Editing IPCC:AR6/WGI/Chapter-4 (section)

==== 4.6.2.2 Consistency Between Shared Socio-economic Pathways and Representative Concentration Pathways ====

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As CMIP5 and CMIP6 employed different scenario sets (RCPs and SSPs, respectively; see [[IPCC:Wg1:Chapter:Chapter-1#1.6.1.1|Section 1.6.1.1]] and Cross-Chapter Box 1.4), we assess how much of the differences in projections are due to the scenario change and how much due to model changes. The CMIP6-simulated GSAT increases tend to be larger than in CMIP5, for nominally comparable scenarios ( [[#4.3.1|Section 4.3.1]] ; [[#Tebaldi--2021|Tebaldi et al., 2021]] ).

The radiative forcing labels on SSP and RCP scenarios is approximate and enables the multiple climate forcings within the scenario to be characterized by a single number. While the scenarios are similar in terms of the stratospheric adjusted radiative forcing ( [[#Tebaldi--2021|Tebaldi et al., 2021]] ), they differ more in their effective radiative forcing (ERF). The combination of component forcings (CO <sub>2</sub> , non-CO <sub>2</sub> greenhouse gases, aerosols) within the scenario also differ ( [[#Meinshausen--2020|Meinshausen et al., 2020]] ). The ERF levels in the RCP and SSP scenarios have been calculated by sampling uncertainty in forcing from a range of different GHG species and aerosols (see 7.SM.1.4 for details). Figure 4.35 shows the time evolution and 2081–2100 mean across the families of scenarios and how this affects projections of GSAT. That the ERFs differ between corresponding SSP and RCP scenarios makes a comparison between CMIP6 and CMIP5 projections challenging ( [[#Tebaldi--2021|Tebaldi et al., 2021]] ). [[#Wyser--2020|Wyser et al. (2020)]] find the EC-Earth3-Veg model exhibits stronger radiative forcing and substantially greater warming under SSP5-8.5 than RCP8.5, and similar, but smaller additional warmings for SSP2-4.5and SSP1-2.6 compared with RCP4.5 and RCP2.6, respectively. In addition to the global response, climate can vary regionally due to non-CO <sub>2</sub> components of forcing ( [[#Samset--2016|Samset et al., 2016]] ; [[#Richardson--2018a|Richardson et al., 2018a]] , b).

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[[File:68e2a399ae3deaecc5953de3fef12860 IPCC_AR6_WGI_Figure_4_35.png]]

'''Figure''' '''4.35 |''' '''Comparison of RCPs and SSPs run by a single emulator to estimate scenario differences.''' Time series with 5–95% ranges and medians of '''(a)''' effective radiative forcings, calculated as described in Annex 7.A.1; and '''(b)''' global surface air temperature projections relative to 1850–1900 for the RCP and SSP scenarios from MAGICC 7.5. Note that the nameplate radiative forcing level refers to stratospheric adjusted radiative forcings in AR5-consistent settings ( [[#Tebaldi--2021|Tebaldi et al., 2021]] ) while ERFs may differ. MAGICC7.5 is here run in the recommended setup for WGIII, prescribing observed GHG concentrations for the historical period and switching to emissions-driven runs in 2015. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

Emulators ( [[#cross-chapter-box-7.1|Cross-Chapter Box 7.1]] ) can be used to aid understanding of differences between generations of scenarios. The AR5 ( [[#Collins--2013|Collins et al., 2013]] ) explored the differences between CMIP3 and CMIP5 (their Figure 12.40). Here we use an emulator calibrated to AR6 assessed GSAT ranges, thus eliminating the effect of differences in the model ensembles, to analyse the differences between SSP and RCP scenarios. MAGICC7.5 in its WGIII-calibrated setup (see [[#cross-chapter-box-7.1|Cross-Chapter Box 7.1]] ) projects differences in 2081–2100 mean warming between the RCP2.6 and SSP1-2.6 scenarios of around 0.2°C, between RCP4.5 and SSP2-4.5 ofaround 0.3°C and between RCP8.5 and SSP5-8.5 of around 0.3°C (Figure 4.35b). The SSP scenarios also have a wider 5–95% range simulated by MAGICC7.5 explaining about half of the increased range seen when comparing CMIP5 and CMIP6 models. Higher climate sensitivity is, though, the primary reason behind the upper end of the warming for SSP5-8.5 reaching 1.5°C higher than the CMIP5 results. Compared with the differences between the CMIP5 and CMIP6 multi-model ensembles for the same scenario pairs (Table A6 in [[#Tebaldi--2021|Tebaldi et al., 2021]] ), the higher ERFs of the SSP scenarios contribute approximately half of the warmer CMIP6 SSP outcomes ( ''medium confidence'' ).

In summary, there is ''medium confidence'' that about half of the warming increase in CMIP6 compared to CMIP5 is due to higher climate sensitivity in CMIP6 models; the other half arises from higher ERF in nominally comparable scenarios (e.g., RCP8.5 and SSP5-8.5).

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