“Site-specific factors play a crucial role in determining which ocean-based CDR and storage methods could realistically be considered. Our analysis helps us to understand what scale we are talking about when discussing the deployment of these methods in German waters—and where along the process chains foreseeable bottlenecks or limitations to feasibility might arise,” said co-lead author Dr. Wanxuan Yao, who was a climate modeler at GEOMAR Helmholtz Centre for Ocean Research Kiel at the time of the study.

Comprehensive Assessment of CO2 Removal Methods and Their Impacts

For their analysis, the team reviewed current scientific literature and incorporated findings from their own research as part of the CDRmare DAM research mission. For each method, they assessed factors such as the amount of water, materials, energy, land or sea area required, possible by-products and waste streams, necessary infrastructure and transport routes required, operating costs, and what is currently known about potential environmental and societal impacts.

“In addition, we looked at whether there are already established processes for measuring and monitoring CO2 removal rates and potential environmental impacts for each method. Without such monitoring frameworks, none of the proposed approaches has a realistic chance of ever being deployed on a large scale,” explained co-lead author Dr. Teresa Morganti, a marine biologist at the Leibniz Institute for Baltic Sea Research Warnemünde at the time of the study.

Ten marine CDR methods shortlisted

At the end of the multi-year selection process, five methods of CO2 removal remained that could potentially be implemented in the German North Sea and Baltic waters. Five further approaches would require deployment in international waters or cooperation with other coastal states.

“The options we’ve outlined are intended to raise specific, practical questions and challenges that would need to be addressed in the event of any large-scale application and to provide a basis for further discussion. It’s important to emphasize, however, that these outlines do not take into account the legal, political, or economic frameworks, nor do they address whether the potential environmental impacts of targeted marine carbon removal are consistent with our societal values and ethical principles. These are essential questions that need to be addressed in follow-up studies,” said Dr Nadine Mengis of GEOMAR Helmholtz Centre for Ocean Research Kiel, co-author of the study and CDRmare co-chair.

Expectations for Marine CDR Are Often Too High

The research team is, therefore, working systematically to develop methods and processes that provide a realistic picture of how feasible marine CDR techniques actually are and what their consequences might be for both people and ecosystems. “Once you scale up marine CDR methods for a particular region, making the true scale of interventions tangible, it becomes clear that earlier expectations were often too high because such practical considerations had not been taken into account. We need many more of these context-specific feasibility studies if we are to arrive at robust estimates of the potential for marine CDR,” said Nadine Mengis.

There is also a risk that high expectations might encourage countries like Germany to place too much hope in future technological solutions while scaling back existing and proven measures to reduce greenhouse gas emissions in the meantime. “This must not be the outcome of our research,” stressed Nadine Mengis.

The marine CDR and storage methods described in the study include:

Methods to increase the CO2 buffering capacity (alkalinity) of the ocean:

• Production of a silicate-based alkaline solution and its distribution in shallow coastal waters along Germany’s North Sea coast
• Production of a lime-based alkaline solution and its distribution along shipping routes in the German North Sea
• Spreading of ground basalt of volcanic origin along the German coastline
• Discharge of sodium hydroxide produced via electrolysis in desalination plants (there are currently no desalination plants in the North Sea or Baltic Sea)

Methods to restore and expand vegetated coastal ecosystems:

• Establishing and expanding kelp forests around the German North Sea island of Heligoland
• Restoration and expansion of mangrove forests in Indonesia
• Artificial upwelling of nutrient-rich deep water to enhance plankton production in the North Atlantic (strengthening the ocean’s biological carbon pump)
• Offshore Sargassum (algae) farming in the South Atlantic subtropical gyre, followed by biomass sinking

Methods for storing captured biogenic CO2:

• Cultivation of large macroalgae, with subsequent conversion of the biomass into biomethane; the CO2 released during combustion would be captured and stored in saline aquifers in the German North Sea
• Direct air capture of CO2 with subsequent storage in subsea basalt crust off the coast of Norway