Carbon dioxide removal
Carbon dioxide removal (CDR), also known as greenhouse gas removal, is a process in which carbon dioxide gas (CO
2) is removed from the atmosphere and sequestered for long periods of time. These methods are also known as negative emissions technologies, as they offset greenhouse gas emissions from practices such as the burning of fossil fuels.
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.
Alternatively, some sources use the term "carbon dioxide removal" to refer to any technology that removes carbon dioxide, such as direct air capture, but can be implemented in a way that causes emissions to increase rather than decrease over the lifecycle of the process.
The IPCC's analysis of climate change mitigation pathways that are consistent with limiting global warming to 1.5°C found that all assessed pathways include the use of CDR to offset emissions. A 2019 consensus report by 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.
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The Intergovernmental Panel on Climate Change defines CDR as:
Anthropogenic activities removing CO
2 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 CO
2 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 CO
2 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 CO
2 per year if fully deployed worldwide. This is one-fifth of the 50 gigatons of CO
2 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.
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). Carbon capture technology has yet to reach more than 33% efficiency.
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."
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Carbon farming is a name for a variety of agricultural methods aimed at sequestering atmospheric carbon into the soil and in crop roots, wood and leaves. Increasing soil's carbon content can aid plant growth, increase soil organic matter (improving agricultural yield), improve soil water retention capacity and reduce fertilizer use (and the accompanying emissions of greenhouse gas nitrous oxide (N
2O). 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. Soils can contain up to five per cent carbon by weight, including decomposing plant and animal matter and biochar.
Blue carbon refers to carbon dioxide removed from the atmosphere by the world's coastal ocean ecosystems, mostly mangroves, salt marshes, seagrasses and macroalgae, through plant growth and the accumulation and burial of organic matter in the soil.
Historically the ocean, atmosphere, soil, and terrestrial forest ecosystems have been the largest natural carbon (C) sinks. New research on the role of vegetated coastal ecosystems has highlighted their potential as highly efficient C sinks, and led to the scientific recognition of the term "Blue Carbon". "Blue Carbon" designates carbon that is fixed via coastal ocean ecosystems, rather than traditional land ecosystems, like forests. Although the ocean's vegetated habitats cover less than 0.5% of the seabed, they are responsible for more than 50%, and potentially up to 70%, of all carbon storage in ocean sediments. Mangroves, salt marshes and seagrasses make up the majority of the ocean's vegetated habitats but only equal 0.05% of the plant biomass on land. Despite their small footprint, they can store a comparable amount of carbon per year and are highly efficient carbon sinks. Seagrasses, mangroves and salt marshes can capture carbon dioxide (CO
2) from the atmosphere by sequestering the C in their underlying sediments, in underground and below-ground biomass, and in dead biomass.
Bio-energy with carbon capture & storage
Bio-energy with carbon capture and storage, or BECCS, uses biomass to extract carbon dioxide from the atmosphere, and carbon capture and storage technologies to concentrate and permanently store it in deep geological formations.
BECCS is currently (as of October 2012) the only CDR technology deployed at full industrial scale, with 550 000 tonnes CO2/year in total capacity operating, divided between three different facilities (as of January 2012).
The Imperial College London, the UK Met Office Hadley Centre for Climate Prediction and Research, the Tyndall Centre for Climate Change Research, the Walker Institute for Climate System Research, and the Grantham Institute for Climate Change issued a joint report on carbon dioxide removal technologies as part of the AVOID: Avoiding dangerous climate change research program, stating that "Overall, of the technologies studied in this report, BECCS has the greatest maturity and there are no major practical barriers to its introduction into today's energy system. The presence of a primary product will support early deployment."
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. The offset of greenhouse gas (GHG) emission, if biochar were to be implemented, would be a maximum of 12%. This equates to about 106 metric tons of CO2 equivalents. On a medium conservative level, it would be 23% less than that, at 82 metric tons. 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 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. Enhanced weathering is considered one of the least expensive geoengineering options. One example of a research project on the feasibility of enhanced weathering is the CarbFix project in Iceland.
Direct air capture
Direct air capture (DAC) is the use of chemical or physical processes to extract CO
2 directly from the ambient air. If the extracted CO
2 is then sequestered in safe long-term storage, the overall process will achieve carbon dioxide removal.
A few engineering proposals have been made for DAC, but work in this area is still in its infancy. A private company Global Research Technologies demonstrated a pre-prototype of air capture technology in 2007. A pilot plant owned by Carbon Engineering has operated in British Columbia, Canada since 2015. An economic study of this plant in 2018 estimated the cost at US$94–$232 per tonne of atmospheric CO
2 removed. Several companies are now working on this approach.
DAC relying on amine-based absorption demands significant water input. It was estimated, that to capture 3.3 Gigatonnes of CO
2 a year would require 300 km3 of water, or 4% of the water used for irrigation. On the other hand, using sodium hydroxide needs far less water, but the substance itself is highly caustic and dangerous.
DAC also requires much greater energy input in comparison to traditional capture from point sources, like flue gas, due to the low concentration of CO
2. The theoretical minimum energy required to extract CO
2 from ambient air is about 250 kWh per tonne of CO
2, while capture from natural gas and coal power plants requires respectively about 100 and 65 kWh per tonne of CO
A crucial issue for CDR is the cost, which differs substantially among the different methods: some of these are not sufficiently developed to perform cost assessments. A 2018 study estimated the cost of direct air capture as between $94 and $232 per tonne. However the value of BECCS and CDR generally in integrated assessment models in the long term is highly dependant on the discount rate.
Removal of other greenhouse gases
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