Editing IPCC:AR6/WGIII/Chapter-3 (section)

=== 3.2.3 The Scenario Literature and Scenario Databases ===

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IPCC reports have often used voluntary submissions to a scenario database in its assessments. The database is an ensemble of opportunity, as there is not a well-designed statistical sampling of the hypothetical model or scenario space: the literature is unlikely to cover all possible models and scenarios, and not all scenarios in the literature are submitted to the database. Model intercomparisons are often the core of scenario databases assessed by the IPCC ( [[#Cointe--2019|Cointe et al. 2019]] ; [[#Nikas--2021|Nikas et al. 2021]] ). Single-model studies may allow more detailed sensitivity analyses or address specific research questions. The scenarios that are organised within the scientific community are more likely to enter the assessment process via the scenario database ( [[#Cointe--2019|Cointe et al. 2019]] ), while scenarios from different communities, in the emerging literature, or not structurally consistent with the database may be overlooked. Scenarios in the grey literature may not be assessed even though they may have greater weight in a policy context.

One notable development since AR5 is the Shared Socio-economic Pathways (SSPs), conceptually outlined in [[#Moss--2010|Moss et al. (2010)]] and subsequently developed to support integrated climate research across the IPCC Working Groups ( [[#O’Neill--2014|O’Neill et al. 2014]] ). Initially, a set of SSP narratives were developed, describing worlds with different challenges to mitigation and adaptation ( [[#O’Neill--2017a|O’Neill et al. 2017a]] ): SSP1 (sustainability), SSP2 (middle of the road), SSP3 (regional rivalry), SSP4 (inequality) and SSP5 (rapid growth). The SSPs have now been quantified in terms of energy, land-use, and emission pathways ( [[#Riahi--2017|Riahi et al. 2017]] ), for both no-climate-policy reference scenarios and mitigation scenarios that follow similar radiative-forcing pathways as the Representative Concentration Pathways (RCPs) assessed in AR5 WGI. Since then the SSPs have been successfully applied in thousands of studies ( [[#O’Neill--2020b|O’Neill et al. 2020b]] ) including some critiques on the use and application of the SSP framework ( [[#Pielke--2021|Pielke and Ritchie 2021]] ; [[#Rosen--2021|Rosen 2021]] ). A selection of the quantified SSPs are used prominently in AR6 WGI as they were the basis for most climate modelling since AR5 ( [[#O’Neill--2016|O’Neill et al. 2016]] ). Since 2014, when the first set of SSP data was made available, there has been a divergence between scenario and historic trends ( [[#Burgess--2020|Burgess et al. 2020]] ). As a result, the SSPs require updating ( [[#O’Neill--2020b|O’Neill et al. 2020b]] ). Most of the scenarios in the AR6 database are SSP-based and consider various updates compared to the first release ( [[#Riahi--2017|Riahi et al. 2017]] ).

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