Carbon sequestration is a natural process that stores carbon in a carbon pool. It helps manage the global carbon cycle and reduces climate change by lowering the amount of carbon dioxide in the atmosphere. There are two main types of carbon sequestration: biologic (also called biosequestration) and geologic.

Biologic carbon sequestration is part of the natural carbon cycle. People can improve it through actions and technology. Carbon dioxide (CO₂) is naturally removed from the atmosphere through biological, chemical, and physical processes. These processes can be made faster through changes in land use and farming practices, called carbon farming. Technology has also been developed to capture and store CO₂ from human activities underground or under the ocean floor, a method called carbon capture and storage.

Plants take in carbon dioxide from the air as they grow and store it in their biomass. However, biological storage areas, such as forests and kelp beds, may only hold carbon temporarily. Events like wildfires, disease, economic pressures, or changes in political goals can release stored carbon back into the atmosphere.

Carbon dioxide removed from the atmosphere can be stored in Earth's crust by injecting it underground or forming insoluble carbonate salts. This process is called mineral sequestration. These methods are considered non-volatile because they remove carbon dioxide from the atmosphere and store it permanently. This means the carbon is "locked away" for thousands to millions of years.

To improve carbon sequestration in oceans, scientists have proposed methods such as ocean fertilization, artificial upwelling, basalt storage, mineralization, deep-sea sediments, and adding bases to neutralize acids. However, these methods are not yet used on a large scale. Large-scale seaweed farming is a biological process that could store significant amounts of carbon. Harvested seaweed might be transported to the deep ocean for long-term storage. The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate suggests that more research is needed on seaweed farming as a way to reduce carbon emissions.

Terminology

The term carbon sequestration has different meanings in scientific writing and news articles. According to the IPCC Sixth Assessment Report, carbon sequestration is "The process of storing carbon in a carbon pool." A carbon pool is described as "a part of the Earth system where elements, such as carbon and nitrogen, remain in different forms for some 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 definition of 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 of these similarities, carbon sequestration is sometimes mixed up with CCS.

Roles

Carbon sequestration is part of the natural carbon cycle. In this cycle, carbon moves between Earth's living parts, soil, Earth's layers, water, and the air. Carbon dioxide is naturally taken from the air through natural processes and stored in long-term places.

Plants take in carbon dioxide from the air as they grow and use it to build their bodies. However, natural storage places like forests and kelp beds are not always reliable for long-term storage. Events such as fires, disease, or changes in how land is used can cause stored carbon to return to the air.

Carbon sequestration acts as a carbon sink, which helps reduce the effects of climate change. It slows the buildup of greenhouse gases, like carbon dioxide, in the air and oceans. These gases are mainly released by burning fossil fuels.

To help reduce climate change, carbon sequestration can either improve natural storage places or use technology to capture and store carbon.

In methods that capture and store carbon, sequestration refers to the storage part. Artificial storage methods include storing carbon gas deep underground in saltwater areas or old gas fields. Another method involves mixing carbon dioxide with materials to create stable, solid substances.

For carbon to be stored artificially, it must first be captured or its release into the air must be delayed or stopped. This can be done by using carbon-rich materials in long-lasting products, like buildings, to prevent carbon from being released through burning or decay. Once stored, carbon can remain in these products for many years or even centuries. For example, wood from trees can be used in construction or other long-lasting items, keeping its carbon stored for a long time. In industries, engineers often capture carbon dioxide from smoke released by power plants or factories.

In the United States, a government order from 2021 called "Protecting Public Health and the Environment and Restoring Science to Tackle the Climate Crisis" included actions to improve natural storage places, such as wetlands and forests. The order highlighted the role of farmers, landowners, and coastal communities in storing carbon. It also asked the Treasury Department to use market-based methods to protect these storage areas.

A study from 2025 found that using Earth's underground storage spaces fully could help limit global warming by about 0.7°C (1.3°F). This shows that Earth's ability to store carbon is not endless.

Biological carbon sequestration on land

Biological carbon storage, also known as biosequestration, is the process of capturing and storing carbon dioxide from the atmosphere through natural biological processes. This happens when plants and trees take in carbon dioxide during photosynthesis. Practices like reforestation and managing forests in a way that protects their health help increase this process. Changing how land is used to support natural carbon capture can store large amounts of carbon each year. This includes protecting, managing, and restoring ecosystems like forests, wetlands, peatlands, and grasslands, as well as using farming methods that help store carbon in soil. There are ways to improve how soil stores carbon 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 taking in carbon dioxide from the air, forests act as carbon sinks, storing large amounts of carbon in their roots, stems, branches, and leaves. Forests store about 25% of human-caused carbon emissions each year, which helps control Earth's climate. Over their lifetime, trees continue to store carbon, keeping it out of the atmosphere for a long time. Practices like managing forests carefully, planting new forests, and restoring forests that have been cut down are important ways to help reduce climate change.

A key point is that forests can change from being carbon sinks to sources of carbon. In 2019, forests absorbed about one-third less carbon than they did in the 1990s because of higher temperatures, droughts, and deforestation. Data from 1999 to 2020 shows that some forests are nearing a point where they might start releasing more carbon than they store. Some tropical forests may become carbon sources by the 2060s.

Studies show that preventing deforestation is better for the environment than allowing forests to be destroyed and then regrown later. This is because deforestation can cause lasting harm to biodiversity and soil. Younger boreal forests are more likely to release stored carbon from the soil. Boreal forests support a type of fungus that breaks down wood, increasing the chance of carbon being released. Damage to tropical rainforests may have caused more greenhouse gas emissions than previously thought. It takes many decades for newly planted forests to store as much carbon as mature forests, so protecting existing forests is a major way to fight climate change.

Planting trees on lands used for farming or pastures helps move carbon from the air into plant material. For this to work, the carbon must stay in the plants and not return to the air when the trees die or burn. Some species of Ficus trees, like Ficus wakefieldii, store carbon as calcium oxalate with the help of certain 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 a ton of calcium carbonate in the soil over its lifetime. Cacti, like the Saguaro, also store carbon by forming calcium carbonate in the soil.

There is enough land available to plant an additional 0.9 billion hectares of trees, though some experts say the actual area that helps cool the climate may be smaller. If these trees survive and grow to maturity, they could store 205 billion tons of carbon. This is about 20 years of global carbon emissions as of 2019. This amount of carbon storage would be about 25% of the atmosphere’s carbon in 2019.

The lifespan of forests varies depending on the tree species, environment, and natural events like fires. In some forests, carbon can stay stored for centuries, while in others, fires may release it quickly. When forests are cut down before major fires, some carbon is stored in wood products like lumber. However, much of the carbon from logged forests ends up in products like paper and pallets. If 90% of new construction used wood products, especially mass timber, it could store 700 million tons of carbon each year. This would also reduce emissions from using materials like steel or concrete, which are carbon-heavy to produce.

A study found that planting forests with a mix of tree species can increase carbon storage while providing other benefits.

Although a bamboo forest stores less total carbon than a mature forest, it absorbs carbon faster than a mature forest or a tree plantation. This means growing bamboo for timber could help store carbon quickly.

The Food and Agriculture Organization (FAO) reported that the total carbon stored in forests decreased from 668 gigatonnes in 1990 to 662 gigatonnes in 2020. In Canada’s boreal forests, up to 80% of the total carbon is stored in the soil as dead organic matter.

The IPCC Sixth Assessment Report states that regrowing forests and restoring damaged ecosystems can help store carbon, with high resilience to problems and benefits like better biodiversity.

The effect of forests on temperature depends on where they are planted. For example, planting forests in boreal or subarctic areas has less impact on climate because it replaces snow-covered areas with forests that reflect less sunlight. In contrast, planting forests in tropical regions can create clouds that reflect sunlight, lowering temperatures.

Planting trees in tropical regions with wet seasons has an added benefit. Trees grow faster in these areas because they can grow year-round. Tropical trees have larger, brighter leaves than those in non-tropical regions. A study of 70,000 trees in Africa found that tropical forests absorb more carbon dioxide than previously thought. Research suggests that forests in Africa, the Amazon, and Asia absorb almost one-fifth of fossil fuel emissions. Simon Lewis said, "Tropical forest trees absorb about 18% of the carbon dioxide added to the atmosphere each year from burning fossil fuels, which helps slow the rate of climate change."

Wetland restoration involves…

Geological carbon sequestration

Geological sequestration is the process of storing carbon dioxide (CO₂) deep underground in locations such as empty oil and gas reservoirs, salty rock layers, or coal beds that are too deep to mine.

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, CO₂ can be moved through pipelines to storage sites. Once there, it is injected underground, usually around 1 kilometer (0.6 miles) deep, where it remains stable for hundreds to millions of years. At this depth, the density of supercritical CO₂ is between 600 and 800 kilograms per cubic meter.

Choosing a good storage site depends on several factors: the rock’s porosity (how much space is inside the rock), permeability (how easily fluids can flow through the rock), the absence of faults (cracks in the rock), and the shape of the rock layers. Ideal storage rock, like sandstone or limestone, has high porosity and permeability. For example, sandstone can have a permeability of 1 to 10 Darcy and a porosity up to about 30%. The storage rock must be covered by a layer of low permeability, called a caprock, to prevent CO₂ from escaping. Shale is a strong caprock, with a permeability of 10⁻¹⁰ to 10⁻⁶ Darcy. Once injected, CO₂ rises due to buoyancy until it hits the caprock, where it spreads sideways. If faults are near the injection site, CO₂ might escape through them, potentially leaking into the atmosphere and harming nearby life. Another risk is induced seismicity, where high underground pressures from CO₂ injection could cause fractures and trigger earthquakes.

Structural trapping is the main way CO₂ is stored. Impermeable rocks like mudstone, anhydrite, halite, or carbonates act as barriers, keeping CO₂ trapped in the storage layer. While stored, 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 of nearly 12,000 gigatons of CO₂ that could theoretically be stored underground, only 1,460 gigatons are considered safe, which is much less than earlier estimates.

Mineral sequestration is a method that traps CO₂ as solid carbonate salts. This process happens naturally over long periods, forming limestone. Carbonic acid in groundwater slowly dissolves minerals like calcium and magnesium, which then react with bicarbonate to form carbonates. These reactions are used by organisms to create shells, which eventually become sediment and turn into limestone. Limestones have formed over billions of years and hold much of Earth’s carbon. Scientists are working to speed up these 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," naturally creates surface limestone over years. Olivine is one metal oxide that reacts with CO₂. Rocks rich in metal oxides like MgO and CaO, found in basalts, are effective for storing CO₂. The reaction speed can be increased with catalysts, higher pressures, or mineral treatments, though these methods may require extra energy.

Ultramafic mine tailings, which contain fine metal oxides, are a ready source for CO₂ storage. Microbial processes can also speed up 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 during rock weathering over long periods.

Calcium and magnesium in nature are often found as silicates, like forsterite and serpentinite, not as pure oxides. Reactions for forsterite and serpentine are more favorable at lower temperatures. This process naturally forms much of Earth’s surface limestone. The reaction rate can be increased by using higher temperatures, pressures, or by grinding minerals to increase surface area. Exposing minerals to water and abrasion, such as by spreading olivine on beaches, can also help.

When CO₂ is dissolved in water and injected into hot basaltic rocks underground, it reacts with the basalt to form solid carbonate minerals. A test plant 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

Several companies are working to scale up methods for capturing and storing carbon in the ocean.

The ocean stores carbon through different processes. The solubility pump helps move carbon dioxide from the air into the ocean's surface, where it reacts with water to form carbonic acid. Carbon dioxide dissolves more easily in colder water. Thermohaline circulation moves dissolved carbon dioxide to cooler, deeper waters, where it becomes more soluble, increasing carbon levels in the deep ocean. The biological pump helps move dissolved carbon dioxide from the ocean's surface to the deep ocean by converting inorganic carbon into organic carbon through photosynthesis. Organic matter that survives breakdown can be carried to the deep ocean through sinking particles or the movement of organisms.

In the deep ocean, low temperatures, high pressure, and low oxygen levels slow down the breakdown of organic matter. This prevents carbon from quickly returning to the atmosphere, acting as a long-term storage place.

Blue carbon refers to carbon stored in marine ecosystems that can be managed to help reduce climate change. These ecosystems, such as tidal marshes, mangroves, and seagrass meadows, play a key role in capturing carbon. When these ecosystems are damaged or destroyed, they release stored carbon back into the atmosphere, increasing greenhouse gas emissions.

Seaweed grows in shallow and coastal areas and captures carbon that can be transported to the deep ocean. Seaweed that reaches the deep ocean stores carbon for thousands of years. Growing seaweed offshore and sinking it to the deep ocean has been proposed as a method to capture carbon. Seaweed grows quickly and can be harvested to produce biomethane through anaerobic digestion, generate electricity, or replace natural gas. One study suggests that if seaweed farms covered 9% of the ocean, they could produce enough biomethane to meet global energy needs, remove 53 gigatonnes of CO₂ from the atmosphere annually, and provide enough fish for 10 billion people. Species like Laminaria digitata, Fucus serratus, and Saccharina latissima are ideal for this purpose.

Both large seaweed (macroalgae) and tiny seaweed (microalgae) are being studied for carbon capture. Marine phytoplankton, though making up only about 1% of global plant life, perform half of the world’s photosynthesis and half of the oxygen production.

Algae lack the complex structures found in land plants, so their carbon is released into the atmosphere more quickly. Algae are considered a short-term storage for carbon that can be used to make biogenic fuels.

Large-scale seaweed farming could capture significant amounts of carbon. Naturally, wild seaweed stores carbon as organic matter sinks to the deep ocean and becomes buried for long periods. In carbon farming, harvested seaweed could be transported to the deep ocean for long-term storage. Seaweed farming is growing rapidly in coastal areas of the Asian Pacific. The IPCC recommends further research on seaweed farming as a climate change solution.

Ocean fertilization involves adding nutrients like iron to the ocean to stimulate phytoplankton growth, which captures carbon dioxide through photosynthesis. This mimics natural processes that occurred before ice ages, after volcanic eruptions, and near hydrothermal vents. Adding nutrients increases marine life and reduces atmospheric carbon dioxide.

Iron fertilization can increase phytoplankton growth by up to 30 times, as shown in open-sea experiments. This method is well-researched and supported by some climate scientists, though its long-term effectiveness is uncertain. A 2021 study suggests iron fertilization has high potential for carbon removal.

Artificial upwelling or downwelling involves mixing ocean layers to move nutrients and gases. Large pipes could pump deep, nutrient-rich water to the surface, triggering algae blooms that capture carbon. However, this method may temporarily increase surface carbon dioxide levels and harm marine ecosystems by blocking sunlight or releasing toxins.

Mixing ocean layers can move cold, dense deep water to the surface, where it absorbs more carbon dioxide. This process may also cause surface water to sink and absorb more carbon. However, algae blooms can damage ecosystems and lower ocean pH, harming coral reefs and marine life.

Carbon dioxide can be stored in underwater basalt by injecting it into deep-sea formations. The CO₂ reacts with seawater and basalt, forming stable carbonate minerals. Basalt offers safe storage because it has natural barriers to prevent leakage. Injecting CO₂ below 2,700 meters ensures it sinks due to its higher density than seawater.

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 storage on land is available. For example, the cost for Carbfix is about US$25 per tonne of CO₂. A 2020 report estimated that storing carbon in forests (which includes 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. Research shows that removing the carbon from the 200 largest fossil fuel companies would cost between 11% and 701% of the world's total economic value, depending on carbon prices.