Editing IPCC:AR6/WGI/Chapter-5 (section)

===== 5.6.2.1.3 Removal effectiveness of CDR =====

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It is well understood that land and ocean carbon fluxes are sensitive to the level of atmospheric CO <sub>2</sub> and climate change and differ under varied future scenarios ( [[#5.4|Section 5.4]] ). It is therefore important to establish to what extent the removal effectiveness of CDR – here defined as the fraction of total CO <sub>2</sub> removed that remains out of the atmosphere – is dependent on the scenario in which CDR is applied. Different metrics have been proposed to quantify the removal effectiveness of CDR ( [[#Tokarska--2015|Tokarska and Zickfeld, 2015]] ; [[#Jones--2016b|Jones et al., 2016b]] ; [[#Zickfeld--2016|Zickfeld et al., 2016]] ). One is the airborne fraction (AF) of cumulative CO <sub>2</sub> emissions, defined in the same way as for positive emissions (i.e., as the fraction of total CO <sub>2</sub> emissions remaining in the atmosphere), with its use extended to periods of declining and net negative CO <sub>2</sub> emissions. This metric, however, has not proven to be useful to quantify the removal effectiveness of CDR in simulations where CDR is applied from a trajectory of increasing atmospheric CO <sub>2</sub> concentration. This is because it measures the carbon cycle response to CDR as well as to the prior atmospheric CO <sub>2</sub> trajectory ( [[#Tokarska--2015|Tokarska and Zickfeld, 2015]] ; [[#Jones--2016b|Jones et al., 2016b]] ). A more useful metric is the perturbation airborne fraction (PAF; [[#Jones--2016b|Jones et al., 2016b]] ), which measures the AF of the perturbation (in this case the CO <sub>2</sub> removal) relative to a reference scenario ( [[#Tokarska--2015|Tokarska and Zickfeld, 2015]] ; [[#Jones--2016b|Jones et al., 2016b]] ). The advantage of this metric is that it isolates the response to a CO <sub>2</sub> removal from the response to atmospheric CO <sub>2</sub> prior to when the removal is applied. A disadvantage is that the PAF cannot be calculated from a single model simulation, but instead requires a reference simulation to evaluate the effect of the CO <sub>2</sub> removal. When CDR is applied from an equilibrium state, the PAF and AF are equivalent measures.

In scenario simulations and idealized simulations with instantaneous CO <sub>2</sub> removals applied from an equilibrium state, the removal effectiveness of CDR is found to be slightly dependent on the rate and amount of CDR ( [[#Tokarska--2015|Tokarska and Zickfeld, 2015]] ; [[#Jones--2016b|Jones et al., 2016b]] ; [[#Zickfeld--2021|Zickfeld et al., 2021]] ), and to be strongly dependent on the emissions scenario from which CDR is applied ( [[#Jones--2016b|Jones et al., 2016b]] ; [[#Zickfeld--2021|Zickfeld et al., 2021]] ). The fraction of CO <sub>2</sub> removed remaining out of the atmosphere decreases slightly for larger removals and decreases strongly when CDR is applied from a lower background atmospheric CO <sub>2</sub> concentration (Figure 5.34), due to state dependencies and climate–carbon cycle feedbacks that lead to a stronger overall response to CO <sub>2</sub> removal ( [[#Zickfeld--2021|Zickfeld et al., 2021]] ). Based on the ''high agreement'' between studies, we assess with ''medium confidence'' that the removal effectiveness of CDR is only slightly dependent on the rate and magnitude of removal and is smaller at lower background atmospheric CO <sub>2</sub> concentrations. Simulations with Earth system models of intermediate complexity (EMIC) with instantaneous CO <sub>2</sub> removal from different equilibrium initial states suggest that the smaller removal effectiveness of CDR at lower background CO <sub>2</sub> levels results in greater cooling per unit CO <sub>2</sub> removed ( [[#Zickfeld--2021|Zickfeld et al., 2021]] ). However, there is ''low confidence'' in the robustness of this result as climate sensitivity has been shown to exhibit opposite state dependence in EMICs and ESMs ( [[IPCC:Wg1:Chapter:Chapter-7#7.4.3.1|Section 7.4.3.1]] ).

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[[File:27eab2cc63805dfdc8394b5b275340e4 IPCC_AR6_WGI_Figure_5_34.png]]

'''Figure 5.34 |''' '''Removal effectiveness of carbon dioxide removal (CDR).''' '''(a)''' Fraction of carbon dioxide (CO <sub>2</sub> ) remaining out of the atmosphere for idealized model simulations with CDR applied instantly (pulse removals) from climate states in equilibrium with different atmospheric CO <sub>2</sub> concentration levels (one to four times the pre-industrial atmospheric CO <sub>2</sub> concentration; shown on the horizontal axis). The fraction is calculated 100 years after pulse removal. The black triangle and error bar indicate the multi-model mean and standard deviation for the seven Earth system models shown in Figure 5.32 forced with a 100 PgC pulse removal. Other symbols illustrate results with the UVic ESCM model of intermediate complexity for different magnitudes of pulse removals (triangles: –100 PgC; circles: –500 PgC; squares: –1000 PgC). Data for the UVic ESCM is from [[#Zickfeld--2021|Zickfeld et al. (2021)]] . '''(b)''' Perturbation airborne fraction (see text for definition) for model simulations where CDR is applied from four Representative Concentration Pathways (RCPs) (shown on the horizontal axis in terms of their cumulative CO <sub>2</sub> emissions during 2020–2099). Symbols indicate results for four CDR scenarios, which differ in terms of the magnitude and rate of CDR (see [[#Jones--2016b|Jones et al. (2016b)]] for details). Results are based on simulations with the Hadley Centre Simple Climate-Carbon Model and are shown for the year 2100. Data from [[#Jones--2016b|Jones et al. (2016b)]] . Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

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