As the urgency of climate mitigation intensifies, carbon dioxide removal (CDR) strategies have been put forward as a promising complement to emissions reduction efforts. The ocean’s large capacity to absorb atmospheric carbon dioxide (CO₂) has promoted the development of marine-based CDR (mCDR) strategies. However, their responsible deployment at climatically relevant scales remains challenging as the environmental risks, trade-offs, and uncertainties across approaches, scales, and regions have yet to be comprehensively understood.
This work presents a comparative synthesis of the technology, carbon removal potential, environmental impacts and deployment constraints of leading mCDR approaches, namely ocean alkalinity enhancement (OAE), iron fertilization, artificial upwelling and macroalgae cultivation. These approaches can be broadly classified as biotic, which enhance biological carbon uptake through increased primary productivity, and abiotic, which increase inorganic carbon storage by modifying seawater carbonate chemistry. While each approach presents distinct environmental trade-offs, OAE serves as a case study as it offers one of the highest CDR potentials though presents both chemical and biological limitations. It also illustrates how seemingly simple changes to seawater chemistry can propagate through marine ecosystems, highlighting thre need for careful ecological and carbon monitoring, reporting and verification.
Biotic approaches are primarily constrained by ecological feedbacks, including nutrient redistribution, hypoxia, altered food-web dynamics, and uncertainties surrounding the permanence of biologically sequestered carbon. In contrast, abiotic approaches, and OAE in particular, are influenced by local carbonate chemistry, feedstocks properties, mineral dissolution and precipitation kinetics, while also raising questions about ecosystem responses to prolonged changes in seawater chemistry. Here, environmental considerations of each approach are compared alongside their CDR potential, technology readiness, scalability, and costs.
Using OAE as an illustrative example, chemical limitations and biological uncertainties are discussed to help guide deployment strategies and improve CDR efficiency. Evidence from laboratory experiments, mesocosm studies and field trials highlights the importance of evaluating both direct geochemical effects, including localised shifts in carbonate chemistry and trace-metal release from alkaline feedstocks, and the ecological responses that they may induce. Understanding the chemical processes that drive biological responses is therefore of prime importance and reinforces the idea that OAE’s environmental impacts cannot be considered solely as chemical or biological processes. Instead, they arise from interactions between the two, including the amount of alkaline material added, the persistence of critical alkalinity periods, and impacts on individual species that may cascade through higher trophic levels.