UnEarth Magazine.

Words by Ash Brennan, Art by Daisy Kelly

The rhythms of tides and waves in the ocean make it endlessly fascinating to watch. Standing on the shore and looking off into the vastness of the wild blue, it feels like nothing could alter something so immense. Yet we all know that people have been changing this place for centuries, whether it be commercial whaling, plastic pollution or ocean acidification, our impacts are felt in the water. But could we exert the same scale of impact with positive effect? In recent years, work in this area has been moving past research. There is now a small ecosystem of test projects and start-ups, each with their own methods for exploring how the natural link between ocean and atmosphere could be harnessed to fight climate change.  

The carbon we pump into the sky doesn’t all stay there. Approximately a third of total human emissions have been absorbed by the ocean. Once under the waves, carbon dioxide no longer contributes to the greenhouse effect, but it does undergo a chemical reaction with water to produce carbonic acid, leading to ocean acidification. It is by taking advantage of these natural equilibria that the power of the waves can be put to work sequestering carbon and reducing acidification to boot.  

There are three broad approaches: ocean capture, alkalinity enhancement and growth of photosynthesisers. Each comes with its own list of advantages, drawbacks and uncertainties, but there are start-ups and trials in each area.  

Captura and Equatic are making waves with ocean capture technology – analogous to direct air capture (DAC), but from water. These companies both have pilot plants in California and Captura has a contract to build a plant in Norway from Equinor, Norway’s state-owned energy firm. The two businesses are chasing the advantages which ocean capture has over DAC: Carbon dioxide is more concentrated in seawater than air by volume, and seawater makes a convenient electrolyte for electrochemical processes. These advantages have the potential to make the process cheaper per tonne of carbon dioxide than DAC. Ocean capture also comes with the advantage of capturing carbon within the plant, where it is easily measurable.  

The principal disadvantage of ocean capture is the energy costs involved. Electrolysis is a very energy intensive process. Equatic uses 2 MWh (approximately equivalent to running 6 household refrigerators for 1 year) to capture 1 tonne of carbon dioxide, but the company makes efficiency gains by coupling carbon capture to hydrogen production. Equatic also has plans to open a plant in Singapore coupled to a desalination plant, further improving efficiency. However, no amount of optimisation can overcome the energy intensive nature of electrolysis which is the basis of both Captura’s and Equatic’s processes. Getting the most out of ocean capture means careful planning in the location and energy sources of plants.  

Alkalinity enhancement, on the other hand, has the potential to use far less energy because it does not rely on electrolysis. Vesta is a US corporation working in this space, having a pilot project running in Southampton, New York, where it mixes finely ground olivine (a mineral common on Earth’s surface) into the sand on a beach. The olivine slowly dissolves in the water and increases the pH (decreases the acidity), leading to more carbon dioxide from the atmosphere dissolving in the water, where it is sequestered. Thus, the project reduces ocean acidification at the same time as capturing carbon dioxide. The only energy costs of this process are the sourcing, grinding and transport of minerals, and ideally these processes are located nearby for maximum efficiency.  

Despite the low energy costs however, there are some drawbacks of alkalinity enhancement. Carbon captured by this process is stored in the ocean, making it difficult to quantify. The process also involves a large-scale environmental intervention with possible flow on effects. Vesta has ongoing monitoring at their test site in Southampton for this reason, and these types of environmental studies would be necessary to track the impact of any such project.  

Both ocean capture and alkalinity enhancement are chemical processes, however there are also a variety of organisations working to bring biological solutions to the ocean. Brilliant Planet is a UK based company which pumps seawater through algal ponds in order to convert carbon dioxide to biomass, which is then buried. Sea-Up has a process by which fertile deep waters in the ocean are pumped to the surface, leading to phytoplankton growth and carbon capture. These techniques will also need detailed environmental studies, and some suffer from difficulty in carbon capture quantification. 

The ocean feels unchanging when we gaze at the never-ending breakers hitting our coastlines. Really it is a dynamic system, with levers that can be pulled or pushed to tweak its inner mechanics in ways that might benefit the planet. As with any complex mechanism though, pulling one lever may have more than one outcome. In practice, the sorts of schemes proposed by carbon capture companies will need oversight and careful monitoring for unforeseen flow on effects. In their best possible form though, they have the potential to make valuable contributions to the fight against climate change.