The need to explore the potential of marine CDR – A guide for policy makers

Limiting global warming to 1.5°C or 2°C in the midst of a climate emergency requires rapid, deep, and sustained emission reductions, alongside annual CDR at the billion-tonne (Gt) scale. CDR is essential for addressing hard-to-abate, residual emissions and reducing atmospheric CO₂. Achieving the billion-tonne CDR target demands a holistic approach that includes both land and ocean – which we term One-Earth CDR.  One-Earth CDR is critical because all CDR methods face a "CDR tax" due to feedbacks from the human-altered Earth System. These feedbacks release stored anthropogenic CO₂ from land and ocean reservoirs, which partially offsets the effectiveness of CDR. Therefore, to reach the billion-tonne goal, CDR must be applied sustainably in all feasible environments.  One-Earth CDR also serves as a safeguard against over-reliance on land-based CDR, which faces challenges such as side effects (e.g., mega-fires) and sustainability limits (e.g., land and water use).  Marine CDR (mCDR) using innovative methods offers a large potential for carbon storage.  Proving the effectiveness and safety of mCDR will likely take at least a decade. Ensuring its integrity is crucial for verifiable CDR. Before large-scale deployment, knowledge gaps must be addressed, including risks, sustainability, scalability, cost, permanence, side effects, monitoring, verification, social acceptance, and governance frameworks.