Editing IPCC:AR6/WGI/Chapter-1 (section)

==== 1.6.1.4 The Likelihood of Reference Scenarios, Scenario Uncertainty and Storylines ====

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In general, no likelihood is attached to the scenarios assessed in this Report. The use of different scenarios for climate change projections allows the exploration of ‘scenario uncertainty’ ([[#1.4.4|Section 1.4.4]]; SR1.5; [[#Collins--2013|Collins et al., 2013]]). Scenario uncertainty is fundamentally different from geophysical uncertainties, which result from limitations in the understanding and predictability of the climate system ([[#Smith--2011|Smith and Stern, 2011]]). In scenarios, by contrast, future emissions depend to a large extent on the collective outcome of choices and processes related to population dynamics and economic activity, or on choices that affect a given activity’s energy and emissions intensity ([[#Jones--2000|Jones, 2000]]; [[#Knutti--2008|Knutti et al., 2008]]; [[#Kriegler--2012|Kriegler et al., 2012]]; [[#van%20Vuuren--2014|van Vuuren et al., 2014]]). Even if identical socio-economic futures are assumed, the associated future emissions still face uncertainties, since different experts and model frameworks diverge in their estimates of future emissions ranges ([[#Ho--2019|Ho et al., 2019]]).

When exploring various climate futures, scenarios with no, or no additional, climate policies are often referred to as ‘baseline’ or ‘reference scenarios’ ([[#1.6.1.1|Section 1.6.1.1]] and Glossary). Among the five core scenarios used most in this report, SSP3-7.0 and SSP5-8.5 are explicit ‘no-climate-policy’ scenarios (Cross-Chapter Box 1.4, Table 1; [[#Gidden--2019|Gidden et al., 2019]]), assuming a carbon price of zero. These future ‘baseline’ scenarios are hence counterfactuals that include fewer climate policies compared to ‘business-as-usual’ scenarios – given that ‘business-as-usual’ scenarios could be understood to imply a continuation of existing climate policies. Generally, future scenarios are meant to cover a broad range of plausible futures, due, for example to unforeseen discontinuities in development pathways ([[#Raskin--2020|Raskin and Swart, 2020]]), or to large uncertainties in underlying long-term projections of economic drivers ([[#Christensen--2018|Christensen et al., 2018]]). However, the likelihood of high-emissions scenarios such as RCP8.5 or SSP5-8.5 is considered low in light of recent developments in the energy sector ([[#Hausfather--2020a|Hausfather and Peters, 2020a]], b). Studies that consider possible future emissions trends in the absence of additional climate policies, such as the recent IEA 2020 World Energy Outlook ‘stated policy’ scenario ([[#IEA--2020|IEA, 2020]]), project approximately constant fossil fuel and industrial CO <sub>2</sub> emissions out to 2070, approximately in line with the intermediate RCP4.5, RCP6.0 and SSP2-4.5 scenarios ([[#Hausfather--2020b|Hausfather and Peters, 2020b]]) and the 2030 global emissions levels that are pledged as part of the Nationally Determined Contributions (NDCs) under the Paris Agreement ([[#1.2.2|Section 1.2.2]]; [[#Fawcett--2015|Fawcett et al., 2015]]; [[#Rogelj--2016|Rogelj et al., 2016]]; [[#UNFCCC--2016|UNFCCC, 2016]]; [[#IPCC--2018|IPCC, 2018]]). On the other hand, the default concentrations aligned with RCP8.5 or SSP5-8.5 and resulting climate futures derived by ESMs could be reached by lower emissions trajectories than RCP8.5 or SSP5-8.5. That is because the uncertainty range on carbon cycle feedbacks includes stronger feedbacks than assumed in the default derivation of RCP8.5 and SSP5-8.5 concentrations (Section 5.4; [[#Ciais--2013|Ciais et al., 2013]]; [[#Friedlingstein--2014|Friedlingstein et al., 2014]]; [[#Booth--2017|Booth et al., 2017]]).

To address long-term scenario uncertainties, scenario storylines (or ‘narratives’) are often used (see [[#1.4.4|Section 1.4.4]] for a more general discussion on ‘storylines’, also covering ‘physical climate storylines’; [[#Rounsevell--2010|Rounsevell and Metzger, 2010]]; [[#O’Neill--2014|O’Neill et al., 2014]]). Scenario storylines are descriptions of a future world, and the related large-scale socio-economic development pathways towards that world that are deemed plausible within the current state of knowledge and historical experience ([[#1.2.3|Section 1.2.3]]; WGIII). Scenario storylines attempt to ‘stimulate, provoke, and communicate visions of what the future could hold for us’ ([[#Rounsevell--2010|Rounsevell and Metzger, 2010]]) in settings where either limited knowledge or inherent unpredictability in social systems prevent a forecast or numerical prediction. Scenario storylines have been used in previous climate research, and they are the explicit or implicit starting point of any scenario exercise, including for the SRES scenarios ([[#IPCC--2000|IPCC, 2000]]) and the SSPs (e.g., [[#O’Neill--2017a|O’Neill et al., 2017a]]).

Recent technological or socio-economic trends might be informative for bounding near-term future trends, for example, if technological progress renders a mitigation technology cheaper than previously assumed. However, short-term emissions trends alone do not generally rule out an opposite trend in the future ([[#van%20Vuuren--2010|van Vuuren et al., 2010]]). The ranking of individual RCP emissions scenarios from the IAMs with regard to emissions levels is different for different time horizons, for example, 2020 compared with longer-term emissions levels. For example, the strongest climate change mitigation scenario, RCP2.6, was in fact the second highest CO <sub>2</sub> emissions scenario (jointly with RCP4.5) before 2020 in the set of RCPs and the strong global emissions decline in RCP2.6 only followed after 2020. Implicitly, this scenario feature was cautioning against the assumption that short-term trends predicate particular long-term trajectories. This is also the case in relation to the COVID-19 related drop in 2020 emissions. Potential changes in underlying drivers of emissions, such as those potentially incentivized by COVID-19 recovery stimulus packages, are more significant for longer-term emissions than the short-term deviation from recent emissions trends (Cross-Chapter Box 6.1 on COVID-19).

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