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

=== 4.2.2 Scenarios ===

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The AR5 drew heavily on four main scenarios, known as Representative Concentration Pathways (RCPs: [[#Meinshausen--2011|Meinshausen et al., 2011]] ; [[#van%20Vuuren--2011|van Vuuren et al., 2011]] ), and simulation results from CMIP5 ( [[#4.2.1|Section 4.2.1]] ; [[#Taylor--2012|Taylor et al., 2012]] ). The RCPs were labelled by the approximate radiative forcing reached at the year 2100, going from 2.6, 4.5, 6.0 to 8.5 W m <sup>–2</sup> .

This chapter draws on model simulations from CMIP6 ( [[#Eyring--2016|Eyring et al., 2016]] ) using a new range of scenarios based on Shared Socio-economic Pathways (SSPs; [[#O’Neill--2016|O’Neill et al., 2016]] ). The set of SSPs is described in detail in [[IPCC:Wg1:Chapter:Chapter-1|Chapter 1]] ( [[IPCC:Wg1:Chapter:Chapter-1#1.6|Section 1.6]] ) and recognizes that global radiative forcing levels can be achieved by different pathways of CO <sub>2</sub> , non-CO <sub>2</sub> greenhouse gases (GHGs), aerosols ( [[#Amann--2013|Amann et al., 2013]] ; [[#Rao--2017|Rao et al., 2017]] ) and land use; the set of SSPs therefore establishes a matrix of global forcing levels and socio-economic storylines. ScenarioMIP ( [[#O’Neill--2016|O’Neill et al., 2016]] ) identifies four priority (tier-1) scenarios that participating modelling groups are asked to perform, SSP1-2.6 for sustainable pathways, SSP2-4.5 for middle-of-the-road, SSP3-7.0 for regional rivalry, and SSP5-8.5 for fossil fuel-rich development. This chapter focuses its assessment on these, plus the SSP1-1.9 scenario, which is directly relevant to the assessment of the 1.5°C Paris Agreement goal. Further, this chapter discusses these scenarios and their extensions past 2100 in the context of the very long-term climate change in [[#4.7.1|Section 4.7.1]] . Projections of short-lived climate forcers (SLCFs) are assessed in more detail in [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] (Section 6.7).

In presenting results and evidence, this chapter tries to be as comprehensive as possible. In tables we show multi-model mean change and 5–95% range for all five SSPs, while in time series figures we show multi-model mean change for all five SSPs but for clarity 5–95% range only for SSP1-2.6 and SSP3-7.0. Where maps are presented, due to space restrictions we focus on showing multi-model mean change for SSP1-2.6 and SSP3-7.0. SSP1-2.6 is preferred over SSP1-1.9 because the latter has far fewer simulations available. The high-end scenarios RCP8.5 or SSP5-8.5 have recently been argued to be implausible to unfold (e.g., [[#Hausfather--2020|Hausfather and Peters, 2020]] ; see Chapter 3 of the AR6 WGIII). However, where relevant we show results for SSP5-8.5, for example to enable backwards compatibility with AR5, for comparison between emissions-driven and concentration-driven simulations, and because there is greater data availability of daily output for SSP5-8.5. When presenting low-likelihood, high-warming storylines we also show results from the high-end SSP5-8.5 scenario.

ScenarioMIP simulations include advances in techniques to better harmonize with historical forcings relative to CMIP5. For example, projected changes in the solar cycle include long-term modulation rather than a repeating solar cycle ( [[#Matthes--2017|Matthes et al., 2017]] ). Background natural aerosols are ramped down to an average historical level used in the control simulation by 2025 and background volcanic forcing is ramped up from the value at the end of the historical simulation period (2015) over 10 years to the same constant value prescribed for the pre-industrial control (piControl) simulations in the DECK, and then kept fixed – both changes are intended to avoid inconsistent model treatment of unknowable natural forcing to affect the near-term projected warming.

Complete backward comparability between CMIP5 and CMIP6 scenarios cannot be established for detailed regional assessments, because the SSP scenarios include regional forcings – especially from land use and aerosols – that are different from the CMIP5 RCPs. Even at a global level, a quantitative comparison is challenging between corresponding SSP and RCP radiative forcing levels due to differing contributions to the forcing ( [[#Meinshausen--2020|Meinshausen et al., 2020]] ) and evidence of differing model responses ( [[#4.6.2.2|Section 4.6.2.2]] ; [[#Wyser--2020|Wyser et al., 2020]] ). The RCP scenarios assessed in AR5 all showed similar, rapid reductions in SLCFs and emissions of SLCF precursor species over the 21st century; the CMIP5 projections hence did not sample a wide range of possible trajectories for future SLCFs ( [[#Chuwah--2013|Chuwah et al., 2013]] ). The SSP scenarios assessed in the AR6 offer more scope to explore SLCF pathways as they sample a broader range of air quality policy options ( [[#Gidden--2019|Gidden et al., 2019]] ) and relationships of CO <sub>2</sub> to non-CO <sub>2</sub> greenhouse gases ( [[#Meinshausen--2020|Meinshausen et al., 2020]] ). [[#4.6.2.2|Section 4.6.2.2]] assesses RCP and SSP differences. Other MIPs (see [[#4.2.1|Section 4.2.1]] ) have been designed to explicitly explore some of the implications of the different socio-economic storylines for a given radiative forcing level.

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