Carbon capture use and storage (CCUS) and carbon dioxide removal (CDR) techniques are often discussed together, but differ significantly. CCUS involves 3 steps: firstly, capturing CO2 emissions from industrial processes, such as power generation or the production of cement, steel or hydrogen; secondly, transporting CO2 or using some of it in a production process (e.g., for enhanced oil recovery, or in the production of chemicals, plastics, food and drink); and thirdly, storing CO2 permanently underground. The amount of CO2 that is eventually stored will mainly be a function of the capture rate, i.e. the percentage of CO2 emissions captured, typically estimated to be in the region of 85-90% (although this will differ depending on the process). Therefore, even in the best case scenario, CCUS does not mitigate all emissions from an industrial process. 

CDR techniques capture CO2 from the air and store it permanently. This can happen through different processes—biological, industrial chemical, or rock weathering. Potential storage locations include in trees, plant matter, the soil, deep underground, the oceans, and long-lived products. Some of the most prominent techniques include afforestation/reforestation, peatland restoration, enhanced rock weathering, bioenergy with Carbon Capture and Storage (BECCS), and Direct Air Carbon Capture and Storage (DACCS). See here for a useful overview of CDR. 

As CO2 is removed from the atmosphere, these techniques generate what are known as negative emissions, which can offset emissions from other emission sources. BECCS and DACCS overlap with CCUS; BECCS is a form of CCUS that not only sequesters CO2 from the combustion of fuels (in this case bioenergy) but also removes it from the atmosphere based on the assumption that sequestration will also happen when the bioenergy regrows. DACCS removes CO2 directly from the atmosphere and then uses similar processes to CCUS for transport, use and storage. 

Some of the above approaches are either at the early stage of development, or are more mature technologies that have yet to scale (e.g., CCUS). In view of this, the extent to which they are likely to be deployed for climate mitigation purposes is highly uncertain. However, in normative energy-economic scenarios for achieving ambitious global mitigation pathways, they often form a critical lever to achieve emission reduction goals. The extent to which these technologies are deployed also has an important impact on the role of fossil fuels in the energy system, allowing increased levels of such fuels in mitigation pathways that achieve global goals (see Achakulwisut et al., 2023). This section of the database includes two papers by Grant et al. that highlight the uncertainty of CCUS/CDR in modelling approaches, and a paper by Achakulwisut et al. that identifies the implications of CCUS/CDR for the role of fossil fuels. 

Grant et al. (2021a) recognise that there is still large uncertainty regarding the future potential of these technologies to deliver the promised effects. As a result of these uncertainties, near-term policies should not rely on removing greenhouse gases in the future but focus on reducing fossil fuel production and consumption right now. The risk that anticipated future CDR could dilute incentives to reduce emissions now is also known as “mitigation deterrence”. Grant et al. (2021b) warn against this phenomenon. They challenge the common assumptions that emissions reduction and removals are perfect substitutes and that future CDR availability is known with certainty. These studies caution against optimism about future emission-removal technologies and advocate for stronger efforts to reduce emissions now. 

These findings are important for fossil fuel-related litigation given that fossil fuel companies may rely on scenarios with high levels of future carbon-capture deployment to justify new emissions-intensive projects now (for instance, in environmental impact assessments).