In the waters of the North Sea and the Baltic Sea, the scope for capturing carbon dioxide and storing it geologically at sea is limited, and only a few approaches are available, say researchers at the German Marine Research Alliance (DAM) working on the CDRmare research mission.

The ocean covers 70% of the Earth's surface, it is the largest long-term sink for anthropogenic CO₂ and can store far more carbon than the atmosphere and the terrestrial biosphere combined, notes the GEOMAR Helmholtz Centre for Ocean Research in Kiel. Even so, ocean-based carbon removal technologies are still only in their early stages, and many of them have yet to be tested on a larger scale.

The research team assessed ten options, six of which could in theory remove at least 10 million tonnes of CO₂ a year (8-22% of Germany's projected hard-to-abate and residual emissions in 2045). Of those six options, however, only three appear feasible in Germany – producing a silicate-based alkaline solution and dispersing it in shallow coastal waters along the North Sea, producing a lime-based alkaline solution and dispersing it along shipping routes in the North Sea, and growing macroalgae biomass to produce biomethane, combined with carbon storage in saline aquifers in the North Sea.

“We expect that marine carbon dioxide removal could make a significant contribution to carbon dioxide removal, and it should be considered among the range of options for reaching net zero in Germany,” the report states.

The researchers took into account both local marine conditions and where the materials, infrastructure and energy needed for operations on such a scale might come from. “It is important to stress, however, that this overview does not take account of the legal, political or economic framework, nor does it examine whether the potential environmental impacts of deliberate marine carbon dioxide removal are consistent with our society's values and ethical principles,” said researcher Nadine Mengis of GEOMAR.

Climate scientists argue that negative emissions will be needed to balance out unavoidable residual emissions, such as methane from livestock farming, and to bring global temperatures back down after the 1.5 °C limit set in the Paris Agreement is, in all likelihood, exceeded. That makes this a key issue for the decades ahead, yet the technologies and the markets that go with them have to be built today.

The previous German government had drawn up a long-term strategy on negative emissions, but it was never completed. It had proposed introducing a net negative greenhouse gas emissions target by 2060, along with interim targets for technical carbon sinks. The issue will now fall to the next coalition, made up of the conservative CDU/CSU alliance and the Social Democrats (SPD). The coalition has already announced that it intends to introduce legislation enabling carbon capture and storage as well as utilisation (CCS/CCU), where certain technologies are also suitable for generating negative emissions.