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Ocean fertilization: we don't need this for two reasons: firstly it falls below "carbon sequestration" which is already mentioned above. Secondly it is not a promising pathway.
Carbon sequestration: these are all part of carbon sequestration so I've moved them down a level.
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{{excerpt|Carbon sequestration|paragraphs=1-2|file=no}}
{{excerpt|Carbon sequestration|paragraphs=1-2|file=no}}


=== Biosequestration ===
==== Biosequestration ====
{{Main|Biosequestration}}
{{Main|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|land-use]] practices such as [[reforestation]], [[Sustainability|sustainable]] [[forest management]], and [[genetic engineering]]. The [[Salk Institute for Biological Studies|SALK]] [[Harnessing Plants Initiative]] led by [[Joanne Chory]] is an example of an [[Biosequestration#Enhanced photosynthesis|enhanced photosynthesis]] initiative<ref name="Beerling">{{cite book|author=Beerling, David|title=The Emerald Planet: How Plants Changed Earth's History|publisher=Oxford University Press|year=2008|isbn=978-0-19-954814-9|pages=194–5|author-link=David Beerling}}</ref><ref>{{Cite book|last1=National Academies Of Sciences|first1=Engineering|title=Negative Emissions Technologies and Reliable Sequestration: A Research Agenda|publisher=National Academies of Sciences, Engineering, and Medicine|year=2019|isbn=978-0-309-48452-7|location=Washington, D.C.|pages=45–136|language=en|doi=10.17226/25259|pmid=31120708|s2cid=134196575}}</ref> Carbon sequestration through [[biological processes]] affects the global [[carbon cycle]].
[[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|land-use]] practices such as [[reforestation]], [[Sustainability|sustainable]] [[forest management]], and [[genetic engineering]]. The [[Salk Institute for Biological Studies|SALK]] [[Harnessing Plants Initiative]] led by [[Joanne Chory]] is an example of an [[Biosequestration#Enhanced photosynthesis|enhanced photosynthesis]] initiative<ref name="Beerling">{{cite book|author=Beerling, David|title=The Emerald Planet: How Plants Changed Earth's History|publisher=Oxford University Press|year=2008|isbn=978-0-19-954814-9|pages=194–5|author-link=David Beerling}}</ref><ref>{{Cite book|last1=National Academies Of Sciences|first1=Engineering|title=Negative Emissions Technologies and Reliable Sequestration: A Research Agenda|publisher=National Academies of Sciences, Engineering, and Medicine|year=2019|isbn=978-0-309-48452-7|location=Washington, D.C.|pages=45–136|language=en|doi=10.17226/25259|pmid=31120708|s2cid=134196575}}</ref> Carbon sequestration through [[biological processes]] affects the global [[carbon cycle]].


===Agricultural practices===
==== Agricultural practices ====
{{See also|Climate change and agriculture#Impact of agriculture on climate change}}
{{See also|Greenhouse gas emissions from agriculture}}
{{excerpt|Carbon farming}}
{{excerpt|Carbon farming}}


===Wetland restoration===
==== Wetland restoration ====
{{excerpt|blue carbon}}
{{excerpt|blue carbon}}


===Bioenergy with carbon capture & storage===
==== Bioenergy with carbon capture & storage ====
{{Excerpt|Bioenergy with carbon capture and storage}}
{{Excerpt|Bioenergy with carbon capture and storage}}


===Biochar===
==== Biochar ====
{{Main|Biochar}}
{{Main|Biochar}}


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[[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.<ref>{{cite web|title=What is biochar?|url=http://www.biochar.ac.uk/what_is_biochar.php|website=UK Biochar research center|publisher=University of Edinburgh Kings Buildings Edinburgh|access-date=25 April 2016|archive-date=October 1, 2019|archive-url=https://web.archive.org/web/20191001070549/https://www.biochar.ac.uk/what_is_biochar.php|url-status=live}}</ref> Biomass is organic matter produced by living organisms or recently living organisms, most commonly plants or plant based material.<ref>{{cite web|title=What is Biomass?|url=http://www.biomassenergycentre.org.uk/portal/page?_pageid=76,15049&_dad=portal|website=Biomass Energy Center|publisher=Direct.gov.uk|access-date=25 April 2016|archive-url=https://web.archive.org/web/20161003092000/http://www.biomassenergycentre.org.uk/portal/page?_pageid=76,15049&_dad=portal|archive-date=October 3, 2016|url-status=dead}}</ref> 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.<ref>{{cite web|url=http://www.ierm.ed.ac.uk/homes/sshackle/WP2.pdf|title=Biochar reducing and removing CO2 while improving soils: A significant sustainable response to climate change|website=UKBRC|publisher=UK Biochar research Center|access-date=25 April 2016|archive-date=November 5, 2016|archive-url=https://web.archive.org/web/20161105203806/http://www.ierm.ed.ac.uk/homes/sshackle/WP2.pdf|url-status=live}}</ref>{{Better source needed|date=June 2021}}
[[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.<ref>{{cite web|title=What is biochar?|url=http://www.biochar.ac.uk/what_is_biochar.php|website=UK Biochar research center|publisher=University of Edinburgh Kings Buildings Edinburgh|access-date=25 April 2016|archive-date=October 1, 2019|archive-url=https://web.archive.org/web/20191001070549/https://www.biochar.ac.uk/what_is_biochar.php|url-status=live}}</ref> Biomass is organic matter produced by living organisms or recently living organisms, most commonly plants or plant based material.<ref>{{cite web|title=What is Biomass?|url=http://www.biomassenergycentre.org.uk/portal/page?_pageid=76,15049&_dad=portal|website=Biomass Energy Center|publisher=Direct.gov.uk|access-date=25 April 2016|archive-url=https://web.archive.org/web/20161003092000/http://www.biomassenergycentre.org.uk/portal/page?_pageid=76,15049&_dad=portal|archive-date=October 3, 2016|url-status=dead}}</ref> 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.<ref>{{cite web|url=http://www.ierm.ed.ac.uk/homes/sshackle/WP2.pdf|title=Biochar reducing and removing CO2 while improving soils: A significant sustainable response to climate change|website=UKBRC|publisher=UK Biochar research Center|access-date=25 April 2016|archive-date=November 5, 2016|archive-url=https://web.archive.org/web/20161105203806/http://www.ierm.ed.ac.uk/homes/sshackle/WP2.pdf|url-status=live}}</ref>{{Better source needed|date=June 2021}}


===Enhanced weathering===
==== Enhanced weathering ====
{{Main|Enhanced weathering}}
{{Main|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 CO<sub>2</sub> emissions, according to estimates.<ref>{{Cite web|url=https://archive.nytimes.com/www.nytimes.com/gwire/2009/03/09/09greenwire-maps-show-rocks-ideal-for-sequestering-carbon-10041.html|title=Maps show rocks ideal for sequestering carbon|website=The New York Times|access-date=2018-05-15|archive-date=May 16, 2018|archive-url=https://web.archive.org/web/20180516103729/https://archive.nytimes.com/www.nytimes.com/gwire/2009/03/09/09greenwire-maps-show-rocks-ideal-for-sequestering-carbon-10041.html|url-status=live}}</ref><ref>{{Cite journal|last=U.S. Department of the Interior|title=Mapping the Mineral Resource Base for Mineral Carbon-Dioxide Sequestration in the Conterminous United States|url=https://pubs.usgs.gov/ds/414/downloads/DS414_text_508.pdf|journal=U.S. Geological Survey|volume=Data Series 414|access-date=May 15, 2018|archive-date=July 27, 2020|archive-url=https://web.archive.org/web/20200727095133/https://pubs.usgs.gov/ds/414/downloads/DS414_text_508.pdf|url-status=live}}</ref> Ocean-based techniques involve alkalinity enhancement, such as grinding, dispersing, and dissolving olivine, limestone, silicates, or calcium hydroxide to address ocean acidification and CO<sub>2</sub> sequestration.<ref>{{Cite web|date=2021-06-23|title=Cloud spraying and hurricane slaying: how ocean geoengineering became the frontier of the climate crisis|url=http://www.theguardian.com/environment/2021/jun/23/cloud-spraying-and-hurricane-slaying-could-geoengineering-fix-the-climate-crisis|access-date=2021-06-23|website=The Guardian|language=en|archive-date=June 23, 2021|archive-url=https://web.archive.org/web/20210623071321/https://www.theguardian.com/environment/2021/jun/23/cloud-spraying-and-hurricane-slaying-could-geoengineering-fix-the-climate-crisis|url-status=live}}</ref> One example of a research project on the feasibility of enhanced weathering is the [[CarbFix]] project in Iceland.<ref>{{Cite web|url=https://www.globalccsinstitute.com/projects/carbfix-sulfix-project|title=CarbFix Project {{!}} Global Carbon Capture and Storage Institute|website=www.globalccsinstitute.com|language=en|access-date=2018-05-15|archive-url=https://web.archive.org/web/20180703133804/https://www.globalccsinstitute.com/projects/carbfix-sulfix-project|archive-date=July 3, 2018|url-status=dead}}</ref><ref>{{Cite news|url=https://www.or.is/english/carbfix/carbfix-project|title=The CarbFix Project|date=2017-08-22|work=www.or.is|access-date=2018-05-15|language=is|archive-date=May 16, 2018|archive-url=https://web.archive.org/web/20180516174158/https://www.or.is/english/carbfix/carbfix-project|url-status=dead}}</ref><ref>{{Cite news|url=https://www.nytimes.com/2015/02/10/science/burying-a-mountain-of-co2.html|title=Turning Carbon Dioxide Into Rock, and Burying It|date=2015-02-09|work=The New York Times|access-date=2018-05-15|language=en-US|issn=0362-4331|archive-date=May 16, 2018|archive-url=https://web.archive.org/web/20180516103143/https://www.nytimes.com/2015/02/10/science/burying-a-mountain-of-co2.html|url-status=live}}</ref>
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 CO<sub>2</sub> emissions, according to estimates.<ref>{{Cite web|url=https://archive.nytimes.com/www.nytimes.com/gwire/2009/03/09/09greenwire-maps-show-rocks-ideal-for-sequestering-carbon-10041.html|title=Maps show rocks ideal for sequestering carbon|website=The New York Times|access-date=2018-05-15|archive-date=May 16, 2018|archive-url=https://web.archive.org/web/20180516103729/https://archive.nytimes.com/www.nytimes.com/gwire/2009/03/09/09greenwire-maps-show-rocks-ideal-for-sequestering-carbon-10041.html|url-status=live}}</ref><ref>{{Cite journal|last=U.S. Department of the Interior|title=Mapping the Mineral Resource Base for Mineral Carbon-Dioxide Sequestration in the Conterminous United States|url=https://pubs.usgs.gov/ds/414/downloads/DS414_text_508.pdf|journal=U.S. Geological Survey|volume=Data Series 414|access-date=May 15, 2018|archive-date=July 27, 2020|archive-url=https://web.archive.org/web/20200727095133/https://pubs.usgs.gov/ds/414/downloads/DS414_text_508.pdf|url-status=live}}</ref> Ocean-based techniques involve alkalinity enhancement, such as grinding, dispersing, and dissolving olivine, limestone, silicates, or calcium hydroxide to address ocean acidification and CO<sub>2</sub> sequestration.<ref>{{Cite web|date=2021-06-23|title=Cloud spraying and hurricane slaying: how ocean geoengineering became the frontier of the climate crisis|url=http://www.theguardian.com/environment/2021/jun/23/cloud-spraying-and-hurricane-slaying-could-geoengineering-fix-the-climate-crisis|access-date=2021-06-23|website=The Guardian|language=en|archive-date=June 23, 2021|archive-url=https://web.archive.org/web/20210623071321/https://www.theguardian.com/environment/2021/jun/23/cloud-spraying-and-hurricane-slaying-could-geoengineering-fix-the-climate-crisis|url-status=live}}</ref> One example of a research project on the feasibility of enhanced weathering is the [[CarbFix]] project in Iceland.<ref>{{Cite web|url=https://www.globalccsinstitute.com/projects/carbfix-sulfix-project|title=CarbFix Project {{!}} Global Carbon Capture and Storage Institute|website=www.globalccsinstitute.com|language=en|access-date=2018-05-15|archive-url=https://web.archive.org/web/20180703133804/https://www.globalccsinstitute.com/projects/carbfix-sulfix-project|archive-date=July 3, 2018|url-status=dead}}</ref><ref>{{Cite news|url=https://www.or.is/english/carbfix/carbfix-project|title=The CarbFix Project|date=2017-08-22|work=www.or.is|access-date=2018-05-15|language=is|archive-date=May 16, 2018|archive-url=https://web.archive.org/web/20180516174158/https://www.or.is/english/carbfix/carbfix-project|url-status=dead}}</ref><ref>{{Cite news|url=https://www.nytimes.com/2015/02/10/science/burying-a-mountain-of-co2.html|title=Turning Carbon Dioxide Into Rock, and Burying It|date=2015-02-09|work=The New York Times|access-date=2018-05-15|language=en-US|issn=0362-4331|archive-date=May 16, 2018|archive-url=https://web.archive.org/web/20180516103143/https://www.nytimes.com/2015/02/10/science/burying-a-mountain-of-co2.html|url-status=live}}</ref>


===Direct air capture===
==== Direct air capture ====
[[File:2010- Direct Air Capture - global - International Energy Agency (IEA) - bar chart.svg|thumb|upright=1.5 | The [[International Energy Agency]] reported growth in [[direct air capture]] global operating capacity.<ref name=IEA_202204>{{cite web |title=Direct Air Capture / A key technology for net zero |url=https://iea.blob.core.windows.net/assets/78633715-15c0-44e1-81df-41123c556d57/DirectAirCapture_Akeytechnologyfornetzero.pdf |website=International Energy Agency (IEA) |archive-url=https://web.archive.org/web/20220410210408/https://iea.blob.core.windows.net/assets/78633715-15c0-44e1-81df-41123c556d57/DirectAirCapture_Akeytechnologyfornetzero.pdf |archive-date=10 April 2022 |page=18 |date=April 2022 |url-status=live }}</ref>]]
[[File:2010- Direct Air Capture - global - International Energy Agency (IEA) - bar chart.svg|thumb|upright=1.5 | The [[International Energy Agency]] reported growth in [[direct air capture]] global operating capacity.<ref name=IEA_202204>{{cite web |title=Direct Air Capture / A key technology for net zero |url=https://iea.blob.core.windows.net/assets/78633715-15c0-44e1-81df-41123c556d57/DirectAirCapture_Akeytechnologyfornetzero.pdf |website=International Energy Agency (IEA) |archive-url=https://web.archive.org/web/20220410210408/https://iea.blob.core.windows.net/assets/78633715-15c0-44e1-81df-41123c556d57/DirectAirCapture_Akeytechnologyfornetzero.pdf |archive-date=10 April 2022 |page=18 |date=April 2022 |url-status=live }}</ref>]]
{{Excerpt|Direct air capture}}
{{Excerpt|Direct air capture}}

Revision as of 18:04, 30 May 2022

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.[1][2][3] 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.[4] In the context of net zero greenhouse gas emissions targets,[5] CDR is increasingly integrated into climate policy, as a new element of mitigation strategies.[6] CDR and GGR methods are also known as negative emissions technologies (NET), and may be cheaper than preventing some agricultural greenhouse gas emissions.[7]

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.[2][8][9] 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 (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.[7] This would offset greenhouse gas emissions at about a fifth of the rate at which they are being produced.

In 2021 the Intergovernmental Panel on Climate Change (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.[10][11]

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.[1]

The U.S.-based National Academies of Sciences, Engineering, and Medicine uses the term "negative emissions technology" with a similar definition.[7]

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[contradictory] and assumed to be intrinsically risky.[7][need quotation to verify] 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.[2][12]

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.[1] 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) has been publicly expressed by a range of individuals and organizations involved with climate change issues, including IPCC chief Dr. Hoesung Lee,[13] the UNFCCC executive secretary Christiana Figueres,[14] and the World Watch Institute.[15] Institutions with major programs focusing on CDR include the Lenfest Center for Sustainable Energy at the Earth Institute, Columbia University,[16] and the Climate Decision Making Center,[17] 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.[7] 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.[7] This is one-fifth of the 50 gigatons of CO2 emitted per year by human activities.[7] 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.[18]

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.[7] 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.[7]

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.[18] Strategies for mitigating climate change that rely less on CDR and more on sustainable use of energy carry less of this risk.[18][19] 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.[20][7] 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.[7]

Methods

Afforestation, reforestation, and forestry management

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."[21]

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 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.[22]

Increased use of wood in construction is being considered.[23]

Carbon sequestration

Carbon sequestration is the process of storing carbon in a carbon pool.[24]: 2248  It plays a crucial role in limiting climate change by reducing the amount of carbon dioxide in the atmosphere. There are two main types of carbon sequestration: biologic (also called biosequestration) and geologic.[25]

Biologic carbon sequestration is a naturally occurring process as part of the carbon cycle. Humans can enhance it through deliberate actions and use of technology. Carbon dioxide (CO
2
) is naturally captured from the atmosphere through biological, chemical, and physical processes. These processes can be accelerated for example through changes in land use and agricultural practices, called carbon farming. Artificial processes have also been devised to produce similar effects. This approach is called carbon capture and storage. It involves using technology to capture and sequester (store) CO
2
that is produced from human activities underground or under the sea bed.

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[26][27] Carbon sequestration through biological processes affects the global carbon cycle.

Agricultural practices

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.[28] 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[29] and reduce fertilizer use.[30] Sustainable forest management is another tool that is used in carbon farming.[31] Carbon farming is one component of climate-smart agriculture. It is also one way to remove carbon dioxide from the atmosphere.

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.[32] 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.[33] Carbon farming has some disadvantages because some of its methods can affect ecosystem services. For example, carbon farming could cause an increase of land clearing, monocultures and biodiversity loss.[34] It is important to maximize environmental benefits of carbon farming by keeping in mind ecosystem services at the same time.[34]

Wetland restoration

Ways one blue carbon habitat can influence the carbon concentration and future carbon sequestration in an adjacent blue carbon habitat[35]

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".[36]: 2220  Most commonly, it refers to the role that tidal marshes, mangroves and seagrass meadows can play in carbon sequestration.[36]: 2220  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.[36]: 2220 

The methods for blue carbon management fall into the category of "ocean-based biological carbon dioxide removal (CDR) methods".[37]: 764  They are a type of biological carbon fixation.

Scientists are looking for ways to further develop the blue carbon potential of ecosystems.[38] However, the long-term effectiveness of blue carbon as a carbon dioxide removal solution is under debate.[39][38][40]

The term deep blue carbon is also in use and refers to storing carbon in the deep ocean waters.[41]

Bioenergy with carbon capture & storage

Example of BECCS: Diagram of bioenergy power plant with carbon capture and storage.[42]

Bioenergy with carbon capture and storage (BECCS) is the process of extracting bioenergy from biomass and capturing and storing the carbon dioxide (CO2) that is produced.

Greenhouse gas emissions from bioenergy can be low because when vegetation is harvested for bioenergy, new vegetation can grow that will absorb CO2 from the air through photosynthesis.[43] After the biomass is harvested, energy ("bioenergy") is extracted in useful forms (electricity, heat, biofuels, etc.) as the biomass is utilized through combustion, fermentation, pyrolysis or other conversion methods. Using bioenergy releases CO2. In BECCS, some of the CO2 is captured before it enters the atmosphere, and stored underground using carbon capture and storage technology.[44] Under some conditions, BECCS can remove carbon dioxide from the atmosphere.[44]

The potential range of negative emissions from BECCS was estimated to be zero to 22 gigatonnes per year.[45] 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.[46] Wide deployment of BECCS is constrained by cost and availability of biomass.[47][48]: 10  Since biomass production is land-intensive, deployment of BECCS can pose major risks to food production, human rights, and biodiversity.[49]

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.[50] Biomass is organic matter produced by living organisms or recently living organisms, most commonly plants or plant based material.[51] 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.[52][better source needed]

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.[53][54] 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.[55] One example of a research project on the feasibility of enhanced weathering is the CarbFix project in Iceland.[56][57][58]

Direct air capture

The International Energy Agency reported growth in direct air capture global operating capacity.[59]
Flow diagram of direct air capture process using sodium hydroxide as the absorbent and including solvent regeneration.
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.[60] 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.[61] After the capture, DAC generates a concentrated stream of CO2 for sequestration or utilization. Carbon dioxide removal is achieved when ambient air makes contact with chemical media, typically an aqueous alkaline solvent[62] or sorbents.[63] 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[64]). DACCS can function as both a carbon dioxide removal mechanism or a carbon negative technology. As of 2023, DACCS has yet to be integrated into emissions trading because, at over US$1000,[65] the cost per ton of carbon dioxide is many times the carbon price on those markets.[66] For the end-to-end process to remain net carbon negative, DAC machines must be powered by renewable energy sources, since the process can be quite energy expensive. Future innovations may reduce the energy intensity of this process.

DAC was suggested in 1999 and is still in development.[67][68] 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, DAC can be used to capture emissions that originated in non-stationary sources such as airplanes.[61]

Magnesium silicate/oxide in cement

Lifecycle amounts are not yet fully understood.[23]

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.[69] 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.[70][71] 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.[72] 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.”[73] 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.[74]

In 2021, businessman Elon Musk announced he was donating $100m for a prize for best carbon capture technology.[75]

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.[76][77] CDR might also in future be added to the UK Emissions Trading Scheme.[23] 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.[78][79]

In April 2022, a private sector aliance led by Stripe with prominent members including Meta, Google and Shopify, 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.” [80]

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.[81]

See also

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.)].

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