An increasing number of carbon removal initiatives are looking to increase the CO₂ removal potential of the ocean or make use of its carbon sink capacities. Since industrialization, our oceans have been the second largest carbon sink for excessive human-made CO₂ emissions, increasing the acidity of ocean waters by 30% and hampering the formation and survival of marine life like corals and shellfish. Proposed approaches to engineered ocean carbon dioxide removal include increasing the alkalinity of the ocean, sinking carbon-rich materials to the deep seabeds where carbon decomposition is minimal, or fertilizing the oceans to speed up natural nutrient cycling flows. Ocean carbon removal is considered to have the largest potential for carbon removal and its cost is expected to be much lower than other carbon dioxide removal (CDR) approaches, but all these technologies are early-stage, and their long-term economics are still not very well known.

‍

Potential and scalability:

Ocean CDR approaches are still in the early phases of development and the marine science community has called for additional research to be carried out to identify and assess the impacts of ocean CDR approaches on marine ecosystems before any large-scale deployments are allowed. Groups like Ocean Visions have created research initiatives specific to ocean carbon removal, such as CDR-specific roadmaps as well as creating links between CDR companies and researchers.

‍

Another main challenge for ocean CDR approaches is the measurement, reporting, and verification (MRV) component of the removal activities. Given the size of oceans, the diffuse aspect of some of the approaches proposed, as well as complex marine jurisdiction schemes, it will be especially crucial to establish robust mechanisms to monitor the potential ecosystem effects of ocean carbon removal activities.

‍

Here is a brief overview of the main approaches in development:

‍

• Ocean alkalinity enhancement (OAE) consists of injecting a basic (high pH) solution such as sodium hydroxide into the ocean, increasing the alkalinity of seawater and hence countering the harm of ocean acidification and increasing its natural capacity to sequester CO₂. Durable carbon removal occurs through the creation of carbonate precipitates, which are naturally channelled to the seabed where they remain for geological timescales (10000+ years). OAE can be deployed in coastal areas, making use of existing effluent fluxes into our seas and oceans (from wastewater treatment of desalinization processes for instance) or directly into the ocean, discharging from cargo ships or dedicated vessels. The challenge for the deployment of OAE relates to the characterization of ocean dilution mechanisms and current fluxes, ensuring that alkalinity pockets are not accidentally created following OAE activities. The need for detailed characterization of potential ecosystem impacts has been highlighted by experts and research is currently underway in this area to enable the definition of detailed risk assessments.

‍

• Biomass sinking proposes to take terrestrial or ocean biomass and sink it into the deep ocean where it is sequestered for thousands of years. This can be accomplished by large-scale seaweed farming, which incorporates atmospheric CO₂ as it grows, and then is sunk to the ocean floor. The premise for biomass sinking as a CDR approach is that microbial activity and carbon degradation are minimal in the deep ocean.Further research on the potential ecosystem impacts on the deep ocean from biomass sinking is required before large-scale deployment of this approach can be rolled out. Projects that use terrestrial biomass for ocean sinking face an additional hurdle of ensuring that the biomass sourcing is done sustainably and does not represent significant competition with existing terrestrial biomass use and doesn’t lead to land use practice changes.

‍

• Microalgae Cultivation and Sequestration proposes to boost the natural carbon cycle of the oceans by providing artificial fertilization of phytoplankton in nutrient-poor waters. This can be achieved either by direct fertilization at the surface or by artificial upwelling of deeper nutrient-rich waters. The aim is to increase the natural sequestration of carbon through the ocean food web by increasing the growth of phytoplankton, which are the first link in the ocean food chain, and hence boost its activity.

‍

The economic feasibility of ocean fertilization approaches remains to be proven and questions have been raised about their potential ecosystem impacts. It appears essential to clearly identify nutrient-depleted zones in the ocean and limit ocean fertilization approaches to these areas to avoid risks of creating harmful ecosystem impacts such as the creation of anoxic zones or algal blooms.