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Public interest-oriented state control of research into climate engineering techniques - the Model London Protocol: Climate Engineering, including Carbon Dioxide Removal- and/or Solar Radiation Management-techniques, increasingly gets again into the public discussion on effective climate change policies. As most of these techniques are still in infancy, the question needs to be answered whether and how research and development should be controlled and governed by states. In this context, public funding may play an important role next to regulatory approaches.

The Paris Climate Agreement and the Importance of climate engineering: Stopping global warming requires Net-Zero CO2 emissions. Since it is not yet foreseeable that all emissions can be avoided by mid-century, the Net-Zero target set in the Paris Climate Agreement implies the use of CO2 removal methods that can be described as a category of climate engineering. Optimistic emission reduction scenarios assume that by mid-century, 5 to 15% of today's emissions will have to be offset by CO2 removal as difficult-to-avoid residual emissions. So-called "nature-based" solutions alone will not be sufficient to achieve this. Technical approaches will also have to be applied at large scale if the climate targets are to be achieved. None of these methods is ready for large-scale deployment today. Public research is needed and the societal debate on how to deal with residual emissions must begin quickly so that appropriate methods can be developed in time, well-informed societal decisions can be made about their eventual deployment, and mechanisms can be devised to regulate such deployment responsibly.

Owing to its cool temperatures and vigorous water mass formation, the strongly eddying Southern Ocean is a key region of ocean CO2uptake. In this study we assess the role of 1) wind stress and buoyancy forcing and 2) the representation of mesoscale eddies, in affecting the mean and temporal variations of the Southern Ocean carbon uptake in the past 60 years. We analyze global ocean biogeochemistry simulations based on the NEMO-MOPS and FESOM-REcoM models and ranging from 1° and 0.5° resolutions (where eddies are parameterized) to eddy-rich 0.25° and 0.1° resolutions.The 0.25° model is also used to perform sensitivityexperiments to unravel the relative role of wind stress and of buoyancy forcing for the carbon uptake variations.We find that eddy-rich models have steeper isopycnals across the Antarctic Circumpolar Current, which results in higher anthropogenic carbon uptake and storage than in models where eddies are parameterized. This, in combination with a somewhat lower outgassing of natural CO2, gives rise to a steeper trend of the Southern Ocean carbon uptake in the eddy-rich than in the eddy-parameterized models. Wind stress and buoyancy forcing are the main drivers of an increased outgassing of natural CO2over the past decades and drive most of its interannual and decadal variability, with wind stress dominating at subpolar latitudes, and buoyancy forcingin water mass formation regions. However, our experiments indicate that the stalling of the Southern Ocean carbon uptake in the 1990s was mostly driven by a reduction of its anthropogenic carbon uptake.&nbsp, The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Global biogeochemical ocean models help to investigate the present and potential future state of the ocean, its productivity and cascading effects on higher trophic levels such as fish. They are often subjectively tuned against data sets of inorganic tracers and surface chlorophyll and only very rarely against organic components such as particulate organic carbon or zooplankton. The resulting uncertainty in biogeochemical model parameters (and parameterisations) associated with these components can explain some of the large spread of global model solutions with regard to the cycling of organic matter and its impacts on biogeochemical tracer distributions, such as oxygen minimum zones (OMZs). A second source of uncertainty arises from differences in the model spin-up length as, so far, there seems to be no agreement on the required simulation time that should elapse before a global model is assessed against observations. We investigated these two sources of uncertainty by optimising a global biogeochemical ocean model against the root-mean-squared error (RMSE) of six different combinations of data sets and different spin-up times. Besides nutrients and oxygen, the observational data sets also included phyto- and zooplankton, as well as dissolved and particulate organic phosphorus (DOP and POP, respectively). We further analysed the optimised model performance with regard to global biogeochemical fluxes, oxygen inventory and OMZ volume. Following the optimisation procedure, we evaluated the RMSE for all tracers located in the upper 100 m (except for POP, for which we considered the entire vertical domain), regardless of their consideration during optimisation. For the different optimal model solutions, we find a narrow range of the RMSE, between 14 % of the average RMSE after 10 years and 24 % after 3000 years of simulation. Global biogeochemical fluxes, global oxygen bias and OMZ volume showed a much stronger divergence among the models and over time than RMSE, indicating that even models that are similar with regard to local surface tracer concentrations can perform very differently when assessed against the global diagnostics for oxygen. Considering organic tracers in the optimisation had a strong impact on the particle flux exponent (Martin b) and may reduce much of the uncertainty in this parameter and the resulting deep particle flux. Independent of the optimisation setup, the OMZ volume showed a particularly sensitive response with strong trends over time, even after 3000 years of simulation time (despite the constant physical forcing); a high sensitivity to simulation time; and the highest sensitivity to model parameters arising from the tuning strategy setup (variation of almost 80 % of the ensemble mean). In conclusion, calibration against observations of organic tracers can help to improve global biogeochemical models even after short spin-up times; here especially, observations of deep particle flux could provide a powerful constraint. However, a large uncertainty remains with regard to global OMZ volume and its evolution over time, which can show very dynamic behaviour during the model spin-up, which renders temporal extrapolation to a final equilibrium state difficult if not impossible. Given that the real ocean shows variations on many timescales, the assumption of observations representing a steady-state ocean may require some reconsideration.

The Paris Agreement to limit global warming to well below 2 °C requires ambitious emission reduction and the balancing of remaining emissions through carbon sinks, i.e. the deployment of carbon dioxide removal (CDR). While ambitious climate protection scenarios until now consider primarily land-based CDR methods, there is growing concern about their potential to deliver sufficient CDR, and marine CDR options receive more and more interest. Based on idealized theoretical studies, Ocean Alkalinity Enhancement (OAE) appears as a promising marine CDR method. However, the knowledge base is insufficient for a robust assessment of its practical feasibility, of its side effects, social and governance aspects as well as monitoring, reporting and verification issues. A number of research efforts aim to improve this in a timely manner. We provide an overview on the current situation of developing OAE as marine CDR method, and describe the history that has led to the creation of the OAE research Best Practices Guide.

The deliberate increase of ocean alkalinity (referred to as Ocean Alkalinity Enhancement or OAE) has been proposed as a method for removing CO2 from the atmosphere. Before OAE can be implemented safely, efficiently, and at scale several research questions have to be addressed including: 1) which alkaline feedstocks are best suited and in what doses can they be added safely, 2) how can net carbon uptake be measured and verified, and 3) what are the potential ecosystem impacts. These research questions cannot be addressed by direct observation alone but will require skillful and fit-for-purpose models. This chapter provides an overview of the most relevant modeling tools, including turbulence-, regional- and global-scale biogeochemical models, and techniques including approaches for model validation, data assimilation, and uncertainty estimation. Typical biogeochemical model assumptions and their limitations are discussed in the context of OAE research, which leads to an identification of further development needs to make models more applicable to OAE research questions. A description of typical steps in model validation is followed by proposed minimum criteria for what constitutes a model that is fit for its intended purpose. After providing an overview of approaches for sound integration of models and observations via data assimilation, the application of Observing System Simulation Experiments (OSSEs) for observing system design is described within the context of OAE research. Criteria for model validation and intercomparison studies are presented. The article concludes with a summary of recommendations and potential pitfalls to be avoided.

EMB Future Science Brief No. 10 highlights the most recent science on Ocean oxygen, including causes, impacts and mitigation strategies of Ocean oxygen loss, and discusses whether “every second breath we take comes from the Ocean”. It closes with key policy, management and research recommendations to address Ocean deoxygenation and communicate more accurately about the role of the Ocean in Earth’s oxygen. The sentence “every second breath you take comes from the Ocean” is commonly used in Ocean Literacy and science communication to highlight the importance of Ocean oxygen. However, despite its widespread use, it is often not phrased correctly. In contrast, there is little awareness about the threat of the global oxygen loss in the Ocean, called deoxygenation, particularly in comparison with other important stressors, such as Ocean acidification or increasing seawater temperatures. Deoxygenation is increasing in the coastal and open Ocean, primarily due to human-induced global warming and nutrient run-off from land, and projections show that the Ocean will continue losing oxygen as global warming continues. The consequences of oxygen loss in the Ocean are extensive and include decreased biodiversity, shifts in species distributions, displacement or reduction in fisheries resources, changes in biogeochemical cycling and mass mortalities. Low oxygen conditions also drive other chemical processes which produce greenhouse gases, toxic compounds and further degrade water quality. The degraded water quality directly affects marine ecosystems, but also indirectly impacts ecosystem services supporting local communities, regional economies and tourism. Although there are still gaps in our knowledge, we know enough to be very concerned about the consequences: the impacts might even be larger than from Ocean acidification or heat waves, and three out of the five global mass extinctions were linked to Ocean deoxygenation. The sense of urgency to improve Ocean health is reflected in the UN Decade of Ocean Science for Sustainable Development (Ocean Decade) and the EU Mission: Restore our Ocean and Waters (Mission Ocean), and tackling the loss of oxygen in the Ocean is critical to achieving the aims of these two initiatives.&nbsp

In this study, we investigate the maximum physical and biogeochemical potential of macroalgae open-ocean mariculture and sinking (MOS) as an ocean-based carbon dioxide removal (CDR) method. Embedding a macroalgae model into an Earth system model, we simulate macroalgae mariculture in the open-ocean surface layer followed by fast sinking of the carbon-rich macroalgal biomass to the deep seafloor (depth>3000 m), which assumes no remineralization of the harvested biomass during the quick sinking. We also test the combination of MOS with artificial upwelling (AU), which fertilizes the macroalgae by pumping nutrient-rich deeper water to the surface. The simulations are done under RCP 4.5, a moderate-emissions pathway. When deployed globally between years 2020 and 2100, the carbon captured and exported by MOS is 270 PgC, which is further boosted by AU of 447 PgC. Because of feedbacks in the Earth system, the oceanic carbon inventory only increases by 171.8 PgC (283.9 PgC with AU) in the idealized simulations. More than half of this carbon remains in the ocean after cessation at year 2100 until year 3000. The major side effect of MOS on pelagic ecosystems is the reduction of phytoplankton net primary production (PNPP) due to the competition for nutrients with macroalgae and due to canopy shading. MOS shrinks the mid-layer oxygen-minimum zones (OMZs) by reducing the organic matter export to, and remineralization in, subsurface and intermediate waters, while it creates new OMZs on the seafloor by oxygen consumption from remineralization of sunken biomass. MOS also impacts the global carbon cycle by reducing the atmospheric and terrestrial carbon reservoirs when enhancing the ocean carbon reservoir. MOS also enriches dissolved inorganic carbon in the deep ocean. Effects are mostly reversible after cessation of MOS, though recovery is not complete by year 3000. In a sensitivity experiment without remineralization of sunken MOS biomass, the whole of the MOS-captured carbon is permanently stored in the ocean, but the lack of remineralized nutrients causes a long-term nutrient decline in the surface layers and thus reduces PNPP. Our results suggest that MOS has, theoretically, considerable CDR potential as an ocean-based CDR method. However, our simulations also suggest that such large-scale deployment of MOS would have substantial side effects on marine ecosystems and biogeochemistry, up to a reorganization of food webs over large parts of the ocean.

The Humboldt Upwelling System is of global interest due to its importance to fisheries, though the origin of its high productivity remains elusive. In regional physical‐biogeochemical model simulations, the seasonal amplitude of mesozooplankton net production exceeds that of phytoplankton, indicating “seasonal trophic amplification.” An analytical approach identifies amplification to be driven by a seasonally varying trophic transfer efficiency due to mixed layer variations. The latter alters the vertical distribution of phytoplankton and thus the zooplankton and phytoplankton encounters, with lower encounters occurring in a deeper mixed layer where phytoplankton are diluted. In global model simulations, mixed layer depth appears to affect trophic transfer similarly in other productive regions. Our results highlight the importance of mixed layer depth for trophodynamics on a seasonal scale with potential significant implications, given mixed layer depth changes projected under climate change., Plain Language Summary: The Humboldt Upwelling System is a fishery‐important region. A common assumption is that a certain amount of phytoplankton supports a proportional amount of fish. However, we find that a small seasonal change in phytoplankton can trigger a larger variation in zooplankton. This implies that one may underestimate changes in fish solely based on phytoplankton. Using ecosystem model simulations, we investigate why changes of phytoplankton are not proportionally reflected in zooplankton. The portion of phytoplankton that ends up in zooplankton is controlled by the changing depth of the surface ocean “mixed layer.” The “mixed layer” traps both the phytoplankton and zooplankton in a limited amount of space. When the “mixed layer” is shallow, zooplankton can feed more efficiently on phytoplankton as both are compressed in a comparatively smaller space. We conclude that in the Humboldt System, and other “food‐rich” regions, feeding efficiently, determined by the “mixed layer,” is more important than how much food is available., Key Points: Environmental factors strongly affect plankton trophodynamics on a seasonal scale. Seasonal trophic amplification in the Humboldt system is driven by mixed layer dynamics. Mixed layer depth and food chain efficiency correlate also in other productive regions., China Sponsorship Council, Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347

promet, 105, Beitrag 9

In this paper, we outline the need for a coordinated international effort toward the building of an open-access Global Ocean Oxygen Database and ATlas (GO 2DAT) complying with the FAIR principles (Findable, Accessible, Interoperable, and Reusable). GO 2DAT will combine data from the coastal and open ocean, as measured by the chemical Winkler titration method or by sensors (e.g., optodes, electrodes) from Eulerian and Lagrangian platforms (e.g., ships, moorings, profiling floats, gliders, ships of opportunities, marine mammals, cabled observatories). GO 2DAT will further adopt a community-agreed, fully documented metadata format and a consistent quality control (QC) procedure and quality flagging (QF) system. GO 2DAT will serve to support the development of advanced data analysis and biogeochemical models for improving our mapping, understanding and forecasting capabilities for ocean O 2 changes and deoxygenation trends. It will offer the opportunity to develop quality-controlled data synthesis products with unprecedented spatial (vertical and horizontal) and temporal (sub-seasonal to multi-decadal) resolution. These products will support model assessment, improvement and evaluation as well as the development of climate and ocean health indicators. They will further support the decision-making processes associated with the emerging blue economy, the conservation of marine resources and their associated ecosystem services and the development of management tools required by a diverse community of users (e.g., environmental agencies, aquaculture, and fishing sectors). A better knowledge base of the spatial and temporal variations of marine O 2 will improve our understanding of the ocean O 2 budget, and allow better quantification of the Earth’s carbon and heat budgets. With the ever-increasing need to protect and sustainably manage ocean services, GO 2DAT will allow scientists to fully harness the increasing volumes of O 2 data already delivered by the expanding global ocean observing system and enable smooth incorporation of much higher quantities of data from autonomous platforms in the open ocean and coastal areas into comprehensive data products in the years to come. This paper aims at engaging the community (e.g., scientists, data managers, policy makers, service users) toward the development of GO 2DAT within the framework of the UN Global Ocean Oxygen Decade (GOOD) program recently endorsed by IOC-UNESCO. A roadmap toward GO 2DAT is proposed highlighting the efforts needed (e.g., in terms of human resources).

4 pages, 5 figures.-- Correction Global variability in seawater Mg:Ca and Sr:Ca ratios in the modern ocean; Proceedings of the National Academy of Sciences of the USA 117(36): 22281-22292 (2020); doi: 10.1073/pnas.1918943117; http://hdl.handle.net/10261/221953, The authors wish to note the following: “This study’s seawater Sr:Ca values were systematically low as a consequence of normalization to another published low value for the International Association for the Physical Sciences of the Oceans (IAPSO) (1). IAPSO has been used at the Ocean Drilling Program, Texas A&M University (ODP-TAMU) (http://www-odp.tamu.edu/), and is still being used as the primary standard for elemental composition of seawater/interstitial water. Consequently, our seawater value of Sr:Ca = 8.28 mmol:mol was systematically low by approx. 3.70%, if we accept seawater Sr:Ca 8.60 mmol:mol as the recommended value for IAPSO North Atlantic surface water salinity standard. The uncertainty budget should be expanded including the uncertainty of IAPSO composition. The largest contribution to expanded uncertainty of our data comes from the uncertainty of the IAPSO reference composition, which is 3.29% using all published values. This will result in 3.30% (1 SD) expanded uncertainty for seawater Sr:Ca (and 0.5%, for seawater Mg:Ca) of the entire data set with respect to accuracy. We have corrected all seawater Sr:Ca values with a factor of 1.0243 in all our tables (e.g., SI Appendix, Table S1 averages) and in the figures (Fig. 4, Fig. 5), where a ratio was used. Note that the seawater Sr:Ca % changes are small, thus changes are hardly noticeable on large displays (e.g., Figures), but they can be seen in the tables and averages/SD calculations. Seawater Sr:Ca ratios are also corrected in the main text where relevant

In this paper, we outline the need for a coordinated international effort toward the building of an open-access Global Ocean Oxygen Database and ATlas (GO2DAT) complying with the FAIR principles (Findable, Accessible, Interoperable, and Reusable). GO2DAT will combine data from the coastal and open ocean, as measured by the chemical Winkler titration method or by sensors (e.g., optodes, electrodes) from Eulerian and Lagrangian platforms (e.g., ships, moorings, profiling floats, gliders, ships of opportunities, marine mammals, cabled observatories). GO2DAT will further adopt a community-agreed, fully documented metadata format and a consistent quality control (QC) procedure and quality flagging (QF) system. GO2DAT will serve to support the development of advanced data analysis and biogeochemical models for improving our mapping, understanding and forecasting capabilities for ocean O2 changes and deoxygenation trends. It will offer the opportunity to develop quality-controlled data synthesis products with unprecedented spatial (vertical and horizontal) and temporal (sub-seasonal to multi-decadal) resolution. These products will support model assessment, improvement and evaluation as well as the development of climate and ocean health indicators. They will further support the decision-making processes associated with the emerging blue economy, the conservation of marine resources and their associated ecosystem services and the development of management tools required by a diverse community of users (e.g., environmental agencies, aquaculture, and fishing sectors). A better knowledge base of the spatial and temporal variations of marine O2 will improve our understanding of the ocean O2 budget, and allow better quantification of the Earth’s carbon and heat budgets. With the ever-increasing need to protect and sustainably manage ocean services, GO2DAT will allow scientists to fully harness the increasing volumes of O2 data already delivered by the expanding global ocean observing system and enable smooth incorporation of much higher quantities of data from autonomous platforms in the open ocean and coastal areas into comprehensive data products in the years to come. This paper aims at engaging the community (e.g., scientists, data managers, policy makers, service users) toward the development of GO2DAT within the framework of the UN Global Ocean Oxygen Decade (GOOD) program recently endorsed by IOC-UNESCO. A roadmap toward GO2DAT is proposed highlighting the efforts needed (e.g., in terms of human resources).

The Net-Zero-2050 cluster aims for a national roadmap for net zero CO2 emissions by 2050, including integrated scenario analyses and negative emission technology assessment (see fact sheet Net-Zero-2050 Structure Project 1). This national target to substantially reduce national CO2 emissions by 2050 stems from the objective to comply with the global long-term temperature goal of well below 2°C of the Paris Agreement (UNFCCC, 2015). Within the cluster it is therefore important to decide on an approach for deriving a national remaining carbon budget from global emissions trajectories in agreement with the Paris Climate Agreement’s longterm temperature goal (UNFCCC, 2015). Allocating national carbon budgets is a balance of environmental effectiveness, equity, national capacity and ability, political feasibility, economic efficiency and technical requirements (Gignac and Matthews, 2015; Höhne et al., 2003; 2014). Given Germany’s capacity and abilities, we decided to follow a sustainable growth trajectory with a convergence phase to equal-per-capita CO2 emissions by 2035, and a net zero CO2 emissions trajectory after 2050 until the end of the century. This approach leads to a remaining Germany CO2 budget of 9 GtCO2 (from 1st January 2018 to 2050 and 2100), which we propose to be used across the Net-Zero-2050 cluster. The remaining carbon budget will serve as a target to be used in all work packages in a concerted way, either qualitatively or quantitatively, and in accordance with other work packages (see also fact sheet Net-Zero-2050 Energy Scenario Approach). The calculated budget is at the lower end of the national budget if allocated by the grandfathering approach (emissions are allocated with respect to today’s emissions shares: 5.5-13.1 GtCO2), but slightly higher than the highest estimate of an equal-per-capita remaining carbon budget (emissions are allocated with respect to Germany’s share of the global population: 3.5-8.4 GtCO2) The 9 GtCO2 national remaining CO2 budget, 6.9 GtCO2 from 1st January 2021, will need to be broken down by category (e.g. energy, land use, industrial processes, and man-made sinks and sources; see Gap Analysis Report) in order to provide a consistent approach across work packages.

Previous studies have suggested that enhanced weathering and benthic phosphorus (P) fluxes, triggered by climate warming, can increase the oceanic P inventory on millennial timescales, promoting ocean productivity and deoxygenation. In this study, we assessed the major uncertainties in projected P inventories and their imprint on ocean deoxygenation using an Earth system model of intermediate complexity for the same business-as-usual carbon dioxide (CO2) emission scenario until the year 2300 and subsequent linear decline to zero emissions until the year 3000. Our set of model experiments under the same climate scenarios but differing in their biogeochemical P parameterizations suggest a large spread in the simulated oceanic P inventory due to uncertainties in (1) assumptions for weathering parameters, (2) the representation of bathymetry on slopes and shelves in the model bathymetry, (3) the parametrization of benthic P fluxes and (4) the representation of sediment P inventories. Considering the weathering parameters closest to the present day, a limited P reservoir and prescribed anthropogenic P fluxes, we find a +30 % increase in the total global ocean P inventory by the year 5000 relative to pre-industrial levels, caused by global warming. Weathering, benthic and anthropogenic fluxes of P contributed +25 %, +3 % and +2 %, respectively. The total range of oceanic P inventory changes across all model simulations varied between +2 % and +60 %. Suboxic volumes were up to 5 times larger than in a model simulation with a constant oceanic P inventory. Considerably large amounts of the additional P left the ocean surface unused by phytoplankton via physical transport processes as preformed P. In the model, nitrogen fixation was not able to adjust the oceanic nitrogen inventory to the increasing P levels or to compensate for the nitrogen loss due to increased denitrification. This is because low temperatures and iron limitation inhibited the uptake of the extra P and growth by nitrogen fixers in polar and lower-latitude regions. We suggest that uncertainties in P weathering, nitrogen fixation and benthic P feedbacks need to be reduced to achieve more reliable projections of oceanic deoxygenation on millennial timescales.

Im Fokus der siebten Ausgabe des »World Ocean Review« stehen die Auswirkungen des Klimawandels auf die Physik des Meeres und auf seine Lebensgemeinschaften; die Folgen von Fischerei, Schifffahrt, Ressourcenabbau, Energiegewinnung und Meeresverschmutzung sowie die Fragen, wie sich Wirkstoffe aus dem Meer nutzen lassen und wie der Ozean künftig so verwaltet werden kann, dass sowohl sein Schutz als auch die Teilhabe möglichst aller Menschen an seinen Leistungen und Gütern gewährleistet sind.

Uncertainties in projections from Earth system models (ESMs) are associated to a large degree with the imperfect representation of the marine plankton ecosystem, in particular the physiology of primary and secondary producers. Here we describe the implementation of an optimality-based plankton-ecosystem model (OPEM) with variable C:N:P stoichiometry in the University of Victoria ESM (UVic) and the behaviour of two calibrated reference configurations, which differ in the assumed temperature dependence of diazotrophs. Predicted tracer distributions of oxygen and dissolved inorganic nutrients are similar to those of an earlier fixed-stoichiometry model (Keller et al., 2012). Compared to the classic fixed-stoichiometry model, OPEM is closer to recent satellite-based estimates of net community production (NCP), despite overestimating net primary production (NPP), can better reproduce deep-ocean gradients in the NO3:PO43− ratio, and partially explains observed patterns of particulate C:N:P in the surface ocean. Allowing diazotrophs to grow (but not necessarily fix N2) at similar temperatures as other phytoplankton results in a better representation of surface Chl and NPP in the Arctic and Antarctic Oceans. Deficiencies of our calibrated OPEM configurations may serve as a magnifying glass for shortcomings in global biogeochemical models and hence guide future model development. The overestimation of NPP at low latitudes indicates the need for improved representations of temperature effects on biotic processes, as well as phytoplankton community composition, which may be represented by locally-varying parameters based on suitable trade-offs. Discrepancies between observed and predicted vertical gradients in particulate C:N:P ratios suggest the need to include preferential P remineralisation, which could also benefit the representation of N2 fixation. While OPEM yields a much improved distribution of surface N* (NO3− 16·PO43− + 2.9 mmol m−3), it still fails to reproduce observed N* in the Arctic, possibly related to a mis-representation of the phytoplankton community there and the lack of benthic denitrification in the model. Coexisting ordinary and diazotrophic phytoplankton can exert strong control on N* in our simulations, which questions the interpretation of N* as reflecting the balance of N2 fixation and denitrification.

Uncertainties in projections of marine biogeochemistry from Earth system models (ESMs) are associated to a large degree with the imperfect representation of the marine plankton ecosystem, in particular the physiology of primary and secondary producers. Here, we describe the implementation of an optimality-based plankton–ecosystem model (OPEM) version 1.1 with variable carbon : nitrogen : phosphorus (C:N:P) stoichiometry in the University of Victoria ESM (UVic; Eby et al., 2009; Weaver et al., 2001) and the behaviour of two calibrated reference configurations, which differ in the assumed temperature dependence of diazotrophs. Predicted tracer distributions of oxygen and dissolved inorganic nutrients are similar to those of an earlier fixed-stoichiometry formulation in UVic (Nickelsen et al., 2015). Compared to the classic fixed-stoichiometry UVic model, OPEM is closer to recent satellite-based estimates of net community production (NCP), despite overestimating net primary production (NPP), can better reproduce deep-ocean gradients in the NO3-:PO43- ratio and partially explains observed patterns of particulate C:N:P in the surface ocean. Allowing diazotrophs to grow (but not necessarily fix N2) at similar temperatures as other phytoplankton results in a better representation of surface Chl and NPP in the Arctic and Antarctic oceans. Deficiencies of our calibrated OPEM configurations may serve as a magnifying glass for shortcomings in global biogeochemical models and hence guide future model development. The overestimation of NPP at low latitudes indicates the need for improved representations of temperature effects on biotic processes, as well as phytoplankton community composition, which may be represented by locally varying parameters based on suitable trade-offs. The similarity in the overestimation of NPP and surface autotrophic particulate organic carbon (POC) could indicate deficiencies in the representation of top-down control or nutrient supply to the surface ocean. Discrepancies between observed and predicted vertical gradients in particulate C:N:P ratios suggest the need to include preferential P remineralisation, which could also benefit the representation of N2 fixation. While OPEM yields a much improved distribution of surface N* (NO3--16⋅PO43-+2.9 mmol m−3), it still fails to reproduce observed N* in the Arctic, possibly related to a misrepresentation of the phytoplankton community there and the lack of benthic denitrification in the model. Coexisting ordinary and diazotrophic phytoplankton can exert strong control on N* in our simulations, which questions the interpretation of N* as reflecting the balance of N2 fixation and denitrification.

Every year, about four percent of the plastic waste generated worldwide ends up in the ocean. What happens to the plastic there is poorly understood, though a growing body of evidence suggests it is rapidly spreading throughout the global ocean. The mechanisms of this spread are straightforward for buoyant larger plastics that can be accurately modelled using Lagrangian particle models. But the fate of the smallest size fractions (the microplastics) are less straightforward, in part because they can aggregate in sinking marine snow and faecal pellets. This biologically-mediated pathway is suspected to be a primary surface microplastic removal mechanism, but exactly how it might work in the real ocean is unknown. We search the parameter space of a new microplastic model embedded in an earth system model to show that biological uptake can significantly shape global microplastic inventory and distributions and even account for the budgetary “missing” fraction of surface microplastic, despite being an inefficient removal mechanism. While a lack of observational data hampers our ability to choose a set of “best” model parameters, our effort represents a first tool for quantitatively assessing hypotheses for microplastic interaction with ocean biology at the global scale.

The University of Victoria Earth System Climate Model (UVic ESCM) of intermediate complexity has been a useful tool in recent assessments of long-term climate changes, including both paleo-climate modelling and uncertainty assessments of future warming. Since the last official release of the UVic ESCM 2.9 and the two official updates during the last decade, considerable model development has taken place among multiple research groups. The new version 2.10 of the University of Victoria Earth System Climate Model presented here will be part of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). More precisely it will be used in the intercomparison of Earth system models of intermediate complexity (EMIC), such as the C4MIP, the Carbon Dioxide Removal and Zero Emissions Commitment model intercomparison projects (CDR-MIP and ZECMIP, respectively). It now brings together and combines multiple model developments and new components that have come about since the last official release of the model. The main additions to the base model are (i) an improved biogeochemistry module for the ocean, (ii) a vertically resolved soil model including dynamic hydrology and soil carbon processes, and (iii) a representation of permafrost carbon. To set the foundation of its use, we here describe the UVic ESCM 2.10 and evaluate results from transient historical simulations against observational data. We find that the UVic ESCM 2.10 is capable of reproducing changes in historical temperature and carbon fluxes well. The spatial distribution of many ocean tracers, including temperature, salinity, phosphate and nitrate, also agree well with observed tracer profiles. The good performance in the ocean tracers is connected to an improved representation of ocean physical properties. For the moment, the main biases that remain are a vegetation carbon density that is too high in the tropics, a higher than observed change in the ocean heat content (OHC) and an oxygen utilization in the Southern Ocean that is too low. All of these biases will be addressed in the next updates to the model.

Ocean deoxygenation is a threat to marine ecosystems. We evaluated the potential of two ocean intervention technologies, that is, “artificial downwelling (AD)” and “artificial upwelling (AU),” for remedying the expansion of Oxygen Deficient Zones (ODZs). The model‐based assessment simulated AD and AU implementations for 80 years along the eastern Pacific ODZ. When AD was simulated by pumping surface seawater to the 178–457 m‐depth range of the ODZ, vertically integrated oxygen increased by up to 4.5% in the deployment region. Pumping water from 457 m depth to the surface (i.e., AU), where it can equilibrate with the atmosphere, increased the vertically integrated oxygen by 1.03%. However, both simulated AD and AU increased biological production via enhanced nutrient supply to the sea surface, resulting in enhanced export production and subsequent aerobic remineralization also outside of the actual implementation region, and an ultimate net decline of global oceanic oxygen., Key Points: Artificial downwelling (AD) and upwelling (AU) in the eastern Pacific oxygen deficient region are simulated in a global model Both technologies can effectively mitigate local expansion of intermediate‐depth oceanic oxygen deficient zones under climate change Global deoxygenation is however enhanced due to increased export production and aerobic respiration resulting from AD and AU, Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659, German Research Foundation (DFG)

The aim of this Project Briefing is a clear definition of the various dimensions of our scenario approach in Net-Zero-2050. Starting from the overarching framework, we then describe, how scenarios are applied in the various projects. We define the scope and focus of the energy scenarios and the scenarios for Carbon Dioxide Removal measures, as well as the interface between both approaches.

Net-Zero-2050 aims for a national roadmap for net-zero CO2 emissions by 2050, including integrated scenario analyses and negative emission technology assessment. The aim of this project briefing is to clarify the overall carbon budget available for Germany to comply with the global long-term temperature limit of well below 2°C of the Paris Agreement.

Clarification of the work focus and the connection of the different elements of the Helmholtz Climate Initiative‘s Cluster I Net-Zero-2050

Numerical simulations and emissions estimates of plastic in and to the ocean consistently over-predict the surface inventory, particularly in the case of microplastic (MP), i.e. fragments less than 5 mm in length. Sequestration in the sediments has been both predicted and, to a limited extent, observed. It has been hypothesized that biology may be exporting a significant fraction of surface MP by way of marine snow aggregation and zooplankton faecal pellets. We apply previously published data on MP concentrations in the surface ocean to an earth system model of intermediate complexity to produce a first estimate of the potential global sequestration of MP by marine aggregates, including faecal pellets. We find a MP seafloor export potential of between 7.3E3-4.2E5 metric tons per year, or about 0.06-8.8% of estimated total annual plastic ocean pollution rates. We find that presently, aggregates alone would have the potential to remove most existing surface ocean MP to the seafloor within less than 2 years if pollution ceases. However, the observed accumulation of MP in the surface ocean, despite this high potential rate of removal, suggests that detrital export is an ineffective pathway for permanent MP removal. We theorize a prominent role of MP biological fouling and de-fouling in the rapid recycling of aggregate-associated MP in the upper ocean. We also present an estimate of how the potential detrital MP sink might change into the future, as climate change (and projected increasing MP pollution) alters the marine habitat. The polar regions, and the Arctic in particular, are projected to experience increasing removal rates as export production increases faster than MP pollution. Northern hemisphere subtropical gyres are projected to experience slowing removal rates as stratification and warming decrease export production, and MP pollution increases. However, significant uncertainty accompanies these results.

We investigate the climate mitigation potential and collateral effects of direct injections of captured CO2 into the deep ocean as a possible means to close the gap between an intermediate CO2 emissions scenario and a specific temperature target, such as the 1.5 ∘C target aimed for by the Paris Agreement. For that purpose, a suite of approaches for controlling the amount of direct CO2 injections at 3000 m water depth are implemented in an Earth system model of intermediate complexity. Following the representative concentration pathway RCP4.5, which is a medium mitigation CO2 emissions scenario, cumulative CO2 injections required to meet the 1.5 ∘C climate goal are found to be 390 Gt C by the year 2100 and 1562 Gt C at the end of simulations, by the year 3020. The latter includes a cumulative leakage of 602 Gt C that needs to be reinjected in order to sustain the targeted global mean temperature. CaCO3 sediment and weathering feedbacks reduce the required CO2 injections that comply with the 1.5 ∘C target by about 13 % in 2100 and by about 11 % at the end of the simulation. With respect to the injection-related impacts we find that average pH values in the surface ocean are increased by about 0.13 to 0.18 units, when compared to the control run. In the model, this results in significant increases in potential coral reef habitats, i.e., the volume of the global upper ocean (0 to 130 m depth) with omega aragonite > 3.4 and ocean temperatures between 21 and 28 ∘C, compared to the control run. The potential benefits in the upper ocean come at the expense of strongly acidified water masses at depth, with maximum pH reductions of about −2.37 units, relative to preindustrial levels, in the vicinity of the injection sites. Overall, this study demonstrates that massive amounts of CO2 would need to be injected into the deep ocean in order to reach and maintain the 1.5 ∘C climate target in a medium mitigation scenario on a millennium timescale, and that there is a trade-off between injection-related reductions in atmospheric CO2 levels accompanied by reduced upper-ocean acidification and adverse effects on deep-ocean chemistry, particularly near the injection sites.

Iron is represented in biogeochemical ocean models by a variety of structurally different approaches employing generally poorly constrained empirical parameterizations. Increasing the structural complexity of iron modules also increases computational costs and introduces additional uncertainties, with as yet unclear benefits. In order to demonstrate the benefits of explicitly representing iron, we calibrate a hierarchy of iron modules and evaluate the remaining model-data misfit. The first module includes a complex iron cycle with major processes resolved explicitly, the second module applies iron limitation in primary production using prescribed monthly iron concentration fields, and the third module does not explicitly include iron effects at all. All three modules are embedded into the same circulation model. Models are calibrated against global data sets of NO3, PO4 and O2 applying a state-of-the-art multi-variable constraint parameter optimization. The model with fully resolved iron cycle is marginally (up to 4.8%) better at representing global distributions of NO3, PO4 and O2 compared to models with implicit or absent parameterizations of iron. We also found a slow down of global surface nutrient cycling by about 30% and a shift of productivity from the tropics to temperate regions for the explicit iron module. The explicit iron model also reduces the otherwise overestimated volume of suboxic waters, yielding results closer to observations.

Summary • According to the most recent observational estimate, the ocean lost 2% of its oxygen inventory between 1960 and 2010. • About 15% (range 10-30%) of the oxygen loss is attributed to warming-induced decline in solubility (high confidence). • Less than 15% of the oxygen decline can be attributed to warming-induced changes in respiration of particulate and dissolved organic matter. Enhanced respiration will tend to generate oxygen deficits close to the sea surface. The increased near-surface vertical oxygen gradient may even increase ocean uptake of oxygen from the atmosphere (low confidence). • The majority of oxygen loss has been caused by changes in ocean circulation and associated ventilation with oxygen from the ocean surface (medium confidence). • Current state-of-the-art models simulate deoxygenation rates more than 2 times smaller than the most recent data-based global estimate.

This report comprehensively examines a wide range o marine geoengineering techniques to remove carbon dioxide from the atmosphere or boost the reflection of incoming solar radiation to space (albedo modification) or in some cases both. Further, the report recommends a) that a coordinated framework for proposing marine geoengineering activities, submitting supporting evidence and integrating independent expert assessment must be developed and b) that a greater expertise on wider societal issues is sought with the aim to establish a knowledge base and provide a subsequent analysis of the major gaps in socio-economics and geopolitics.

Summary • The equilibrium state of the ocean-atmosphere system has been perturbed these last few decades with the ocean becoming a source of oxygen for the atmosphere even though its oxygen inventory is only ~0.6% of that of the atmosphere. Different analyses conclude that the global ocean oxygen content has decreased by 1-2% since the middle of the 20th century. Global warming is expected to have contributed to this decrease, directly because the solubility of oxygen in warmer waters decreases, and indirectly through changes in the physical and biogeochemical dynamics. • Since the middle of the 20th century, the increased river export of nitrogen and phosphorus has resulted in eutrophication in coastal areas world-wide. Eutrophication implies huge oxygen consumption, and when combined with a low ventilation, often due to vertical stratification, this leads to the occurrence of oxygen deficiencies near the sea bed. The number of reported sites affected by low oxygen conditions (>500) has dramatically increased in the last few decades. Climate warming is expected to exacerbate the decrease of oxygen by reducing the ventilation and extending the stratification period. • The volume of anoxic zones has expanded since 1960 altering biogeochemical pathways by allowing processes that consume fixed nitrogen and release phosphate and iron, and possibly nitrous oxide (N2O). The relatively small inventory of essential elements, like nitrogen and phosphorus, makes such alterations capable of perturbing the chemical composition equilibrium of the ocean. Positive feedback loops (e.g. remobilization of phosphorus and iron from sediment particles) may speed up the run away from this equilibrium in ways that we hardly know or understand. • Deoxygenation affects many aspects of the ecosystem services provided by the ocean and coastal waters. For example, deoxygenation effects on fisheries include low oxygen affecting populations through reduced recruitment and population abundance, and also through altered spatial distributions of the harvested species causing changes in the dynamics of the fishing vessels. This can lead to changes in the profitability of the fisheries and can affect the interpretation of the monitoring data leading to misinformed management advice. • Model simulations for the end of this century project a decrease of oxygen in the high and low emission scenarios, while the projections of river exports to the coastal ocean indicate that eutrophication will likely continue in many regions of the world. Warming is expected to further amplify the deoxygenation issue in coastal areas influenced by eutrophication by strengthening and extending the stratification.

This report comprehensively examines a wide range o marine geoengineering techniques to remove carbon dioxide from the atmosphere or boost the reflection of incoming solar radiation to space (albedo modification) or in some cases both. Further, the report recommends a) that a coordinated framework for proposing marine geoengineering activities, submitting supporting evidence and integrating independent expert assessment must be developed and b) that a greater expertise on wider societal issues is sought with the aim to establish a knowledge base and provide a subsequent analysis of the major gaps in socio-economics and geopolitics.

Eine rasche Reduktion der Treibhausgasemissionen ist essentiell, wenn ambitionierter Klimaschutz erreicht werden soll. Bei der Abschätzung der dafür notwendigen Anstrengungen und der Bewertung des zukünftigen Beitrags von Technologien, die es erlauben, der Atmosphäre CO2 zu entziehen (negative Emissionstechnologien, NETs), gehen die Meinungen und die Interpretationen des aktuellen Sonderberichts des Weltklimarats stark auseinander. Interpretationen, die sich auf eher große verbleibende CO2-Budgets stützen und damit gleichzeitig die Rolle von NETs für die Erreichung des Temperaturziels herunterspielen, führen nicht zu verantwortungsvollen oder realistischen Einschätzungen der zukünftigen (Forschungs-)Herausforderung: Wir müssen bereits jetzt die Wirksamkeit verschiedener NETs, ihre Grenzen und ihre Wechselwirkungen verstehen, wenn die international angestrebten CO2-Konzentrationspfade realistisch sein sollen. Eine verfrühte Festlegung auf bestimmte NETs sollte vermieden werden. Sobald die Technologien, die sich als effizient erweisen, ausgereift sind, sollte der Umfang ihres Einsatzes durch die Einbeziehung in CO2-Emissionshandelssysteme oder CO2-Emissionssteuerregime bestimmt werden.

Eine rasche Reduktion der Treibhausgasemissionen ist essentiell, wenn ambitionierter Klimaschutz erreicht werden soll. Bei der Abschätzung der dafür notwendigen Anstrengungen und der Bewertung des zukünftigen Beitrags von Technologien, die es erlauben, der Atmosphäre CO2 zu entziehen (negative Emissionstechnologien, NETs), gehen die Meinungen und die Interpretationen des aktuellen Sonderberichts des Weltklimarats stark auseinander. Interpretationen, die sich auf eher große verbleibende CO2-Budgets stützen und damit gleichzeitig die Rolle von NETs für die Erreichung des Temperaturziels herunterspielen, führen nicht zu verantwortungsvollen oder realistischen Einschätzungen der zukünftigen (Forschungs-)Herausforderung: Wir müssen bereits jetzt die Wirksamkeit verschiedenen NETs, ihre Grenzen und ihre Wechselwirkungen verstehen, wenn die international angestrebten CO2-Konzentrationspfade realistisch sein sollen. Eine verfrühte Festlegung auf bestimmte NETs sollte vermieden werden. Sobald die Technologien, die sich als effizient erweisen, ausgereift sind, sollte der Umfang ihres Einsatzes durch die Einbeziehung in CO2-Emissionshandelssysteme oder CO2-Emissionssteuerregime bestimmt werden.

Current mitigation efforts and existing future commitments are inadequate to accomplish the Paris Agreement temperature goals. In light of this, research and debate are intensifying on the possibilities of additionally employing proposed climate geoengineering technologies, either through atmospheric carbon dioxide removal or farther-reaching interventions altering the Earth’s radiative energy budget. Although research indicates that several techniques may eventually have the physical potential to contribute to limiting climate change, all are in early stages of development, involve substantial uncertainties and risks, and raise ethical and governance dilemmas. Based on present knowledge, climate geoengineering techniques cannot be relied on to significantly contribute to meeting the Paris Agreement temperature goals.

Fixed nitrogen (N) limits productivity across much of the low-latitude ocean. The magnitude of its inventory results from the balance of N input and N loss, the latter largely occurring in regionally well-defined low-oxygen waters and sediments (denitrification and anammox). The rate and distribution of N input by biotic N-2 fixation, the dominant N source, is not well known. Here we compile N-2 fixation estimates from experimental measurements, tracer-based geochemical and modeling approaches, and discuss their limitations and uncertainties. The lack of adequate experimental data coverage and the insufficient understanding of the controls of marine N-2 fixation result in high uncertainties, which make the assessment of the current N-balance a challenge. We suggest that a more comprehensive understanding of the environmental and ecological interaction of marine N-2 fixers is required to advance the field toward robust N-2 fixation rates estimates and predictions.iiiii

Increasing atmospheric CO2 is having detrimental effects on the Earth system. Societies have recognized that anthropogenic CO2 release must be rapidly reduced to avoid potentially catastrophic impacts. Achieving this via emissions reductions alone will be very difficult. Carbon dioxide removal (CDR) has been suggested to complement and compensate for insufficient emissions reductions, through increasing natural carbon sinks, engineering new carbon sinks, or combining natural uptake with engineered storage. Here, we review the carbon cycle responses to different CDR approaches and highlight the often-overlooked interaction and feedbacks between carbon reservoirs that ultimately determines CDR efficacy. We also identify future research that will be needed if CDR is to play a role in climate change mitigation, these include coordinated studies to better understand (i) the underlying mechanisms of each method, (ii) how they could be explicitly simulated, (iii) how reversible changes in the climate and carbon cycle are, and (iv) how to evaluate and monitor CDR.

To maintain the chance of keeping the average global temperature increase below 2 degrees C and to limit long-term climate change, removing carbon dioxide from the atmosphere (carbon dioxide removal, CDR) is becoming increasingly necessary. We analyze optimal and cost-effective climate policies in the dynamic integrated assessment model (IAM) of climate and the economy (DICE2016R) and investigate (1) the utilization of (ocean) CDR under different climate objectives, (2) the sensitivity of policies with respect to carbon cycle feedbacks, and (3) how well carbon cycle feedbacks are captured in the carbon cycle models used in state-of-the-art IAMs. Overall, the carbon cycle model in DICE2016R shows clear improvements compared to its predecessor, DICE2013R, capturing much better long-term dynamics and also oceanic carbon outgassing due to excess oceanic storage of carbon from CDR. However, this comes at the cost of a (too) tight short-term remaining emission budget, limiting the model suitability to analyze low-emission scenarios accurately. With DICE2016R, the compliance with the 2 degrees C goal is no longer feasible without negative emissions via CDR. Overall, the optimal amount of CDR has to take into account (1) the emission substitution effect and (2) compensation for carbon cycle feedbacks.

Oxygen is critical to the health of the ocean. It structures aquatic ecosystems and is a fundamental requirement for marine life from the intertidal zone to the greatest depths of the ocean. Oxygen is declining in the ocean. Since the 1960s, the area of low oxygen water in the open ocean has increased by 4.5 million km2, and over 500 low oxygen sites have been identified in estuaries and other coastal water bodies. Human activities are a major cause of oxygen decline in both the open ocean and coastal waters. Burning of fossil fuels and discharges from agriculture and human waste, which result in climate change and increased nitrogen and phosphorus inputs, are the primary causes. Published Refereed

The chapter discusses sources of uncertainties in climate models and their possible impacts on the model results. The three criteria “adequacy”, “consistency” and “representativeness” are suggested for a comprehensive assessment of the quality of climate models. The fit to data determines the model’s representativeness. For many climate variables, such as precipitation, cloudiness and the climate sensitivity, this has not significantly improved from the second-to-last to the last assessment report of the Intergovernmental Panel on Climate Change (IPCC). However, the level of detailed mechanistic descriptions has increased for a number of processes included in the models, yielding an improved adequacy of these models. Still, with current climate models being still unable to consistently reproduce glacial cycles driven only by orbital parameters, and with the amplitude of climate change expected until the end of the century being of similar amplitude as glacial-interglacial changes, there is still considerable uncertainty regarding how reliable current models’ projections of 21st century climate change can be. However, uncertainty must not hinder society to make informed decisions, and it is the responsibility of climate research to provide relevant information regarding the uncertainty of climate model projections.

The potential of Coastal Ocean Alkalinization (COA), a carbon dioxide removal (CDR) climate engineering strategy that chemically increases ocean carbon uptake and storage, is investigated with an Earth system model of intermediate complexity. The CDR potential and possible environmental side effects are estimated for various COA deployment scenarios, assuming olivine as the alkalinity source in ice-free coastal waters (about 8.6% of the global ocean's surface area), with dissolution rates being a function of grain size, ambient seawater temperature and pH. Our results indicate that for a large-enough olivine deployment of small-enough grain sizes (10 μm), atmospheric CO2 could be reduced by more than 800 GtC by the year 2100. However, COA with coarse olivine grains (1000 μm) has little CO2 sequestration potential on this time scale. Ambitious CDR with fine olivine grains would increase coastal aragonite saturation Ω to levels well beyond those that are currently observed. When imposing upper limits for aragonite saturation levels (Ωlim) in the grid boxes subject to COA (Ωlim = 3.4 and 9 chosen as examples), COA still has the potential to reduce atmospheric CO2 by 265 GtC (Ωlim=3.4) to 790 GtC (Ωlim=9) and increase ocean carbon storage by 290 Gt (Ωlim=3.4) to 913 Gt (Ωlim=9) by year 2100.

In this pilot study we link the yield of industrial fisheries to changes in the zooplankton mortality in an idealized way accounting for different target species (planktivorous fish—decreased zooplankton mortality; large predators—increased zooplankton mortality). This indirect approach is used in a global coupled biogeochemistry circulation model to estimate the range of the potential impact of industrial fisheries on marine biogeochemistry. The simulated globally integrated response on phytoplankton and primary production is in line with expectations—a high (low) zooplankton mortality results in a decrease (increase) of zooplankton and an increase (decrease) of phytoplankton. In contrast, the local response of zooplankton and phytoplankton depends on the region under consideration: In nutrient-limited regions, an increase (decrease) in zooplankton mortality leads to a decrease (increase) in both zooplankton and phytoplankton biomass. In contrast, in nutrient-replete regions, such as upwelling regions, we find an opposing response: an increase (decrease) of the zooplankton mortality leads to an increase (decrease) in both zooplankton and phytoplankton biomass. The results are further evaluated by relating the potential fisheries-induced changes in zooplankton mortality to those driven by CO2 emissions in a business-as-usual 21st century emission scenario. In our idealized case, the potential fisheries-induced impact can be of similar size as warming-induced changes in marine biogeochemistry.

The natural abundance of 14C in total CO2 dissolved in seawater (DIC) is a property applied to evaluate the water age structure and circulation in the ocean and in ocean models. In this study we use three different representations of the global ocean circulation augmented with a suite of idealised tracers to study the potential and limitations of using natural 14C to determine water age, which is the time elapsed since a body of water has been in contact with the atmosphere. We find that, globally, bulk 14C-age is dominated by two equally important components, one associated with ageing, i.e. the time component of circulation, and one associated with a "preformed 14C-age". The latter quantity exists because of the slow and incomplete atmosphere–ocean equilibration of 14C particularly in high latitudes where many water masses form. In the ocean's interior, preformed 14C-age behaves like a passive tracer. The relative contribution of the preformed component to bulk 14C-age varies regionally within a given model, but also between models. Regional variability in the Atlantic Ocean is associated with the mixing of waters with very different end members of preformed 14C-age. Here, variations in the preformed component over space and time mask the circulation component to an extent that its patterns are not detectable from bulk 14C-age. Between models, the variability of preformed 14C-age can also be considerable (factor of 2), related to the combination of physical model parameters, which influence circulation dynamics or gas exchange. The preformed component was found to be very sensitive to gas exchange and moderately sensitive to ice cover. In our model evaluation, the choice of the gas-exchange constant from within the currently accepted range of uncertainty had such a strong influence on preformed and bulk 14C-age that if model evaluation would be based on bulk 14C-age, it could easily impair the evaluation and tuning of a model's circulation on global and regional scales. Based on the results of this study, we propose that considering preformed 14C-age is critical for a correct assessment of circulation in ocean models.