Cyclic and single-shot approaches to carbon dioxide removal (CDR) differ in their advantages and disadvantages, particularly in terms of energy intensity, environmental impact, and scalability.

Cyclic CDR systems:

• Description: Cycle systems, such as chemical direct air capture (DAC), repeatedly use the same materials to capture atmospheric CO₂. After the CO₂ is captured, the CO₂ is extracted and the material is reused.
• Energy intensity: These systems are inherently energy-intensive. The second law of thermodynamics sets a lower bound on the energy required. It takes a large amount of energy to capture one gigatonne of CO₂ from the air using cyclic CDR processes. The minimum energy required is comparable to the combined electricity consumption of 11 million average US households in 2021 (14 gigawatt-years). Existing cyclic CDR approaches require at least several times the theoretical minimum.
• Environmental impact: Cyclic systems can enable carbon sequestration that is easier to verify and has a lower environmental impact than other approaches to CDR. They have a smaller land area impact than most terrestrial single-use or biological CDR systems of comparable capacity.
• Scalability: Cyclic systems, particularly chemical direct air capture with carbon storage (DACCS), can in principle be scaled up to capture and sequester atmospheric CO₂ on a gigatonnes-per-year scale. The key to scaling such systems is the development of integrated carbon-free energy sources.
• Advantages: Simple output, smaller land area affected than many other CDR systems, manageable measurement, reporting and verification (MRV). The location of the facility can be chosen to optimize the availability of waste heat or renewable energy sources, water needs, proximity to CO₂ sequestration sites and the operating environment.
• Disadvantages: Energy requirements.

Disposable CDR systems:

• Description: One-off systems, such as accelerated rock weathering (ERW), involve a one-off use of a resource. Many approaches use minerals extracted from the Earth to directly or indirectly absorb CO₂.
• Energy intensity: These systems may use less energy. Some of this energy has been supplied by natural processes over millennia.
• Environmental impact: They generally require extensive resource extraction and can have a significant environmental impact. The effectiveness of disposable systems is difficult to measure and verify.
• Scalability: To reduce atmospheric CO₂ on a large scale, a large amount of materials needs to be processed.
• Advantages: They may require less energy than cyclic processes.
• Disadvantages: They require extensive resource extraction and can have a significant environmental impact. Their effectiveness is difficult to measure and verify.

General considerations:

• Measurement, Reporting and Verification (MRV): For an effective CO₂ capture program, accurate MRV mechanisms must be in place. Standards are also needed to quantify and minimize the risks and environmental impacts associated with CDR.
• Costs: A full cost analysis must include costs for all aspects of the life cycle, including material extraction, system construction, energy and other inputs, transportation, storage, and externalities.
• Policy recommendations: CDR in the range of Gt CO₂/year may be desirable in the coming decades to meet specific climate targets, therefore research and development of various CDR approaches is recommended despite their extensive energy and material needs. Any substantial development of CDR systems, especially cyclical systems such as DAC, should be undertaken in combination with dedicated low-carbon or carbon-free energy sources.

In short, cyclical CDR systems, such as DAC, are energy intensive but allow for easier verification and less environmental impact, while one-off systems, such as ERW, may require less energy but have significant environmental impact due to extensive resource extraction. An effective CDR program requires accurate MRV mechanisms and standards to quantify and minimize environmental risks and impacts. Spring

Glossary of key terms

• CDR (Carbon Dioxide Removal): Carbon dioxide removal refers to anthropogenic activities that remove CO₂ from the atmosphere and permanently store it in geological, terrestrial or oceanic reservoirs, or in products.
• DAC (Direct Air Capture): Direct air extraction, primarily refers to chemical direct air extraction, a specific class of carbon capture approaches that can be used in combination with sequestration/storage for CDR (DACCS).
• DACCS (Direct Air Capture with Carbon Storage): Direct air extraction with carbon storage, see also DAC.
• DIC (Dissolved Inorganic Carbon): Dissolved inorganic carbon refers to all inorganic carbon forms in aqueous solution.
• DOC (Direct Ocean Capture): Direct ocean extraction, refers to the direct removal of CO₂ from the oceans, a specific class of approaches to CDR.
• ERW (Enhanced Rock Weathering): Improved rock weathering.
• LULUCF (Land Use, Land Use Change, and Forestry): Land use, land use change and forestry.
• mCDR (Marine CDR): Marine CDR.
• MRV (Measurement, Reporting, Verification): Measurement, reporting, verification.
• NET (Negative Emission Technologies): Negative emissions technologies, see CDR.
• OAE (Ocean Alkalinity Enhancement): Increasing ocean alkalinity.
• SOC (Soil Organic Carbon): Organic carbon in soil.