Carbon dioxide removal (CDR) is a process where humans intentionally take carbon dioxide (CO₂) from Earth's atmosphere and store it in places like underground areas, forests, oceans, or in products. This process is also called carbon removal, greenhouse gas removal, or negative emissions. CDR is increasingly being used in climate policies as part of plans to reduce climate change. To reach net zero emissions, it is most important to greatly and continuously reduce emissions. After that, CDR can help balance any remaining emissions that are hard to eliminate, such as some emissions from farming and industry.

CDR includes methods that take place on land or in water. On land, these methods include planting trees, restoring forests, farming practices that store carbon in soil, using plants to create energy and capturing the carbon (called BECCS), and capturing CO₂ directly from the air and storing it. In water, methods include adding nutrients to oceans to help plants grow, increasing ocean alkalinity, restoring wetlands, and using ocean-based methods to store carbon (called blue carbon). To determine how much carbon is removed by a specific method, a detailed analysis is needed. This includes looking at the entire process from start to finish and tracking, reporting, and checking the results (called MRV). Carbon capture and storage (CCS) is not considered CDR because it does not remove CO₂ already in the atmosphere.

As of 2023, CDR is estimated to remove about 2 gigatons of CO₂ each year. This is about 4% of the greenhouse gases humans release annually. Some CDR methods can safely and economically remove up to 10 gigatons of CO₂ per year. However, it is difficult to measure exactly how much CO₂ CDR removes from the atmosphere.

Definition

Carbon dioxide removal (CDR) is described by the IPCC as: "Human activities that take carbon dioxide from the air and store it safely in places like underground areas, forests, oceans, or products. It includes ways to improve natural processes that absorb carbon dioxide, as well as technologies like direct air capture, but does not include natural carbon dioxide absorption that is not caused by humans."

Other names for CDR are greenhouse gas removal (GGR), negative emissions technology, and carbon removal. Scientists have suggested methods to remove other gases, like methane, from the air, but only carbon dioxide can be removed in large amounts right now. Because of this, in most situations, greenhouse gas removal refers to carbon dioxide removal.

The term "geoengineering" (or "climate engineering") is sometimes used in scientific writing to describe both CDR and SRM (solar radiation management) when these methods are used worldwide. However, the terms "geoengineering" and "climate engineering" are no longer used in IPCC reports.

Categories

Carbon dioxide removal (CDR) methods can be grouped into different categories based on specific factors:

• Their role in the carbon cycle (such as land-based biological, ocean-based biological, geochemical, or chemical); or
• How long carbon is stored (from decades to centuries, centuries to millennia, or thousands of years or longer).

CDR is sometimes confused with carbon capture and storage (CCS), a process that collects carbon dioxide from specific sources, like gas-fired power plants, where CO₂ is released in a concentrated stream from smokestacks. The CO₂ is then compressed and either stored or used. When CCS is used to store carbon from a fossil fuel-powered plant, it reduces emissions from the plant’s ongoing operation. However, CCS does not remove the carbon dioxide that is already present in the atmosphere.

Role in climate change mitigation

The use of CDR helps reduce how much carbon dioxide humans add to the atmosphere. Earth's surface temperature will only stop rising when global emissions reach net zero, which means emissions must be cut sharply and CDR must be used. It is not possible to reach net zero without CDR because some emissions are hard to eliminate completely. These include nitrous oxide from farming, emissions from airplanes, and some industrial emissions. In plans to fight climate change, CDR helps balance these hard-to-remove emissions.

After net zero emissions are achieved, CDR can help lower the amount of carbon dioxide in the atmosphere, which might reduce some of the warming that has already happened. All plans to limit global warming to 1.5°C or 2°C by 2100 include using CDR along with reducing emissions.

Restoring the climate to a healthier state depends on CDR because reducing carbon dioxide levels below current amounts is needed. However, experts warn that CDR should not be seen as a way to avoid cutting greenhouse gas emissions. Oceanographer David Ho said in 2023, "We must stop thinking of CDR as a solution now, when emissions are still high—as if it replaces urgent cuts in emissions."

In 2018, relying heavily on CDR was called a "major risk" to keeping warming below 1.5°C because it is unclear how quickly CDR can be used at a large scale. Plans that focus more on reducing emissions and using energy wisely carry less risk.

Using CDR on a large scale in the future could create a "moral hazard," meaning people might work less hard to cut emissions now, thinking CDR will solve the problem later. However, a 2019 report said, "Saying that CDR can delay action because it might help in the future is wrong, since CDR is not yet ready at that scale."

CDR should support efforts in sectors where cutting emissions is very hard, not replace them. Limiting warming to 1.5°C and reaching net zero by mid-century would require removing a large amount of carbon dioxide from the air. However, it is unclear how much each country should do, and some countries may not have enough land or underground space to help fairly.

Experts also note that using CDR has limits, such as the land needed. For example, plans from 2023 to remove carbon dioxide require 1.2 billion hectares of land, which is about the size of all the world's farmland combined.

Plants like forests and kelp absorb carbon dioxide as they grow and store it in their bodies. However, these natural storage places are unreliable because events like wildfires or disease, or changes in how land is used, can release the stored carbon back into the air.

Carbon from plants can be stored underground, and carbon dioxide removed from the air can be injected deep underground or turned into stable minerals. These methods keep carbon out of the atmosphere for thousands to millions of years.

In 2023, CDR removes about 2 gigatons of carbon dioxide each year, mostly through simple methods like planting forests. This is about 4% of the carbon dioxide humans add to the atmosphere yearly. A 2019 study said other safe and affordable CDR methods could remove up to 10 gigatons of carbon dioxide yearly if used widely. All plans to prevent more than 1.5°C of warming by 2100 include CDR.

Some plans suggest using one technology to remove carbon quickly, but this would need converting large areas of farmland to grow biofuel crops. More research into methods like capturing carbon directly from the air, storing it underground, and turning it into minerals could make larger CDR efforts more affordable. Using natural solutions, like planting trees, can help buy time for new technologies to be developed and used widely by the middle of the 21st century.

Methods

The following is a list of known Carbon Dioxide Removal (CDR) methods, ordered by their Technology Readiness Level (TRL). Methods at the top have a high TRL of 8 to 9 (the highest level, meaning the technology is proven). Methods at the bottom have a low TRL of 1 to 2 (meaning the technology is not proven or only tested in labs).

• Afforestation / reforestation
• Soil carbon storage in farmland and grasslands
• Restoring peatlands and coastal wetlands
• Agroforestry and better forest management
• Biochar Carbon Removal (BCR)
• Direct Air Carbon Capture and Storage (DACCS)
• Bioenergy with Carbon Capture and Storage (BECCS)
• Enhanced weathering (increasing alkalinity)
• Managing blue carbon in coastal wetlands (restoring ecosystems like mangroves, salt marshes, and seagrass beds)
• Ocean fertilization and increasing ocean alkalinity to boost carbon storage

The CDR methods with the greatest potential to help reduce climate change, based on scientific studies, are land-based biological methods (like afforestation/reforestation) and Bioenergy with Carbon Capture and Storage (BECCS). Some studies also include Direct Air Capture and Storage (DACCS) as a key method.

Trees use photosynthesis to take in carbon dioxide from the air and store it in wood and soil. Afforestation means planting trees in areas that had no forest before. Reforestation means replanting trees in areas that were once forests but were cleared. Forests are important for humans, animals, and plants because they clean the air, help control the climate, and provide homes for many species.

As trees grow, they absorb carbon dioxide and store it in living parts of the tree, dead plant matter, and soil. Afforestation and reforestation help this process by creating or restoring forests. It takes about 10 years for forests to start absorbing carbon at the highest rate.

Depending on the type of tree, it may take 20 to 100 years for trees to reach full maturity. Once mature, trees store carbon but no longer actively remove it from the air. Carbon can stay in forests for a long time, but it can also be lost if trees are cut down, burned, or damaged by disease or drought. Mature trees can be used for wood products, bioenergy, or biochar. When forests regrow, they continue to remove carbon from the air.

Challenges for planting new forests include finding enough land, competing with other uses for land, and the long time it takes for trees to grow.

Carbon farming is a group of farming practices that help store carbon in soil, crops, and plants. The goal is to reduce the amount of carbon in the atmosphere by increasing how much carbon is stored in soil and plants. This can be done by improving soil health, which also helps plants grow better, keeps water in the soil, and reduces the need for fertilizers. Managing forests sustainably is another part of carbon farming.

Farming methods for carbon farming include changing how soil is tilled or how livestock graze, using organic mulch or compost, working with biochar, and growing different types of crops. In forests, methods like reforestation and growing bamboo are used. However, carbon farming has challenges, such as possibly harming ecosystems by increasing land clearing, reducing biodiversity, or creating large areas of the same plant species.

Biomass Carbon Removal and Storage (BiCRS) is a group of technologies that remove carbon dioxide from the air by collecting plant material (like agricultural waste) and storing it permanently or for a long time. These methods are often compared to Direct Air Capture (DAC), which uses machines to remove carbon from the air (a process that is expensive and uses a lot of energy). BiCRS relies on plants absorbing carbon through photosynthesis and then using technology to store the carbon-rich material.

BiCRS has challenges, such as difficulty in measuring how much carbon is stored in buried plant material and the need for more agricultural land to grow biomass. Experts recommend rules to limit which types of biomass can be used for these processes.

BiCRS includes companies like Charm Industrial and Vaulted Deep.

Bioenergy with Carbon Capture and Storage (BECCS) is a process where energy is made from biomass (like plants), and the carbon dioxide produced during this process is captured and stored.

Biochar is made by heating biomass in a low-oxygen environment, a process called pyrolysis. Biochar is a type of charcoal used in farming that helps store carbon in soil. A study by the UK Biochar Research Center says biochar could store 1 gigaton of carbon per year. With more use, it could store 5–9 gigatons annually. However, biochar storage is limited by how much carbon soil can hold and needs rules to prevent carbon from escaping.

Direct Air Capture (DAC) uses chemical or physical methods to remove carbon dioxide directly from the air. If the captured carbon is stored safely for a long time, the process is called Direct Air Carbon Capture and Sequestration (DACCS), which removes carbon dioxide from the atmosphere. These systems are also called negative emissions technologies (NET).

Some methods remove carbon from the ocean by adding nutrients to the water, a process called ocean fertilization. This could store carbon for 10–100 years. However, this process might increase acidity in deep ocean water. A 2021 report says ocean fertilization could be efficient and scalable at a low cost, but it has medium environmental risks. It could store 0.1 to 1 gigaton of carbon dioxide per year at a cost of $8 to $80 per ton.

Ocean alkalinity enhancement involves grinding minerals like olivine or limestone, spreading them in the ocean, and dissolving them to increase the ocean’s ability to store carbon. This method is still being studied.

Costs and economics

The cost of carbon dioxide removal (CDR) varies greatly based on the technology used and the economics of carbon removal markets. For example, pyrolysis of biomass creates biochar, which can be used for soil improvement and wastewater treatment. Direct air capture (DAC) costs between $94 and $600 per tonne, biochar costs between $200 and $584 per tonne, and nature-based solutions like reforestation cost less than $50 per tonne. Biochar is more valuable in carbon markets because it stores carbon for hundreds or thousands of years, unlike nature-based solutions, which face risks such as forest fires, pests, and changes in land use.

Some CDR technologies have different advantages. While nature-based solutions are inexpensive, a DAC plant capturing 1 million tons of CO2 annually requires 0.4–1.5 km² (99–371 acres) of land, which is equivalent to the CO2 capture rate of about 46 million trees, needing 3,098–4,647 km² (765,494–1,148,241 acres) of land. The Oxford Principles for Net Zero Aligned Carbon Offsetting state that organizations must increase the use of carbon removal offsets over time, aiming to rely solely on carbon removal by mid-century. New standards, such as the Puro Standard, will help grow the carbon removal market.

As of 2021, CDR was not included in the EU Allowance, but the European Commission is working on certification for carbon removal and exploring carbon contracts for difference. CDR may also be added to the UK Emissions Trading Scheme in the future. Carbon prices in these cap-and-trade systems, which focus on carbon reductions rather than removals, remained below $100 as of late 2021. After many countries set net-zero goals, CDR has become more important in emerging economies like Brazil, China, and India.

By early 2023, funding for high-tech CDR methods was still far below what is needed to make a major impact on climate change. However, private sector funding has grown significantly, such as a $1 billion fund launched in April 2022 by Stripe, with support from companies like Meta, Google, and Shopify. This fund was described as about 30 times larger than the carbon removal market in 2021 but still 1,000 times smaller than what will be needed by 2050. Concerns remain about relying on voluntary markets, which have historically been much smaller than those driven by government policies.

In 2023, several governments, including Sweden, Switzerland, and the United States, increased their support for CDR. The U.S. government, for example, announced plans to fund the Bipartisan Infrastructure Law’s $3.5 billion CDR program and passed the Inflation Reduction Act of 2022, which includes the 45Q tax credit to boost the CDR market.

Removal of other greenhouse gases

Some researchers have suggested ways to remove methane. However, others believe that studying nitrous oxide might be more important because it remains in the atmosphere for a longer time.