₂ uptake to climate variability and lead to improved climate projections and decadal climate predictions.

The recent IPCC reports state that continued anthropogenic greenhouse gas emissions are changing the climate, threatening severe, pervasive and irreversible impacts. Slow progress in emissions reduction to mitigate climate change is resulting in increased attention to what is called geoengineering, climate engineering, or climate intervention – deliberate interventions to counter climate change that seek to either modify the Earth's radiation budget or remove greenhouse gases such as CO2 from the atmosphere. When focused on CO2, the latter of these categories is called carbon dioxide removal (CDR). Future emission scenarios that stay well below 2 °C, and all emission scenarios that do not exceed 1.5 °C warming by the year 2100, require some form of CDR. At present, there is little consensus on the climate impacts and atmospheric CO2 reduction efficacy of the different types of proposed CDR. To address this need, the Carbon Dioxide Removal Model Intercomparison Project (or CDRMIP) was initiated. This project brings together models of the Earth system in a common framework to explore the potential, impacts, and challenges of CDR. Here, we describe the first set of CDRMIP experiments, which are formally part of the 6th Coupled Model Intercomparison Project (CMIP6). These experiments are designed to address questions concerning CDR-induced climate reversibility, the response of the Earth system to direct atmospheric CO2 removal (direct air capture and storage), and the CDR potential and impacts of afforestation and reforestation, as well as ocean alkalinization.>

Novel carbon dioxide removal (CDR) approaches are essential to address climate change. Ocean alkalinity enhancement (OAE) is being considered to enhance the ocean’s capacity as an atmospheric carbon sink. It is widely recognised that responsible development and deployment of marine-based CDR requires effective engagement with local stakeholders. This article considers how stakeholders could actively participate in OAE, enhancing their agency and identifying pathways to incorporate their skills and knowledge into research and deployment activities. Adopting a place-based approach in Tasmania, Australia, we interviewed 23 participants and asked them how they might contribute to future OAE activities in/around this island state. Participants saw themselves contributing to one or more activity-based themes: knowledge creation, regulatory oversight, safeguarding values, and/or stakeholder networking. While some responses were predictable (i.e. government representatives adopting regulatory roles), others were unexpected, highlighting the importance of proactively engaging with stakeholders. Several participants identified the need for further support to effectively perform their roles, including expert advice and access to resources. Capacity assessments can evaluate the ability of local stakeholders to play an active role in OAE activities and thus inform strategies to bolster capacity through learning and infrastructure development. We recommend integrating capacity assessments into OAE/marine CDR stakeholder engagement strategies. Further research is needed to develop and implement capacity assessments for OAE and other marine CDR approaches.

The oceans slow the rate of climate change by absorbing about 25% of anthropogenic carbon dioxide emissions annually. The Southern Ocean makes a substantial contribution to this oceanic carbon sink: more than 40% of the anthropogenic carbon dioxide in the ocean has entered south of 40° S. The rate-limiting step in the oceanic sequestration of anthropogenic carbon dioxide is the transfer of carbon across the base of the surface mixed layer into the ocean interior, a process known as subduction. However, the physical mechanisms responsible for the subduction of anthropogenic carbon dioxide are poorly understood. Here we use observationally based estimates of subduction and anthropogenic carbon concentrations in the Southern Ocean to determine the mechanisms responsible for carbon sequestration. We estimate that net subduction amounts to 0.42 ± 0.2 Pg C yr −1 between 35° S and the marginal sea-ice zone. We show that subduction occurs in specific locations as a result of the interplay of wind-driven Ekman transport, eddy fluxes and variations in mixed-layer depth. The zonal distribution of the estimated subduction is consistent with the distribution of anthropogenic carbon dioxide in the ocean interior. We conclude that oceanic carbon sequestration depends on physical properties, such as mixed-layer depth, ocean currents, wind and eddies, which are potentially sensitive to climate variability and change. The Southern Ocean makes a substantial contribution to the oceanic carbon sink. Observationally based estimates of carbon subduction suggest that carbon sequestration depends on physical properties, such as mixed layer depth, ocean currents, wind and eddies, that are potentially sensitive to climate variability and change.

The Great Barrier Reef (GBR) is a globally significant coral reef system supporting productive and diverse ecosystems. The GBR is under increasing threat from climate change and local anthropogenic stressors, with its general condition degrading over recent decades. In response to this, a number of techniques have been proposed to offset or ameliorate environmental changes. In this study, we use a coupled hydrodynamic-biogeochemical model of the GBR and surrounding ocean to simulate artificial ocean alkalinisation (AOA) as a means to reverse the impact of global ocean acidification on GBR reefs. Our results demonstrate that a continuous release of 90 000 t of alkalinity every 3 d over one year along the entire length of the GBR, following the Gladstone-Weipa bulk carrier route, increases the mean aragonite saturation state ( Ω a r ) across the GBR’s 3860 reefs by 0.05. This change offsets just over 4 years (∼4.2) of ocean acidification under the present rate of anthropogenic carbon emissions. The injection raises Ω a r in the 250 reefs closest to the route by ⩾ 0.15 , reversing further projected Ocean Acidification. Following cessation of alkalinity injection Ω a r returns to the value of the waters in the absence of AOA over a 6 month period, primarily due to transport of additional alkalinity into the Coral Sea. Significantly, our study provides for the first time a model of AOA applied along existing shipping infrastructure that has been used to investigate shelf scale impacts. Thus, amelioration of decades of OA on the GBR is feasible using existing infrastructure, but is likely to be extremely expensive, include as yet unquantified risks, and would need to be undertaken continuously until such time, probably centuries in the future, when atmospheric CO 2 concentrations have returned to today’s values.

Grazing dynamics are one of the most poorly constrained components of the marine carbon cycle. We use inverse modeling to infer the distribution of community‐integrated zooplankton grazing dynamics based on the ability of different grazing formulations to recreate the satellite‐observed seasonal cycle in phytoplankton biomass after controlling for physical and bottom‐up controls. We find large spatial variability in the optimal community‐integrated half saturation concentration for grazing (K1/2), with lower (higher) values required in more oligotrophic (eutrophic) biomes. This leads to a strong sigmoidal relationship between observed mean‐annual phytoplankton biomass and the optimally inferred grazing parameterization. This relationship can be used to help constrain, validate and/or parameterize next‐generation biogeochemical models. Plain Language Summary To improve predictions of the ocean's ability to feed a growing human population and buffer a changing climate, we need to improve our understanding of what happens to carbon once it is absorbed into the surface ocean. One of the largest knowledge gaps in marine carbon cycling is the role of zooplankton grazing. The rate at which zooplankton graze phytoplankton modifies the size and seasonal evolution of phytoplankton populations and in turn, the associated rates of net primary production at the base of the food‐web, secondary production of grazers (an indicator of fisheries potential) and export production (the biological sequestration of carbon). However, regional differences in grazing, which are difficult to measure outside of the laboratory, remain poorly constrained by observations and thus difficult to model. Here, we run a suite of model simulations, which each simulate grazing differently, then compare the results to infer which grazing dynamics best match observations. We find that there is dramatic spatial variability in how zooplankton, as a community, appear to be grazing and that this variability maps well onto observed phytoplankton abundance, suggesting that the type of zooplankton present may be determined by the amount of prey available. Key Points Oligotrophic (eutrophic) biomes exhibit more (less) efficient community‐integrated grazing, characteristic of micro‐ (meso‐) zooplankton We find a strong link between observed mean‐annual phytoplankton biomass and the grazing dynamics required to recreate its seasonal cycle A type III functional response typically does a better job recreating observed phytoplankton seasonal cycles than a type II response

The current generation of Earth system models used by the United Nations to project future climate scenarios (CMIP6) relies heavily on marine biogeochemical models to track the fate of carbon absorbed into the oceans. Here we compare 11 CMIP6 marine biogeochemical models and find the largest source of inter-model uncertainty in their representation of the marine carbon cycle is phytoplankton-specific loss rates to zooplankton grazing. This uncertainty is over three times larger than that of net primary production and driven by large differences in prescribed zooplankton grazing dynamics. We run a controlled sensitivity experiment in a global marine biogeochemical model and find that small changes in prescribed grazing dynamics (roughly 5% of what is used across CMIP6 models) can increase secondary and export production by 5 and 2 PgC yr −1 , respectively, even when tuned to identical net primary production, likely biasing predictions of future climate states and food security.

Based on the 2019 assessment of the Global Carbon Project, the ocean took up on average, 2.5+/-0.6PgCyr-1 or 23+/-5% of the total anthropogenic CO2 emissions over the decade 2009-2018. This sink estimate is based on global ocean biogeochemical models (GOBMs) and is compared to data-products based on surface ocean pCO2 (partial pressure of CO2) observations accounting for the outgassing of river-derived CO2. Here we evaluate the GOBM simulations by comparing the simulated pCO2 to observations. The simulations are well suited for quantifying the global ocean carbon sink on the time-scale of the annual mean and its multi-decadal trend (RMSE <20 μatm), as well as on the time-scale of multi-year variability (RMSE <10 μatm), despite the large model-data mismatch on the seasonal time-scale (RMSE of 20-80 μatm). Biases in GOBMs have a small effect on the global mean ocean sink (0.05 PgC yr−1), but need to be addressed to improve the regional budgets and model-data comparison. Accounting for non-mapped areas in the data-products reduces their spread as measured by the standard deviation by a third. There is growing evidence and consistency among methods with regard to the patterns of the multi-year variability of the ocean carbon sink, with a global stagnation in the 1990s and an extra-tropical strengthening in the 2000s. GOBMs and data-products point consistently to a shift from a tropical CO2 source to a CO2 sink in recent years. On average, the GOBMs reveal less variations in the sink than the data-based products. Despite the reasonable simulation of surface ocean pCO2 by the GOBMs, there are discrepancies between the resulting sink estimate from GOBMs and data-products. These discrepancies are within the uncertainty of the river flux adjustment, increase over time, and largely stem from the Southern Ocean. Progress in our understanding of the global ocean carbon sink necessitates significant advancement in modelling and observing the Southern Ocean including (i) a game-changing increase in high-quality pCO2 observations, and (ii) a critical re-evaluation of the regional river flux adjustment.