Editing IPCC:AR6/SR15/Chapter-4 (section)

== FAQ 4.2 What are Carbon Dioxide Removal and Negative Emissions? ==

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'''''Summary: ''''' ''Carbon dioxide removal (CDR) refers to the process of removing CO <sub>2</sub>  from the atmosphere. Since this is the opposite of emissions, practices or technologies that remove CO <sub>2</sub>  are often described as achieving ‘negative emissions’. The process is sometimes referred to more broadly as greenhouse gas removal if it involves removing gases other than CO <sub>2</sub> . There are two main types of CDR: either enhancing existing natural processes that remove carbon from the atmosphere (e.g., by increasing its uptake by trees, soil, or other ‘carbon sinks’) or using chemical processes to, for example, capture CO <sub>2</sub>  directly from the ambient air and store it elsewhere (e.g., underground). All CDR methods are at different stages of development and some are more conceptual than others, as they have not been tested at scale.''

Limiting warming to 1.5°C above pre-industrial levels would require unprecedented rates of transformation in many areas, including in the energy and industrial sectors, for example. Conceptually, it is possible that techniques to draw CO <sub>2 </sub> out of the atmosphere (known as carbon dioxide removal, or CDR) could contribute to limiting warming to 1.5°C. One use of CDR could be to compensate for greenhouse gas emissions from sectors that cannot completely decarbonize, or which may take a long time to do so.

If global temperature temporarily overshoots 1.5°C, CDR would be required to reduce the atmospheric concentration of CO <sub>2</sub>  to bring global temperature back down. To achieve this temperature reduction, the amount of CO <sub>2 </sub> drawn out of the atmosphere would need to be greater than the amount entering the atmosphere, resulting in ‘net negative emissions’. This would involve a greater amount of CDR than stabilizing atmospheric CO <sub>2</sub>  concentration – and, therefore, global temperature – at a certain level. The larger and longer an overshoot, the greater the reliance on practices that remove CO <sub>2</sub>  from the atmosphere.

There are a number of CDR methods, each with different potentials for achieving negative emissions, as well as different associated costs and side effects. They are also at differing levels of development, with some more conceptual than others. One example of a CDR method in the demonstration phase is a process known as bioenergy with carbon capture and storage (BECCS), in which atmospheric CO <sub>2</sub>  is absorbed by plants and trees as they grow, and then the plant material (biomass) is burned to produce bioenergy. The CO <sub>2</sub>  released in the production of bioenergy is captured before it reaches the atmosphere and stored in geological formations deep underground on very long time scales. Since the plants absorb CO <sub>2</sub>  as they grow and the process does not emit CO <sub>2</sub> , the overall effect can be to reduce atmospheric CO <sub>2</sub> .

Afforestation (planting new trees) and reforestation (replanting trees where they previously existed) are also considered forms of CDR because they enhance natural CO <sub>2</sub>  ‘sinks’. Another category of CDR techniques uses chemical processes to capture CO <sub>2 </sub> from the air and store it away on very long time scales. In a process known as direct air carbon capture and storage (DACCS), CO <sub>2 </sub> is extracted directly from the air and stored in geological formations deep underground. Converting waste plant material into a charcoal-like substance called biochar and burying it in soil can also be used to store carbon away from the atmosphere for decades to centuries.

There can be beneficial side effects of some types of CDR, other than removing CO <sub>2 </sub> from the atmosphere. For example, restoring forests or mangroves can enhance biodiversity and protect against flooding and storms. But there could also be risks involved with some CDR methods. For example, deploying BECCS at large scale would require a large amount of land to cultivate the biomass required for bioenergy. This could have consequences for sustainable development if the use of land competes with producing food to support a growing population, biodiversity conservation or land rights. There are also other considerations. For example, there are uncertainties about how much it would cost to deploy DACCS as a CDR technique, given that removing CO <sub>2</sub>  from the air requires considerable energy.

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'''FAQ 4.2, Figure 1'''

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'''Carbon dioxide removal (CDR) refers to the process of removing CO2 from the atmosphere.'''
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There are a number of CDR techniques, each with different potential for achieving ‘negative emissions’, as well as different associated costs and side effects.

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