Carbon dioxide removal

This article is about removing carbon dioxide from the atmosphere. For technologies that remove carbon dioxide from point sources, see Carbon capture and storage.

Planting trees is a means of carbon dioxide removal.

Carbon dioxide removal (CDR), also known as negative CO₂ emissions, is a process in which carbon dioxide gas (CO₂) is removed from the atmosphere and sequestered for long periods of time. Similarly, greenhouse gas removal (GGR) or negative greenhouse gas emissions is the removal of greenhouse gases (GHGs) from the atmosphere by deliberate human activities, i.e., in addition to the removal that would occur via natural carbon cycle or atmospheric chemistry processes. In the context of net zero greenhouse gas emissions targets, CDR is increasingly integrated into climate policy, as a new element of mitigation strategies. CDR and GGR methods are also known as negative emissions technologies, (NET) and may be cheaper than preventing some agricultural greenhouse gas emissions.

CDR methods include afforestation, agricultural practices that sequester carbon in soils, bio-energy with carbon capture and storage, ocean fertilization, 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.

A 2019 consensus report by the US National Academies of Sciences, Engineering, and Medicine concluded that using existing CDR methods at scales that can be safely and economically deployed, there is potential to remove and sequester up to 10 gigatons of carbon dioxide per year. This would offset greenhouse gas emissions at about a fifth of the rate at which they are being produced.

In 2021 the IPCC said that emission pathways that limit globally averaged warming to 1.5 °C or 2 °C by the year 2100 assume the use of CDR approaches in combination with emission reductions.

Definitions

The Intergovernmental Panel on Climate Change defines CDR as:

Anthropogenic activities removing CO₂ 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 CO₂ uptake not directly caused by human activities.

The U.S.-based National Academies of Sciences, Engineering, and Medicine (NASEM) uses the term "negative emissions technology" with a similar definition.

The concept of deliberately reducing the amount of CO₂ in the atmosphere is often mistakenly classified with solar radiation management as a form of climate engineering and assumed to be intrinsically risky. In fact, CDR addresses the root cause of climate change and is part of strategies to reduce net emissions.

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 CO₂ in a concentrated stream. The CO₂ 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

Using CDR in parallel with other efforts to reduce greenhouse gas emissions, such as deploying renewable energy, is likely to be less expensive and disruptive than using other efforts alone. 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 CO₂ per year if fully deployed worldwide. This is one-fifth of the 50 gigatons of CO₂ emitted per year by human activities. In the IPCC's 2018 analysis of ways to limit climate change, 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.

The IPCC's 2018 report said that reliance on large-scale deployment of CDR would be 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. 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.

Carbon sequestration

Main article: Carbon sequestration

Forests, kelp beds, and other forms of plant life absorb carbon dioxide from the air as they grow, and bind it into biomass. As the use of plants as carbon sinks can be undone by events such as wildfires, the long-term reliability of these approaches has been questioned.

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 (mineral sequestration). This is because they are removing carbon from the atmosphere and sequestering it indefinitely and presumably for a considerable duration (thousands to millions of years).

Methods

Afforestation, reforestation, and forestry management

See also: Deforestation and climate change and Reforestation

According to the International Union for Conservation of Nature: "Halting the loss and degradation of natural systems and promoting their restoration have the potential to contribute over one-third of the total climate change mitigation scientists say is required by 2030."

Forests are vital for human society, animals and plant species. This is because trees keep our air clean, regulate the local climate and provide a habitat for numerous species. Trees and plants convert carbon dioxide back into oxygen, using photosynthesis. They are important for regulating CO₂ levels in the air, as they remove and store carbon from the air. Without them, the atmosphere would heat up quickly and destabilise the climate.

Biosequestration

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 photosynthesis via land-use practices such as reforestation, sustainable forest management, and genetic engineering. The SALK Harnessing Plants Initiative led by Joanne Chory is an example of an enhanced photosynthesis initiative Carbon sequestration through biological processes affects the global carbon cycle.

Agricultural practices

See also: Climate change and agriculture § Impact of agriculture on climate change

This section is an excerpt from Carbon farming.[edit]

Measuring soil respiration on agricultural land. Carbon farming enhances carbon sequestration in the soil.

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. 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 and reduce fertilizer use. Sustainable forest management is another tool that is used in carbon farming. Carbon farming is one component of climate-smart agriculture. It is also one way to remove carbon dioxide from the atmisphere.

Agricultural methods for carbon farming include adjusting how tillage and livestock grazing is done, using organic mulch or compost, working with biochar and terra preta, and changing the crop types. Methods used in forestry include reforestation and 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. 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. 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. It is important to maximize environmental benefits of carbon farming by keeping in mind ecosystem services at the same time.

Wetland restoration

This section is an excerpt from Blue carbon.[edit]

Ways one blue carbon habitat can influence carbon processing in an adjacent blue carbon habitat

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". Most commonly, it refers to the role that tidal marshes, mangroves and seagrasses can play in carbon sequestration. 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.

The methods for blue carbon management fall into the category of "ocean-based biological carbon dioxide removal (CDR) methods". They are a type of biological carbon fixation.

Scientists are looking for ways to further develop the blue carbon potential of ecosystems. However, the long-term effectiveness of blue carbon as a carbon dioxide removal solution is under debate.

The term deep blue carbon is also in use and refers to storing carbon in the deep ocean waters.

Bioenergy with carbon capture & storage

This section is an excerpt from Bioenergy with carbon capture and storage.[edit]

Diagram-of-Bioenergie power plant with carbon capture and storage (cropped).jpg (description page)

Bioenergy with carbon capture and storage (BECCS) is the process of extracting bioenergy from biomass and capturing and storing the carbon, thereby removing it from the atmosphere. BECCS can theoretically be a "negative emissions technology" (NET), although its deployment at the scale considered by many governments and industries can "also pose major economic, technological, and social feasibility challenges; threaten food security and human rights; and risk overstepping multiple planetary boundaries, with potentially irreversible consequences". The carbon in the biomass comes from the greenhouse gas carbon dioxide (CO₂) which is extracted from the atmosphere by the biomass when it grows. Energy ("bioenergy") is extracted in useful forms (electricity, heat, biofuels, etc.) as the biomass is utilized through combustion, fermentation, pyrolysis or other conversion methods.

Some of the carbon in the biomass is converted to CO₂ or biochar which can then be stored by geologic sequestration or land application, respectively, enabling carbon dioxide removal (CDR).

The potential range of negative emissions from BECCS was estimated to be zero to 22 gigatonnes per year. As of 2019, five facilities around the world were actively using BECCS technologies and were capturing approximately 1.5 million tonnes per year of CO₂. Wide deployment of BECCS is constrained by cost and availability of biomass.

Biochar

Main article: Biochar

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.

Enhanced weathering

Main article: Enhanced weathering

Enhanced weathering is a chemical approach to remove carbon dioxide involving land- or ocean-based techniques. One example of a land-based enhanced weathering technique is in-situ carbonation of silicates. Ultramafic rock, for example, has the potential to store from hundreds to thousands of years' worth of CO₂ emissions, according to estimates. Ocean-based techniques involve alkalinity enhancement, such as grinding, dispersing, and dissolving olivine, limestone, silicates, or calcium hydroxide to address ocean acidification and CO₂ sequestration. One example of a research project on the feasibility of enhanced weathering is the CarbFix project in Iceland.

Direct air capture

This section is an excerpt from Direct air capture.[edit]

Flow diagram of direct air capture process using sodium hydroxide as the absorbent and including solvent regeneration

An example of what Direct Air Capture could look like and how the process works.

Direct air capture (DAC) is the use of chemical or physical processes to extract carbon dioxide directly from the ambient air. If the extracted CO₂ is then sequestered in safe long-term storage (called direct air carbon capture and sequestration (DACCS)), the overall process will achieve carbon dioxide removal and be a "negative emissions technology" (NET).

The carbon dioxide (CO₂) is captured directly from the ambient air; this is contrast to carbon capture and storage (CCS) which captures CO₂ from point sources, such as a cement factory or a bioenergy plant. After the capture, DAC generates a concentrated stream of CO₂ for sequestration or utilization or production of carbon-neutral fuel. Carbon dioxide removal is achieved when ambient air makes contact with chemical media, typically an aqueous alkaline solvent or sorbents. These chemical media are subsequently stripped of CO₂ through the application of energy (namely heat), resulting in a CO₂ stream that can undergo dehydration and compression, while simultaneously regenerating the chemical media for reuse.

When combined with long-term storage of CO₂, DAC is known as direct air carbon capture and storage (DACCS or DACS). It would require sustainable energy to power since approximately 400kJ of energy is needed per mole of CO₂ capture. DACCS can act as a carbon dioxide removal mechanism (or a carbon negative technology), although as of 2023 it has yet to be integrated into emissions trading because, at over US$1000, the cost per tonne of carbon dioxide is many times the carbon price on those markets.

DAC was suggested in 1999 and is still in development. Several commercial plants are planned or in operation in Europe and the US. Large-scale DAC deployment may be accelerated when connected with economical applications or policy incentives.

In contrast to carbon capture and storage (CCS) which captures emissions from a point source such as a factory, DAC reduces the carbon dioxide concentration in the atmosphere as a whole. Thus, CCS is recommended for large and stationary sources of CO₂ rather than distributed and movable ones. On the contrary, DAC has no limitation on sources.

Ocean fertilization

This section is an excerpt from Ocean fertilization.[edit]

CO
₂ sequestration in the ocean

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. 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.

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 yearsdependent 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. 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.

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.

Issues

Economic issues

Further information: Economics of climate change mitigation and Economics of climate change

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.

The value of BECCS and CDR generally in integrated assessment models in the long term is highly dependent on the discount rate. In 2021, businessman Elon Musk announced he was donating $100m for a prize for best carbon capture technology.

In late 2021 the EU Allowance was trading around 80 euros - but the cost of DAC was not projected to fall below the EUA for some years. Although not covered by the EUA as of 2021, the European Commission is preparing for carbon removal certification and considering carbon contracts for difference.

Other issues

CDR faces issues common to all forms of climate engineering, including moral hazard.

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.

See also

• Carbon dioxide scrubber
• Carbon-neutral fuel
• Climate change mitigation scenarios
• Climate engineering
• List of emerging technologies
• Lithium peroxide
• Low-carbon economy
• Virgin Earth Challenge

Bibliography

• IPCC, 2018: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [V. Masson-Delmotte, P. Zhai, H. O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J. B. R. Matthews, Y. Chen, X. Zhou, M. I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, T. Waterfield (eds.)].
  • Report website, chapters I–V
  • Summary for policymakers, 32 pp.
  • Headline statements, 2 pp.
  • Technical summary, 22 pp.
  • FAQ, 24 pp.
  • Glossary, 24 pp.

External links

• Deep Dives by Carbon180. Info about carbon removal solutions.

• Krupp, Fred; Keohane, Nathaniel; Pooley, Eric (September 25, 2019). "Less Than Zero; Can Carbon-Removal Technologies Curb Climate Change?". Foreign Affairs (March/April 2019): 142–152.
• The Road to Ten Gigatons - Carbon Removal Scale Up Challenge Game.