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

==== 1.6.1.3 History of Scenarios within the IPCC ====

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Scenario modelling experiments have been a core element of physical climate science since the first transient simulations with a general circulation model in 1988 ( [[#1.3|Section 1.3]] ; [[#Hansen--1988|Hansen et al., 1988]] ). Scenarios and modelling experiments assessed in IPCC reports have evolved over time, which provides a ‘history of how the future was seen’. The starting time for the scenarios moves as actual emissions supersede earlier emissions assumptions, while new scientific insights into the range of plausible population trends, behavioural changes and technology options and other key socio-economic drivers of emissions also emerge (see WGIII; [[#Leggett--1992|Leggett et al., 1992]] ; [[#IPCC--2000|IPCC, 2000]] ; [[#Moss--2010|Moss et al., 2010]] ; [[#Riahi--2017|Riahi et al., 2017]] ). Many different sets of climate projections have been produced over the past several decades, using different sets of scenarios. Here, we compare those earlier scenarios against the most recent ones.

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'''Figure 1.28 |''' '''Comparison of the range of fossil fuel and industrial CO''' <sub>2</sub> '''emissions from scenarios used in previous assessments up to AR6.''' Previous assessments are the IS92 scenarios from 1992 '''(top)''' , the Special Report on Emissions Scenarios (SRES) scenarios from the year 2000 '''(second panel)''' , the Representative Concentration Pathway (RCP) scenarios designed around 2010 '''(third panel)''' and the Shared Socio-economic Pathways (SSP) scenarios '''(fourth panel)''' . In addition, historical emissions are shown (black line; Figure 5.5); a more complete set of scenarios is assessed in SR1.5 '''(bottom)''' ; ( [[#Huppmann--2018|Huppmann et al., 2018]] ). Further details on data sources and processing are available in the chapter data table (Table 1.SM.1).
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Climate science research involving scenarios necessarily follows a series of consecutive steps (Figure 1.27). As each step waits for input from the preceding one, delays often occur that result in the impact literature basing its analyses on earlier scenarios than those most current in the climate change mitigation and climate system literature. It is therefore important to provide an approximate comparison across the various scenario generations (Chapter 4, Figure 1.28, and Cross-Chapter Box 1.4, Table 1).

The first widely used set of IPCC emissions scenarios was the IS92 scenarios in 1992 ( [[#Leggett--1992|Leggett et al., 1992]] ). Apart from reference scenarios, IS92 also included a set of stabilization scenarios, the so-called ‘S’ scenarios. Those ‘S’ pathways were designed to lead to CO <sub>2</sub> stabilization levels such as 350 ppm or 450 ppm. By 1996, those latter stabilization levels were complemented in the scientific literature by alternative trajectories that assumed a delayed onset of climate change mitigation action (Figure 1.28; [[#Wigley--1996|Wigley et al., 1996]] ).

By 2000, the IPCC Special Report on Emissions Scenarios (SRES) produced the SRES scenarios ( [[#IPCC--2000|IPCC, 2000]] ), albeit without assuming any climate policy-induced mitigation. The four broad groups of SRES scenarios (scenario ‘families’) – A1, A2, B1 and B2 – were the first scenarios to emphasize socio-economic scenario storylines, and also first to emphasize other GHGs, land-use change and aerosols. Represented by three scenarios for the high-growth A1 scenario family, those 6 SRES scenarios (A1FI, A1B, A1T, A2, B1, and B2) can still sometimes be found in today’s climate impact literature. The void of missing climate change mitigation scenarios was filled by a range of community exercises, including the so-called ‘post-SRES scenarios’ ( [[#Swart--2002|Swart et al., 2002]] ).

The RCP scenarios ( [[#van%20Vuuren--2011|van Vuuren et al., 2011]] ) then broke new ground by providing low-emissions pathways that implied strong climate change mitigation, including an example with negative CO <sub>2</sub> emissions on a large scale, namely RCP2.6. As shown in Figure 1.28, the upper end of the scenario range has not substantially shifted. Building on the SRES multi-gas scenarios, the RCPs include time series of emissions and concentrations of the full suite of GHGs, aerosols and chemically active gases, as well as land use and land cover ( [[#Moss--2010|Moss et al., 2010]] ). The word ‘representative’ signifies that each RCP is only one of many possible scenarios that would lead to the specific radiative forcing characteristics. The term ‘pathway’ emphasizes that not only the long-term concentration levels are of interest, but also the trajectory taken over time to reach that outcome ( [[#Moss--2010|Moss et al., 2010]] ). RCPs usually refer to the concentration pathway extending to 2100, for which IAMs produced corresponding emissions scenarios. Four RCPs produced from IAMs were selected from the published literature and are used in AR5 as well as in this report, spanning approximately the range from below 2°C warming to high (above 4°C) warming best-estimates by the end of the 21st century: RCP2.6, RCP4.5 and RCP6.0 and RCP8.5 (Cross-Chapter Box 1.4, Table 1). Extended Concentration Pathways (ECPs) describe extensions of the RCPs from 2100 to 2300 that were calculated using simple rules generated by stakeholder consultations; these do not represent fully consistent scenarios ( [[#Meinshausen--2011b|Meinshausen et al., 2011b]] ).

By design, theRCP emissions and concentrations pathways were originally developed using particular socio-economic development pathways, but those are no longer considered ( [[#Moss--2010|Moss et al., 2010]] ). The different levels of emissions and climate change represented in the RCPs can hence be explored against the backdrop of different socio-economic development pathways (SSP1 to SSP5; [[#1.6.1.1|Section 1.6.1.1]] and Cross-Chapter Box 1.4). This integrative SSP-RCP framework (‘SSPX-RCPY’ in Table 1.4) is now widely used in the climate impact and policy analysis literature (e.g., [[#ICONICS--2021|ICONICS, 2021]] ; [[#Green--2020|Green et al., 2020]] ; [[#O’Neill--2020|O’Neill et al., 2020]] ), where climate projections obtained under the RCP scenarios are analysed against the backdrop of various SSPs. Considering various levels of future emissions and climate change for each socio-economic development pathway was an evolution from the previous SRES framework ( [[#IPCC--2000|IPCC, 2000]] ), in which socio-economic and emissions futures were closely aligned.

The new set of scenarios (SSP1-1.9 to SSP5-8.5) now features a higher top level of CO <sub>2</sub> emissions (SSP5-8.5 compared to RCP8.5), although the most significant change is again the addition of a very low climate change mitigation scenario (SSP1-1.9, compared to the previous low scenario, RCP2.6). Also, historically, none of the previous scenario sets featured a scenario that involves a very pronounced peak-and-decline emissions trajectory, but SSP1-1.9 does so now. The full set of nine SSP scenarios now includes a high-aerosol-emissions scenario (SSP3-7.0). The RCPs featured more uniformly low aerosol trajectories across all scenarios (Cross-Chapter Box 1.4, Figure 2). More generally, the SSP scenarios feature a later peak of global emissions for the lower scenarios, simply as a consequence of historical emissions not having followed the trajectory projected by previous low scenarios (Figure 1.28).

Over the last decades, discussions around scenarios have often focussed on whether recent trends make certain future scenarios more or less probable or whether all scenarios are too high or too low. When the SRES scenarios first appeared, the debate was often whether the scenarios were overestimating actual world emissions developments (e.g., Castles and Henderson, 2003). With the strong emissions increase throughout the 2000s, that debate then shifted towards the question of whether the lower future climate change mitigation scenarios were rendered unfeasible ( [[#Pielke--2008|Pielke et al., 2008]] ; [[#van%20Vuuren--2008|van Vuuren and Riahi, 2008]] ). Historical emissions between 2000 and 2010 approximately track the upper half of SRES and RCP projections (Figure 1.28). More generally, the global fossil fuel and industrial CO <sub>2</sub> emissions of recent decades tracked approximately the middle of the projected scenario ranges (Figure 1.28), although with regional differences ( [[#Pedersen--2020|Pedersen et al., 2020]] ).

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