When it comes to ocean carbon removal, there are two ways we can boost the ocean’s uptake of carbon: biological and chemical. Both involve removing carbon from the surface ocean and storing it in ocean sediments or other stable forms (like seaweed) in the deep ocean or underground, which then allows the ocean to absorb more CO₂ from the atmosphere. The sheer scale of the ocean makes this pathway an exciting one to pursue, but each of these overarching methods comes with its own risks and unknowns.
Those knowledge gaps are significant enough that many experts recommend additional research before we go all in. If we’re going to alter the chemistry of the Earth’s biosphere — home to 78% of animals on the planet and core to the livelihood of millions of people in coastal communities — we simply need to know more.
How ocean carbon removal works
Within the two categories of ocean carbon removal, biological and chemical, there are a range of approaches that make use of the same basic principles. On the biological side, the uptake work is done by micro- and macro-algae (for example, phytoplankton and seaweed), which use photosynthesis to take in carbon at the surface and sequester it by sinking. Chemical pathways, on the other hand, either directly extract CO₂ from the ocean for storage on land, or increase the ocean’s alkalinity, which causes the ocean to absorb more CO₂.
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Ocean fertilization
Adding nutrients to surface ocean waters to increase photosynthesis and CO2 uptake by phytoplankton.
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Artificial upwelling
Using pipes or other methods to transport cold, CO2-rich, and nutrient-dense deep seawater to the surface to promote the growth of algae.
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Artificial downwelling
Using pipes or other methods to transport CO2-rich surface waters to the deep ocean to store carbon.
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Macro-algae cultivation
Large-scale seaweed farming that removes CO2 from the atmosphere by sinking seaweed to the deep ocean.
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Ocean alkalinity enhancement
Increasing ocean alkalinity by distributing alkaline minerals in the ocean or adding alkalinity via electrochemical reactions.
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Direct ocean capture
Using electrochemical approaches to “swing” seawater towards producing CO2, which is removed and stored. CO2-lean seawater is reintroduced to the ocean to increase the ocean’s capacity to remove CO2 from the atmosphere.
Each of these pathways varies in technological readiness, cost, and climate and non-climate risks and benefits. Biological pathways can have impacts on marine ecosystems — we need to consider how growing algae can affect light, oxygen levels, pH values, and available nutrients. Chemical pathways can have impacts on marine and terrestrial environments, including air pollution from mining and transporting minerals, introduction of trace toxins contained in alkaline minerals, and potential risks to marine life from changes to the ocean’s biogeochemistry.
Benefits, costs, and challenges
Before we can see the benefits of ocean carbon removal, we need to address four major challenges currently facing the field: insufficient governance, a small knowledge base, underdeveloped monitoring and verification processes, and uncertain environmental and social impacts. By overcoming these challenges, experts and communities will be able to make better-informed decisions about their next steps for pursuing ocean carbon removal.
If we can resolve those concerns, ocean carbon removal pathways have the potential to provide environmental and social co-benefits like ocean de-acidification, job creation, and new economic opportunities for the aquaculture sector. The potential to de-acidify the ocean, in particular, is a rare chance to begin redressing past harms and injustices, as ocean acidification threatens food security, economies, and culture. Because there are currently no meaningful ways to counteract acidification at scale, it’s worth seriously considering and exploring.
The ocean is a vast, complex, and delicate system, and there’s a lot we don’t know yet. In order to avoid damaging the very system we are trying to protect, we must minimize uncertainties around the effectiveness, community and ecosystem impacts, and governance of ocean carbon removal.
Sifang ChenManaging Science and Innovation Advisor
Ocean carbon removal policy outlook
In a fledgling industry like this, federal policy has a major role to play. The first step will be to invest in research that can reduce uncertainties in the space, with the goal of identifying ocean CDR pathways that maximize climate and ecosystem co-benefits and remove CO₂ from the atmosphere. Policy can also support the development of ocean CDR technologies under a framework of responsible innovation and equitable distribution of benefits. Because ocean carbon removal could have far-reaching consequences, good policies will be critically important to safeguarding our ocean’s ecosystems and coastal communities.
