Marine carbon dioxide removal (mCDR) and geological carbon storage in the marine environment (mCS) promise to contribute to the mitigation of global climate change in combination with drastic emission reductions. However, the implementable potential of mCDR and mCS depends, apart from technology readiness, also on site-specific conditions. In this paper, we explore different options for mCDR and mCS, using the German context as a case study. We challenge each option to remove 10 Mt CO2 yr-1, which accounts for 8-22% of projected hard-to-abate and residual emissions of Germany in 2045. We focus on the environmental, resource, and infrastructure requirements of individual mCDR and mCS options at a specific site, within the German jurisdiction when possible. Furthermore, we discuss main uncertainty factors and research needs, and, where possible, cost estimates, expected environmental effects, and monitoring approaches. In total, we describe ten mCDR and mCS options; four aim at enhancing the chemical carbon uptake of the ocean through alkalinity enhancement, four aim at enhancing blue carbon ecosystems’ sink capacity, and two employ geological off-shore storage. Our results indicate that five out of ten options would potentially be implementable within German jurisdiction, and three of them could potentially rise to the challenge. This exercise provides a basis for further studies to assess the socio-economic, ethical, political, and legal aspects for such implementations.

A document by Malgorzata Borchers. Click on the document to view its contents.

Achieving global climate goals and securing future food supplies poses significant challenges, especially if efforts are limited to land-based solutions. Given that the ocean covers over 70% of the Earth’s surface and plays a critical role in CO2 sequestration, exploring ocean-based climate mitigation strategies will be essential. One promising approach is Ocean Alkalinity Enhancement (OAE), a form of marine geoengineering aimed at accelerating the ocean’s natural carbon sink, reducing atmospheric CO2 levels, and mitigating ocean acidification. However, the implications of OAE for global fisheries, which are vital for food security and livelihoods worldwide, remain underexplored. This study develops and analyzes future scenarios for global fisheries under different socioeconomic and climate trajectories, utilizing the Shared Socioeconomic Pathways (SSPs) and Representative Concentration Pathways (RCPs) framework. Specifically, we focus on three combined pathways: SSP1-2.6, SSP3-7.0, and SSP5-8.5, to explore the potential impacts of OAE implementation. Through an integrated narrative approach, we (semi-)quantify changes in key bio-economic parameters such as technological progress, fishing costs, fisheries management, marine aquaculture, and carrying capacity, providing an explorative assessment of how OAE could influence these under varying global conditions. With this approach, we contribute to the development of sector-specific and long-term interdisciplinary models that are crucial for future policy and management strategies aimed at climate change mitigation and the sustainable use of marine ecosystems. Our framework aligns with global scenarios that are being applied internationally.

To reach their net-zero targets, countries will have to compensate hard-to-abate CO2 emissions through carbon dioxide removal (CDR). Yet, current assessments rarely include socio-cultural or institutional aspects or fail to contextualize CDR options for implementation.Here we present a context-specific feasibility assessment of CDR options for the example of Germany. We assess fourteen CDR options, including three chemical carbon capture options, six options for bioenergy combined with carbon capture and storage (BECCS), and five options that aim to increase ecosystem carbon uptake. The assessment addresses technological, economic, environmental, institutional, social-cultural and systemic considerations using a traffic-light system to evaluate implementation opportunities and hurdles.We find that in Germany CDR options like cover crops or seagrass restoration currently face comparably low implementation hurdles in terms of technological, economic, or environmental feasibility and low institutional or social opposition but show comparably small CO2 removal potentials. In contrast, some BECCS options that show high CDR potentials face significant techno-economic, societal and institutional hurdles when it comes to the geological storage of CO2. While a combination of CDR options is likely required to meet the net-zero target in Germany, the current climate protection law includes a limited set of options. Our analysis aims to provide comprehensive information on CDR hurdles and possibilities for Germany for use in further research on CDR options, climate, and energy scenario development, as well as an effective decision support basis for various actors.

Quantifying changes in oceanic aerobic respiration is essential for understanding marine deoxygenation. Here we use an Earth system model to investigate if and to what extent oxygen utilization rate (OUR) can be used to track the temporal change of true respiration ($R_\mathrm{true}$). $R_\mathrm{true}$ results from the degradation of particulate and dissolved organic matter in the model ocean, acting as ground truth to evaluate the accuracy of OUR. Results show that in thermocline and intermediate waters of the North Atlantic Subtropical Gyre (200m-1000m), vertically integrated OUR and $R_\mathrm{true}$ both decrease by 0.2 $\mathrm{mol O_{2}/m^{2}/yr}$ from 1850 to 2100 under global warming. However, in the mesopelagic Tropical South Atlantic, integrated OUR increases by 0.2 $\mathrm{mol O_{2}/m^{2}/yr}$, while the $R_\mathrm{true}$ integral decreases by 0.3 $\mathrm{mol O_{2}/m^{2}/yr}$. A possible reason for the diverging OUR and $R_\mathrm{true}$ is ocean mixing, which affects water mass composition and maps remote respiration changes to the study region.

This study introduces an ocean-based carbon dioxide removal (CDR) approach: Nearshore Macroalgae Aquaculture for Carbon Sequestration (N-MACS). By cultivating macroalgae in nearshore ocean surface areas, N-MACS aims to sequester CO2 with subsequent carbon storage. Utilizing an Earth System Model with intermediate complexity (EMIC), we explore the CDR potential of N-MACS alongside its impacts on the global carbon cycle, marine biogeochemistry and marine ecosystems. Our investigations unveil that coastal N-MACS could potentially sequester 0.7 to 1.1 GtC yr-1. However, it also significantly suppresses marine phytoplankton net primary productivity because of nutrient removal and canopy shading, counteracting approximately 30% of the N-MACS CDR capacity. This suppression of surface NPP, in turn, reduces carbon export out of the euphotic zone to the ocean interior, leading to elevated dissolved oxygen levels and diminished denitrification in present-day oxygen minimum zones. Effects due to harvesting-induced phosphorus removal continue for centuries even beyond the cessation of N-MACS.

The current narrative of artificial upwelling (AU) is to use ocean pipes to pump nutrient rich deep water to the ocean surface, thereby stimulating the biological carbon pump. This simplistic concept of AU does not take the response of the solubility pump or the CO2 emission scenario into account. Using global ocean-atmosphere model experiments and several idealized model tracers we show that the effectiveness of almost globally applied AU from the year 2020 to 2100 to draw down CO2 from the atmosphere is strongly dependent on the CO2 emission scenario and ranges from 1.01 Pg C / year under RCP 8.5 to 0.32 Pg C / year under RCP 2.6. The solubility pump becomes equally effective compared to the biological carbon pump under the highest emission scenario (RCP 8.5), but responds with CO2 outgassing under low CO2emission scenarios.

Iron is a key micronutrient controlling phytoplankton growth in vast regions of the global ocean. Despite its importance, uncertainties remain high regarding external iron source fluxes and internal cycling on a global scale. In this study, we used a global dissolved iron dataset, including GEOTRACES measurements, to constrain source and scavenging fluxes in the marine iron component of a global ocean biogeochemical model. Our model simulations tested three key uncertainties: source inputs of atmospheric soluble iron deposition (varying from 1.4 - 3.4 Gmol/yr), reductive sedimentary iron release (14 - 117 Gmol/yr), and compare a variable ligand parameterization to a constant distribution. In each simulation, scavenging rates were adjusted to reproduce the observed global mean iron inventory for consistency. The apparent oxygen utilization term in the variable ligand parameterization significantly improved the model-data misfit, suggesting that heterotrophic bacteria are an important source of ligands to the ocean. Model simulations containing high source fluxes of atmospheric soluble iron deposition (3.4 Gmol/yr) and reductive sedimentary iron release (114 Gmol/yr) further improved the model, which then required high scavenging rates to maintain the observed iron inventory in these high source scenarios. Our model-data analysis suggests that the global marine iron cycle operates with high source fluxes and high scavenging rates, resulting in relatively short surface and global ocean mean residence times of 0.83 and 7.5 years, respectively, which are on the low-end of previous model estimates. Model biases and uncertainties remain high and are discussed to help improve global marine iron cycle models.

In a widely-held conception, the biological carbon pump (BCP) is equal to the export of organic matter out of the euphotic zone. Using global ocean-atmosphere model experiments we show that the change in export production is a poor measure of the biological pump’s feedback to the atmosphere. The change in global true oxygen utilization (TOU), an integrative measure of the imprint of the biological carbon pump on marine oxygen, however, is in good agreement with the net change in the biogenic air-sea flux of oxygen. Since, TOU correlates very well with apparent oxygen utilization (AOU) in our experiments, we propos to measure the change of AOU from data of global float programs to monitor the feedback of the BCP to the atmosphere. For the current ocean we estimate that BCP changes lead to an uptake of CO by the ocean in the range of 0.07 to 0.14 GtC/yr.