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

===== 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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