Ensuring responsible research on ocean-based CO2 removal

Transporting nutrient-rich deep ocean waters with the use of pipes or wave pumps to the upper layer of the ocean can have a fertilizing effect. Nutrients stimulate growth of phytoplankton and more CO₂ can be fixed in their biomass. When phytoplankton die and their biomass sinks into the deep ocean, the fixed carbon can be stored at depth for decades to thousands of years, depending on the location. Uncertainties exist about how much CO₂-rich deep water would be pumped and about biological and other side-effects of the approach. More information. 

This approach aims to harvest and dump terrestrial biomass in the deep ocean, anoxic waters, or bury it in coastal sediments. Terrestrial biomass contains the carbon, removed from the atmosphere by vegetation during growth. Side-effects include burying seabed fauna, leading to suffocation and death of organisms, disrupting sediment biogeochemistry, and potentially creating new areas without oxygen. The released carbon and nutrients may also return to the surface on timescales of decades to thousands of years, depending on the location.

Ocean fertilization approaches aim to add certain elements (e.g. iron sulfate) to the upper sunlit layers of the ocean, which stimulate phytoplankton growth. The process of photosynthesis fixes carbon, turning it into phytoplankton’s biomass. When phytoplankton die and their biomass sinks into the deep ocean, the fixed carbon can be stored at depth for decades to 1000s of years depending on the location. Uncertainties about how much carbon dioxide can be sequestered through these approaches exist. Known side-effects include altered plankton biodiversity and productivity (both locally and even remotely if done at a large scale). If done at a massive scale, modelling studies have shown that this can affect ocean oxygen. More information.

Coastal ecosystems such as mangroves, seagrass beds, salt marshes, and kelp are extremely efficient at absorbing CO₂ and storing carbon in their biomass and soil. Maintaining, restoring and engineering of coastal ecosystems can increase the amount of carbon stored in coastal sediments. This approach has positive ecological co-benefits: it promotes biodiversity and provides important habitats for many species. It also helps to protect coasts from erosion, storms, and sea level rise. The sequestration rates of this approach are highly variable and depend on species, geography, and environmental conditions. More information.

Alkalinity is the capacity of a solution to neutralize acid. Increasing the alkalinity of seawater can enhance its carbon storage capacity. As a result, the ocean can remove more CO₂ from the atmosphere and permanently store it. Minerals from crushed silicate, carbonate rocks or manufactured minerals such as lime, when discharged into the surface water of the ocean induce the oceanic uptake of CO₂ when they dissolve and increase alkalinity (if the alkaline waters remain in contact with the atmosphere). Alkaline solutions can also be generated from water itself via electrochemistry in reactors, and then added to the surface ocean. For mineral based approaches, this chemical neutralization requires more than one ton of rock per ton of CO₂ removed. 

To achieve a global effect, a whole new mining and industrial manufacturing industry on a large scale would have to be established for the amount of minerals needed. For electrochemical approaches new infrastructure would have to created, e.g., as an add-on to existing desalination plants. So far, it is unclear how the alkalinity increases via these approaches would affect marine ecosystems. In addition to risks from potentially toxic contaminants of some rock dust, ecological effects, for example the effect of increased pH value and decreased CO₂ partial pressure, still need to be studied. More information. 

This approach aims to transport carbon that has been taken up at the surface ocean into the deep ocean where it will be stored for hundreds to thousands of years. So far, this engineering approach exists only in theory and according to calculations would require large amounts of energy with numerous uncertainties about side-effects of the approach.  

These approaches aim to chemically or electro-chemically remove CO₂ from the seawater and concentrate it for geological storage. Direct removal may then increase the uptake of CO2 in the surface waters of the ocean when water with a lower CO₂ concentration is added back to the ocean and re-equilibrates with the atmosphere. Side-effects include the need to filter massive amounts of water with possible impacts on plankton, associated risks of carbon capture and storage, and altered oceanic carbonate chemistry.  

These approaches aim to grow algae or macroalgae in order (a) to pyrolyze the biomass to form biochar to spread on land or (b) to create biofuels from the biomass that can be burned in conjunction with carbon capture and storage technology or (c) to sink the biomass into the deep ocean. Macroalgae farming may also lead to natural carbon sequestration via dissolved and particulate organic carbon. Uncertainties exist regarding how much carbon gets sequestered and about side-effects on the biology of the ocean via competition for nutrients and shading of the water column by macroalgae farms.