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

===== 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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'''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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'''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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