As the global urgency to combat climate change escalates, the pursuit of carbon dioxide removal (CDR) methods gains critical momentum. While reducing emissions remains the cornerstone of climate mitigation strategies, attention increasingly turns to techniques capable of capturing and storing CO₂ from the atmosphere, particularly those employing the vast options presented by the ocean. This emergent field of marine-based carbon removal and storage is fraught with both promise and complexity. A groundbreaking study led by a consortium of German researchers has, for the first time, systematically assessed the potential for large-scale marine CDR within Germany’s territorial waters. Their findings shed new light on the practicalities, limitations, and ecological considerations inherent in deploying oceanic CDR solutions in the North Sea and Baltic Sea regions.
Oceans cover more than 70% of the Earth’s surface and play a pivotal role in the global carbon cycle. Their ability to absorb CO₂ is fundamental, yet this capacity is finite and susceptible to perturbation from ongoing emissions. Enhancing the ocean’s natural CO₂ buffering capacity via engineered interventions could, in theory, significantly augment atmospheric CO₂ removal. The study recognizes that site-specific factors—such as local water chemistry, ocean currents, ecosystem functionality, and available infrastructure—critically determine which marine CDR methods can be feasibly scaled. Comprehensive regional analyses ensure that proposed solutions are not only scientifically sound but practically executable, avoiding overly ambitious expectations detached from reality.
One key dimension explored is the potential to increase ocean alkalinity, a process that boosts the seawater’s ability to absorb more CO₂ by chemically shifting the carbonate system toward bicarbonate ions. This can be achieved through deploying silicate- or lime-based alkaline solutions directly into coastal waters or shipping lanes. Additionally, spreading finely ground basalt, a volcanic rock rich in silicate minerals, along shorelines can trigger natural weathering reactions that sequester CO₂ over long timescales. However, the study reveals that these approaches require vast quantities of raw materials and energy-intensive processing, raising questions about their lifecycle emissions and logistical feasibility within Germany’s existing marine and industrial infrastructure.
Beyond chemical methods, the restoration and expansion of vegetated coastal ecosystems such as kelp forests represent another promising avenue. Kelp, a fast-growing macroalgae, actively fixes CO₂ through photosynthesis and can be cultivated extensively in the North Sea, particularly around islands like Heligoland. The biological sequestration potential of such ecosystems hinges on enhancing biomass accumulation and ensuring long-term carbon storage, possibly through sinking harvested algae into deep ocean layers. However, ecosystem-based approaches demand careful management to avoid adverse effects on biodiversity and fisheries, stressing the need for robust environmental monitoring frameworks.
The study also examines more geographically expanded approaches, including mangrove restoration projects outside Germany’s immediate waters—such as in Indonesia—and artificial upwelling in the North Atlantic. Artificial upwelling involves pumping nutrient-rich deep waters to the surface to stimulate phytoplankton blooms, which can enhance the biological carbon pump—the natural process by which CO₂ fixed by plankton is transported to the deep ocean. Offshore farming of Sargassum algae in subtropical gyres, followed by biomass sinking, emerges as yet another strategy with potential but entails significant operational complexity and uncertain ecological impacts, highlighting the necessity for site-specific assessments.
In terms of carbon storage, the study highlights innovative techniques involving captured biogenic CO₂. One example is the cultivation of large macroalgae whose biomass is converted to biomethane, a renewable energy source. The carbon dioxide released during subsequent combustion is anticipated to be captured and injected into saline aquifers beneath the German North Sea. Such carbon capture and storage (CCS) infrastructures are already under development, and coupling them with biomass-based energy generation could create integrated carbon negative energy systems. Another highlighted technology involves direct air capture of CO₂, followed by its storage in subsea basalt formations off Norway’s coast. Both methods require international cooperation and substantial investments but illustrate the multi-layered nature of marine CDR potential and its intertwining with geopolitical and economic frameworks.
Crucially, the study cautions against overinflated expectations of marine CDR. Scaling these technologies reveals significant resource demands—in terms of water, energy, materials, and infrastructure—that present formidable challenges. Many proposals have remained largely theoretical, often overlooking practical bottlenecks such as transport logistics, monitoring requirements, and unforeseen environmental consequences. This underscores the indispensable need for rigorous, context-specific feasibility research to realistically appraise marine CDR’s capacity and risks, rather than relying on speculative optimism.
A recurring theme is the imperative for robust measurement, monitoring, and verification (MMV) frameworks compatible with each marine CDR method. Without reliable systems to quantify net CO₂ removal and track ecological impacts, deployment on a meaningful scale remains infeasible. The study emphasizes that the establishment of MMV protocols must precede or accompany any large-scale project development, ensuring transparency, credibility, and societal trust. Such frameworks will also facilitate adaptive management, allowing interventions to be calibrated based on real-world feedback.
Furthermore, the research stresses that marine CDR development must not detract from urgent emission reductions already achievable with existing technologies and policy instruments. There is a latent risk that hope in future marine CDR breakthroughs could unintentionally diminish political and social commitment to nearer-term mitigation efforts. The authors highlight that the research community and policymakers alike must treat marine CDR as a complement—not a substitute—in the broader climate strategy, maintaining a clear ethical stance on sustainable and equitable environmental stewardship.
Legal, political, and economic dimensions are acknowledged as critical yet unresolved factors influencing the scalability of marine CDR. International waters pose jurisdictional complexities, necessitating diplomatic collaboration for approaches requiring extended deployment beyond German territorial boundaries. Moreover, the acceptability of potential environmental impacts involves intricate societal value judgments, pointing to the need for cross-disciplinary approaches integrating social science insights, public participation, and science communication.
This landmark meta-analysis propels the discourse on marine carbon removal forward by systematically synthesizing scientific evidence and integrating empirical research from missions such as CDRmare DAM. It situates marine CDR in a regional context, stipulating realistic operational scales aligned with Germany’s marine environmental conditions and industrial capabilities. As marine carbon removal technologies inch closer to practical realization, such studies will serve as foundational references guiding responsible innovation, policymaking, and investment toward carbon neutrality.
In conclusion, while marine-based CO₂ removal remains a complex and multifaceted challenge, the study elucidates a promising portfolio of approaches with varying geographical applicability and technical readiness. Their meticulous characterization of physical, ecological, and logistical factors equips researchers, policymakers, and stakeholders with vital knowledge needed to navigate the nuanced pathway toward harnessing the ocean’s carbon sequestration potential. This path demands sober evaluation, comprehensive governance, and unwavering commitment to sustainability to transform scientific ideas into climate solutions that are both effective and socially aligned.
Subject of Research: Ocean-based carbon dioxide removal (CDR), marine carbon sequestration, feasibility assessment for marine CDR in German waters.
Article Title: Exploring site-specific carbon dioxide removal options with storage or sequestration in the marine environment – the 10 Mt CO₂ yr⁻¹ removal challenge for Germany
News Publication Date: 14-Apr-2025
Web References: 10.1029/2024EF004902
Keywords: Carbon sequestration, Carbon capture, Seawater, Marine plants, Alkalinity, Ocean pH, Environmental monitoring, Environmental stresses, International cooperation, Marine ecosystems, Seawater desalinization, Carbon biomass, Ocean currents, Ecological processes
