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

===== 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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