5Ocean-Based Carbon Dioxide Removal

What is Ocean-Based Carbon Dioxide Removal?

Ocean-based carbon dioxide removal (CDR) solutions aim to increase and accelerate the ocean’s capacity to absorb CO₂ and store it long-term. While doing so, such solutions also seek to address governance implications, achieve equitable outcomes, and avoid negative impacts.

Whereas land-based CDR approaches are limited by the quantity of land available for carbon removal and storage, climate scientists and innovators see our oceans as a far more scalable, natural resource for addressing the climate crisis.

Can ocean-based CDR practices restore the climate?

The approaches to ocean-based CDR are still novel, and most require further research to understand how to maximize their benefits while minimizing risks. However, given that our oceans have already absorbed 30% of human-caused emissions, it’s clear that the ocean’s capacity to absorb massive amounts of CO₂ is unparalleled. The questions that remain are:

• How much more CO₂ might our oceans safely absorb?
• How might we facilitate CO₂ absorption safely, effectively, and equitably?
• Which pathways are best suited for scaling ocean-based CDR to restore our climate?

Oceans have three main mechanisms by which they absorb and store CO₂, each described below, and there are several approaches to enhancing or accelerating each of these mechanisms.

Source: National Academies / Ocean-based CDR and sequestration

1. The solubility pump moves atmospheric CO₂ into the deep ocean as surface water cools. The two main approaches to enhancing this CO₂ uptake through the solubility pump are alkalinity enhancement (adding minerals to ocean water to make it less acidic and increase its ability to take up CO₂) and electrochemical CDR (using technology to separate the CO₂ out of seawater). It’s useful to note that both of these methods allow ocean waters to take up more CO₂ without increasing acidity.
2. The biological pump converts CO₂ into biocarbon using photosynthesis (absorbing CO₂ using sunlight) and sinks some of that biocarbon into the deep ocean. Both seaweed (macroalgal) cultivation and ocean nutrient fertilization seek to increase ocean primary productivity — that is, the amount of photosynthesis conducted in the ocean — through the biological pump.
3. Lastly, physical transport mimics ocean currents, which transport CO₂ throughout the oceans and allow large quantities to be stored in the deep ocean for decades to centuries. Artificial upwelling and downwelling are two ways in which physical transport could increase ocean carbon removal and storage, but further research on these two methods is necessary to determine whether they are effective CDR strategies.

The several approaches toward ocean-based CDR described above each vary in their durability, financeability, scalability, and equity. The capacity of any one solution or mechanism to shoulder a significant load of the necessary CDR depends on many variables including local ocean acidity levels, temperatures, wind conditions, currents, etc. These characteristics must be taken into account when selecting each approach and designing for effective ocean-based CDR interventions.

Durability

As with any solution, ocean-based CDR approaches must keep carbon locked away for centuries or longer to measurably contribute to climate restoration. Current research indicates some ocean-based approaches, such as sub-seafloor geologic carbon storage, can lock away CO₂ for over 10,000 years, longer than many land-based CDR approaches. Still, most ocean-based CDR methods are novel and do not yet have robust evidence of their durable long-term storage. Further research into each approach is needed to develop more definitive ocean-based CDR durability estimates.

Scalability

Ocean-based CDR includes different approaches toward CO₂ absorption with varying levels of scalability. For example, electrochemical ocean CDR uses electricity to rearrange water and salt molecules into an acidic and basic solution, which may be functionally limitless but has practical limitations. Other approaches, such as ocean nutrient fertilization require further research to better understand their scalability, due to the complex and interdependent variables in ocean ecosystems. While ocean-based CDR is very promising in terms of its potential, further research is needed to maximize the solutions’ scale.

Financeability

Because ocean-based CDR approaches are still in their infancy, their implementation costs remain uncertain. Ocean nutrient fertilization is currently the most cost-effective approach, costing less than $25–50 per ton of CO₂. Certain approaches that enhance ocean health, like nutrient fertilization and seaweed cultivation, can also generate revenue through sales of fish, kelp, or other byproducts. Other approaches involve new technology and are more financially demanding, such as electrochemical CDR. Although electrochemical CDR currently costs anywhere from $150-$2,500 per ton of CO₂, this could be reduced to less than $100 per ton of CO₂ with further public and private research and development to gain scale and financeability.

Equity

As with all CDR methods, the social implications and equity concerns of ocean-based CDR will emerge with the deployment processes, policies, and motivations of those involved. Historically, the distribution of ocean benefits has been inequitable, with coastal communities seeing few of the benefits of ocean-based enterprises. To avoid exacerbating these inequities in the deployment of ocean-based CDR, we must understand stakeholders’ varying needs and concerns, address the impacts of existing power dynamics, and commit to inclusive governance coupled with long-term planning that promotes equitable outcomes.