A message The American Physical Society (APS) January 2025 paper on Atmospheric Carbon Dioxide Removal (CDR) provides an overview of CDR methods and their underlying physical limitations. It focuses on CDR approaches that have potential to contribute to CO₂ removal on a gigatonne/year (Gt/year) scale.

Key points of the report:

• Motivation for CDR: Human activity adds approximately 35 Gt of CO₂ to the Earth's atmosphere annually from fossil fuel sources. Limiting overall warming caused by anthropogenic CO₂ to 1.5 or 2°C above pre-industrial levels would require removal of several gigatonnes of CO₂ per year through CDR.
• Classification of CDR methods:
  • Cyclic vs. one-off processes: CDR systems can be divided into two basic types: cyclic and single-use. Cyclic systems, such as chemical direct air extraction (DAC), use a single batch of material to repeatedly capture CO₂ in a cyclic process. Single-use systems use material that is discarded after the CO₂ is captured.
  • CDR approaches: The main approaches to CDR include chemical DAC, biological carbon capture (ecosystem-based, bioenergy sCCS, and other biological approaches), rock weathering, and increasing ocean alkalinity.
• Direct Air Capture (DAC) Chemical Systems: Chemical DAC systems remove CO₂ from the air using a working material, such as a solid sorbent or a liquid solvent. The largest energy consumption in DAC systems is for extracting the CO₂ from the sorbent or solvent.
• Biological carbon capture: Biological systems naturally capture atmospheric carbon dioxide through photosynthesis. Ecosystem-based CDR generally involves modifying land use and management practices to promote increased carbon storage in soil or biomass. Bioenergy with CCS (BECCS) involves growing crops or trees that capture biomass, using that biomass in power plants with carbon capture directly at the source, and storing CO₂.
• Carbon storage and sequestration: A direct approach to carbon storage and sequestration involves compressing CO₂ into a supercritical fluid and injecting it into an underground reservoir, where it remains trapped.
• Measurement, Reporting and Verification (MRV): If serious efforts are made to implement CDR on a large scale, it is essential to have accurate and reliable methods to verify that carbon has actually been removed from the atmosphere and is permanently stored with minimal leakage, and that the process has not had serious negative impacts on ecosystems or human well-being.
• Economic aspects: Large-scale carbon capture has enormous energy and material demands and would be quite expensive. There is currently no clear domestic or international framework for paying for large-scale CDR.
• Recommendations for policymakers: Large-scale CDR requires significant energy inputs and a large-scale material infrastructure. It is important to develop reliable measurement, reporting and verification (MRV) systems to quantify the effectiveness of CDR systems and confirm that they reliably capture and permanently sequester atmospheric CO₂.

The report highlights that a portfolio of different methods is needed to implement CDR on a large scale, taking into account energy requirements, material infrastructure, environmental impacts and economic costs. It is also important to develop reliable MRV systems to verify the effectiveness of CDR systems. 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.