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

=== 3.2.5 Illustrative Mitigation Pathways ===

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Successive IPCC Assessment Reports (ARs) have used scenarios to illustrate key characteristics of possible climate (policy) futures. In AR5 four RCPs made the basis of climate modelling in WGI and WGII, with WGIII assessing over 1000 scenarios spanning those RCPs (Clarke et al. 2014). Of the over 400 scenarios assessed in SR1.5, four scenarios were selected to highlight the trade-off between short-term emission reductions and long-term deployment of BECCS ( [[#Rogelj--2018|Rogelj et al. 2018]] a), referred to as ‘Illustrative Pathways’ (IPs). AR6 WGI and WGII rely on the scenarios selected for CMIP6, called ScenarioMIP ( [[#O’Neill--2016|O’Neill et al. 2016]] ), to assess warming levels. In addition to the full set of scenarios, AR6 WGIII also uses selected Illustrative Mitigation Pathways (IMPs).

In WGIII, IMPs were selected to denote the implications of different societal choices for the development of future emissions and associated transformations of main GHG-emitting sectors (Figure 3.5a and Box 3.1). The most important function of the IMPs is to illustrate key themes that form a common thread in the report, both with a storyline and a quantitative illustration. The storyline describes the key characteristics that define an IMP. The quantitative versions of the IMPs provide numerical values that are internally consistent and comparable across chapters of the report. The quantitative IMPs have been selected from the AR6 scenario database. No assessment of the likelihood of each IMP has been made.

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'''Figure 3.5 |''' '''(a) Process for creating the AR6 scenario database and selecting the illustrative (mitigation) pathways.''' The compiled scenarios in the AR6 scenarios database were vetted for consistency with historical statistics and subsequently a temperature classification was added using climate model emulators. The illustrative (mitigation) pathways were selected from the full set of pathways based on storylines of critical mitigation strategies that emerged from the assessment. '''(b)''' An overview of the Illustrative Pathways selected for use in IPCC AR6 WGIII, consisting of pathways illustrative of higher emissions, Current Policies ( ''CurPol'' ) and Moderate Action ( ''ModAct'' ), and Illustrative Mitigation Pathways (IMPs): gradual strengthening of current policies ( ''IMP-GS'' ), extensive use of net negative emissions ( ''IMP-Neg'' ), renewables ( ''IMP-Ren'' ), low demand ( ''IMP-LD'' ), and shifting pathways ( ''IMP-SP'' ). The Ren2.0 and Neg2.0 scenarios are alternative scenarios to the IMPs. These pathways are based on renewables and extensive use of negative emissions, respectively, but leading to temperature levels comparable to the C3 category and have sometimes been used for comparison.

The selected scenarios (IPs) are divided into two sets (Figures 3.5 and 3.6): two reference pathways illustrative of high emissions and five Illustrative Mitigation Pathways (IMPs). The narratives are explained in full in Annex III.II.2.4. The two reference pathways explore the consequences of current policies and pledges: Current Policies ( ''CurPol'' ) and Moderate Action ( ''ModAct'' ). The ''CurPol'' pathway explores the consequences of continuing along the path of implemented climate policies in 2020 and only a gradual strengthening after that. The scenario illustrates the outcomes of many scenarios in the literature that project the trend from implemented policies until the end of 2020. The ''ModAct'' pathway explores the impact of implementing the Nationally Determined Contributions (NDCs) as formulated in 2020 and some further strengthening after that. In line with current literature, these two reference pathways lead to an increase in global mean temperature of more than 2°C ( [[#3.3|Section 3.3]] ).

The Illustrative Mitigation Pathways (IMPs) properly explore different pathways consistent with meeting the long-term temperature goals of the Paris Agreement. They represent five different pathways that emerge from the overall assessment. The IMPs differ in terms of their focus, for example, placing greater emphasis on renewables (IMP-Ren), deployment of carbon dioxide removal that results in net negative global GHG emissions (IMP-Neg), and efficient resource use and shifts in consumption patterns, leading to low demand for resources, while ensuring a high level of services (IMP-LD). Other IMPs illustrate the implications of a less rapid introduction of mitigation measures followed by a subsequent gradual strengthening (IMP-GS), and how shifting global pathways towards sustainable development, including by reducing inequality, can lead to mitigation (IMP-SP) In the IMP framework, ''IMP-GS'' is consistent with limiting warming to 2°C (&gt;67%) (C3), ''IMP-Neg'' shows a strategy that also limits warming to 2°C (&gt;67%) but returns to nearly 1.5°C (&gt;50%) by the end of the century (hence indicated as C2*). The other variants that can limit warming to 1.5°C (&gt;50%) (C1) were selected. In addition to these IMPs, sensitivity cases that explore alternative warming levels (C3) for ''IMP-Neg'' and ''IMP-Ren'' are assessed ( ''IMP-Neg-2.0'' and ''IMP-Ren-2.0'' ).

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'''Figure 3.6 | Overview of the net CO''' 2 '''emissions and Kyoto greenhouse gas (GHG) emissions for each Illustrative Mitigation Pathway (IMP).'''

The IMPs are selected to have different mitigation strategies, which can be illustrated looking at the energy system and emission pathways (Figure 3.7 and Figure 3.8). The mitigation strategies show the different options in emission reduction (Figure 3.7). Each panel shows the key characteristics leading to total GHG emissions, consisting of residual (gross) emissions (fossil CO 2 emissions, CO 2 emissions from industrial processes, and non-CO 2 emissions) and removals (net land-use change, bioenergy with carbon capture and storage – BECCS, and direct air carbon capture and storage – DACCS), in addition to avoided emissions through the use of carbon capture and storage on fossil fuels. The ''IMP-Neg'' and ''IMP-GS'' scenarios were shown to illustrate scenarios with a significant role of CDR. The energy supply (Figure 3.8) shows the phase-out of fossil fuels in the ''IMP-LD'' , ''IMP-Ren'' and ''IMP-SP'' cases, but a less substantial decrease in the ''IMP-Neg'' case. The ''IMP-GS'' case needs to make up its slow start by (i) rapid reductions mid-century and (ii) massive reliance on net negative emissions by the end of the century. The ''CurPol'' and ''ModAct'' cases both result in relatively high emissions, showing a slight increase and stabilisation compared to current emissions, respectively.

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'''Figure 3.7 | The residual fossil fuel and industry emissions, carbon dioxide removal (CDR) {LUC, DACCS, BECCS} , and non-CO''' 2 '''emissions (using AR6''' '''GWP-100''' ''') for each of the seven illustrative pathways (IPs).''' Fossil CCS is also shown, though this does not lead to emissions to the atmosphere ( [[#3.2.5|Section 3.2.5]] ).

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'''Figure 3.8 |The energy system in each of the illustrative pathways (IPs).'''

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