Carbon dioxide removal
Carbon dioxide removal, also known as greenhouse gas removal, usually refers to human-driven methods of removing carbon dioxide from the atmosphere and sequestering it for long periods, such that more carbon dioxide is sequestered in the process than emitted. 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.
Definitions
The Intergovernmental Panel on Climate Change defines CDR as:The U.S.-based National Academies of Sciences, Engineering, and Medicine uses the term “negative emissions technology” with a similar definition.
The concept of deliberately reducing the amount of 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. Whether CDR would satisfy common definitions of "climate engineering" or "geoengineering" usually depends upon the scale at which it would be undertaken.
Concepts using similar terminology
CDR can be confused with carbon capture and storage, a process in which carbon dioxide is collected from point-sources such as coal-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 coal-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 reducing net emissions
Using CDR in parallel with other efforts to reduce 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 per year if fully deployed worldwide. This is one-fifth of the 50 gigatons of emitted per year by human activities.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 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:
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. This is because they are removing carbon from the atmosphere and sequestering it indefinitely and presumably for a considerable duration. Carbon capture technology has yet to reach more than 33% efficiency. Furthermore, this process could be rapidly undone, for example by earthquakes or mining.
Methods
Afforestation, reforestation, and forestry management
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 the only CDR technology deployed at full industrial scale, with 550 000 tonnes CO2/year in total capacity operating, divided between three different facilities.
The Imperial College London, the UK Met Office Hadley Centre for Climate Prediction and Research, the Tyndall Centre for Climate Change Research, the , 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."
According to the OECD, "Achieving lower concentration targets depends significantly on the use of BECCS".
Agricultural practices
Wetland restoration
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 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
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 (DAC)
Carbon dioxide can be removed from ambient air through chemical processes, sequestered, and stored. Traditional modes of carbon capture such as precombustion and postcombustion capture from large point sources can help slow the rate of increase of the atmospheric concentration, but the direct removal of from the air, or direct air capture, can actually reduce the global atmospheric concentration if combined with long-term storage of.DAC relying on amine-based absorption demands significant water input. It was estimated, that to capture 3.3 Gigatonnes of 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. The theoretical minimum energy required to extract from ambient air is about 250 kWh per tonne of, while capture from natural gas and coal power plants requires respectively about 100 and 65 kWh per tonne of.
Ocean fertilization
Ocean fertilization or ocean nourishment is a type of climate engineering based on the purposeful introduction of nutrients to the upper ocean to increase marine food production and to remove carbon dioxide from the atmosphere. A number of techniques, including fertilization by iron, urea and phosphorus have been proposed.Economic issues
A crucial issue for CDR methods is their cost, which differs substantially among the different technologies: some of these are not sufficiently developed to perform cost assessments. In 2011 the American Physical Society estimated the costs for direct air capture to be $600/tonne with optimistic assumptions. A 2018 study found this estimate lowered to between $94 and $232 per tonne. The IEA Greenhouse Gas R&D Programme and Ecofys provides an estimate that 3.5 billion tonnes could be removed annually from the atmosphere with BECCS at carbon prices as low as €50 per tonne, while a report from Biorecro and the Global Carbon Capture and Storage Institute estimates costs "below €100" per tonne for large scale BECCS deployment.Risks, problems and criticisms
CDR is slow to act, and requires a long-term political and engineering program to effect. CDR is even slower to take effect on acidified oceans. In a Business as usual concentration pathway, the deep ocean will remain acidified for centuries, and as a consequence many marine species are in danger of extinction.The Special 1.5°C IPCC report was very clear about CDR: "CDR deployed at scale is unproven and reliance on such technology is a major risk in the ability to limit warming to 1.5°C."
These objections are at least partly based on a straw man, as CDR has never been proposed as a sole solution, claiming to solve the climate crisis by itself. The Environmental Defense Fund now favors its use in conjunction with renewable electricity, electric vehicles, and other strategies to reduce emissions.
However, the heavy reliance on CDR in the integrated assessment models currently informing climate mitigation policy has been questioned by scientists, arguing that it may well contribute to locking in high-temperature pathways.