History of the term (etymology)

The term sequestration is based on the Latin sequestrare, which means set aside or surrender. It is derived from sequester, a depositary or trustee, one in whose hands a thing in dispute was placed until the dispute was settled. In English "sequestered" means secluded or withdrawn.^[9]

In law, sequestration is the act of removing, separating, or seizing anything from the possession of its owner under process of law for the benefit of creditors or the state.^[9]

In nature

Carbon sequestration is part of the natural carbon cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of Earth.

Carbon dioxide is naturally captured from the atmosphere through biological, chemical or physical processes, and stored in long-term reservoirs.

In climate change mitigation

Carbon sequestration - when acting as a carbon sink - helps to mitigate climate change and thus reduce harmful effects of climate change. It helps to slow the atmospheric and marine accumulation of greenhouse gases, which are released by burning fossil fuels and industrial livestock production.^[10]

Carbon sequestration, when applied for climate change mitigation, can either build on enhancing naturally occurring carbon sequestration or by applying artificial carbon sequestration processes.

Within the carbon capture and storage approaches, carbon sequestration refers to the "storage" component. Here, artificial carbon storage technologies are applied, such as gaseous storage in deep geological formations (including saline formations and exhausted gas fields), and solid storage by reaction of CO₂ with metal oxides to produce stable carbonates.^[11]

For carbon to be sequestered artificially (i.e. not using the natural processes of the carbon cycle) it must first be captured, or it must be significantly delayed or prevented from being re-released into the atmosphere (by combustion, decay, etc.) from an existing carbon-rich material, by being incorporated into an enduring usage (such as in construction). Thereafter it can be passively stored or remain productively utilized over time in a variety of ways. For instance, upon harvesting, wood (as a carbon-rich material) can be incorporated into construction or a range of other durable products, thus sequestering its carbon over years or even centuries.^[12]

Coastal Carbon Capture is an upcoming technology that seeks to remove carbon dioxide from the atmosphere through the deployment of carbon-removing sand, which increases the alkalinity of seawater.^[13] This enhances the capacity of seawater to absorb CO2. Specifically, an increase in alkalinity results in the conversion of carbonic acid to bicarbonate and the consequent and subsequent uptake of CO2 from the atmosphere into storage in seawater. This process aims to expedite the natural chemical weathering of the mineral olivine by distributing significant quantities of ground olivine-containing rock along coastlines, where it can dissolve in seawater, thus enhancing the rate of CO2 absorption by the ocean.

Forestry

Trees absorb carbon dioxide (CO2) from the atmosphere through the process of photosynthesis. Throughout this biochemical process, chlorophyll in the tree's leaves harnesses sunlight to convert CO2 and water into glucose and oxygen.^[18] While glucose serves as a source of energy for the tree, oxygen is released into the atmosphere as a byproduct. Trees store carbon in the form of biomass, encompassing roots, stems, branches, and leaves. Throughout their lifespan, trees continue to sequester carbon, acting as long-term storage units for atmospheric CO2.^[19] Sustainable forest management, afforestration, reforestation and proforestation are therefore important contributions to climate change mitigation. Afforestation is the establishment of a forest in an area where there was no previous tree cover. Proforestation is the practice of growing an existing forest intact toward its full ecological potential.^[20] An important consideration in such efforts is that the carbon sink potential of forests will saturate^[21] and forests can turn from sinks to carbon sources.^[22][23] The Intergovernmental Panel on Climate Change (IPCC) concluded that a combination of measures aimed at increasing forest carbon stocks, andsustainable timber offtake will generate the largest carbon sequestration benefit.^[24]

In terms of carbon retention on forest land, it is better to avoid deforestation than to remove trees and subsequently reforest, as deforestation leads to irreversible effects e.g. biodiversity loss and soil degradation.^[25] Additionally, the effects of af- or reforestation will be farther in the future compared to keeping existing forests intact.^[26] It takes much longer − several decades − for reforested areas to return to the same carbon sequestration levels found in mature tropical forests.^[27]

There are four primary ways in which reforestation and reducing deforestation can increase carbon sequestration. First, by increasing the volume of existing forest. Second, by increasing the carbon density of existing forests at a stand and landscape scale.^[28] Third, by expanding the use of forest products that will sustainably replace fossil-fuel emissions. Fourth, by reducing carbon emissions that are caused from deforestation and degradation.^[29]

The planting of trees on marginal crop and pasture lands helps to incorporate carbon from atmospheric CO
₂ into biomass.^[30][31] For this carbon sequestration process to succeed the carbon must not return to the atmosphere from biomass burning or rotting when the trees die.^[32] To this end, land allotted to the trees must not be converted to other uses and management of the frequency of disturbances might be necessary in order to avoid extreme events. Alternatively, the wood from them must itself be sequestered, e.g., via biochar, bio-energy with carbon storage (BECS), landfill or stored by use in construction.

Reforestation with long-lived trees (>100 years) will sequester carbon for substantial periods and be released gradually, minimizing carbon's climate impact during the 21st century. Earth offers enough room to plant an additional 1.2 trillion trees.^[33] Planting and protecting them would offset some 10 years of CO₂ emissions and sequester 205 billion tons of carbon.^[34] This approach is supported by the Trillion Tree Campaign. Restoring all degraded forests world-wide would capture about 205 billion tons of carbon in total, which is about two-thirds of all carbon emissions.^[35][36]

During a 30-year period to 2050 if all new construction globally utilized 90% wood products, largely via adoption of mass timber in low rise construction, this could sequester 700 million net tons of carbon per year,^[37][38] thus negating approximately 2% of annual carbon emissions as of 2019.^[39] This is in addition to the elimination of carbon emissions from the displaced construction material such as steel or concrete, which are carbon-intense to produce.

Wetlands

An example of a healthy wetland ecosystem

Global distribution of blue carbon (rooted vegetation in the coastal zone): tidal marshes, mangroves and seagrasses.^[42]

Wetland restoration involves restoring a wetland's natural biological, geological, and chemical functions through re-establishment or rehabilitation.^[43] It has also been proposed as a potential climate change mitigation strategy.^[44] Wetland soil, particularly in coastal wetlands such as mangroves, sea grasses, and salt marshes,^[44] is an important carbon reservoir; 20–30% of the world's soil carbon is found in wetlands, while only 5–8% of the world's land is composed of wetlands.^[45] Studies have shown that restored wetlands can become productive CO₂ sinks^[46][47][48] and many restoration projects have been enacted in the US and around the world.^[49][50] Aside from climate benefits, wetland restoration and conservation can help preserve biodiversity, improve water quality, and aid with flood control.^[51]

The plants that make up wetlands absorb carbon dioxide (CO2) from the atmosphere and convert it into organic matter. The waterlogged nature of the soil slows down the decomposition of organic material, leading to the accumulation of carbon-rich peat, acting as a long-term carbon sink.^[52] Additionally, anaerobic conditions in waterlogged soils hinder the complete breakdown of organic matter, promoting the conversion of carbon into more stable forms.^[53]

As with forests, for the sequestration process to succeed, the wetland must remain undisturbed. If it is disturbed somehow, the carbon stored in the plants and sediments will be released back into the atmosphere and the ecosystem will no longer function as a carbon sink.^[54] Additionally, some wetlands can release non-CO₂ greenhouse gases, such as methane^[55] and nitrous oxide^[56] which could offset potential climate benefits. The amounts of carbon sequestered via blue carbon by wetlands can also be difficult to measure.^[51]

Wetlands are created when water overflows into heavily vegetated soil causing plants to adapt to a flooded ecosystem.^[57] Wetlands can occur in three different regions.^[58] Marine wetlands are found in shallow coastal areas, tidal wetlands are also coastal but are found farther inland, and non-tidal wetlands are found inland and have no effects from tides. Wetland soil is an important carbon sink; 14.5% of the world's soil carbon is found in wetlands, while only 5.5% of the world's land is composed of wetlands.^[59] Not only are wetlands a great carbon sink, they have many other benefits like collecting floodwater, filtering air and water pollutants, and creating a home for numerous birds, fish, insects, and plants.^[58]

Climate change could alter soil carbon storage changing it from a sink to a source.^[60] With rising temperatures comes an increase in greenhouse gasses from wetlands especially locations with permafrost. When this permafrost melts it increases the available oxygen and water in the soil.^[60] Because of this, bacteria in the soil would create large amounts of carbon dioxide and methane to be released into the atmosphere.^[60]

The link between climate change and wetlands is still not fully known.^[60] It is also not clear how restored wetlands manage carbon while still being a contributing source of methane. However, preserving these areas would help prevent further release of carbon into the atmosphere.^[61]

Agriculture

Panicum virgatum switchgrass, valuable in biofuel production, soil conservation, and carbon sequestration in soils.

Compared to natural vegetation, cropland soils are depleted in soil organic carbon (SOC). When soil is converted from natural land or semi-natural land, such as forests, woodlands, grasslands, steppes, and savannas, the SOC content in the soil reduces by about 30–40%.^[64] This loss is due to the removal of plant material containing carbon, in terms of harvests. When land use changes, the carbon in the soil will either increase or decrease, and this change will continue until the soil reaches a new equilibrium. Deviations from this equilibrium can also be affected by variated climate.^[65] The decreasing of SOC content can be counteracted by increasing the carbon input. This can be done with several strategies, e.g. leave harvest residues on the field, use manure as fertiliser, or including perennial crops in the rotation. Perennial crops have a larger below ground biomass fraction, which increases the SOC content.^[64] Perennial crops reduce the need for tillage and thus help mitigate soil erosion, and may help increase soil organic matter. Globally, soils are estimated to contain >8,580 gigatons of organic carbon, about ten times the amount in the atmosphere and much more than in vegetation.^[66] Researchers have found that rising temperatures can lead to population booms in soil microbes, converting stored carbon into carbon dioxide. In laboratory experiments heating soil, fungi-rich soils released less carbon dioxide than other soils.^[67]

Modification of agricultural practices is a recognized method of carbon sequestration as soil can act as an effective carbon sink offsetting as much as 20% of 2010 carbon dioxide emissions annually.^[68] (See No-till farming). Restoration of organic farming and earthworms may entirely offset CO₂ annual carbon excess of 4 Gt per year and drawdown the residual atmospheric excess.^[69] (See Compost).

Carbon emission reduction methods in agriculture can be grouped into two categories: reducing and/or displacing emissions and enhancing carbon removal from the atmosphere. Some of these reductions involve increasing the efficiency of farm operations (e.g. more fuel-efficient equipment) while some involve interruptions in the natural carbon cycle. Also, some effective techniques (such as the elimination of stubble burning^[70]) can negatively impact other environmental concerns (increased herbicide use to control weeds not destroyed by burning).

As enforcement of forest protection may not sufficiently address the drivers behind deforestation – the largest of which being the production of beef in the case of the Amazon rainforest^[71] – it may also need policies. These could effectively ban and/or progressively discourage deforestation-associated trade via e.g. product information requirements, satellite monitoring like the Global Forest Watch, related eco-tariffs, and product certifications.^[72][73][74]

Burial of biomass

Burying biomass (such as trees) directly, mimics the natural processes that created fossil fuels.^[105] The global potential for carbon sequestration using wood burial is estimated to be 10 ± 5 GtC/yr and largest rates in tropical forests (4.2 GtC/yr), followed by temperate (3.7 GtC/yr) and boreal forests (2.1 GtC/yr).^[12] In 2008, Ning Zeng of the University of Maryland estimated 65 GtC lying on the floor of the world's forests as coarse woody material which could be buried and costs for wood burial carbon sequestration run at 50 USD/tC which is much lower than carbon capture from e.g. power plant emissions.^[12] CO₂ fixation into woody biomass is a natural process carried out through photosynthesis. This is a nature-based solution and suggested methods include the use of "wood vaults" to store the wood-containing carbon under oxygen-free conditions.^[106]

In 2022 a certification organisation published methodologies for biomass burial.^[107] Other biomass storage proposals have included the burial of biomass deep underwater, including at the bottom of the Black Sea.^[108]

Geological sequestration

Geological sequestration refers to the storage of CO₂ underground in depleted oil and gas reservoirs, saline formations, or deep, un-minable coal beds.

Once CO₂ is captured from a point source, such as a cement factory,^[109] it can be compressed to ≈100 bar into a supercritical fluid. In this form, the CO₂ could be transported via pipeline to the place of storage. The CO₂ could then be injected deep underground, typically around 1 km, where it would be stable for hundreds to millions of years.^[7] Under these storage conditions, the density of supercritical CO₂ is 600 to 800 kg/m³.^[110]

The important parameters in determining a good site for carbon storage are: rock porosity, rock permeability, absence of faults, and geometry of rock layers. The medium in which the CO₂ is to be stored ideally has a high porosity and permeability, such as sandstone or limestone. Sandstone can have a permeability ranging from 1 to 10⁻⁵ Darcy, with a porosity as high as ≈30%. The porous rock must be capped by a layer of low permeability which acts as a seal, or caprock, for the CO₂. Shale is an example of a very good caprock, with a permeability of 10⁻⁵ to 10⁻⁹ Darcy. Once injected, the CO₂ plume will rise via buoyant forces, since it is less dense than its surroundings. Once it encounters a caprock, it will spread laterally until it encounters a gap. If there are fault planes near the injection zone, there is a possibility the CO₂ could migrate along the fault to the surface, leaking into the atmosphere, which would be potentially dangerous to life in the surrounding area. Another risk related to carbon sequestration is induced seismicity. If the injection of CO₂ creates pressures underground that are too high, the formation will fracture, potentially causing an earthquake.^[111]

Structural trapping is considered the principal storage mechanism, impermeable or low permeability rocks such as mudrocks, anhydrite, halite, or tight carbonates act as a barrier to the upward buoyant migration of CO2, resulting in the retention of CO2 within a storage formation.^[112] While trapped in a rock formation, CO₂ can be in the supercritical fluid phase or dissolve in groundwater/brine. It can also react with minerals in the geologic formation to precipitate carbonates.

Two most respected ways of geological sequestration according to most research:

Injecting CO2 into saline aquifers is a key method of geological sequestration. Saline aquifers are underground layers of porous sediments filled with brackish (saline) water, typically located beneath freshwater reservoirs. The process involves capturing CO2 from industrial sources, liquefying it, and then injecting it into these deep geological formations. The CO2 is injected in a supercritical state, which means it has properties of both a liquid and a gas. This allows it to displace the denser brine in the aquifer. Once. This creates a multiphase, multicomponent environment within the aquifer. This allows it to displace the denser brine in the aquifer. Once in the aquifer, the CO2 is sequestered both hydrodynamically and by reacting with other dissolved salts to form carbonates. This creates a multiphase, multicomponent environment within the aquifer. The process is economically viable and can also enhance oil recovery when used in oil reservoirs.^[113]

Injecting CO2 into oil wells and coal seams is a significant method of geological sequestration. In oil wells, CO2 is injected to displace oil, enhancing the recovery of declining oil and gas fields, a process known as Enhanced Oil Recovery (EOR). This strategy is employed in places like Texas, USA, and offshore oil wells in Norway. However, this is not considered sequestration when the injected CO2 is extracted from underground wells. In coal seams, CO2 is injected where it is absorbed into coal, displacing methane (CH4). The process enhances the recovery of coal bed methane (CBM), with the injected CO2 being absorbed into coal twice as much as CH4.^[113]

Worldwide storage capacity in oil and gas reservoirs is estimated to be 675–900 Gt CO₂, and in un-minable coal seams is estimated to be 15–200 Gt CO₂. Deep saline formations have the largest capacity, which is estimated to be 1,000–10,000 Gt CO₂.^[110] In the US, there is estimated to be at least 2,600 Gt and at most 22,000 Gt total CO₂ storage capacity.^[114]

There are a number of large-scale carbon capture and sequestration projects that have demonstrated the viability and safety of this method of carbon storage, which are summarized by the Global CCS Institute.^[115] The dominant monitoring technique is seismic imaging, where vibrations are generated that propagate through the subsurface. The geologic structure can be imaged from the refracted/reflected waves.^[111]

In September 2020, the US Department of Energy awarded $72 million in federal funding to support the development and advancement of carbon capture technologies.^[116]

CO
₂ has been used extensively in enhanced crude oil recovery operations in the United States beginning in 1972.^[10] There are in excess of 10,000 wells that inject CO
₂ in the state of Texas alone. The gas comes in part from anthropogenic sources, but is principally from large naturally occurring geologic formations of CO
₂. It is transported to the oil-producing fields through a large network of over 5,000 kilometres (3,100 mi) of CO
₂ pipelines. The use of CO
₂ for enhanced oil recovery (EOR) methods in heavy oil reservoirs in the Western Canadian Sedimentary Basin (WCSB) has also been proposed.^[117] However, transport cost remains an important hurdle. An extensive CO
₂ pipeline system does not yet exist in the WCSB. Athabasca oil sands mining that produces CO
₂ is hundreds of kilometers north of the subsurface Heavy crude oil reservoirs that could most benefit from CO
₂ injection.

Mineral sequestration

Mineral sequestration aims to trap carbon in the form of solid carbonate salts. This process occurs slowly in nature and is responsible for the deposition and accumulation of limestone over geologic time. Carbonic acid in groundwater slowly reacts with complex silicates to dissolve calcium, magnesium, alkalis and silica and leave a residue of clay minerals. The dissolved calcium and magnesium react with bicarbonate to precipitate calcium and magnesium carbonates, a process that organisms use to make shells. When the organisms die, their shells are deposited as sediment and eventually turn into limestone. Limestones have accumulated over billions of years of geologic time and contain much of Earth's carbon. Ongoing research aims to speed up similar reactions involving alkali carbonates.^[118]

Several serpentinite deposits are being investigated as potentially large scale CO₂ storage sinks such as those found in NSW, Australia, where the first mineral carbonation pilot plant project is underway.^[119] Beneficial re-use of magnesium carbonate from this process could provide feedstock for new products developed for the built environment and agriculture without returning the carbon into the atmosphere and so acting as a carbon sink.^[120]

One proposed reaction is that of the olivine-rich rock dunite, or its hydrated equivalent serpentinite with carbon dioxide to form the carbonate mineral magnesite, plus silica and iron oxide (magnetite).

Serpentinite sequestration is favored because of the non-toxic and stable nature of magnesium carbonate. The ideal reactions involve the magnesium endmember components of the olivine (reaction 1) or serpentine (reaction 2), the latter derived from earlier olivine by hydration and silicification (reaction 3). The presence of iron in the olivine or serpentine reduces the efficiency of sequestration, since the iron components of these minerals break down to iron oxide and silica (reaction 4).

Mineral carbonation

CO₂ exothermically reacts with metal oxides, producing stable carbonates (e.g. calcite, magnesite). This process (CO₂-to-stone) occurs naturally over periods of years and is responsible for much surface limestone. Olivine is one such metal oxide.^[123] Rocks rich in metal oxides that react with CO₂, such as MgO and CaO as contained in basalts, have been proven as a viable means to achieve carbon-dioxide mineral storage.^[124][125] The reaction rate can in principle be accelerated with a catalyst^[126] or by increasing temperatures and/or pressures, or by mineral pre-treatment, although this method can require additional energy. The IPCC estimates that a power plant equipped with CCS using mineral storage would need 60–180% more energy than one without.^[127] Theoretically, up to 22% of crustal mineral mass is able to form carbonates. Formation of Carbonates is considered to be the safest capturing mechanism of CO₂.^[128]

Ultramafic mine tailings are a readily available source of fine-grained metal oxides that could serve this purpose.^[129] Accelerating passive CO₂ sequestration via mineral carbonation may be achieved through microbial processes that enhance mineral dissolution and carbonate precipitation.^{[130][131][132]}

Carbon, in the form of CO
₂ can be removed from the atmosphere by chemical processes, and stored in stable carbonate mineral forms. This process (CO
₂-to-stone) is known as "carbon sequestration by mineral carbonation" or mineral sequestration. The process involves reacting carbon dioxide with abundantly available metal oxides – either magnesium oxide (MgO) or calcium oxide (CaO) – to form stable carbonates. These reactions are exothermic and occur naturally (e.g., the weathering of rock over geologic time periods).^[133][134]

CaO + CO
₂ → CaCO
₃

MgO + CO
₂ → MgCO
₃

Calcium and magnesium are found in nature typically as calcium and magnesium silicates (such as forsterite and serpentinite) and not as binary oxides. For forsterite and serpentine the reactions are:

Mg
₂SiO
₄ + 2 CO
₂ → 2 MgCO
₃ + SiO
₂

Mg
₃Si
₂O
₅(OH)
₄+ 3 CO
₂ → 3 MgCO
₃ + 2 SiO
₂ + 2 H
₂O

These reactions are slightly more favorable at low temperatures.^[133] This process occurs naturally over geologic time frames and is responsible for much of the Earth's surface limestone. The reaction rate can be made faster however, by reacting at higher temperatures and/or pressures, although this method requires some additional energy. Alternatively, the mineral could be milled to increase its surface area, and exposed to water and constant abrasion to remove the inert Silica as could be achieved naturally by dumping Olivine in the high energy surf of beaches.^[135] Experiments suggest the weathering process is reasonably quick (one year) given porous basaltic rocks.^[136][137]

The reaction yield, that is, the amount of CO₂ mineralized per unit mass of the target material, is rarely achieved as per stoichiometry and as such, higher temperature, pressure and even chemical reagents will have to be used to achieve a better yield in a short time. As mineralized products occupy more volume than the originally excavated rocks, the environmental impacts associated with landfilling more material than was excavated in the first place must be considered.^[138]

CO
₂ naturally reacts with peridotite rock in surface exposures of ophiolites, notably in Oman. It has been suggested that this process can be enhanced to carry out natural mineralisation of CO
₂.^[139][140]

When CO
₂ is dissolved in water and injected into hot basaltic rocks underground it has been shown that the CO
₂ reacts with the basalt to form solid carbonate minerals.^[141] A test plant in Iceland started up in October 2017, extracting up to 50 tons of CO₂ a year from the atmosphere and storing it underground in basaltic rock.^[142]

Researchers from British Columbia, developed a low cost process for the production of magnesite, also known as magnesium carbonate, which can sequester CO₂ from the air, or at the point of air pollution, e.g. at a power plant. The crystals are naturally occurring, but accumulation is usually very slow.^[143]

Concrete is a promising destination of captured carbon dioxide. Several advantages that concrete offers include, but not limited to: a source of plenty of calcium due to its substantial production all over the world; a thermodynamically stable condition for carbon dioxide to be stored as calcium carbonates; and its long-term capability of storing carbon dioxide as a material widely used in infrastructure.^[144][145] Demolished concrete waste or recycled concrete could be also used aside from newly produced concrete.^[146] Studies at HeidelbergCement show that carbon sequestration can turn demolished and recycled concrete into a supplementary cementitious material, which can act as a secondary binder in tandem with Portland cement, in new concrete production.^[147][148]

Marine carbon pumps

The pelagic food web, showing the central involvement of marine microorganisms in how the ocean imports carbon and then exports it back to the atmosphere and ocean floor

The ocean naturally sequesters carbon through different processes. The solubility pump moves carbon dioxide from the atmosphere into the surface ocean where it reacts with water molecules to form carbonic acid. The solubility of carbon dioxide increases with decreasing water temperatures. Thermohaline circulation moves dissolved carbon dioxide to cooler waters where it is more soluble, increasing carbon concentrations in the ocean interior. The biological pump moves dissolved carbon dioxide from the surface ocean to the ocean's interior through the conversion of inorganic carbon to organic carbon by photosynthesis. Organic matter that survives respiration and remineralization can be transported through sinking particles and organism migration to the deep ocean.

The low temperatures, high pressure, and reduced oxygen levels in the deep sea slow down decomposition processes, preventing the rapid release of carbon back into the atmosphere and acting as a long-term storage reservoir.^[149]

Vegetated coastal ecosystems

Seaweed farming and algae

Kelp forest

Seagrass meadow

Seaweed grow in shallow and coastal areas, and capture significant amounts of carbon that can be transported to the deep ocean by oceanic mechanisms; seaweed reaching the deep ocean sequester carbon and prevent it from exchanging with the atmosphere over millennia.^[151] Growing seaweed offshore with the purpose of sinking the seaweed in the depths of the sea to sequester carbon has been suggested.^[152] In addition, seaweed grows very fast and can theoretically be harvested and processed to generate biomethane, via anaerobic digestion to generate electricity, via cogeneration/CHP or as a replacement for natural gas. One study suggested that if seaweed farms covered 9% of the ocean they could produce enough biomethane to supply Earth's equivalent demand for fossil fuel energy, remove 53 gigatonnes of CO₂ per year from the atmosphere and sustainably produce 200 kg per year of fish, per person, for 10 billion people.^[153] Ideal species for such farming and conversion include Laminaria digitata, Fucus serratus and Saccharina latissima.^[154]

Both macroalgae and microalgae are being investigated as possible means of carbon sequestration.^[155][156] Marine phytoplankton perform half of the global photosynthetic CO₂ fixation (net global primary production of ~50 Pg C per year) and half of the oxygen production despite amounting to only ~1% of global plant biomass.^[157]

Because algae lack the complex lignin associated with terrestrial plants, the carbon in algae is released into the atmosphere more rapidly than carbon captured on land.^[155][158] Algae have been proposed as a short-term storage pool of carbon that can be used as a feedstock for the production of various biogenic fuels.^[159]

Women working with seaweed

Large-scale seaweed farming (called "ocean afforestation") could sequester huge amounts of carbon.^[160] Wild seaweed will sequester large amount of carbon through dissolved particles of organic matter being transported to deep ocean seafloors where it will become buried and remain for long periods of time.^[161] Currently seaweed farming is carried out to provide food, medicine and biofuel.^[161] In respect to carbon farming, the potential growth of seaweed for carbon farming would see the harvested seaweed transported to the deep ocean for long-term burial.^[161] Seaweed farming has gathered attention given the limited terrestrial space available for carbon farming practices.^[161] Currently seaweed farming occurs mostly in the Asian Pacific coastal areas where it has been a rapidly increasing market.^[161] The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate recommends "further research attention" on seaweed farming as a mitigation tactic.^[162]

Ocean fertilisation

An oceanic phytoplankton bloom in the South Atlantic Ocean, off the coast of Argentina. Encouraging such blooms with iron fertilization could lock up carbon on the seabed. However, this approach is currently (2022) no longer being actively pursued.

Artificial upwelling

Artificial upwelling or downwelling is an approach that would change the mixing layers of the ocean. Encouraging various ocean layers to mix can move nutrients and dissolved gases around, offering avenues for geoengineering.^[168] Mixing may be achieved by placing large vertical pipes in the oceans to pump nutrient rich water to the surface, triggering blooms of algae, which store carbon when they grow and export carbon when they die.^{[168][169][170]} This produces results somewhat similar to iron fertilization. One side-effect is a short-term rise in CO
₂, which limits its attractiveness.^[171]

Mixing layers involve transporting the denser and colder deep ocean water to the surface mixed layer. As the ocean temperature decreases with depth, more carbon dioxide and other compounds are able to dissolve in the deeper layers.^[172] This can be induced by reversing the oceanic carbon cycle through the use of large vertical pipes serving as ocean pumps,^[173] or a mixer array.^[174] When the nutrient rich deep ocean water is moved to the surface, algae bloom occurs, resulting in a decrease in carbon dioxide due to carbon intake from phytoplankton and other photosynthetic eukaryotic organisms. The transfer of heat between the layers will also cause seawater from the mixed layer to sink and absorb more carbon dioxide. This method has not gained much traction as algae bloom harms marine ecosystems by blocking sunlight and releasing harmful toxins into the ocean.^[175] The sudden increase in carbon dioxide on the surface level will also temporarily decrease the pH of the seawater, impairing the growth of coral reefs. The production of carbonic acid through the dissolution of carbon dioxide in seawater hinders marine biogenic calcification and causes major disruptions to the oceanic food chain.^[176]

Basalt storage

Carbon dioxide sequestration in basalt involves the injecting of CO
₂ into deep-sea formations. The CO
₂ first mixes with seawater and then reacts with the basalt, both of which are alkaline-rich elements. This reaction results in the release of Ca²⁺ and Mg²⁺ ions forming stable carbonate minerals.^[177]

Underwater basalt offers a good alternative to other forms of oceanic carbon storage because it has a number of trapping measures to ensure added protection against leakage. These measures include "geochemical, sediment, gravitational and hydrate formation." Because CO
₂ hydrate is denser than CO
₂ in seawater, the risk of leakage is minimal. Injecting the CO
₂ at depths greater than 2,700 meters (8,900 ft) ensures that the CO
₂ has a greater density than seawater, causing it to sink.^[178]

One possible injection site is Juan de Fuca plate. Researchers at the Lamont–Doherty Earth Observatory found that this plate at the western coast of the United States has a possible storage capacity of 208 gigatons. This could cover the entire current U.S. carbon emissions for over 100 years.^[178]

This process is undergoing tests as part of the CarbFix project, resulting in 95% of the injected 250 tonnes of CO₂ to solidify into calcite in two years, using 25 tonnes of water per tonne of CO₂.^[137][179]

Adding bases to neutralize acids

Carbon dioxide forms carbonic acid when dissolved in water, so ocean acidification is a significant consequence of elevated carbon dioxide levels, and limits the rate at which it can be absorbed into the ocean (the solubility pump). A variety of different bases have been suggested that could neutralize the acid and thus increase CO
₂ absorption.^{[187][188][189][190][191]} For example, adding crushed limestone to oceans enhances the absorption of carbon dioxide.^[192] Another approach is to add sodium hydroxide to oceans which is produced by electrolysis of salt water or brine, while eliminating the waste hydrochloric acid by reaction with a volcanic silicate rock such as enstatite, effectively increasing the rate of natural weathering of these rocks to restore ocean pH.^{[193][194][195]}

Abandoned ideas

Applications in climate change policies