Editing IPCC:AR6/WGI/TS (section)

==== TS.3.3.2 Carbon Dioxide Removal ====

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'''Deliberate carbon dioxide removal (CDR) from the atmosphere has the potential to compensate for residual CO <sub>2</sub> emissions to reach net zero CO <sub>2</sub> emissions or to generate net negative CO <sub>2</sub> emissions. In the same way that part of current anthropogenic net CO <sub>2</sub> emissions are taken up by land and ocean carbon stores, net CO <sub>2</sub> removal will be partially counteracted by CO <sub>2</sub> release from these stores ( ''very high confidence'' ). Asymmetry in the carbon cycle response to simultaneous CO <sub>2</sub> emissions and removals implies that a larger amount of CO <sub>2</sub> would need to be removed to compensate for an emission of a given magnitude to attain the same change in atmospheric CO <sub>2</sub> ( ''medium confidence'' ). CDR methods have wide-ranging side-effects that can either weaken or strengthen the carbon sequestration and cooling potential of these methods and affect the achievement of sustainable development goals ( ''high confidence'' ). Links to chapters 4.6.3, 5.6'''
Carbon dioxide removal (CDR) refers to anthropogenic activities that deliberately remove CO <sub>2</sub> from the atmosphere and durably store it in geological, terrestrial or ocean reservoirs, or in products. Carbon dioxide is removed from the atmosphere by enhancing biological or geochemical carbon sinks or by direct capture of CO <sub>2</sub> from air. Emissions pathways that limit global warming to 1.5°C or 2°C typically assume the use of CDR approaches in combination with GHG emissions reductions. CDR approaches could be used to compensate for residual emissions from sectors that are difficult or costly to decarbonize. CDR could also be implemented at a large scale to generate global net negative CO <sub>2</sub> emissions (i.e., anthropogenic CO <sub>2</sub> removals exceeding anthropogenic emissions), which could compensate for earlier emissions as a way to meet long-term climate stabilization goals after a temperature overshoot. This Report assesses the effects of CDR on the carbon cycle and climate. Co-benefits and trade-offs for biodiversity, water and food production are briefly discussed for completeness, but a comprehensive assessment of the ecological and socio-economic dimensions of CDR options is left to the WGII and WGIII reports. Links to chapters 4.6.3, 5.6

CDR methods have the potential to sequester CO <sub>2</sub> from the atmosphere ( ''high confidence'' ). In the same way part of current anthropogenic net CO <sub>2</sub> emissions are taken up by land and ocean carbon stores, net CO <sub>2</sub> removal will be partially counteracted by CO <sub>2</sub> release from these stores, such that the amount of CO <sub>2</sub> sequestered by CDR will not result in an equivalent drop in atmospheric CO <sub>2</sub> ( ''very'' ''high confidence'' ). The fraction of CO <sub>2</sub> removed from the atmosphere that is not replaced by CO <sub>2</sub> released from carbon stores – a measure of CDR effectiveness – decreases slightly with increasing amounts of removal ( ''medium confidence'' ) and decreases strongly if CDR is applied at lower atmospheric CO <sub>2</sub> concentrations ( ''medium confidence'' ). The reduction in global surface temperature is approximately linearly related to cumulative CO <sub>2</sub> removal ( ''high confidence'' ). Because of this near-linear relationship, the amount of cooling per unit CO <sub>2</sub> removed is approximately independent of the rate and amount of removal ( ''medium confidence'' ). Links to chapters 4.6.3, 5.6.2.1, Figure 5.32, Figure 5.34

Due to non-linearities in the climate system, the century-scale climate–carbon cycle response to a CO <sub>2</sub> removal from the atmosphere is not always equal and opposite to its response to a simultaneous CO <sub>2</sub> emission ( ''medium'' confidence). For CO <sub>2</sub> emissions of 100 PgC released from a state in equilibrium with pre-industrial atmospheric CO <sub>2</sub> levels, CMIP6 models simulate that 27± 6% (mean ± 1 standard deviation) of emissions remain in the atmosphere 80–100 years after the emissions, whereas for removals of 100 PgC only 23 ± 6% of removals remain out of the atmosphere. This asymmetry implies that an extra amount of CDR is required to compensate for a positive emission of a given magnitude to attain the same change in atmospheric CO <sub>2</sub> . Due to ''low agreement'' between models, there is ''low confidence'' in the sign of the asymmetry of the temperature response to CO <sub>2</sub> emissions and removals. Links to chapters 4.6.3, 5.6.2.1, Figure 5.35

Simulations with ESMs indicate that under scenarios where CO <sub>2</sub> emissions gradually decline, reach net zero and become net negative during the 21st century (e.g., SSP1-2.6), land and ocean carbon sinks begin to weaken in response to declining atmospheric CO <sub>2</sub> concentrations, and the land sink eventually turns into a source (Figure TS.19). This sink-to-source transition occurs decades to a few centuries after CO <sub>2</sub> emissions become net negative. The ocean remains a sink of CO <sub>2</sub> for centuries after emissions become net negative. Under scenarios with large net negative CO <sub>2</sub> emissions (e.g., SSP5-3.4-OS) and rapidly declining CO <sub>2</sub> concentrations, the land source is larger than for SSP1-2.6 and the ocean also switches to a source. While the general response is robust across models, there is ''low confidence'' in the timing of the sink-to-source transition and the magnitude of the CO <sub>2</sub> source in scenarios with net negative CO <sub>2</sub> emissions. Carbon dioxide removal could reverse some aspects climate change if CO <sub>2</sub> emissions become net negative, but some changes would continue in their current direction for decades to millennia. For instance, sea level rise due to ocean thermal expansion would not reverse for several centuries to millennia ( ''high confidence'' ) (Box TS.4). Links to chapters 4.6.3, 5.4.10, 5.6.2.1, Figure 5.30, Figure 5.33

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'''Figure TS.19 | Carbon sink response in a scenario with net carbon dioxide (CO''' 2 ''') removal from the atmosphere.''' ''The intent of this figure is to show how atmospheric CO'' 2 ''evolves under negative emissions and its dependence on the negative emissions technologies. It also shows the evolution of the ocean and land sinks.'' Shown are CO 2 flux components from concentration-driven Earth system model (ESM) simulations during different emissions stages of SSP1–2.6 and its long-term extension. (a) Large net positive CO <sub>2</sub> emissions, (b) small net positive CO <sub>2</sub> emissions, (c–d) net negative CO <sub>2</sub> emissions, and (e) net zero CO <sub>2</sub> emissions. Positive flux components act to raise the atmospheric CO <sub>2</sub> concentration, whereas negative components act to lower the CO <sub>2</sub> concentration. Net CO <sub>2</sub> emissions and land and ocean CO <sub>2</sub> fluxes represent the multi-model mean and standard deviation (error bar) of four ESMs (CanESM5, UKESM1, CESM2-WACCM, IPSL-CM6a-LR) and one Earth system model of intermediate complexity (Uvic ESCM). Net CO <sub>2</sub> emissions are calculated from concentration-driven ESM simulations as the residual from the rate of increase in atmospheric CO <sub>2</sub> and land and ocean CO <sub>2</sub> fluxes. Fluxes are accumulated over each 50-year period and converted to concentration units (parts per million, or ppm). Links to chapters 5.6.2.1, Figure 5.33

Carbon dioxide removal methods have a range of side effects that can either weaken or strengthen the carbon sequestration and cooling potential of these methods and affect the achievement of sustainable development goals ( ''high confidence'' ). Biophysical and biogeochemical side-effects of CDR methods are associated with changes in surface albedo, the water cycle, emissions of CH <sub>4</sub> and N <sub>2</sub> O, ocean acidification and marine ecosystem productivity ( ''high confidence'' ). These side-effects and associated Earth system feedbacks can decrease carbon uptake and/or change local and regional climate and in turn limit the CO <sub>2</sub> sequestration and cooling potential of specific CDR methods ( ''medium confidence'' ). Deployment of CDR, particularly on land, can also affect water quality and quantity, food production and biodiversity ( ''high confidence'' ). These effects are often highly dependent on local context, management regime, prior land use, and scale ( ''high confidence'' ). The largest co-benefits are obtained with methods that seek to restore natural ecosystems or improve soil carbon sequestration ( ''medium confidence'' ). The climate and biogeochemical effects of terminating CDR are expected to be small for most CDR methods ( ''medium confidence'' ). Links to chapters 4.6.3, 5.6.2.2, Figure 5.36, 8.4.3, 8.6.3

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