Carbon sequestration

Carbon sequestration is the process of storing carbon in a

carbon pool.^[2]^: 2248 It plays a crucial role in mitigating climate change by reducing the amount of carbon dioxide in the atmosphere. There are two main types of carbon sequestration: biologic (also called biosequestration) and geologic.^[3] Biologic carbon sequestration is a naturally occurring process as part of the carbon cycle. Humans can enhance it through deliberate actions and use of technology. Carbon dioxide (CO
₂) is naturally captured from the atmosphere through biological, chemical, and physical processes. These processes can be accelerated for example through changes in land use and agricultural practices, called carbon farming. Artificial processes have also been devised to produce similar effects. This approach is called carbon capture and storage

. It involves using technology to capture and sequester (store) CO
₂ that is produced from human activities underground or under the sea bed.

Forests,

kelp beds, and other forms of plant life absorb carbon dioxide from the air as they grow, and bind it into biomass. However, these biological stores are considered impermanent carbon sinks as the long-term sequestration cannot be guaranteed. For example, natural events, such as wildfires or disease, economic pressures and changing political priorities can result in the sequestered carbon being released back into the atmosphere.^[4]

Carbon dioxide that has been removed from the atmosphere can also be stored in the Earth's crust by injecting it into the subsurface, or in the form of

insoluble carbonate salts. The latter process is called mineral sequestration

. These methods are considered non-volatile because they not only remove carbon dioxide from the atmosphere but also sequester it indefinitely. This means the carbon is "locked away" for thousands to millions of years.

To enhance carbon sequestration processes in oceans the following technologies have been proposed:

artificial upwelling, basalt storage, mineralization and deep sea sediments, adding bases to neutralize acids.^[5]

However, none have achieved large scale application so far.

Terminology

The term carbon sequestration is used in different ways in the literature and media. The IPCC Sixth Assessment Report defines it as "The process of storing carbon in a carbon pool".^[2]^: 2248 Subsequently, a pool is defined as "a reservoir in the Earth system where elements, such as carbon and nitrogen, reside in various chemical forms for a period of time".^[2]^: 2244

The United States Geological Survey (USGS) defines carbon sequestration as follows: "Carbon sequestration is the process of capturing and storing atmospheric carbon dioxide."^[3] Therefore, the difference between carbon sequestration and carbon capture and storage (CCS) is sometimes blurred in the media. The IPCC however defines CCS as "a process in which a relatively pure stream of carbon dioxide (CO₂) from industrial sources is separated, treated and transported to a long-term storage location".^[6]^: 2221

Hence CCS is a technology application that utilizes artificial carbon sequestration techniques.^{[citation needed]}

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.^[7]

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.^[7]

Roles

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.^{[citation needed]}

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

Forests,

kelp beds, and other forms of plant life absorb carbon dioxide from the air as they grow, and bind it into biomass. However, these biological stores are considered volatile carbon sinks as the long-term sequestration cannot be guaranteed. Events such as wildfires or disease, economic pressures and changing political priorities can result in the sequestered carbon being released back into the atmosphere.^[8]

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.^[9]

Carbon sequestration, when applied for climate change mitigation, can either build on enhancing naturally occurring carbon sequestration or use technology for carbon sequestration processes.^{[citation needed]}

Within the carbon capture and storage approaches, carbon sequestration refers to the storage component. Artificial carbon storage technologies can be 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.^[10]

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.^[11]

Biological carbon sequestration on land

Biological carbon sequestration (also called biosequestration) is the capture and storage of the atmospheric greenhouse gas carbon dioxide by continual or enhanced biological processes. This form of carbon sequestration occurs through increased rates of

Land-use changes that enhance natural carbon capture have the potential to capture and store large amounts of carbon dioxide each year. These include the conservation, management, and restoration of ecosystems such as forests, peatlands, wetlands, and grasslands, in addition to carbon sequestration methods in agriculture.^[14]

Methods and practices exist to enhance soil carbon sequestration in both sectors of agriculture and forestry.^[15]

Forestry

land rights

to indigenous inhabitants is argued to efficiently conserve forests.

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.^[16] 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.^[17] 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.^[18] An important consideration in such efforts is that the carbon sink potential of forests will saturate^[19] and forests can turn from sinks to carbon sources.^[20]^[21] The Intergovernmental Panel on Climate Change (IPCC) concluded that a combination of measures aimed at increasing forest carbon stocks, and sustainable timber offtake will generate the largest carbon sequestration benefit.^[22]

In terms of carbon retention on forest land, it is better to avoid

soil degradation.^[23] Additionally, the effects of af- or reforestation will be farther in the future compared to keeping existing forests intact.^[24] It takes much longer − several decades − for reforested areas to return to the same carbon sequestration levels found in mature tropical forests.^[25]

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.[26] 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.^[27]

The planting of trees on marginal crop and

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.^[31] Planting and protecting them would offset some 10 years of CO₂ emissions and sequester 205 billion tons of carbon.^[32] 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.^[33]^[34]

Although a bamboo forest stores less total carbon than a mature forest of trees, a bamboo plantation sequesters carbon at a much faster rate than a mature forest or a tree plantation. Therefore, the farming of bamboo timber may have significant carbon sequestration potential.^[35]

During a 30-year period to 2050 if all new construction globally utilized 90% wood products, largely via adoption of

low rise construction, this could sequester 700 million net tons of carbon per year,^[36]^[37] thus negating approximately 2% of annual carbon emissions as of 2019.^[38]

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.

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^[39] – 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.^[40]^[41]^[42]

Wetlands

Global distribution of blue carbon (rooted vegetation in the coastal zone): tidal marshes, mangroves and seagrasses.^[43]

Wetland restoration involves restoring a wetland's natural biological, geological, and chemical functions through re-establishment or rehabilitation.^[44] It has also been proposed as a potential climate change mitigation strategy.^[45] Wetland soil, particularly in coastal wetlands such as mangroves, sea grasses, and salt marshes,^[45] 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.^[46] Studies have shown that restored wetlands can become productive CO₂ sinks^[47]^[48]^[49] and many restoration projects have been enacted in the US and around the world.^[50]^[51] Aside from climate benefits, wetland restoration and conservation can help preserve biodiversity, improve water quality, and aid with flood control.^[52]

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.^[53] Additionally, anaerobic conditions in waterlogged soils hinder the complete breakdown of organic matter, promoting the conversion of carbon into more stable forms.^[54]

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.^[55] Additionally, some wetlands can release non-CO₂ greenhouse gases, such as methane^[56] and nitrous oxide^[57] which could offset potential climate benefits. The amounts of carbon sequestered via blue carbon by wetlands can also be difficult to measure.^[52]

Wetlands are created when water overflows into heavily vegetated soil causing plants to adapt to a flooded ecosystem.^[58] Wetlands can occur in three different regions.^[59] 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.^[60] 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.^[59]

Climate change could alter soil carbon storage changing it from a sink to a source.^[61] 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.^[61] Because of this, bacteria in the soil would create large amounts of carbon dioxide and methane to be released into the atmosphere.^[61]

The link between climate change and wetlands is still not fully known.^[61] 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.^[62]

Peatlands, mires and peat bogs

Peatlands hold approximately 30% of the carbon in our ecosystem.^[62] When they are drained for agricultural land and urbanization, because peatlands are so vast, large quantities of carbon decompose and emit CO₂ into the atmosphere.^[62] The loss of one peatland could potentially produce more carbon than 175–500 years of methane emissions.^[61]

Peat bogs act as a sink for carbon because they accumulate partially decayed biomass that would otherwise continue to decay completely. There is a variance on how much the peatlands act as a carbon sink or carbon source that can be linked to varying climates in different areas of the world and different times of the year.^[63] By creating new bogs, or enhancing existing ones, the amount of carbon that is sequestered by bogs would increase.^[64]

Agriculture

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%.

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.^[66]

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 fertilizer, or including perennial crops in the rotation. Perennial crops have a larger below ground biomass fraction, which increases the SOC content.^[65]

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.^[67]

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.^[68]

Following carbon dioxide (CO2) absorption from the atmosphere, plants deposit organic matter into the soil.

humification.^[70]

On a global basis, it is estimated that soil contains about 2,500 gigatons of carbon. This is greater than 3-fold the carbon found in the atmosphere and 4-fold of that found in living plants and animals.[71] About 70% of the global soil organic carbon in non-permafrost areas is found in the deeper soil within the upper 1 meter and stabilized by mineral-organic associations.^[72]

Carbon farming

Carbon farming is a set of agricultural methods that aim to store carbon in the soil, crop roots, wood and leaves. The technical term for this is carbon sequestration. The overall goal of carbon farming is to create a net loss of carbon from the atmosphere.^[73] This is done by increasing the rate at which carbon is sequestered into soil and plant material. One option is to increase the soil's organic matter content. This can also aid plant growth, improve soil water retention capacity^[74] and reduce fertilizer use.^[75] Sustainable forest management is another tool that is used in carbon farming.^[76] Carbon farming is one component of climate-smart agriculture. It is also one of the methods for carbon dioxide removal (CDR).

Agricultural methods for carbon farming include adjusting how

bamboo farming

.

Carbon farming methods might have additional costs. Some countries have government policies that give financial incentives to farmers to use carbon farming methods.^[77]

As of 2016, variants of carbon farming reached hundreds of millions of hectares globally, of the nearly 5 billion hectares (1.2×10¹⁰ acres) of world farmland.^[78]

Carbon farming is not without its challenges or disadvantages. This is because some of its methods can affect ecosystem services. For example, carbon farming could cause an increase of land clearing, monocultures and biodiversity loss.^[79] It is important to maximize environmental benefits of carbon farming by keeping in mind ecosystem services at the same time.^[79]

Prairies

Prairie restoration is a conservation effort to restore prairie lands that were destroyed due to industrial, agricultural, commercial, or residential development.^[80] The primary aim is to return areas and ecosystems to their previous state before their depletion.^[81] The mass of SOC able to be stored in these restored plots is typically greater than the previous crop, acting as a more effective carbon sink.^[82]^[83]

Biochar

oxidation to CO
₂ and consequential atmospheric release. However concerns have been raised about biochar potentially accelerating release of the carbon already present in the soil.^[87]

Terra preta, an anthropogenic, high-carbon soil, is also being investigated as a sequestration mechanism. By pyrolysing biomass, about half of its carbon can be reduced to charcoal, which can persist in the soil for centuries, and makes a useful soil amendment, especially in tropical soils (biochar or agrichar).^[88]^[89]

Burial of biomass

fossil fuels.^[90] 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).^[11] 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.^[11] 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.^[91]

In 2022 a certification organization published methodologies for biomass burial.[92] Other biomass storage proposals have included the burial of biomass deep underwater, including at the bottom of the Black Sea.^[93]

Geological carbon sequestration

Underground storage in suitable geologic formations

Geological sequestration refers to the storage of CO₂ underground in depleted oil and gas reservoirs, saline formations, or deep, coal beds unsuitable for mining.^{[citation needed]}

Once CO₂ is captured from a point source, such as a cement factory,^[94] 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.^[95] Under these storage conditions, the density of supercritical CO₂ is 600 to 800 kg/m³.^[96]

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.^[97]

Structural trapping is considered the principal storage mechanism, impermeable or low permeability rocks such as mudstone, 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.^[98] 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 become carbonates.

Mineral sequestration

Mineral sequestration aims to trap carbon in the form of solid

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.^[99]

Zeolitic imidazolate frameworks

Zeolitic imidazolate frameworks (ZIFs) are

metal-organic frameworks similar to zeolites. Because of their porosity, chemical stability and thermal resistance, ZIFs are being examined for their capacity to capture carbon dioxide.^[100]

Mineral carbonation

CO₂

catalyst^[104] or by increasing temperatures ^{[dubious – discuss]} 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.^[105] Theoretically, up to 22% of crustal mineral mass is able to form carbonates.^{[citation needed]} Formation of Carbonates is considered to be the safest capturing mechanism of CO₂.^[106]

Selected metal oxides of Earth's crust
Earthen oxide	Percent of crust	Carbonate	Enthalpy change (kJ/mol)
CaO	4.90	CaCO₃	−179
MgO	4.36	MgCO₃	−118
Na₂O	3.55	Na₂CO₃	−322
FeO	3.52	FeCO₃	−85
K₂O	2.80	K₂CO₃	−393.5
Fe₂O₃	2.63	FeCO₃	112
All oxides	21.76	All carbonates

mine tailings are a readily available source of fine-grained metal oxides that could serve this purpose.^[107] Accelerating passive CO₂ sequestration via mineral carbonation may be achieved through microbial processes that enhance mineral dissolution and carbonate precipitation.^[108]^[109]^[110]

Carbon, in the form of CO
₂ can be removed from the atmosphere by chemical processes, and stored in stable

geologic time periods).^[111]^[112]

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.[111] 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.^[113] Experiments suggest the weathering process is reasonably quick (one year) given porous basaltic rocks.^[114]^[115]

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.^[116]

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 mineralization of CO
₂.^[117]^[118]

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.^[119] 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.^[120]

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.^[121]

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

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.^[125]^[126]

Sequestration in oceans

Marine carbon pumps

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.^{[citation needed]} 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.^{[citation needed]}

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.^[127]

Vegetated coastal ecosystems

Blue carbon is a concept within climate change mitigation that refers to "biologically driven carbon fluxes and storage in marine systems that are amenable to management."^[128]^: 2220 Most commonly, it refers to the role that tidal marshes, mangroves and seagrasses can play in carbon sequestration.^[128]^: 2220 These ecosystems can play an important role for climate change mitigation and ecosystem-based adaptation. However, when blue carbon ecosystems are degraded or lost they release carbon back to the atmosphere, thereby adding to greenhouse gas emissions.^[128]^: 2220

Seaweed farming and algae

gigatonnes of CO₂ per year from the atmosphere and sustainably produce 200 kg per year of fish, per person, for 10 billion people.^[131] Ideal species for such farming and conversion include Laminaria digitata, Fucus serratus and Saccharina latissima.^[132]

Both

macroalgae and microalgae are being investigated as possible means of carbon sequestration.^[133]^[134] 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.^[135]

Because

biogenic fuels.^[137]

Large-scale seaweed farming (called "ocean afforestation") could sequester huge amounts of carbon.^[138] 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.^[139] Currently seaweed farming is carried out to provide food, medicine and biofuel.^[139] 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.^[139] Seaweed farming has gathered attention given the limited terrestrial space available for carbon farming practices.^[139] Currently seaweed farming occurs mostly in the Asian Pacific coastal areas where it has been a rapidly increasing market.^[139] The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate recommends "further research attention" on seaweed farming as a mitigation tactic.^[140]

However, seaweed farming, and carbon farming in general, only keeps the carbon within the fast carbon cycle, in intimate contact with the ocean and atmosphere, and once in equilibrium with the ecology, cannot be expected to hold additional carbon.

Ocean fertilization

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.

]

Ocean fertilization or ocean nourishment is a type of technology for carbon dioxide removal from the ocean based on the purposeful introduction of plant nutrients to the upper ocean to increase marine food production and to remove carbon dioxide from the atmosphere.^[141]^[142] Ocean nutrient fertilization, for example iron fertilization, could stimulate photosynthesis in phytoplankton. The phytoplankton would convert the ocean's dissolved carbon dioxide into carbohydrate, some of which would sink into the deeper ocean before oxidizing. More than a dozen open-sea experiments confirmed that adding iron to the ocean increases photosynthesis in phytoplankton by up to 30 times.^[143]

This is one of the more well-researched carbon dioxide removal (CDR) approaches, however this approach would only sequester carbon on a timescale of 10-100 years dependent on ocean mixing times. While surface ocean acidity may decrease as a result of nutrient fertilization, when the sinking organic matter remineralizes, deep ocean acidity will increase. A 2021 report on CDR indicates that there is medium-high confidence that the technique could be efficient and scalable at low cost, with medium environmental risks.^[144] One of the key risks of nutrient fertilization is nutrient robbing, a process by which excess nutrients used in one location for enhanced primary productivity, as in a fertilization context, are then unavailable for normal productivity downstream. This could result in ecosystem impacts far outside the original site of fertilization.^[144]

A number of techniques, including fertilization by the micronutrient iron (called iron fertilization) or with nitrogen and phosphorus (both macronutrients), have been proposed. But research in the early 2020s suggested that it could only permanently sequester a small amount of carbon.^[145]

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[115]

Authority control databases
International	FAST
National	France BnF data Germany Israel United States Latvia
Other	IdRef

Carbon sequestration

Terminology

History of the term (etymology)

Roles

In nature

In climate change mitigation

Biological carbon sequestration on land

Forestry

Wetlands

Peatlands, mires and peat bogs

Agriculture

Carbon farming

Prairies

Biochar

Burial of biomass

Geological carbon sequestration

Underground storage in suitable geologic formations

Mineral sequestration

Zeolitic imidazolate frameworks

Mineral carbonation

Sequestration in oceans

Marine carbon pumps

Vegetated coastal ecosystems

Seaweed farming and algae

Ocean fertilization

Artificial upwelling

Basalt storage

Mineralization and deep sea sediments

Adding bases to neutralize acids

Single-step carbon sequestration and storage

Abandoned ideas

Direct deep-sea carbon dioxide injection

Cost

Applications in climate change policies

United States

See also

References