Editing IPCC:Wg1:Chapter:Chapter-1-comments (section)

==== 1.6.1.1 Shared Socio-economic Pathways ====

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The Shared Socio-economic Pathways SSP1 to SSP5 describe a range of plausible trends in the evolution of society over the 21st century. They were developed in order to connect a wide range of research communities ( [[#Nakicenovic--2014|Nakicenovic et al., 2014]] ) and consist of two main elements: a set of qualitative, narrative storylines describing societal futures ( [[#O’Neill--2017a|O’Neill et al., 2017a]] ) and a set of quantified measures of development at aggregated and/or spatially resolved scales. Each pathway is an internally consistent, plausible and integrated description of a socio-economic future, but these socio-economic futures do not account for the effects of climate change, and no new climate policies are assumed. The SSPs’ quantitative projections of socio-economic drivers include population, gross domestic product (GDP) and urbanization ( [[#Dellink--2017|Dellink et al., 2017]] ; [[#Jiang--2017|Jiang and O’Neill, 2017]] ; [[#Samir--2017|Samir and Lutz, 2017]] ). By design, the SSPs differ in terms of the socio-economic challenges they present for climate change mitigation and adaptation ( [[#Rothman--2014|Rothman et al., 2014]] ; [[#Schweizer--2014|Schweizer and O’Neill, 2014]] ) and the evolution of these drivers within each SSP reflects this design. Broadly, the five SSPs represent ‘sustainability’ (SSP1), a ‘middle-of-the-road’ path (SSP2), ‘regional rivalry’ (SSP3), ‘inequality’ (SSP4), and ‘fossil fuel-intensive’ development (SSP5; Cross-Chapter Box 1.4, Figure 1; [[#O’Neill--2017a|O’Neill et al., 2017a]] ). More specific information on the SSP framework and the assumptions underlying the SSPs will be provided in the IPCC WGIII report (WGIII Chapter 3; see also Box SPM.1 in SRCCL ( [[#IPCC--2019d|IPCC, 2019d]] )).

The SSP narratives and drivers were used to develop scenarios of energy use, air pollution control, land use, and GHG emissions developments using integrated assessment models (IAMs; [[#Riahi--2017|Riahi et al., 2017]] ; [[#Rogelj--2018a|Rogelj et al., 2018a]] ). An IAM can derive multiple emissions futures for each socio-economic development pathway, assuming no new mitigation policies or various levels of additional mitigation action (in the case of reference scenarios and mitigation scenarios, respectively; [[#Riahi--2017|Riahi et al., 2017]] ). By design, the evolution of drivers and emissions within the SSP scenarios do not take into account the effects of climate change.

The SSPX-Y scenarios and the RCP scenarios are categorized similarly, by reference to the approximate radiative forcing levels each one entails at the end of the 21st century. For example, the ‘1.9’ in the SSP1-1.9 scenario stands for an approximate radiative forcing level of 1.9 W m <sup>–2</sup> in 2100. The first number (X) in the ‘SSPX-Y’ acronym refers to one of the five shared socio-economic development pathways (Cross-Chapter Box 1.4, Figure 1 and Table 1.4).
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'''Table 1.4 | Overview of different RCP and SSP acronyms as used in this report.'''

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This SSP scenario categorization, focused on end-of-century radiative forcing levels, reflects how scenarios were conceptualized until recently, namely, to reach a particular climate target in 2100 at the lowest cost and irrespective of whether the target was exceeded over the century. More recently, and in particular since IPCC SR1.5 report focused attention on peak warming scenarios ( [[#Rogelj--2018b|Rogelj et al., 2018b]] ), scenario development started to explicitly consider peak warming, cumulative emissions and the amount of net negative emissions ( [[#Rogelj--2018b|Rogelj et al., 2018b]] ; [[#Fujimori--2019|Fujimori et al., 2019]] ).

The SSP scenarios can be used for either emissions- or concentration-driven model experiments (Cross-Chapter Box 1.4). ESMs can be run with emissions and concentrations data for GHGs and aerosols and land-use or landcover maps and calculate levels of radiative forcing internally. The radiative forcing labels of the RCP and SSP scenarios, such as ‘2.6’ in RCP2.6 or SSP1-2.6, are thus approximate labels for the year 2100 only. The actual global mean effective radiative forcing varies across ESMs due to different radiative transfer schemes, uncertainties in aerosol–cloud interactions, and different feedback mechanisms, among other reasons. Nonetheless, using approximate radiative forcing labels is advantageous because it establishes a clear categorization of scenarios, with multiple climate forcings and different combinations in those scenarios summarized in a single number. The classifications according to cumulative carbon emissions ( [[#1.6.3|Section 1.6.3]] ) and global warming level ( [[#1.6.2|Section 1.6.2]] and Cross-Chapter Box 7.1 on emulators) complement those forcing labels.

A key advance of the SSP scenarios relative to the RCPs is a wider span of assumptions on future air-quality mitigation measures, and hence emissions of short-lived climate forcers (SLCFs; [[#Rao--2017|Rao et al., 2017]] ; [[#Lund--2020|Lund et al., 2020]] ). This allows for a more detailed investigation into the relative roles of GHG and SLCF emissions in future global and regional climate change, and hence the implications of policy choices. For instance, SSP1-2.6 builds on an assumption of stringent air-quality mitigation policy, leading to rapid reductions in particle emissions, while SSP3-7.0 assumes slow improvements, with pollutant emissions over the 21st century comparable to current levels (Figure 6.19 and Cross-Chapter Box 1.4, Figure 2).

One limitation of the SSP scenarios used for CMIP6 and in this Report is that they reduce emissions from all the major ozone-depleting substances controlled under the Montreal Protocol (CFCs, halons, and hydrochlorofluorocarbons (HCFCs)) uniformly, rather than representing a fuller range of possible high- and low-emissions futures ( [[#UNEP--2016|UNEP, 2016]] ). Hydrofluorocarbon (HFC) emissions, on the other hand, span a wider range within the SSPs than in the RCPs (Cross-Chapter Box 1.4, Figure 2).

The SSP scenarios and previous RCP scenarios are not directly comparable. First, the gas-to-gas compositions differ; for example, the SSP5-8.5 scenario has higher CO <sub>2</sub> concentrations but lower CH <sub>4</sub> concentrations compared to RCP8.5. Second, the projected 21st-century trajectories may differ, even if they result in the same radiative forcing by 2100. Third, the overall effective radiative forcing (Chapter 7) may differ, and tends to be higher for the SSPs compared to RCPs that share the same nominal stratospheric-temperature-adjusted radiative forcing label. The stratospheric-temperature-adjusted radiative forcings of the SSPs and RCPs, however, remain relatively close, at least by 2100 ( [[#Tebaldi--2021|Tebaldi et al., 2021]] ). In summary, differences in, for example, CMIP5 RCP8.5 and CMIP6 SSP5-8.5 ESM outputs, are partially due to different scenario characteristics rather than different ESM characteristics only ( [[IPCC:Wg1:Chapter:Chapter-4#4.6.2|Section 4.6.2]] ).

When investigating various mitigation futures, WGIII goes beyond the core set of SSP scenarios assessed in WGI (SSP1-1.9, SSP1-2.6, etc.) to consider the characteristics of more than 1000 scenarios (Cross-Chapter Box 7.1). In addition, while staying within the framework of socio-economic development pathways (SSP1 to SSP5), WGIII also considers various mitigation possibilities through so-called illustrative pathways (IPs). These illustrative pathways help to highlight key narratives in the literature concerning various technological, social and behavioural options for mitigation, various timings for implementation, or varying emphasis on different GHG and land-use options. Just as with the SSPX-Y scenarios considered in this Report, these illustrative pathways can be placed in relation to the matrix of SSP families and approximate radiative forcing levels in 2100 (Cross-Chapter Box 1.4, Figure 1; IPCC WGIII, Chapter 3).

No likelihood is attached to the scenarios assessed in this report, and the feasibility of specific scenarios in relation to current trends is best informed by the WGIII contribution to AR6. In the scenario literature, the plausibility of the high emissions levels underlying scenarios such as RCP8.5 or SSP5-8.5 has been debated in light of recent developments in the energy sector ( [[#1.6.1.4|Section 1.6.1.4]] ).

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