Three Ways the Ocean Can Become a Carbon Removal Titan

Author: Eve Pope, Technology Analyst at IDTechEx

Every year, our oceans absorb 11-15 gigatonnes of CO₂ as part of the Earth's natural carbon cycle. An unsung hero in the face of rising global temperatures, negative emission technology pioneers have begun asking a simple question: What if the power of this immense marine carbon sink could be harnessed even further?

The new IDTechEx report “Carbon Dioxide Removal (CDR) 2024-2044: Technologies, Players, Carbon Credit Markets, and Forecasts” explores many carbon dioxide removal approaches. Interest in ocean-based removals has swelled significantly over the past few years, with start-ups deploying a variety of different technologies surfacing. The three most promising removal methods – ocean alkalinity enhancement, direct ocean capture, and seaweed sinking – are illustrated in this article.

Benchmarking of promising ocean-based carbon dioxide removal technologies. A complete version of this table, including numbers and other carbon removal technologies, can be found in the new IDTechEx CDR report. Source: IDTechEx

Fighting back against ocean acidification

The concept of adding more CO₂ to the ocean may raise immediate alarm bells for some. As anthropogenic CO₂ emissions have risen over the last century, the additional carbon dioxide in our seas has led to increased acidity. Therefore, ocean-based negative emission technologies must not contribute to this ocean acidification.

Enter ocean alkalinity enhancement (OAE). For this carbon dioxide removal method, alkalinity is added to seawater to increase the CO₂ buffering capacity of the ocean via decreased acidity. Ocean alkalinity enhancement shifts the carbonate chemistry equilibrium of our seas, encouraging the ocean to pull in more atmospheric CO₂. The source of alkalinity can be either electrochemically produced brine or minerals such as olivine.

Start-ups such as Vesta, Planetary, and Vycarb are continuing to develop technologies and conduct OAE field studies. Hundreds of tonnes of CO₂ have been removed so far using this approach. Scaling of extraction and distribution of alkaline materials will determine the success of OAE. The safety for ocean ecosystems and effectiveness for carbon removal of the technology needs to be demonstrated before further scale-up occurs.

Carbon dioxide removal from seawater is simpler than capture from the air

While direct air capture has received a lot of attention from the media and investors for its potential role in delivering large-scale carbon dioxide removal, CO₂ removal from the ocean requires less energy than capturing carbon dioxide directly from ambient air. This is because the ocean has CO₂ levels 100 times higher than the air. Direct ocean capture (DOC) has, therefore, seen a surge of players seeking to prove its validity. Technologies being explored for commercialization include electrodialysis (developed by Captura and SeaO2), electrolysis (developed by Equatic – green H₂ is also generated using this approach), calcium looping (developed by CarbonBlue), and photochemical methods (developed by Banyu Carbon).

In the realm of start-ups, Equatic is constructing a 3,500 tpa (tonnes per annum of CO₂) commercial DOC demonstration plant in Singapore. Similarly, Captura has a 1000 tpa pilot planned in Norway, with installation due in late 2024. If successful, direct ocean capture players will have reached the kiloton per annum scale in under three years — a feat that took direct air capture pioneers almost a decade to achieve.

Direct ocean capture players are setting themselves an aggressive pace, with several declaring ambitions to reach the megaton per annum scale. Existing water infrastructure, such as desalination plants, can assist in accelerating scale-up. IDTechEx forecasts direct ocean capture technologies will reach a removal capacity of 40 Mtpa by 2044.

Seaweed sinking will help Caribbean communities

Ocean alkalinity enhancement and direct ocean capture may sound eccentric, but simpler ocean-based carbon dioxide removal methods like seaweed sinking also show promise. The concept behind seaweed sinking is simple: cultivate seaweed, which takes in CO₂ as it grows, then sink it to the ocean floor, locking away carbon dioxide for long durations. While scientists have expressed concerns over potential disruption to deep-sea food webs, there are some situations where the environmental benefits are clear.

Beginning in 2011, a giant nuisance bloom of seaweed has developed in the central Atlantic Ocean every spring and summer. Known as the Great Atlantic Sargassum Belt, it has been caused by rising global temperatures alongside nitrogen and phosphorus nutrients from intensive farming being dumped into rivers. Due to the smell, the attracted insects, and the sheer amount of it piling up on beaches, the Sargassum Belt negatively impacts communities in the Caribbean and West Africa – especially those with economies dependent on tourism. In previous years, the nuisance bloom has caused Guadeloupe to issue a health alert, as well as the US Virgin Islands to declare a state of emergency. The Sargassum Belt infects reefs and mangroves, filling up habitats and preventing wildlife from breathing.

Clearly, sinking nuisance seaweed before it reaches shores, sequestering carbon dioxide in the process, could greatly benefit Caribbean communities. Start-ups pursuing this approach include Seaweed Generation, Seafields, Pull to Refresh.

But barriers remain

It’s not all plain sailing for ocean-based carbon dioxide removal. There are a wide number of local, regional, national, and international laws, regulations, and institutions that govern activities in the various maritime zones, and many of these prohibit “dumping” material in the ocean, restricting CO₂ removal methods like ocean alkalinity enhancement. Given the immense complexity of the ocean, computer modeling is challenging, and scientists are concerned that some ocean negative emission technologies may cause unintended consequences such as damage to marine food webs.

Another obvious question: what is the business model for ocean-based carbon dioxide removal approaches? As these technologies are nascent, they are not included in governmental climate strategies. Private sector investment will be vital for this technology to mature, which can come in the form of carbon removal credit purchases on the voluntary carbon market. In 2023, the voluntary market saw breakout success, with sales of durable, engineered removals, including those generated by ocean-based CO₂ removal technologies, totaling billions of dollars. But voluntary demand is vulnerable, and compliance market incorporation will be vital if engineered removals – ocean-based or otherwise – are to reach an impactful scale. Time will tell if the necessary regulation will be prioritized before the voluntary bubble bursts.

To find out more about the IDTechEx report “Carbon Dioxide Removal (CDR) 2024-2044: Technologies, Players, Carbon Credit Markets, and Forecasts”, including downloadable sample pages, please visit www.IDTechEx.com/CDR.

For the full portfolio of energy and decarbonization market research from IDTechEx, please see www.IDTechEx.com/Research/Energy.