With so many carbon removal technologies popping up, one area with the largest sequestration potential is ocean carbon dioxide removal (Ocean CDR). Many believe that the ocean has the greatest capacity for sequestering atmospheric carbon dioxide due to its large surface area and ability to hold carbon in benign forms. This article provides insight on the different types of ocean CDR, how the science works, and the benefits and challenges of each technique.
Why should we care about ocean CDR?
The ocean has a big surface area and thus has the potential to store CO2 in the form of bicarbonate. In fact, it is already the largest sink of carbon on Earth’s surface. Scientists design technologies that attempt to convert atmospheric CO2 into bicarbonate, where it can remain. Alternatively, the CO2 can be converted into calcium carbonate or organic carbon, which can then be sequestered over longer time scales.
Types of ocean CDR: Biotic & Abiotic
There are several types of ocean CDR, which are laid out nicely in this video from Climateworks. Broadly, there are two types of natural ocean carbon storage that companies are trying to leverage - biotic (short term storage) and abiotic (long term storage).
I. Abiotic Ocean CDR
Abiotic ocean CDR companies are finding innovative ways to enhance ocean alkalinity by converting atmospheric CO2 into bicarbonate which can then be stored for 100-1000 years. Some companies that use electrochemistry techniques to generate alkalinity are EbbCarbon, and PlanetaryTech. They achieve this by using (ideally) renewable energy to split water and salt into an acid and a base. The base is added to the ocean where it reacts with dissolved CO2 to form bicarbonate, all while reducing the effects of ocean acidification. Furthermore, some companies have taken this technique even further by utilizing the acid they create to generate hydrogen,a carbon-free energy source (e.g. Planetary Tech).
Other companies hope to store atmospheric carbon dioxide over longer time scales by precipitating it as calcium carbonate. This is how the earth’s natural climate regulation stores carbon dioxide. However, through natural processes, it takes millions of years for the chemical reaction to occur. SeaChange hopes to accelerate the precipitation of calcium carbonate through electrochemical reactions.
Other methods of creating alkalinity are coastal carbon capture techniques such as Project Vesta. Project Vesta aims to rapidly accelerate chemical weathering by mining and grinding silicate rocks to place on a beach where the ocean can interact with them to generate alkalinity. This additional alkalinity has the added co-benefit of reducing the effects of ocean acidification, while simultaneously storing atmospheric carbon.
II. Biotic Ocean CDR
Biotic ocean CDR companies are utilizing photosynthesizing organisms to create organic carbon, such as algae or seaweed. The organic matter can then be:
Some companies in this space plan to employ robots to help sink organic matter down far enough so that it will not revert back into CO2 (Phykos). Other companies plan on burying organic matter in the desert where it will not decompose (Brilliant Planet). The scope and economic viability of these technologies will become clearer as these technologies scale-up.
Abiotic ocean CDR techniques can initially be energy intensive. It is important to factor in and track the energy usage of electrochemical techniques to determine if they create net negative emissions. Additionally, abiotic CDR companies would like to keep atmospheric CO2 in the ocean in the form of bicarbonate, making it a less efficient sequestration method since eventual precipitation of calcium carbonate will release back some CO2.
On the other hand, biotic ocean CDR only requires the sun and CO2 as an energy source, so acquiring the front-end energy is not a challenge. As organic matter decomposes, however, it releases CO2 back into the ocean, so it is unclear how much CO2 is successfully stored and for how long. The energy intensive portion of this technique involves long term storage.
To summarize, biotic CDR does not require energy to generate organic matter, but it does require human intervention to extend the sequestration of carbon. Whereas abiotic CDR requires humans to supply energy in order to generate bicarbonate, but ensures long term storage without any additional energy requirements.
Based on the pros and cons above, the technologies that we think have the most potential are abiotic electrochemical techniques. When using 100% renewable energy, these technologies can have the most potential to remove carbon dioxide, while limiting residual emissions. However, a common argument against renewable-powered CDR techniques is that renewable energy should be allocated to the grid instead of carbon removal activities to reduce emissions. This logic is flawed according to the most recent IPCC report. The report emphasizes the need for BOTH emission reduction AND carbon dioxide removal if we want to keep global warming to below 2˚C.
The IPCC also warns against overshooting 2˚C and relying on CDR alone to bring us to safer CO2 levels. In these scenarios, historical emissions will still cause irreversible damages because of the build up of emissions over time. The only way to avoid that scenario is to rapidly decarbonize while developing CDR technologies that can scale and bring us to a 2˚C future.