We define negative emissions as ‘Intentional human efforts to remove CO2 emissions from the atmosphere,’ beyond the natural processes of carbon dioxide removal.
Carbon dioxide removal technologies can be distinguished by their capture process and implementation options, the earth system in which they operate, and the storage medium. Five out of the seven technology clusters considered (AR, SCS, BECCS, OF, BC) use photosynthesis for capturing the CO2. Only EW (incl. ocean alkalinization) and DACCS bind the CO2 through chemical processes. A central distinction is whether the technology is land or ocean-based, as the latter can involve transboundary pollution issues and will require higher levels of international coordination—particularly if larger scale applications are intended. Finally, the storage medium is of great interest as there can be a significant variation in the reliability, permanence and overall quantity of available CO2 storage. In principle, the literature highlights that land management approaches such as AR and SCS provide more vulnerable (and less verifiable) storage options, where stored CO2 can be released again within short time frames. In contrast, geological reservoirs used by BECCS and DACCS are thought to provide a larger and less vulnerable storage option.
A taxonomy of negative emissions technologies (NETs). NETs are distinguished by approach to carbon capture, earth system and storage medium. Major implementation options are distinguished for each NET. (Source)
Carbon dioxide removal is fundamentally different from a more generic term that is often used in climate change debates – geoengineering. Geoengineering refers to a broad set of methods that aim to deliberately alter the climate system in order to alleviate the impacts of climate change, most notably solar radiation management. In contrast, carbon dioxide removal addresses a prior intervention point: the presence of CO2 in the atmosphere. Other important differences between CDR and geoengineering include the scale of interventions, the scope of risks, and time-scales on which specific interventions play out. Hence we advocate for a separate classification of CO2 removal and geoengineering, as shown in the following figure.
Human response options to the climate problem. Horizontal arrows in the top row show the causal chain of the climate change problem. Vertical arrows and bottom row define locus and modes of intervention for climate policy. (Source)