![Tree planting closeup](https://upload.wikimedia.org/wikipedia/commons/4/46/Tree_planting_closeup.jpg)
Carbon dioxide removal (CDR), also known as negative emissions, is a process in which
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
gas () 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.
[IPCC, 2021]
Annex VII: Glossary
atthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C. Méndez, S. Semenov, A. Reisinger (eds.) I
Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
[Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022. In the context of net zero
greenhouse gas emissions
Greenhouse gas emissions from human activities strengthen the greenhouse effect, contributing to climate change. Most is carbon dioxide from burning fossil fuels: coal, oil, and natural gas. The largest emitters include coal in China and ...
targets, CDR is increasingly integrated into climate policy, as a new element of
mitigation
Mitigation is the reduction of something harmful or the reduction of its harmful effects. It may refer to measures taken to reduce the harmful effects of hazards that remain ''in potentia'', or to manage harmful incidents that have already occur ...
strategies. CDR and GGR methods are also known as negative emissions technologies (NET), and may be cheaper than preventing some agricultural
greenhouse gas emissions
Greenhouse gas emissions from human activities strengthen the greenhouse effect, contributing to climate change. Most is carbon dioxide from burning fossil fuels: coal, oil, and natural gas. The largest emitters include coal in China and ...
.
CDR methods include
afforestation, agricultural practices that sequester carbon in soils,
bio-energy with carbon capture and storage
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. The carbon in the biomass comes from the greenhouse gas carbon ...
,
ocean fertilization
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 dioxid ...
,
enhanced weathering
Enhanced weathering is a process that aims to accelerate the natural weathering by spreading finely ground silicate rock, such as basalt, onto surfaces which speeds up chemical reactions between rocks, water, and air. It also removes carbon dioxi ...
, and
direct air capture
Direct air capture (DAC) is a process of capturing carbon dioxide () directly from the ambient air (as opposed to capturing from point sources, such as a cement factory or biomass power plant) and generating a concentrated stream of for seque ...
when combined with storage.
To assess whether net negative emissions are achieved by a particular process, comprehensive
life cycle analysis
Life cycle assessment or LCA (also known as life cycle analysis) is a methodology for assessing environmental impacts associated with all the stages of the life cycle of a commercial product, process, or service. For instance, in the case ...
of the process must be performed.
A 2019 consensus report by the US
National Academies of Sciences, Engineering, and Medicine (NASEM) 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.
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 as:
The same definition is commonly used for "net negative greenhouse gas emissions", "net zero CO2 emissions" and "net zero greenhouse gas emissions".
The concept of deliberately reducing the amount of in the atmosphere is often mistakenly grouped with
solar radiation management under the umbrella term "
climate engineering
Climate engineering (also called geoengineering) is a term used for both carbon dioxide removal (CDR) and solar radiation management (SRM), also called solar geoengineering, when applied at a planetary scale.IPCC (2022Chapter 1: Introduction and ...
." 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 CO
2 levels.
Concepts using similar terminology
CDR can be confused with
carbon capture and storage
Carbon capture and storage (CCS) or carbon capture and sequestration is the process of capturing carbon dioxide (CO2) before it enters the atmosphere, transporting it, and storing it (carbon sequestration) for centuries or millennia. Usually th ...
(CCS), a process in which carbon dioxide is collected from point-sources such as
gas-fired power plant
A gas-fired power plant or gas-fired power station or natural gas power plant is a thermal power station which burns natural gas to generate electricity. Natural gas power stations generate almost a quarter of world electricity and a signifi ...
s, 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
The likely need for CDR (carbon dioxide removal) as an element of
climate change mitigation has been publicly expressed by a range of individuals and organizations involved with
climate change
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...
issues, including
IPCC
The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations. Its job is to advance scientific knowledge about climate change caused by human activities. The World Meteorological Organization (WMO) ...
chief Dr.
Hoesung Lee
Hoesung Lee (born December 31, 1945) is a South Korean economist and current chair of the Intergovernmental Panel on Climate Change. He is professor in the economics of climate change, energy and sustainable development in thGraduate School of ...
, the
UNFCCC
The United Nations Framework Convention on Climate Change (UNFCCC) established an international environmental treaty to combat "dangerous human interference with the climate system", in part by stabilizing greenhouse gas concentrations in th ...
executive secretary
Christiana Figueres, and the
World Watch Institute
The Worldwatch Institute was a globally focused environmental research organization based in Washington, D.C., founded by Lester R. Brown. Worldwatch was named as one of the top ten sustainable development research organizations by Globescan Sur ...
. Institutions with major programs focusing on CDR include the Lenfest Center for Sustainable Energy at the
Earth Institute, Columbia University, and the Climate Decision Making Center,
an international collaboration operated out of Carnegie-Mellon University's Department of Engineering and Public Policy.
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
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 dioxid ...
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
Direct air capture (DAC) is a process of capturing carbon dioxide () directly from the ambient air (as opposed to capturing from point sources, such as a cement factory or biomass power plant) and generating a concentrated stream of for seque ...
,
geologic sequestration of carbon dioxide
Carbon sequestration is the process of storing carbon in a carbon pool. Carbon dioxide () is naturally captured from the atmosphere through biological, chemical, and physical processes. These changes can be accelerated through changes in lan ...
, 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.
The possibility of large-scale future CDR deployment has been described as a
moral hazard
In economics, a moral hazard is a situation where an economic actor has an incentive to increase its exposure to risk because it does not bear the full costs of that risk. For example, when a corporation is insured, it may take on higher risk ...
, as it could lead to a reduction in near-term efforts to mitigate climate change.
The 2019 NASEM report concludes:
Methods
Afforestation, reforestation, and forestry management
According to the
International Union for Conservation of Nature
The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of nat ...
: "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 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 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.
Increased use of wood in construction is being considered.
Carbon sequestration
Biosequestration
Biosequestration
Biosequestration or biological sequestration 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 photosyn ...
is the capture and storage of the atmospheric
greenhouse gas carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
by continual or enhanced biological processes. This form of
carbon sequestration
Carbon sequestration is the process of storing carbon in a carbon pool. Carbon dioxide () is naturally captured from the atmosphere through biological, chemical, and physical processes. These changes can be accelerated through changes in lan ...
occurs through increased rates of
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
via
land-use
Land use involves the management and modification of natural environment or wilderness into built environment such as Human settlement, settlements and semi-natural habitats such as Arable land, arable fields, pastures, and managed Woodland, woo ...
practices such as
reforestation
Reforestation (occasionally, reafforestation) is the natural or intentional restocking of existing forests and woodlands (forestation) that have been depleted, usually through deforestation, but also after clearcutting.
Management
A debat ...
,
sustainable forest management Forest management is a branch of forestry concerned with overall administrative, legal, economic, and social aspects, as well as scientific and technical aspects, such as silviculture, protection, and forest regulation. This includes management fo ...
, and
genetic engineering. The
SALK Harnessing Plants Initiative led by
Joanne Chory
Joanne Chory is an American plant biologist and geneticist. Chory is a professor and director of the Plant Molecular and Cellular Biology Laboratory, at the Salk Institute for Biological Studies and an investigator of the Howard Hughes Medical I ...
is an example of an
enhanced photosynthesis initiative
Carbon sequestration through
biological processes
Biological processes are those processes that are vital for an organism to live, and that shape its capacities for interacting with its environment. Biological processes are made of many chemical reactions or other events that are involved in the ...
affects the global
carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major componen ...
.
Agricultural practices
Wetland restoration
Bioenergy with carbon capture & storage
Biochar
Biochar
Biochar is the lightweight black residue, made of carbon and
ashes, remaining after the pyrolysis of biomass. Biochar is defined by the International Biochar Initiative as "the solid material obtained from the thermochemical conversion of ...
is created by the
pyrolysis
The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...
of
biomass, and is under investigation as a method of
carbon sequestration
Carbon sequestration is the process of storing carbon in a carbon pool. Carbon dioxide () is naturally captured from the atmosphere through biological, chemical, and physical processes. These changes can be accelerated through changes in lan ...
.
Biochar is a charcoal that is used for agricultural purposes which also aids in
carbon sequestration
Carbon sequestration is the process of storing carbon in a carbon pool. Carbon dioxide () is naturally captured from the atmosphere through biological, chemical, and physical processes. These changes can be accelerated through changes in lan ...
, 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.
Others
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
Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...
, for example, has the potential to store from hundreds to thousands of years' worth of CO
2 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 CO
2 sequestration. One example of a research project on the feasibility of enhanced weathering is the
CarbFix project in Iceland.
Direct air capture with carbon sequestration
Direct air capture (DAC) is the use of chemical or physical processes to extract directly from the ambient air. If the extracted is then
sequestered in safe long-term storage, the overall process will achieve carbon dioxide removal. As of 2002, it has yet to be profitable because the cost of using DAC to sequester carbon dioxide is several times the
carbon price
Carbon pricing (or pricing), also known as cap and trade (CAT) or emissions trading scheme (ETS), is a method for nations to reduce global warming. The cost is applied to greenhouse gas emissions in order to encourage polluters to reduce the co ...
.
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.
Issues
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
Reforestation (occasionally, reafforestation) is the natural or intentional restocking of existing forests and woodlands (forestation) that have been depleted, usually through deforestation, but also after clearcutting.
Management
A debat ...
initiative.
REDD+ is an example of a carbon credit initiative. Individuals and businesses can purchase carbon credits through verified retailers such a
ACT4
In 2021, businessman
Elon Musk
Elon Reeve Musk ( ; born June 28, 1971) is a business magnate and investor. He is the founder, CEO and chief engineer of SpaceX; angel investor, CEO and product architect of Tesla, Inc.; owner and CEO of Twitter, Inc.; founder of The ...
announced he was donating $100m for a prize for best carbon capture technology.
Although CDR is not covered by the
EU Allowance
EU Allowances (EUA) are climate credits (or carbon credits) used in the European Union Emissions Trading Scheme (EU ETS). EU Allowances are issued by the EU Member States into Member State Registry accounts. By April 30 of each year, operators of ...
as of 2021, the
European Commission
The European Commission (EC) is the executive of the European Union (EU). It operates as a cabinet government, with 27 members of the Commission (informally known as "Commissioners") headed by a President. It includes an administrative body ...
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 The UK Emissions Trading Scheme (UK ETS) is the carbon emission trading scheme of the United Kingdom. It is cap and trade and came into operation on 1 January 2021 following the UK's departure from the European Union. The cap is reduced in line wit ...
.
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.
In April 2022, a private sector alliance led by
Stripe with prominent members including
Meta,
Google
Google LLC () is an American Multinational corporation, multinational technology company focusing on Search Engine, search engine technology, online advertising, cloud computing, software, computer software, quantum computing, e-commerce, ar ...
and
Shopify
Shopify Inc. is a Canadian multinational e-commerce company headquartered in Ottawa, Ontario. Shopify is the name of its proprietary e-commerce platform for online stores and retail point-of-sale systems. The Shopify platform offers online ret ...
, revealed a nearly $1 billion dollar fund to reward companies able to permanently capture & store carbon. According to senior Stripe employee Nan Ransohoff, the new fund "is 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."
According to the National Academies of Sciences, Engineering and Medicine, some of these "negative-emission technologies" are already being used on a large scale. Congress passed the 45Q tax, which gives companies a $50 credit for every ton of carbon dioxide they fix and store. So the study proposes some CO2 fixation technologies that cost between $20 and $100 per ton.
Removal of other greenhouse gases
Although some researchers have suggested methods for removing
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
, others say that
nitrous oxide
Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has ...
would be a better subject for research due to its longer lifetime in the atmosphere.
See also
*
Carbon dioxide scrubber
A carbon dioxide scrubber is a piece of equipment that absorbs carbon dioxide (CO2). It is used to treat exhaust gases from industrial plants or from exhaled air in life support systems such as rebreathers or in spacecraft, submersible craft or ...
*
Carbon-neutral fuel
Carbon-neutral fuel is fuel which produces no net-greenhouse gas emissions or carbon footprint. In practice, this usually means fuels that are made using carbon dioxide (CO2) as a feedstock. Proposed carbon-neutral fuels can broadly be grouped ...
*
Climate change scenario
Climate change scenarios or socioeconomic scenarios are projections of future greenhouse gas (GHG) emissions used by analysts to assess future vulnerability to climate change. Scenarios and pathways are created by scientists to survey any long ...
*
Climate engineering
Climate engineering (also called geoengineering) is a term used for both carbon dioxide removal (CDR) and solar radiation management (SRM), also called solar geoengineering, when applied at a planetary scale.IPCC (2022Chapter 1: Introduction and ...
*
List of emerging technologies
*
Lithium peroxide
Lithium peroxide is the inorganic compound with the formula Li2 O2. It is a white, nonhygroscopic solid. Because of its high oxygen:mass and oxygen:volume ratios, the solid has been used to remove CO2 from the atmosphere in spacecraft.
Preparat ...
*
Low-carbon economy
A low-carbon economy (LCE) or decarbonised economy is an economy based on energy sources that produce low levels of greenhouse gas (GHG) emissions. GHG emissions due to human activity are the dominant cause of observed climate change since the mi ...
*
Virgin Earth Challenge
References
Sources
*
**
*
External links
Deep Divesby
Carbon180. Info about carbon removal solutions.
*
The Road to Ten Gigatons - Carbon Removal Scale Up ChallengeGame.
{{Authority control
Climate engineering
Climate change policy
Carbon dioxide removal