Five ways we can use the ocean to remove CO2 at scale.
A brief summary of ocean acidification and the most promising ways we can use the ocean to remove carbon emissions.
People generally think of water as the thing that gets absorbed by a sponge, but in actual fact, the world's oceans are perhaps the most important sponge on the planet. When carbon dioxide (CO2) in the air mixes with water (H2O) it forms carbonic acid (H2CO3) that stores carbon dioxide from the atmosphere in the water. The total amount of carbon stored in the ocean is approximately 50 times greater than the amount in the atmosphere. The ocean has been able to absorb around 30% (~35 billion metric tonnes) of anthropogenic fossil fuel emissions since the industrial revolution and 90% of the excess heat. Without this the earth's surface temperature would be double its current global average making it completely uninhabitable for most biological lifeforms, including us.
Whilst the ocean’s function as a carbon sponge (or ‘sink’) has been a life-line for our planet, unsurprisingly the formation of so such much extra carbonic acid comes with consequences. Liquids are either acidic or alkaline, this range is well known as the ‘pH scale’. The oceans slightly alkaline pH level is critical for maintaining the balance of its delicate ecosystems, although a great number of research papers, including ‘The Geological Record of Ocean Acidification’, indicate this is changing, with an increase in acidity of ~25% since the industrial revolution. Aside from the threat to ecosystems, scientist also anticipate that the ocean, like any sponge, is approaching saturation where the ability to absorb atmospheric carbon diminishes.
Ocean CO2 Removal: 5 methods
The transition towards green consumption is hugely important in fighting climate change, although to achieve the planet saving milestones set out by the IPCC, carbon capture will also have to play an important role. There are a number of ways to sequester carbon via the ocean, a couple of which have a direct co-benefit in reducing acidity levels too. A research study led by the US National Academies of Science, Engineering and Medicine (NASEM), entitled ‘A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration’, has conducted a benchmarking of five leading ocean carbon dioxide removal (CDR) methods. Below I have summarised the top five of those methods in no particular order;
Ocean Alkalinity Enhancement
There is a natural weathering process where silicate and carbonate rocks slowly break down in the water causing a chemical reaction that restores alkalinity. Ocean alkalinity enhancement (OAE), which is also referred to as ‘enhanced weathering’, utilises this natural process through a variety of methods, namely spreading large quantities of finely ground silica/carbonate rock through the surface ocean. This not only helps balance the increasing acidity, but free’s up the surface water to draw down more carbon from the atmosphere.
Efficacy: High
Scalability: Medium—High (>0.1–1.0 Gt CO2/yr)
Environmental risk: Medium
Cost of scale-up: Medium—High (>$100–$150/t CO2)
Electrochemical Processes
Electrochemical ocean carbon dioxide removal uses electricity to rearrange water and salt molecules into an acidic and basic solution. Essentially this is directly removing carbon and reducing acidity which allows the water to absorb even more carbon. A leading startups in this space is Captura who are working towards gigaton scale and renewably powered electrochemical ocean CDR plants in the future. The image below is a concept rendering of what one of their future plants might look like.
Efficacy: High
Scalability: Medium—High (>0.1–1.0 Gt CO2/yr)
Environmental risk: Medium—High
Cost of scale-up: High (>$150/t CO2)
Ocean Nutrient Fertilisation
Ocean nutriment fertilisation is the deliberate stimulation of organisms at the very bottom of the oceanic food chain such as phytoplankton. These organisms such as phytoplankton are photosynthesising meaning they absorb CO2 and release oxygen, so stimulating their growth will sequester more carbon. Phytoplankton biomass in the world's oceans amounts to ∽1-2% of the total global plant carbon, yet these organisms fix between 30 and 50 billion metric tons of carbon annually. Stimulation of their growth can be done through the addition of nutrients such as nitrogen and phosphorus to surface waters.
Efficacy: Medium—High
Scalability: Medium (>0.1–1.0 Gt CO2/yr)
Environmental risk: Medium
Cost of scale-up: Low (<$50/t CO2)
Seaweed Cultivation
Seaweed can grow extremely quickly and does so via photosynthesis so cultivating it is an opportunity similar to ocean nutrient fertilisation for sequestering carbon. One company, Seafields, plans to use the currents in the South Atlantic Ocean to create a natural pen to cultivate Sargassum. Their vision is to scale the farm modularly until it reaches 94,000 square kms, which would be slightly larger than Portugal. Additionally seaweed can also be a valuable resource for food consumption, fertilisation, cosmetics and more which increases its commercial efficacy.
Efficacy: Medium
Scalability: Medium (Between >0.1 Gt CO2/yr and <1.0 Gt CO2/yr)
Environmental risk: Medium—High
Cost of scale-up: Medium (~$100/t CO2)
Ecosystem Recovery
Collectively, the soils and vegetation in coastal ecosystems store between 10 and 24 billion metric tons of carbon, making them small but very powerful carbon sinks. Not only are these ecosystems such as mangroves, salt marshes and kelp forests hugely important for storing carbon but they’re also critical pillars of biodiversity. Only ~15% of coastal areas around the world remain intact, according to an international study led by the University of Queensland, leaving potentially up to a gigaton of opportunity to sequester carbon through restoration.
Efficacy: Low—Medium
Scalability: (<0.1–1.0 Gt CO2/yr)
Environmental risk: Low
Cost of scale-up: Low (<$50/t CO2)
Thanks for reading! Below I have noted the given definitions for each benchmarking categories and a citation for anyway interested in reading the full report,
James
Efficacy:
What is the confidence level that this approach will remove atmospheric CO2 and lead to net increase in ocean carbon storage (low, medium, high).
Scalability:
Potential scalability at some future date with global-scale implementation(low, <0.1 Gt CO2/ yr; medium, >0.1 Gt CO2/yr and <1.0 Gt CO2/yr; high, >1.0 Gt CO2/yr).
Environmental risk:
Intended and unintended undesirable consequences at scale (unknown, low, medium, high.
Cost of scale-up:
Estimated costs in dollars per metric ton CO2 for future deployment at scale; does not include all of monitoring and verification costs needed for smaller deployments during R&D phases (low, <$50/t CO2; medium, ~$100/t CO2; high, >>$150/t CO2).
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National Academies of Sciences, Engineering, and Medicine. 2022.
A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration.
Washington, DC: The National Academies Press.
https://doi.org/10.17226/26278.