EU Experts Warn Carbon Removal Technologies Not Ready for Large-Scale Ocean Deployment

New technologies designed to harness and accelerate the process of ocean-based carbon dioxide absorption are not yet ready for large-scale deployment, according to a new report from the European Marine Board (EMB). 

The EMB’s report, ‘Monitoring, Reporting and Verification for Marine Carbon Dioxide Removal’, was released in November 2025 to coincide with COP30 in Brazil. It stressed that while oceans can be part of the climate solution, safeguards must be strengthened to ensure these technologies do not cause more harm than good.

The expert group, chaired by Helene Muri of the Norwegian University of Science and Technology, has warned that marine carbon dioxide removal (mCDR) methods must be subject to strict monitoring, reporting and verification before they can be scaled up responsibly.

Marine carbon dioxide removal builds on the ocean’s natural ability to absorb CO₂. Approaches fall into two broad categories: biotic and geochemical methods.

Biotic methods, which rely on photosynthesis by marine organisms such as phytoplankton and seaweed. These organisms absorb CO₂ and, when they die, some carbon sinks to the ocean depths where it can be stored for centuries.

Six biotic approaches are currently being explored:

1. Transferring naturally occurring biomass such as Sargassum to the deep ocean.
2. Cultivating seaweed or kelp and sinking it after harvest.
3. Converting seaweed into durable products like bioplastics or bioenergy with carbon capture.
4. Fertilising phytoplankton with nutrients to enhance growth.
5. Artificial upwelling of nutrient-rich deep water to stimulate plankton blooms.
6. Restoring coastal ecosystems such as mangroves and seagrasses.

Geochemical methods, which use chemical processes to capture and store CO₂, inspired by natural weathering cycles.

Geochemical methods include: 

1. Ocean alkalinity enhancement to take up CO₂ from the atmosphere by changing the CO₂ dissolved into bicarbonates or carbonates.
2. Artificial downwelling to enhance the transport of both dissolved and particulate carbon deeper into the ocean.
3. Direct extraction of CO₂ from seawater using electrochemical processes powered by renewable energy.

The EMB report highlighted a key challenge in proving that these methods actually remove carbon and keep it stored. Unlike land-based approaches such as afforestation, the ocean is dynamic and difficult to govern. Carbon stored in the deep ocean may not remain there indefinitely, and monitoring its fate is complex.

“This is about safeguarding the oceans for a common good. The oceans can be part of the climate solution, but we need to strengthen the way we safeguard them before we scale things up,” said Muri.

The report reiterated that cutting emissions must remain the top priority. UN Secretary-General António Guterres told COP30 leaders: 

“Science now tells us that a temporary overshoot beyond the 1.5°C limit – starting at the latest in the early 2030s – is inevitable,” said UN Secretary-General, António Guterres at the COP30 Leaders’ Summit on 6th November. “Let us be clear: the 1.5°C limit is a red line for humanity. It must be kept within reach. And scientists also tell us that this is still possible”.

Muri echoed this point, stressing that proven methods of reducing emissions must take precedence. Marine carbon removal should only be considered as a complement to emissions cuts, particularly to offset “residual” emissions from hard-to-abate sectors such as aviation and shipping.

Achieving net-zero by 2050 will require balancing emissions with removals. But to limit warming to 1.5°C, societies must go further, achieving net negative emissions. 

According to IPCC scenarios, this means removing 5–10 gigatons of CO₂ per year by the end of the century. With global emissions at 42.4 gigatons in 2024, the scale of the challenge is immense.

Land-based removal methods such as afforestation and direct air capture are already being deployed. Marine approaches remain at the testing stage, but could play a role if standards are established.

Some methods, such as ocean fertilisation, raise concerns about unintended ecological impacts. Large plankton blooms could disrupt ecosystems, while artificial upwelling and downwelling remain technically unfeasible. Even coastal restoration, though beneficial, requires careful governance to ensure long-term carbon storage.

The EMB warned that without stringent monitoring and verification, companies or governments could claim credits for removals that are not real or permanent. 

“If we want to be serious about figuring out if you can do marine carbon dioxide removal in responsible ways that can make meaningful contributions, then we have to get serious about the monitoring, reporting and verification aspects,” said Muri.

“We don’t know all the threats of these immature methods yet, but it’s a bit hard to just take them off the table because they’re uncomfortable to think about”.

Marine carbon dioxide removal is not a “miracle ocean fix,” but it could become part of a broader portfolio of solutions if governance frameworks are established. The EMB has called for international cooperation to develop standards, clarify responsibilities among agencies and treaties, and ensure transparency in crediting systems.

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