FEATURE: Options for ocean-based carbon dioxide removal
Quantum Commodity Intelligence – The UN's Intergovernmental Panel on Climate Change has said that the need for carbon dioxide removals (CDR) technologies is "unavoidable" if the world is to meet the Paris Agreement's climate goals.
A lot of the focus on CDR is on land-based approaches, be they nature-based or geo-engineering, such as direct air capture, but the ocean is perceived by many as offering the greatest potential for carbon sequestration, not least because of its size.
Possible approaches to marine CDR (mCDR) are wide ranging, with differing risks and challenges. Here Quantum outlines a few of the main activities that could be utilised to sequester carbon in the world's oceans.
Ocean alkalinity enhancement
Ocean alkalinity enhancement (OAE), as the name suggests, is a mCDR approach that involves adding alkaline materials to the sea or through electrochemical systems to speed up the ocean's natural carbon sink process. Basically, increasing the alkalinity in the ocean transforms dissolved inorganic carbon dioxide (CO2) in the seawater into bicarbonates and/or carbonates. These converted materials can store the CO2 for thousands of years thus providing stable storage.
The chemical process occurs naturally over time in the ocean through the weathering of rocks and resulting addition of alkaline materials into the sea, which in turn causes the ocean to absorb more CO2 from the air to restore equilibrium. OAE activities aim to accelerate this process, thus absorbing and storing more CO2.
There are a number of ways to add alkalinity to the ocean, according to US-based non-profit Ocean Visions.
"These include spreading finely ground alkaline substances over the open ocean, depositing alkaline sand or gravel on beaches or coastal flats, and reacting seawater with alkaline minerals inside reactors before releasing the modified seawater back into the ocean," it said.
Last month, carbon registry Isometric unveiled what it said is the first standard for OAE, with Canada-based Planetary Technologies the first project developer signed up to utilise it.
The 'Ocean Alkalinity Enhancement from Coastal Outfalls' protocol was certified on May 31. "The guiding principle for this protocol is to provide a high level of scientific rigor and safety guardrails for early stage OAE trials and deployments, while also balancing operational feasibility and leaving flexibility for innovation," according to the protocol's text.
Isometric will review the protocol at least every two years "and/or when there is an update to scientific published literature which would affect net CO2e removal quantification or the monitoring and modeling guidelines" in the protocol".
Other companies in this space include California-based Equatic, which uses an electrochemical process to change the alkalinity of seawater. It, in effect, uses electrochemical reactors, with lots of tanks, plumbing and piping, that suck in millions of gallons of seawater and pass it through an electrically charged mesh to increase its pH.
This induces a series of chemical reactions, including combining dissolved CO2 in the water, and CO2 in the air, with calcium and magnesium. The two elements trap CO2, becoming calcium and magnesium carbonates – materials found in chalk and seashells – while hydrogen is simultaneously produced as a by-product. The seawater then flows back into the ocean as does the carbonate slurry it creates.
Ocean fertilisation
Ocean fertilisation involves approaches that boost the growth in the ocean of phytoplankton, which remove CO2 from the atmosphere through the natural process of photosynthesis and then are transferred to the deep ocean. However, it does not include activities that are the result of conventional aquaculture, mariculture or through the creation of artificial reefs, according to the US Environmental Protection Agency (EPA).
Phytoplankton growth can be enhanced by adding nutrients to the sea, such as iron, phosphorus and nitrogen. Ocean iron fertilisation was put forward as a possible approach to generate carbon credits under the Kyoto Protocol's Clean Development Mechanism more than a decade ago. But companies, such as Climos, were ultimately unsuccessful back then. Other approaches include 'artificial upwelling', which involves pumping cooler, nutrient-rich waters from the deep ocean to stimulate phytoplankton growth,
The EPA says that for ocean fertilisation to mitigate climate change, three processes need to happen. First, the process "must lead to increased growth of phytoplankton, consolidating carbon and nutrients together into organic material".
In the next stage organic material from the phytoplankton bloom has to be transferred into the deep ocean "so that it does not simply get recycled near the surface, releasing its captured carbon back to the atmosphere". Finally, the transfer to the deep ocean "must result in a subsequent transfer of carbon from the atmosphere into the surface ocean".
Biomass sinking
This technique is the sequestration of CO2 in marine or land-based plants and then depositing them in the deep ocean. There are some similarities between this and ocean fertilisation in terms of the processes involved.
US company Running Tide, which announced last month it is ceasing operations mainly due to a lack of funding, aimed to foster carbon capture and removal by deploying floating buoys containing limestone and algae, which are then sunk into the deep ocean. The limestone compound is said to boost the ocean's alkalinity and the algae (biomass) grows and captures more carbon. Once the buoys are close to the ocean floor, gravity and water pressure hold the biomass in place to store the carbon in the ocean. Other approaches could involve seaweed farming that is then sunk to the sea bed.
Germany-based CDR marketplace and verification services provider Carbonfuture has warned projects that use land-based 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".
Another company in this space is Israel-based Rewind, which in May published a carbon protocol to oversee its work. The proposed framework, called Marine Terrestrial Biomass Storage, is the result of two years of work and was open for public consultation until June 20.
Rewind sinks large volumes of biomass at the bottom of the Black Sea, one of the world's deepest, and claims the CDRs as carbon credits. The process occurs naturally as rivers carry a lot of biomass to the bottom of the sea, where organic matter does not degrade because there is little oxygen present, but Rewind says biomass carbon removal and storage or BiCRS can accelerate the process.
Direct ocean removal
Direct ocean removal (DOC) is a technique that, as the name suggests, removes CO2 from seawater resulting in the water absorbing more of the gas from the atmosphere.
A pioneer of the approach is US start-up Captura, which has raised more than $45 million to help commercialise and scale DOC. Captura's DOC technology enhances the ocean's natural CO2 absorption process but does not increase CO2 levels in the sea. It uses renewable energy and electrodialysis technology to capture CO2 from seawater, which allows the CO2-depleted seawater to absorb the same quantity of CO2 from the air that was originally removed.
'Blue' carbon
A range of so-called 'blue carbon' approaches are already well advanced, particularly mangrove restoration, but also tidal salt marshes and seagrass meadows. CO2 is 'fixed' in the roots of these plants and marine sediments for thousands of years.
Major carbon standards, such as US-based Verra and Switzerland-based Gold Standard, already have or are developing methodologies for blue carbon projects. And in Japan there is a scheme for generating J-Blue Credits managed by the Japan Blue Economy Technology Research Association.