⚡️ Ocean-assisted carbon removal⚡️
A deep dive into ocean alkalinity enhancement and direct ocean capture
Readers of this newsletter might recall that our ocean is a grand carbon reservoir. This is because CO2 reacts with seawater to transform the carbon into a form which cannot easily be exchanged back into the air.
It is hard to overstate the ocean’s capacity for storing carbon: today it holds an astonishing 38,000 Gt in the deep ocean interior. According to a ClimateWorks Foundation report, if we could return our atmosphere to preindustrial levels by transforming all excess CO2 into ocean carbon, it would only increase the total ocean carbon content by .7%.
Today I’d like to dive into two other methods within the ocean carbon dioxide removal (oCDR) category: ocean alkalinity enhancement (OAE) and electrochemical direct ocean capture (DOC). Both use renewable energy to power electrochemical processing within their pathway: one to derive alkalinity from minerals on land and the other to strip CO2 directly from seawater. Additional Ventures positions these pathways as among the most permanent and highly scalable ocean CDR approaches currently being explored.
These methods are more technical, so it’s worth laying a bit of ground work on how oCDR methods compare to other forms of carbon capture.
Direct Air Capture companies - like Climeworks - rely on an air contactor where a stream of air is pulled over a chemical sorbent to trigger a chemical reaction and remove the CO2. In ocean CDR methods, the ocean acts as a massive, natural and free air contactor.
The ocean surface is alive with an invisible exchange of chemicals. The back-and-forth sea-air gas exchange — or flux — can be positive or negative, pulling CO2 from the atmosphere at different rates depending on factors like temperature, pH, wind and CO2 saturation. What ocean CDR methods do is manipulate the factors in the top ocean layer — for example, its pH levels — to trigger these natural chemical reactions.
So, how do these methods work exactly?
Ocean Alkalinity Enhancement refers to adding minerals that raise the alkalinity, or pH, of the water’s surface, increasing the water’s ability to drawdown more CO2 from the air. Like olivine weathering, the ocean bicarbonate ions produced by this method cannot be exchanged back into the atmosphere and are thus stored permanently (~10,000 yrs). The graphic above is from an excellent episode of This is CDR, where NOAA’s Dr. Jessica Cross highlights the massive storage potential and durability of this method (see very bottom battery bars).
The most advanced company working on OAE right now is Planetary Technology. Its overall thesis is based on research by ocean carbon cycle expert and founder of the excellent CDR Google Group, Prof. Greg Rau. Their approach is a land-ocean hybrid, where alkalinity is developed through the electrochemical processing of mine waste to provide a very pure form of alkalinity. This alkalinity is then added back into the ocean through municipal partnerships such as wastewater treatment plants.
When evaluated against a range of CDR methods, OAE scores well on duration, effectiveness and cost, but, like direct ocean capture, still faces some open questions on technical readiness:
Though this method has been used in aquaculture to regulate local acidity levels for several years, whether and how to expand this to ocean-scale is still uncertain. The biggest challenges to this method are sourcing an adequate and low-carbon supply of alkalinity, establishing points of distribution and especially, measurement, monitoring, reporting & verification (MRV) that sequestration has taken place.
Electrochemical Direct Ocean Capture (aka electrochemical CO2 stripping) refers to using renewable electricity to split a stream of seawater into acidic and basic solutions. The acid solution is used to trigger the release of CO2 directly from a stream of seawater, and trapping it (imagine collecting the CO2 from a fizzing can of soda).
Seawater is an intuitive place to hunt for CO2, as it’s 50x more present than in the atmosphere. This pathway has a nice elegance to it: no inputs other than seawater and renewable electricity, no byproducts and a stream of captured CO2 that you can point at and measure. The challenge here is that to qualify as carbon removal, it would need to be paired with a long term storage solution, such as pumping it into the deep ocean, mineralizing it beneath ocean basalt or incorporating into long term durable goods.
How do we know these methods work?
Ocean CDR methods capture atmospheric carbon indirectly by relying on well known principle called Henry’s Law. We know with certainty that the ocean pulls CO2 from the atmosphere, the challenge is to measure the quantity, speed and attribution to specific interventions. Although adding alkalinity consumes CO2 from the surface of the ocean, it could take months or even years to replenish the natural balance of CO2. And because there is no current agreed upon MRV standard or framework for certification, the early movers in this category are helping to shape their own MRV protocols and, hopefully, share their findings to expand this field.
With regards to OAE, there is a risk of potential secondary reactions that could limit expected CO2 absorption by half. While the chemistry shorthand might make it tempting to equate 1 ton of alkalinity with 1 ton of sequestered carbon, the reality may be closer to 1 to .5, according to Dr. Jessica Cross.
One advantage for direct ocean capture, is the stream of water returning from processing — or effluent — can be more directly measured and precisely tweaked. This provides a greater control over secondary reactions and increases the efficiency of the gas transfer to nearly 1 to 1, or 1 to .97.
—> If you’re working in ocean MRV, get in touch to be included in an upcoming MRV round-up post.
Notable, Emerging & Ecosystem Players:
Canadian company and X-Prize winner Planetary Technology intends to use fresh water, renewable electricity and mine tailings to produce a very pure form of alkalinity and to disperse it carefully into the sea.
Another X-prize winner and a Frontier advanced purchase recipient, Captura has found interesting ways to reduce operating costs and is expanding its direct ocean capture technology to a 100 ton pilot in Pasadena. Cost estimates for future high capacity plants are $100 - $200/ton with plans to hit $65 at scale.
California-based Ebb Carbon uses an electrochemical process on waste product in desalination plants to create an alkaline solution that turns into bicarbonate over time. Stripe has committed $1.5M to become Ebb’s first customer.
Heimdal has launched the first ocean-assisted carbon removal plant in Hawaii, currently operating renewably energy-powered modular units with 1,000 ton / year capacity at $475/ton sequestered.
Social Implications
Geoengineering is a public good with public consequences, requiring significant effort to build the social license necessary for its success. Sometimes the costs are borne in one place and benefits accrued in another. Planetary has committed to a code of conduct to guide its scientific direction as well as stakeholder engagement.
One potential permitting hurdle for OAE is the London Protocol, or broad prohibitions against ocean-dumping. Organizations seem to be maneuvering around this challenge by pursuing more niche forms of permitting: for example enhanced coastal weathering pilots are working within local beach replenishment permits and alkalinity pilots are working through wastewater effluent permits.
Advantages
Natural process (OAE)
Provides a rare level of permanence (OAE)
Very high capture potential, 1-10+ Gt/yr (NOAA, Table 1)
Not land use-competitive
Local relief from ocean acidity
Cost, energy and land use advantages compared to direct air capture
Multiple revenue streams (carbon credits, tech licensing, sale of acids or hydrogen byproducts)
Builds on existing mining logistics networks and infrastructure (OAE)
Sequestration and storage happen in a single intervention (OAE)
Challenges
Lab testing on ecological impact still in its early stages, and even more important given permanence of chemical changes
MRV and field-testing is difficult in open ocean environments, relying in large part on modeling
Complex permitting requirements to navigate around “ocean dumping”
In the case of OAE, addition of “foreign” materials to the ocean might have adverse consequences
Sourcing alkaline rocks could require mining 7 Gt for every 1Gt CO2 sequestered, the size of the global cement industry (OAE)
Safe and permanent storage of the CO2 (DOC)
Limited scale (e.g. suitable mine tailing sites, desalination plants or intuitive areas to safely distribute alkalinity) (OAE)
Nerds Only (deeper reading that shaped this work):
Frontier has identified several opportunity gaps around ocean CDR
Assessing Ocean Alkalinity for Carbon Sequestration, Phil Renforth 2017
Ocean-based carbon dioxide removal, a primer for philanthropy, Antonius Gagern, Lydia Kapsenberg 2021
You can read through all carbon cycle solutions in depth in the deck linked below, but I will also be following up on each class of solutions with an individual blog post; you can subscribe to get notified when I publish.
Let me know what you think of this pathway, I’d love to hear from you: by email or via DM on twitter.
Thanks Irene, as ever well timed , insightful and helpful.
This is amazing and interesting explanation.