Terrestrial CDR solutions are land-based, the natural habitat of humans and therefore perhaps more practical. We can readily subdivide these CDR approaches into two subcategories – those that are terrestrial for our convenience, specifically direct air capture which theoretically could operate anywhere – and those that are obligatorily terrestrial because they depend on non-marine soil for growing biomass or sequestering carbon. For DAC, we need to provide energy, some maintenance attention, and a means to manage the collected carbon dioxide. Energy is increasingly and necessarily provided by renewable sources – DAC processes that capture carbon dioxide using energy from fossil fuel combustion cannot yet capture more CO2 than the fossil fuel combustion would release (citation needed!). For terrestrial CDR other than DAC, we are primarily considering CDR via photosynthesis as the primary unit operation. After carbon is contained in biomass, we can simply store it (BiCRS or soil carbon), or we can burn it in some controlled fashion to further concentrate the carbon – as in BECCS or biochar. A third route of terrestrial carbon capture is mineralization. Carbon dioxide will combine with silicate minerals to form carbonate minerals – this process occurs naturally in rock weathering, and can be accelerated by enhanced (silicate) rock weathering or combining CO2 in concrete (which is made from fine particles of silicate minerals).
Ocean CDR solutions are marine-based, the natural place to look on a planet that is 70% ocean. We can think of two distinct varieties – the coastal zone approaches, which are collectively “coastal blue carbon”, and deep-ocean concepts – which are being actively investigated but none have yet been implemented because the sea is a harsh mistress. The coastal blue carbon and some deep-ocean concepts like micro-algae and macro-algae growth, rely on CDR as a result of biological activity. The production of biomass sequesters carbon. Normally simple biomass sequestration would be insufficient for our plans (see more below), but if biomass accumulates in marine sediments in coastal zones or sinks into deep ocean basins, the path for carbon to return to the atmosphere is long – perhaps hundreds of years or perhaps millions. Ocean alkalinization and some related electrochemical processes, and some variations of enhanced rock weathering are based on the physical chemistry of seawater. These processes lead to mineralized carbon as stable dissolved inorganic carbon that stays dissolved in the sea or is incorporated into marine organisms or directly forms marine sediments.
For both terrestrial and ocean CDR, we are interested in the permanence of sequestration. Biological sequestration alone is transient because dead organisms are subject to decay and the decomposition of organic matter produces carbon dioxide and sometimes methane, GHGs that are likely to re-enter the atmosphere. We desire to keep carbon within the Earth’s crust as long as practical, whether in engineered materials with long durability, in soils and sediments and in forms that do not readily decay, or in rock formations with low potential to leak or where the CO2 can readily mineralize. Once the carbon is permanently in the crust, it is in the Carbonate-Silicate cycle and will not return to the atmosphere except over millions of years.
There is not always a sharp division between terrestrial and ocean CDR approaches. Several concepts revolve around accelerating the silicate-carbonate conversion, and this requires high silicate mafic rock (which is generally mined terrestrially, but is also present in marine zones). When the rock is mined and crushed, it can be used in either terrestrial or ocean approaches. Instead of mining the rock, especially where it occurs at great depths, the mineralization can be achieved in-situ with geological sequestration (which works equally whether terrestrial or ocean based). Some ocean-based CDR approaches can involve processing of seawater on shore-based facilities. Some terrestrial CDR processes could lead to ocean sequestration.
Although most terrestrial biomes are well studied, many ocean biomes are less well known. CDR approaches that have the potential to cause disturbances to biomes must be implemented with great care, and researching oceanic biomes is challenging and expensive. Similarly, the fluid nature of the sea can make measurement, reporting and verification of carbon removal efforts more difficult.
Although climate change affects us all, terrestrial efforts tend to occur within the boundaries of a single political entity, under a single government’s control. Nations can implement ocean-based CDR efforts within their own territorial waters or exclusive economic zone, but some deep-ocean CDR concepts may be subject to more complex international treaty regulation. Effects of CDR efforts on oceans will not remain confined to a single nation, so these CDR approaches require a higher degree of sensitivity to neighbors’ concerns.