Carbon sequestration is the natural process of storing carbon in places like forests, oceans, or rocks. It helps manage the Earth's carbon cycle and reduces the amount of carbon dioxide in the air, which is important for limiting climate change. There are two main types of carbon sequestration: biological (also called biosequestration) and geological.
Biological carbon sequestration happens naturally as part of the carbon cycle. People can help speed up this process by changing how land is used or using technology. Plants take in carbon dioxide from the air and use it to grow, storing it in their leaves, stems, and roots. However, these natural stores, like forests or kelp beds, may not keep carbon locked away forever. Events like wildfires, disease, or human activities can release stored carbon back into the atmosphere.
Geological sequestration involves capturing carbon dioxide from human activities and storing it underground or beneath the ocean floor. Another method is mineral sequestration, where carbon dioxide is turned into insoluble carbonate salts and stored in rocks. These methods are considered safe because they can keep carbon locked away for thousands to millions of years.
To help store more carbon in the oceans, scientists have proposed ideas like adding nutrients to the water, moving deep ocean water to the surface, or using rocks to trap carbon. However, these methods are not yet used on a large scale. Growing seaweed in the ocean is a biological process that could store large amounts of carbon. Harvested seaweed could be buried in the deep ocean for long-term storage. The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate suggests more research is needed to explore seaweed farming as a way to reduce carbon in the atmosphere.
Terminology
The term carbon sequestration has different meanings in books and articles. The IPCC Sixth Assessment Report defines carbon sequestration as "the process of storing carbon in a carbon pool." A carbon pool is a storage area in Earth's systems where elements like carbon and nitrogen remain in different chemical forms for a period of time.
The United States Geological Survey (USGS) defines carbon sequestration as "the process of capturing and storing atmospheric carbon dioxide." This definition is very similar to the one for carbon capture and storage (CCS). The IPCC 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." Because the wording in these definitions is so similar, carbon sequestration is sometimes confused with CCS.
Roles
Carbon sequestration is part of Earth's natural carbon cycle, where carbon moves between the biosphere (living things), pedosphere (soil), geosphere (Earth's layers), hydrosphere (water), and atmosphere. Carbon dioxide is naturally removed from the air through natural processes and stored in long-term places.
Plants take in carbon dioxide from the air as they grow and store it in their bodies. However, natural storage places like forests and kelp beds are less stable because events such as fires, disease, or changes in land use can release stored carbon back into the air.
Carbon sequestration helps reduce the effects of climate change by acting as a carbon sink, which means it holds carbon that would otherwise be in the atmosphere. It slows the buildup of greenhouse gases, such as carbon dioxide, which come from burning fossil fuels.
To help fight climate change, carbon sequestration can either improve natural storage places or use technology to capture and store carbon. In carbon capture and storage methods, sequestration refers to the storage part. Artificial storage methods include putting carbon dioxide into deep underground spaces, like saltwater layers or old gas fields, or combining it with materials to form stable compounds.
For carbon to be stored artificially, it must first be captured or kept from entering the air. This can be done by using carbon-rich materials in long-lasting products, like buildings, which prevent carbon from being released through burning or decay. These materials can be stored or used for many years. For example, wood from trees can be used in construction, keeping its carbon stored for decades or even centuries. In factories, engineers often capture carbon dioxide from smoke and emissions.
In the United States, a government rule called Executive Order 13990, issued in 2021 and canceled in 2025, mentioned using carbon sequestration through protecting and restoring ecosystems like wetlands and forests. It highlighted the role of farmers, landowners, and coastal communities in storing carbon. It also asked the Treasury Department to support conservation efforts using market-based methods.
A 2025 study found that even if Earth's underground storage spaces were fully used, they could help limit global warming by only about 0.7°C (1.3°F). This shows that Earth's ability to store carbon has limits.
Biological carbon sequestration on land
Biological carbon sequestration is the process of capturing and storing carbon dioxide from the atmosphere through natural biological activities. This happens when land-use practices, such as reforestation and sustainable forest management, increase the rate of photosynthesis. Changes in land use that support natural carbon capture can store large amounts of carbon dioxide each year. These efforts include protecting, managing, and restoring ecosystems like forests, peatlands, wetlands, and grasslands, as well as using methods in agriculture to store carbon in soil. Practices exist to improve soil carbon storage in both farming and forestry.
Forests are important in the global carbon cycle because trees and plants absorb carbon dioxide through photosynthesis. This makes forests a key part of reducing climate change. By removing carbon dioxide from the air, forests act as carbon sinks, storing large amounts of carbon in the form of plant parts like roots, stems, branches, and leaves. Forests sequester about 25% of human carbon emissions each year, playing a major role in Earth's climate. Trees continue to store carbon throughout their lives, keeping atmospheric carbon dioxide locked away for long periods. Practices like sustainable forest management, afforestation, and reforestation help reduce climate change.
A key point is that forests can change from being carbon sinks to carbon sources. In 2019, forests absorbed about one-third less carbon than they did in the 1990s due to higher temperatures, droughts, and deforestation. Data from 1999 to 2020 shows some forests are nearing climate thresholds that could shift them from storing carbon to releasing it. Some tropical forests may become carbon sources by the 2060s.
Research shows that avoiding deforestation is better for the environment than allowing deforestation followed by reforestation. This is because deforestation causes irreversible harm, such as loss of biodiversity and soil damage. Younger boreal forests are more likely to release carbon from the soil. Boreal forests support the growth of Armillaria (honey fungus), a pathogen that breaks down wood, increasing carbon release. Damage to tropical rainforests may have been undercounted in global greenhouse gas emissions until 2019. Keeping existing forests intact provides faster climate benefits than planting new forests, as mature trees store carbon more effectively. Scientists say protecting and restoring carbon-rich ecosystems, especially natural forests, is a major solution to climate change.
Planting trees on unused farmland or pasture helps store carbon in plant biomass. For this to work, the carbon must stay in the biomass and not return to the atmosphere when trees die, such as through burning or rotting. Some Ficus tree species, like Ficus wakefieldii, store carbon dioxide as calcium oxalate with the help of bacteria and fungi. This process creates calcium carbonate in the tree and makes the soil more alkaline. These trees are being studied for use in agroforestry. The Iroko tree can store up to one ton of calcium carbonate in its lifetime. Cacti, like the Saguaro, transfer carbon from biological cycles to geological cycles by forming calcium carbonate.
The Earth has enough space to plant an additional 0.9 billion hectares of tree canopy, though this estimate is debated. When considering factors like albedo (reflectivity), the actual area with a cooling effect is 20-80% smaller. Planting and protecting these trees could store 205 billion tons of carbon if they survive to maturity. This is about 20 years of global carbon emissions (as of 2019) and would account for 25% of the atmosphere's carbon pool in 2019.
The lifespan of forests varies globally, depending on tree species, location, and natural events. Some forests store carbon for centuries, while others release it quickly due to frequent fires. Harvesting forests before fires occur allows carbon to remain in products like lumber. However, most carbon from logged forests becomes by-products like paper or pallets. If 90% of new construction used wood products, especially mass timber, it could store 700 million tons of carbon yearly. This would also reduce emissions from using materials like steel or concrete, which produce more carbon.
A study found that mixed-species forests store more carbon and offer other benefits compared to single-species plantations.
Although bamboo forests store less total carbon than mature forests, they absorb carbon faster. This makes bamboo farming a promising method for carbon storage.
The Food and Agriculture Organization (FAO) reported that total forest carbon decreased from 668 gigatonnes in 1990 to 662 gigatonnes in 2020. In Canada’s boreal forests, up to 80% of carbon is stored in soil as dead organic matter.
The IPCC Sixth Assessment Report states that restoring degraded forests and non-forest ecosystems can greatly help with carbon storage, with high resilience to disturbances and benefits like increased biodiversity.
The climate impact of forests depends on their location. For example, reforestation in boreal or subarctic regions has less effect on climate because forests replace snow-covered areas with darker canopies that absorb more sunlight. In contrast, tropical reforestation can create clouds that reflect sunlight, lowering temperatures.
Planting trees in tropical regions with wet seasons helps them grow faster and store more carbon because they grow year-round. Tropical trees have larger, brighter leaves than those in non-tropical regions. A study of 70,000 trees across Africa found that tropical forests absorb more carbon dioxide than previously thought. Research suggests nearly one-fifth of fossil fuel emissions are absorbed by forests in Africa, the Amazon, and Asia. Simon Lewis noted that tropical forests absorb about 18% of annual carbon dioxide emissions from burning fossil fuels, slowing climate change.
Wetland restoration involves r
Geological carbon sequestration
Geological sequestration is the process of storing carbon dioxide (CO₂) underground in areas such as old oil and gas reservoirs, saltwater-filled rock layers, or deep coal beds that are no longer useful for mining.
After CO₂ is captured from a source, like a factory, it is compressed to about 100 bar, turning it into a supercritical fluid. In this state, the CO₂ can be moved through pipelines to storage locations. Once there, it is injected deep underground, usually around 1 kilometer (0.6 miles) below the surface. At this depth, the CO₂ remains stable for hundreds to millions of years. The density of supercritical CO₂ is between 600 and 800 kilograms per cubic meter.
When choosing a good location for CO₂ storage, important factors include the rock’s porosity (how much space is inside the rock), permeability (how easily fluids can move through the rock), the absence of cracks or faults in the rock, and the shape of the rock layers. The best storage rocks, like sandstone or limestone, have high porosity and permeability. For example, sandstone can have a permeability of 1 to 10 Darcy and a porosity of up to about 30%. The porous rock must be covered by a layer of low permeability, called a caprock, to prevent CO₂ from escaping. Shale is a good example of a caprock, with a permeability of 10⁻¹⁰ to 10⁻⁶ Darcy. After injection, CO₂ rises due to buoyancy because it is less dense than surrounding materials. When it reaches a caprock, it spreads sideways until it finds a gap. If there are faults near the injection site, CO₂ might move along the fault to the surface, potentially causing harm to nearby life. Another risk is induced seismicity, where high underground pressures from CO₂ injection could cause fractures in the rock, possibly leading to earthquakes.
Structural trapping is the main way CO₂ is stored. Impermeable rocks, such as mudstone, anhydrite, halite, or carbonates, act as barriers that stop CO₂ from rising. Once trapped, CO₂ can exist as a supercritical fluid, dissolve in groundwater or brine, or react with minerals in the rock to form carbonates.
In 2025, research found that out of nearly 12,000 gigatons of CO₂ that could theoretically be stored underground, only 1,460 gigatons are considered safe for storage, which is much less than most estimates suggested.
Mineral sequestration involves trapping CO₂ as solid carbonate salts. This process happens slowly in nature and is responsible for forming limestone over long periods. Carbonic acid in groundwater reacts with silicate minerals, dissolving calcium, magnesium, and other elements while leaving behind clay. The dissolved calcium and magnesium then combine with bicarbonate to form calcium and magnesium carbonates, which organisms use to build shells. When these organisms die, their shells settle as sediment and eventually become limestone. Limestones have formed over billions of years and hold much of Earth’s carbon. Scientists are working to speed up similar reactions using alkali carbonates.
Zeolitic imidazolate frameworks (ZIFs) are materials similar to zeolites. Because they are porous, chemically stable, and heat-resistant, ZIFs are being studied for their ability to capture CO₂.
CO₂ reacts with metal oxides to form stable carbonates, such as calcite or magnesite. This process, called "CO₂-to-stone," happens naturally over years and forms much of Earth’s surface limestone. Olivine is one type of metal oxide. Rocks rich in metal oxides, like MgO and CaO found in basalt, have been shown to store CO₂ effectively. The reaction can be faster with catalysts, higher pressures, or mineral treatments, though these methods may require extra energy.
Ultramafic mine tailings, which are fine-grained metal oxides, can be used for CO₂ storage. Microbial processes can also speed up natural CO₂ sequestration by dissolving minerals and forming carbonates.
CO₂ can be removed from the atmosphere through chemical processes and stored as stable carbonate minerals. This method, called "carbon sequestration by mineral carbonation," involves reacting CO₂ with metal oxides like MgO or CaO to form carbonates. These reactions release heat and occur naturally, such as through the weathering of rocks over long periods.
Calcium and magnesium in nature are usually found as silicates, like forsterite and serpentinite, not as pure oxides. For forsterite and serpentine, the reactions are:
These reactions are more favorable at lower temperatures. Naturally, this process forms much of Earth’s surface limestone over geologic time. The reaction can be made faster with higher temperatures and pressures, though this requires extra energy. Alternatively, grinding minerals to increase surface area and exposing them to water and abrasion, like in beach environments, can also help.
When CO₂ dissolved in water is injected into hot basaltic rocks underground, it reacts with the basalt to form solid carbonate minerals. A test facility in Iceland, started in October 2017, removes up to 50 tons of CO₂ annually from the atmosphere and stores it in basaltic rock underground.
Sequestration in oceans
The ocean stores carbon through several processes. The solubility pump moves carbon dioxide from the air into the surface ocean, where it combines with water to form carbonic acid. Cooler water holds more carbon dioxide than warmer water. Thermohaline circulation moves dissolved carbon dioxide to deeper, colder waters, where it becomes more soluble and stays in the ocean longer. The biological pump moves carbon dioxide from the surface ocean to the deep ocean by converting inorganic carbon into organic carbon through photosynthesis. Organic matter that survives breakdown can sink to the deep ocean, where it remains for long periods.
Deep ocean conditions, such as cold temperatures, high pressure, and low oxygen, slow the breakdown of organic matter. This prevents carbon from quickly returning to the atmosphere and helps store it for many years.
Blue carbon refers to carbon stored in marine ecosystems like tidal marshes, mangroves, and seagrass meadows. These ecosystems help reduce carbon in the atmosphere. However, if these areas are destroyed, they release stored carbon back into the air, increasing greenhouse gas emissions.
Seaweed grows in shallow coastal areas and captures carbon. Some of this carbon can be carried to the deep ocean, where it stays for thousands of years. Scientists suggest growing seaweed offshore and sinking it to the deep ocean to store carbon. Seaweed grows quickly and can be used to produce energy, such as biomethane, through digestion or used as a fuel source. One study says that if seaweed farms covered 9% of the ocean, they could produce enough energy to meet global needs, remove 53 gigatonnes of CO₂ yearly, and provide enough fish for 10 billion people. Species like Laminaria digitata, Fucus serratus, and Saccharina latissima are ideal for farming.
Both large seaweed (macroalgae) and tiny algae (microalgae) are being studied for carbon storage. Marine phytoplankton perform about half of the world’s photosynthesis and oxygen production, even though they make up only 1% of all plant life. Algae lack the tough material found in land plants, so their carbon is released into the atmosphere faster. Algae can act as a short-term carbon storage source for making fuels.
Large-scale seaweed farming could store significant carbon. Naturally, wild seaweed releases carbon into the deep ocean, where it remains buried for long periods. For carbon farming, harvested seaweed could be sent to the deep ocean for long-term storage. Seaweed farming is growing in coastal areas of the Asian Pacific. A report by the IPCC suggests more research on seaweed farming as a way to reduce carbon.
Ocean fertilization adds nutrients like iron to the ocean’s surface to help phytoplankton grow. This process mimics natural events like volcanic eruptions or whale activity. Adding nutrients increases marine life and removes carbon dioxide from the air. Experiments show that adding iron can boost phytoplankton growth by up to 30 times.
Ocean iron fertilization is a well-studied method for removing carbon. Scientists believe it has strong potential for reducing atmospheric CO₂. However, it is unclear how long the stored carbon remains in the ocean. A 2021 study says this method has high potential among ocean-based carbon removal strategies.
Artificial upwelling uses large pipes to bring deep, nutrient-rich water to the surface. This encourages algae growth, which stores carbon during its life and releases it when it dies. This method is similar to iron fertilization but may temporarily increase surface CO₂ levels, which limits its use. Moving deep water to the surface can also harm marine life by blocking sunlight and releasing toxins. It may lower seawater pH, harming coral reefs and disrupting ocean ecosystems.
Storing carbon in underwater basalt involves injecting CO₂ into deep-sea rock formations. The CO₂ mixes with seawater and reacts with basalt, forming stable carbonate minerals. Basalt is a good storage option because it has natural barriers that prevent CO₂ from leaking. Injecting CO₂ below 2,700 meters ensures it sinks due to its higher density than seawater, reducing the risk of leakage.
Costs
The cost of storing carbon (not including capturing or moving it) can vary. In some cases, it is less than US$10 per tonne when onshore storage is available. For example, a method called Carbfix costs about US$25 per tonne of CO₂. A 2020 report estimated that storing carbon in forests (including capturing it) costs between US$35 and US$280 per tonne, depending on the amount needed to limit warming to 1.5°C. However, there is a risk that forest fires could release stored carbon back into the air. Research shows that offsetting the total carbon reserves of the 200 largest fossil fuel companies could cost between 11% and 701% of global GDP, depending on the price of carbon in the market.