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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[[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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