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

==== 3.2.1.1 Reference Scenarios ====

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It is common to define a reference scenario (also called a baseline scenario). Depending on the research question, a reference scenario could be defined in different ways ( [[#Grant--2020|Grant et al. 2020]] ): (i) a hypothetical world with no climate policies or climate impacts ( [[#Kriegler--2014b|Kriegler et al. 2014b]] ), (ii) assuming current policies or pledged policies are implemented ( [[#Roelfsema--2020|Roelfsema et al. 2020]] ), or (iii) a mitigation scenario to compare sensitivity with other mitigation scenarios ( [[#Kriegler--2014a|Kriegler et al. 2014a]] ; [[#Sognnaes--2021|Sognnaes et al. 2021]] ).

No-climate-policy reference scenarios have often been compared with mitigation scenarios (Clarke et al. 2014). A no-climate-policy scenario assumes that no future climate policies are implemented, beyond what is in the model calibration, effectively implying that the carbon price is zero. No-climate-policy reference scenarios have a broad range depending on socio-economic assumptions and model characteristics, and consequently are important when assessing mitigation costs ( [[#Riahi--2017|Riahi et al. 2017]] ; [[#Rogelj--2018|Rogelj et al. 2018]] b). As countries move forward with climate policies of varying stringency, no-climate-policy baselines are becoming increasingly hypothetical ( [[#Hausfather--2020|Hausfather and Peters 2020]] ). Studies clearly show current policies are having an effect, particularly when combined with the declining costs of low-carbon technologies ( [[#IEA--2020a|IEA 2020a]] ; [[#Roelfsema--2020|Roelfsema et al. 2020]] ; [[#Sognnaes--2021|Sognnaes et al. 2021]] ; [[#UNEP--2020|UNEP 2020]] ), and, consequently, realised trajectories begin to differ from earlier no-climate-policy scenarios ( [[#Burgess--2020|Burgess et al. 2020]] ). High-end emission scenarios, such as RCP8.5 and SSP5-8.5, are becoming less likely with climate policy and technology change (Box 3.3), but high-end concentration and warming levels may still be reached with the inclusion of strong carbon or climate feedbacks ( [[#Hausfather--2020|Hausfather and Peters 2020]] ; [[#Pedersen--2020|Pedersen et al. 2020]] ).

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