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

==== 5.6.2.1 Global Carbon Cycle Responses to CDR ====

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This subsection assesses evidence about the response of the global carbon cycle to CDR from idealized model simulations which assume that CO <sub>2</sub> is removed from the atmosphere directly and stored permanently in the geologic reservoir, which is analogous to direct air carbon capture with carbon storage (DACCS) (Table 5.9). The carbon cycle response to specific land and ocean-based CDR methods is assessed in [[#5.6.2.2.2|Section 5.6.2.2.2]] . At the time of AR5 there were very few studies about the global carbon cycle response to CDR. Based on these studies and general understanding of the carbon cycle, AR5 WGI [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] assessed that it is ''virtually certain'' that deliberate removal of CO <sub>2</sub> from the atmosphere will be partially offset by outgassing of CO <sub>2</sub> from the ocean and land carbon sinks. ''Low confidence'' was placed on any quantification of effects. Since AR5 WGI Chapter 6, several studies have investigated the carbon cycle response to CDR in idealized ‘pulse’ removal simulations, whereby a specified amount of CO <sub>2</sub> is removed instantly from the atmosphere, and scenario simulations with CO <sub>2</sub> emissions and removals following a plausible trajectory. In addition, a dedicated carbon dioxide removal model intercomparison project was initiated (CDRMIP; [[#Keller--2018b|Keller et al., 2018b]] ) which includes a range of CDR experiments from idealized simulations to simulations of deployment of specific CDR methods (afforestation and ocean alkalinization).

This subsection assesses three aspects of the climate–carbon cycle response to CDR: the time-dependent behaviour of CO <sub>2</sub> fluxes in scenarios with CDR, the effectiveness of CDR in drawing down atmospheric CO <sub>2</sub> and cooling global mean temperature, and the symmetry of the climate–carbon cycle response to positive and negative CO <sub>2</sub> emissions.

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===== 5.6.2.1.1 Carbon cycle response to instantaneous CDR =====

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Idealized ‘pulse’ removal Earth system model simulations are useful for understanding the carbon cycle response to CDR. Figure 5.32 illustrates the response of atmospheric CO <sub>2</sub> , land and ocean carbon sinks to an instantaneous CO <sub>2</sub> removal applied from a pre-industrial equilibrium state. Following CO <sub>2</sub> removal from the atmosphere, the atmospheric CO <sub>2</sub> concentration declines rapidly at first and then rebounds (Figure 5.32a). This rebound is due to CO <sub>2</sub> release by the terrestrial biosphere and the ocean in response to declining atmospheric CO <sub>2</sub> levels (Figure 5.32b,c; M. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ). For the model simulations shown in Figure 5.32, 23 ± 6% (mean ± 1 standard deviation) of the 100 PgC removed remains out of the atmosphere 80–100 years after the instantaneous removal. The remainder is offset by CO <sub>2</sub> outgassing from the land (49 ± 12%) and ocean (29 ± 7%). While the direction of the CO <sub>2</sub> flux is robust across models, the relative contribution of the outgassing from land and ocean reservoirs to the atmospheric CO <sub>2</sub> rebound after removal varies. These results corroborate the ''high confidence'' placed by AR5 WGI (Chapter 6) on the partial compensation of CO <sub>2</sub> removal from the atmosphere by CO <sub>2</sub> outgassing from the land and ocean. Due to disagreement between models, the magnitude of this outgassing and the relative contribution of land and ocean fluxes remains ''low confidence'' .

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[[File:f3780888188c488285b91664a404d52e IPCC_AR6_WGI_Figure_5_32.png]]

'''Figure 5.32 |''' '''Carbon cycle response to instantaneous carbon dioxide (CO''' <sub>2</sub> ''') removal from''' '''the atmosphere.''' '''(a)''' Atmospheric CO <sub>2</sub> concentration; '''(b)''' change in land carbon reservoir; '''(c)''' change in ocean carbon reservoir. Results are shown for simulations with seven CMIP6 Earth system models and the University of Victoria Earth System Climate Model (UVic ESCM) of intermediate complexity forced with 100 PgC instantaneously removed from the atmosphere. The ‘pulse’ removal is applied from a model state in equilibrium with a pre-industrial atmospheric CO <sub>2</sub> concentration (CDRMIP experiment CDR-pi-pulse; [[#Keller--2018b|Keller et al., 2018b]] ). Changes in land and ocean carbon reservoirs are calculated relative to a pre-industrial control simulation. Data for the UVic ESCM is from [[#Zickfeld--2021|Zickfeld et al. (2021)]] . Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

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===== 5.6.2.1.2 Carbon cycle response over time in scenarios with CDR =====

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Since AR5 WGI (Chapter 6), studies with ESMs have explored the land and ocean carbon sink response to scenarios with CO <sub>2</sub> emissions gradually declining during the 21st century. As CDR and other mitigation activities are ramped up, CO <sub>2</sub> emissions in these scenarios reach net zero and, as removals exceed emissions, become net negative. Studies exploring the carbon sink response to such scenarios (e.g., RCP2.6, SSP1-2.6) show that, when net CO <sub>2</sub> emissions are positive but start to decline, uptake of CO <sub>2</sub> by the land and ocean begins to weaken (compare land and ocean CO <sub>2</sub> fluxes in panels a and b of Figure 5.33; [[#Tokarska--2015|Tokarska and Zickfeld, 2015]] ; [[#Jones--2016b|Jones et al., 2016b]] ). During the first decades after CO <sub>2</sub> emissions become net negative, both the ocean and land carbon sinks continue to take up CO <sub>2</sub> , albeit at a lower rate. For the land carbon sink, the sink-to-source transition occurs decades to a century after CO <sub>2</sub> emissions become net negative (Figure 5.33c). The ocean remains a sink of CO <sub>2</sub> for centuries after emissions become net negative (Figure 5.33c–e; [[#5.4.9|Section 5.4.9]] ; Figure 5.30). Whether the transition to source occurs at all, the timing of the transition and the magnitude of the CO <sub>2</sub> source are determined by the magnitude of the removal and the rate and amount of net CO <sub>2</sub> emissions prior to emissions becoming net negative ( ''medium confidence'' ) ( [[#Tokarska--2015|Tokarska and Zickfeld, 2015]] ; [[#Jones--2016b|Jones et al., 2016b]] ). For scenarios with large amounts of CO <sub>2</sub> removal, such as SSP5-3.4-overshoot, the land source is larger than for SSP1-2.6 and the ocean also turns into a source ( [[#5.4.10|Section 5.4.10]] , Figure 5.30). While the qualitative response to scenarios with net-negative emissions is largely robust across models, the timing of the sink-to-source transition and the magnitude of the CO <sub>2</sub> source vary between models, particularly for the land sink. Due to ''low agreement'' between 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.

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[[File:cc8957d18d88db9d40ad241c093f7844 IPCC_AR6_WGI_Figure_5_33.png]]

'''Figure 5.33 |''' '''Carbon sink response in a scenario with net carbon dioxide (CO''' <sub>2</sub> ''') removal from the atmosphere.''' Shown are CO <sub>2</sub> 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; '''(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, 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; [[#Mengis--2020|Mengis et al., 2020]] ). 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 (ppm). Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

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===== 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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===== 5.6.2.1.4 Symmetry of carbon cycle response to positive and negative CO 2 emissions =====

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It is commonly assumed that the climate–carbon cycle response to a negative CO <sub>2</sub> emission (i.e., removal from the atmosphere) is equal in magnitude and opposite in sign to the response to a positive CO <sub>2</sub> emission of equal magnitude – that is, symmetric. If the response were symmetric, a positive CO <sub>2</sub> emission could be offset by a negative emission of equal magnitude. This subsection assesses the symmetry in the coupled climate–carbon cycle response in model simulations with positive and negative CO <sub>2</sub> emission pulses applied from a pre-industrial climate state. Simulations with seven CMIP6 ESMs and the UVic Earth System Climate Model (ESCM) of intermediate complexity suggest that the carbon cycle response is asymmetric for pulse emissions/removals of ±100 PgC (Figure 5.35). For all models, the fraction of CO <sub>2</sub> remaining in the atmosphere after an emission is larger than the fraction of CO <sub>2</sub> remaining out of the atmosphere after a removal (by 4 ± 3%; mean ± standard deviation). In other words, an emission of CO <sub>2</sub> into the atmosphere is more effective at raising atmospheric CO <sub>2</sub> than an equivalent CO <sub>2</sub> removal is at lowering it. Sensitivity experiments with the UVic ESCM suggest that the asymmetry increases for larger amounts of emissions/removals and is insensitive to the background atmospheric CO <sub>2</sub> concentration from which the emissions/removals are applied (Figure 5.35). This asymmetry in the atmospheric CO <sub>2</sub> response originates from asymmetries in the land and ocean carbon fluxes due to non-linearities in the carbon cycle response to CO <sub>2</sub> and temperature ( [[#5.4|Section 5.4]] ) ( [[#Zickfeld--2021|Zickfeld et al., 2021]] ). Given ''medium evidence'' and ''high agreement'' , there is ''medium confidence'' in the sign of the asymmetry of the carbon cycle response to positive and negative CO <sub>2</sub> emissions. The sign of the symmetry of the temperature response differs between models, with three out of seven examined ESMs showing a smaller temperature response to a 100 PgC emission than to an equivalent CO <sub>2</sub> removal. Therefore, there is ''low confidence'' in the sign of the asymmetry of the temperature response to positive and negative CO <sub>2</sub> emissions.

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[[File:0d06ac2654ccbbcf807e96c9a11b2902 IPCC_AR6_WGI_Figure_5_35.png]]

'''Figure 5.35 |''' '''Asymmetry in the atmospheric carbon dioxide (CO''' <sub>2</sub> ''') response to CO''' <sub>2</sub> '''emissions and removals.''' Shown are the fractions of total CO <sub>2</sub> emissions remaining in the atmosphere (right-hand side) and CO <sub>2</sub> removals remaining out of the atmosphere (left-hand side) 80–100 after a pulse emission/removal. Triangles and green circles denote results for seven Earth system models (ESMs) and the UVic ESCM model of intermediate complexity forced with ±100 PgC pulses applied from a pre-industrial state (1 × CO <sub>2</sub> ) (Carbon Dioxide Removal Model Intercomparison Project (CDRMIP) experiment CDR-pi-pulse; [[#Keller--2018b|Keller et al., 2018b]] ). Yellow circles and diamonds indicate UVic ESCM results for CO <sub>2</sub> emissions/removals applied at 1.5 times (1.5 × CO <sub>2</sub> ) and 2 times (2 × CO <sub>2</sub> ) the pre-industrial CO <sub>2</sub> concentration, respectively. Pulses applied from a 2 × CO <sub>2</sub> state span the magnitude ±100 PgC to ±500 PgC. UVic ESCM data is from [[#Zickfeld--2021|Zickfeld et al. (2021)]] . Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

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'''Box 5.3 | Carbon Cycle Response to CO''' <sub>2</sub> '''Removal from''' '''the Atmosphere'''

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During the industrial era, CO <sub>2</sub> emitted by the combustion of fossil fuels and land-use change has been redistributed between atmosphere, land, and ocean carbon reservoirs due to carbon cycle processes (Box 5.3, Figure 1b and Figure 5.13). Over the past decade (2010–2019), 46% of the emitted CO <sub>2</sub> remained in the atmosphere, 23% was taken up by the ocean, and 31% by the terrestrial biosphere ( [[#5.2.1.5|Section 5.2.1.5]] ). When carbon dioxide removal (CDR) is applied during periods in which human activities are net CO <sub>2</sub> sources to the atmosphere, and the amount of emissions removed by CDR is smaller than the net source (net positive CO <sub>2</sub> emissions), CDR acts to reduce the net emissions (Box 5.3 Figure 1c). In this scenario, part of the CO <sub>2</sub> emissions in the atmosphere are removed by the land and ocean sinks, as has been the case historically.

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[[File:b0f01dff95c42721b9c68bcb0a161c4c IPCC_AR6_WGI_Box_5_3_Figure_1.png]]

'''Box 5.3, Figure 1 |'''

'''Schematic representation of carbon fluxes between atmosphere, land, ocean and geological reservoirs.''' Different system conditions are shown: '''(a)''' an unperturbed Earth system; and changes in carbon fluxes for '''(b)''' an Earth system perturbed by fossil fuel carbon dioxide (CO <sub>2</sub> ) emissions; '''(c)''' an Earth system in which fossil fuel CO <sub>2</sub> emissions are partially offset by carbon dioxide removal (CDR); '''(d)''' an Earth system in which CDR exceeds CO <sub>2</sub> emissions from fossil fuels (‘net negative’ CO <sub>2</sub> emissions). Carbon fluxes depicted in (a) (solid and dashed black lines) also occur in (b–d). The question mark in the land-to-ocean carbon flux perturbation in (c) and (d) indicates that the effect of CDR on this flux is unknown. Note that box sizes do not scale with the size of carbon reservoirs. Adapted from [[#Keller--2018a|Keller et al. (2018a)]] . Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

When CDR removes more CO <sub>2</sub> emissions than human activities emit (net negative CO <sub>2</sub> emissions), and atmospheric CO <sub>2</sub> declines, the land and ocean sinks initially continue to take up CO <sub>2</sub> from the atmosphere. This is because carbon sinks, particularly the ocean, exhibit inertia and continue to respond to the prior trajectory of rising atmospheric CO <sub>2</sub> concentration. After some time, which is determined by the magnitude of the removal and the rate and amount of CO <sub>2</sub> emissions prior to the CDR application, land and ocean carbon reservoirs begin to release CO <sub>2</sub> into the atmosphere, making CDR less effective (Box 5.3, Figure 1d).

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