This sections informs about the various technologies availabe for CO₂ removal.
Afforestation and reforestation (AR)
… consists of restoring areas where trees have been cut down or degraded (reforestation) or planting trees where there were previously none (afforestation). Trees remove CO2 from the atmosphere during their growth and store it in living biomass.
… is created via the pyrolysis of biomass (i.e. the degrading biomass with heat in the absence of oxygen). Biochar is added to soils, increasing carbon stocks and holding carbon for hundreds to thousands of years.
Soil carbon sequestration (SCS)
… refers to the increase in soil organic carbon content that occurs from land management changes such as modern farming methods, grassland restoration and creation of wetlands. The additional CO2 sequestered remains in the soil if adequately managed.
Bioenergy with carbon capture and storage (BECCS)
… consists of converting biomass (e.g. farmed bioenergy crops, forests, residual biomass or industrial organic waste) into energy (usually through burning), capturing the resulting CO2 and storing it underground in geological reservoirs. The bioenergy needs to be provided at zero or low carbon emissions (i.e. as much additional CO2 needs to be sequestered when growing the feedstock as is released during conversion). Because CO2 is captured during the biomass growth and subsequently stored underground, BECCS leads to a removal of CO2 from the atmosphere.
Enhanced weathering (EW)
… artificially stimulates the natural process of rock decomposition to increase the natural absorption of CO2 by mineral rocks (carbonate, silicate). Crushed rocks are spread on fields and over the ocean, the increased surface area speeds up the natural weathering process). The reaction stores the CO2 in mineral form.
Direct air capture and carbon capture and storage (DACCS)
… uses chemical processes to absorb CO2 from the atmosphere. The CO2 is then stored underground in geological reservoirs.
Ocean fertilization (OF)
… consists of adding otherwise limited nutrients (iron or nitrogen) to upper ocean waters in order to increase biological production. The availability of nutrients stimulates phytoplankton growth, therefore increasing CO2 sequestration. As they die, plankton sink to the deep ocean and permanently sequester the CO2.
Further technologies not considered
The following technologies were not part of our review, but should be considered in further work.
… consists of the management and restoration of coastal and marine ecosystems – particularly sea grasses, mangroves and salt marshes – in order to expand their carbon sinks. CO2 is sequestered in the form of living biomass and sediment.
Carbon capture and utilisation
… refers to using CO2 as a synthetic feedstock for chemical materials. Storage medium, stability and permanence depend on the use. Potential products include Poly Propylene Carbonate (Qin et al 2015), carbon mineralization, Enhanced Oil Recovery (EOR), biodiesel and synfuel production (Abanades et al 2017) and other chemical applications providing economic incentives and opportunities for technological learning for carbon capture.