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

=== 5.4.10 Long-term Response Past 2100 ===

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The AR5 assessed with ''very high confidence'' that the carbon cycle in the ocean and on land will continue to respond to climate change and rising atmospheric CO <sub>2</sub> concentrations created during the 21 <sup>st</sup> century (WGI, Chapter 6, Executive Summary). Since AR5, experiments with the Community Earth System Model version 1 (CESM1) under the RCP8.5 extension scenario to 2300, suggest that both land and ocean carbon–climate feedbacks strengthen in time, land and ocean carbon-concentration feedbacks weaken, and the relative importance of ocean sinks versus land sinks increases ( [[#Randerson--2015|Randerson et al., 2015]] ). Under high emissions scenarios, this relative strengthening of land carbon–climate feedbacks leads the terrestrial biosphere to shift from sink to source at some point after 2100 in all of the CMIP5 ESMs and CMIP5-era Earth system models of intermediate complexity (EMICs) ( [[#Tokarska--2016|Tokarska et al., 2016]] ). The strengthening of land and ocean carbon–climate feedbacks projected beyond 2100 under high emissions scenarios offsets the declining climate sensitivity to incremental increases of CO <sub>2</sub> , leading to a net strengthening of carbon cycle feedbacks, as measured by the gain parameter, from one century to the next ( [[#Randerson--2015|Randerson et al., 2015]] ).

Figure 5.30 shows carbon cycle changes to 2300 under three SSP scenarios with long-term extensions: SSP5-8.5, SSP5-3.4-overshoot, and SSP1-2.6, for four CMIP6 ESMs and one EMIC. Under all three scenarios, all five models project a reversal of the terrestrial carbon cycle from a sink to a source. However, the reasons for these reversals under very high emissions and low/negative emissions are very different. Under the SSP5-8.5 scenario, the terrestrial carbon–climate feedback is projected to strengthen, while the carbon-concentration feedbacks weaken after emissions peak at 2100, which together drives the land to become a net carbon source after 2100 ( [[#Tokarska--2016|Tokarska et al., 2016]] ). The difference in both timing and magnitude of this transition across the ensemble, leads to an assessment of ''medium confidence'' in the likelihood and ''low confidence'' in the timing and strength, of the land transitioning from a net sink to a net source under such a scenario. Based on ''high agreement'' across all available models, we assess with ''high confidence'' that the ocean sink strength would weaken but not reverse under a long-term high emissions scenario. In the SSP5-3.4-overshoot scenario, both the terrestrial and ocean reservoirs act as transient carbon sources during the overshoot period, when net anthropogenic CO <sub>2</sub> emissions are negative and CO <sub>2</sub> concentrations are falling, and then revert to near-zero (land) or weak sink (ocean) fluxes after stabilization of atmospheric CO <sub>2</sub> . The SSP1-2.6 scenario, characterized by lower peak CO <sub>2</sub> concentrations, a smaller overshoot, and much less carbon loss from land-use change, shows instead a relaxation towards a neutral biosphere on land, and a sustained weak sink in the ocean (see also [[#5.6.2.2.1.2|Section 5.6.2.2.1.2]] ).

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'''Figure 5.30 |''' '''Trajectories of carbon cycle dynamics for models beyond 2100.''' Shown are three scenarios: SSP5-8.5; SSP5-3.4-overshoot; and SSP1-2.6, from four ESMs (CanESM5, UKESM1, CESM2-WACCM, IPSL-CM6a-LR) and one EMIC (UVIC-ESCM, [[#Mengis--2020|Mengis et al., 2020]] ) for which extensions beyond 2100 are available. Solid lines represent the median flux value across the ensemble, and shading represents 15 <sup>th</sup> –85 <sup>th</sup> percentiles across the ensemble. Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

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