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Carbon dioxide removal (CDR), also known as greenhouse gas removal (GGR) or negative emissions, is a process in which carbon dioxide gas  is removed from the atmosphere by deliberate human activities and durably stored in geological, terrestrial, or ocean reservoirs, or in products. CDR methods are also known as negative emissions technologies. In the context of net zero greenhouse gas emissions targets, CDR is increasingly integrated into climate policy, as a new element of climate change mitigation strategies. Use of CDR can be less expensive than reducing some sources of emissions.

CDR methods include afforestation, agricultural practices that sequester carbon in soils (carbon farming), bioenergy with carbon capture and storage (BECCS), enhanced weathering, and direct air capture when combined with storage. To assess whether net negative emissions are achieved by a particular process, comprehensive life cycle analysis of the process must be performed.

There is potential to remove and sequester up to 10 gigatons of carbon dioxide per year by using those existing CDR methods which can be safely and economically deployed now. This would offset greenhouse gas emissions at about a fifth of the rate at which they are being produced (as of 2018).

All emission pathways that limit global warming to 1.5 °C or 2 °C by the year 2100 assume the use of CDR approaches in combination with emission reductions.

Definitions
Carbon dioxide removal (CDR) is defined by the IPCC as:

"Anthropogenic activities removing from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage, but excludes natural uptake not directly caused by human activities."

Synonyms for CDR include greenhouse gas removal (GGR), negative emissions technology,  and carbon removal. Technologies have been proposed for removing non- greenhouse gases such as methane from the atmosphere, but only carbon dioxide is currently feasible to remove at scale. Therefore in most contexts, greenhouse gas removal means carbon dioxide removal.

The term geoengineering (or climate engineering) is sometimes used in the scientific literature for both CDR or SRM (solar radiation management), if the techniques are used at a global scale. The terms geoengineering or climate engineering are no longer used in IPCC reports.

Categories
CDR methods can be placed in different categories that are based on different criteria:


 * Role in the carbon cycle (land-based biological; ocean-based biological; geochemical; chemical); or
 * Timescale of storage (decades to centuries; centuries to millennia; thousand years or longer)

Concepts using similar terminology
CDR can be confused with carbon capture and storage (CCS), a process in which carbon dioxide is collected from point-sources such as gas-fired power plants, whose smokestacks emit in a concentrated stream. The is then compressed and sequestered or utilized. When used to sequester the carbon from a gas-fired power plant, CCS reduces emissions from continued use of the point source, but does not reduce the amount of carbon dioxide already in the atmosphere.

Potential for climate change mitigation
Use of CDR reduces the overall rate at which humans are adding carbon dioxide to the atmosphere. All emission pathways that limit global warming to 1.5 °C or 2 °C by the year 2100 assume the use of CDR approaches in combination with emission reductions. CDR can be used to address certain types of emissions that are technically difficult to eliminate, such as nitrous oxide emissions from agriculture. Offsetting these types of emissions through CDR may be less expensive than eliminating them directly.

The Earth’s surface temperature will stabilize only after global emissions reach net zero, which will require both aggressive efforts to reduce emissions and deployment of CDR. After the warming trend stops, CDR could be used to reduce atmospheric CO2 concentrations, partially reversing the warming.

The possibility of large-scale future CDR deployment has been described as a moral hazard, as it could lead to a reduction in near-term efforts to mitigate climate change. The 2019 NASEM report concludes:

"Any argument to delay mitigation efforts because NETs will provide a backstop drastically misrepresents their current capacities and the likely pace of research progress."

Feasibility
A 2019 consensus study report by NASEM assessed the potential of all forms of CDR other than ocean fertilization that could be deployed safely and economically using current technologies, and estimated that they could remove up to 10 gigatons of per year if fully deployed worldwide. This is one-fifth of the 50 gigatons of emitted per year by human activities. In 2018, all analyzed mitigation pathways that would prevent more than 1.5 °C of warming included CDR measures.

Some mitigation pathways propose achieving higher rates of CDR through massive deployment of one technology, however these pathways assume that hundreds of millions of hectares of cropland are converted to growing biofuel crops. Further research in the areas of direct air capture, geologic sequestration of carbon dioxide, and carbon mineralization could potentially yield technological advancements that make higher rates of CDR economically feasible.

Reliance on large-scale deployment of CDR was regarded in 2018 as a "major risk" to achieving the goal of less than 1.5 °C of warming, given the uncertainties in how quickly CDR can be deployed at scale. Strategies for mitigating climate change that rely less on CDR and more on sustainable use of energy carry less of this risk.

Public perception
When CDR is framed as a form of climate engineering, people tend to view it as intrinsically risky. In fact, CDR addresses the root cause of climate change and is part of strategies to reduce net emissions and manage risks related to elevated atmospheric CO2 levels.

Overview listing based on technology readiness level
The following is a list of known CDR methods in the order of their technology readiness level. The ones at the top have a high TDR of 8 to 9 (9 being the maximum possible value, meaning the technology is proven), the ones at the bottom have a low TDR of 1 to 2, meaning the technology is not proven or only validated at laboratory scale.

The CDR methods with the greatest potential to contribute to climate change mitigation efforts as per illustrative mitigation pathways are the land-based biological CDR methods (primarily afforestation/reforestation (A/R)) and/or bioenergy with carbon capture  and storage (BECCS). Some of the pathways also include direct air capture and storage (DACCS).
 * 1) Afforestation/ reforestation
 * 2) Soil carbon sequestration in croplands and grasslands
 * 3) Peatland and coastal wetland restoration
 * 4) Agroforestry, improved forest management
 * 5) Biochar
 * 6) Direct air carbon capture and storage (DACCS), bioenergy with carbon capture and storage (BECCS)
 * 7) Enhanced weathering (EW)
 * 8) ‘Blue carbon management’ in coastal wetlands (restoration of vegetated coastal ecosystems; an ocean-based biological CDR method which encompasses mangroves, salt marshes and seagrass beds)
 * 9) Ocean fertilisation, ocean alkalinity enhancement

Afforestation, reforestation, and forestry management
Trees use photosynthesis to absorb carbon dioxide and store the carbon in wood and soils. Afforestation is the establishment of a forest in an area where there was previously no forest. Reforestation is the re-establishment of a forest that has been previously cleared. Forests are vital for human society, animals and plant species. This is because trees keep air clean, regulate the local climate and provide a habitat for numerous species.

As trees grow they absorb CO2 from the atmosphere and store it in living biomass, dead organic matter and soils. Afforestation and reforestation – sometimes referred to collectively as ‘forestation’ – facilitate this process of carbon removal by establishing or re-establishing forest areas. Once a forest reaches maturity the net uptake of CO2 slows, though additional gains can be made through forest management, such as by optimising thinning and improved rotation. Once mature, forest products can be harvested and the biomass stored in long-lived wood products, or used for bioenergy or biochar. Consequent forest regrowth then allows continuing CO2 removal.

Biochar Carbon Removal (BCR)
Biochar is created by the pyrolysis of biomass, and is under investigation as a method of carbon sequestration. Biochar is a charcoal that is used for agricultural purposes which also aids in carbon sequestration, the capture or hold of carbon. It is created using a process called pyrolysis, which is basically the act of high temperature heating biomass in an environment with low oxygen levels. What remains is a material known as char, similar to charcoal but is made through a sustainable process, thus the use of biomass. Biomass is organic matter produced by living organisms or recently living organisms, most commonly plants or plant based material. A study done by the UK Biochar Research Center has stated that, on a conservative level, biochar can store 1 gigaton of carbon per year. With greater effort in marketing and acceptance of biochar, the benefit could be the storage of 5–9 gigatons per year of carbon in biochar soils.

Magnesium silicate/oxide in cement
The replacement of carbonate in cement allows for the potential absorption of carbon dioxide over concrete lifecycle. However, lifecycle amounts are not yet fully understood.

Economic issues
The cost of CDR differs substantially depending on the maturity of the technology employed as well as the economics of both voluntary carbon removal markets and the physical output; for example, the pyrolysis of biomass produces biochar that has various commercial applications, including soil regeneration and wastewater treatment. In 2021 DAC cost from $250 to $600 per ton, compared to $100 for biochar and less than $50 for nature-based solutions, such as reforestation and afforestation. The fact that biochar commands a higher price in the carbon removal market than nature-based solutions reflects the fact that it is a more durable sink with carbon being sequestered for hundreds or even thousands of years while nature-based solutions represent a more volatile form of storage, which risks related to forest fires, pests, economic pressures and changing political priorities. The Oxford Principles for Net Zero Aligned Carbon Offsetting states that to be compatible with the Paris Agreement: "...organizations must commit to gradually increase the percentage of carbon removal offsets they procure with the view of exclusively sourcing carbon removals by mid-century." These initiatives along with the development of new industry standards for engineered carbon removal, such as the Puro Standard, will help to support the growth of the carbon removal market.

Forests can be used to create carbon credits, often involving the use of geospatial analytical systems to calculate carbon offsets by conserving a forest area or a reforestation initiative. REDD+ is an example of a carbon credit initiative. Individuals and businesses can purchase carbon credits through verified retailers such as ACT4

Although CDR is not covered by the EU Allowance as of 2021, the European Commission is preparing for carbon removal certification and considering carbon contracts for difference. CDR might also in future be added to the UK Emissions Trading Scheme. As of end 2021 carbon prices for both these cap-and-trade schemes currently based on carbon reductions, as opposed to carbon removals, remained below $100.

As of early 2023, financing has fell short of the sums required for CDR to contribute significantly to climate change mitigation, though available funds have recently increased substantially. Most of this increase has been from voluntary private sector initiatives. Such as a private sector alliance led by Stripe with prominent members including Meta, Google and Shopify, which in April 2022 revealed a nearly $1 billion fund to reward companies able to permanently capture & store carbon. According to senior Stripe employee Nan Ransohoff, the fund was "roughly 30 times the carbon-removal market that existed in 2021. But it’s still 1,000 times short of the market we need by 2050." The predominance of private sector funding has raised concerns as historically, voluntary markets have proved "orders of magnitude" smaller than those brought about by government policy. As of 2023 however, various governments have increased their support for CDR; these include Sweden, Switzerland, and the US. Recent activity from the US government includes the June 2022 Notice of Intent to fund the Bipartisan Infrastructure Law's $3.5 billion CDR program, and the signing into law of the Inflation Reduction Act of 2022, which contains the 45Q tax to enhance the CDR market.

Removal of other greenhouse gases
Although some researchers have suggested methods for removing methane, others say that nitrous oxide would be a better subject for research due to its longer lifetime in the atmosphere.