Editing IPCC:AR6/SR15/Chapter-2 (section)

=== 2.1.3 New Scenario Information since AR5 ===

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In this chapter, we extend the AR5 mitigation pathway assessment based on new scenario literature. Updates in understanding of climate sensitivity, transient climate response, radiative forcing, and the cumulative carbon budget consistent with 1.5°C are discussed in Sections 2.2.

Mitigation pathways developed with detailed process-based integrated assessment models (IAMs) covering all sectors and regions over the 21st century describe an internally consistent and calibrated (to historical trends) way to get from current developments to meeting long-term climate targets like 1.5°C (Clarke et al., 2014) <sup>[[#fn:r7|7]]</sup> . The overwhelming majority of available 1.5°C pathways were generated by such IAMs, and these pathways can be directly linked to climate outcomes and their consistency with the 1.5°C goal evaluated. The AR5 similarly relied upon such studies, which were mainly discussed in Chapter 6 of Working Group III (WGIII) (Clarke et al., 2014) <sup>[[#fn:r8|8]]</sup> .

Since the AR5, several new, integrated multimodel studies have appeared in the literature that explore specific characteristics of scenarios more stringent than the lowest scenario category assessed in AR5 that was assessed to limit warming below 2°C with greater than 66% likelihood (Rogelj et al., 2015b, 2018; Akimoto et al., 2017; Marcucci et al., 2017; Su et al., 2017; Bauer et al., 2018; Bertram et al., 2018; Grubler et al., 2018; Holz et al., 2018b; Kriegler et al., 2018a; Liu et al., 2018; Luderer et al., 2018; Strefler et al., 2018a; van Vuuren et al., 2018; Vrontisi et al., 2018; Zhang et al., 2018) <sup>[[#fn:r9|9]]</sup> . Those scenarios explore 1.5°C-consistent pathways from multiple perspectives (see Supplementary Material 2.SM.1.3), examining sensitivity to assumptions regarding:

* socio-economic drivers and developments including energy and food demand as, for example, characterized by the Shared Socio-Economic Pathways (SSPs; Cross-Chapter Box 1 in Chapter 1)
* near-term climate policies describing different levels of strengthening the NDCs
* the use of bioenergy and the availability and desirability of carbon dioxide removal (CDR) technologies

A large number of these scenarios were collected in a scenario database established for the assessment of this Special Report (Supplementary Material 2.SM.1.3). Mitigation pathways were classified by four factors: consistency with a temperature increase limit (as defined by Chapter 1), whether they temporarily overshoot that limit, the extent of this potential overshoot, and the likelihood of falling within these bounds. Specifically, they were put into classes that either kept surface temperature increases below a given threshold throughout the 21st century or returned to a value below 1.5°C above pre-industrial levels at some point before 2100 after temporarily exceeding that level earlier – referred to as an overshoot (OS). Both groups were further separated based on the probability of being below the threshold and the degree of overshoot, respectively (Table 2.1). Pathways are uniquely classified, with 1.5°C-related classes given higher priority than 2°C classes in cases where a pathway would be applicable to either class.

The probability assessment used in the scenario classification is based on simulations using two reduced-complexity carbon cycle, atmospheric composition, and climate models: the ‘Model for the Assessment of Greenhouse Gas-Induced Climate Change’ (MAGICC) (Meinshausen et al., 2011a) <sup>[[#fn:r10|10]]</sup> , and the ‘Finite Amplitude Impulse Response’ (FAIRv1.3) model (Smith et al., 2018) <sup>[[#fn:r11|11]]</sup> . For the purpose of this report, and to facilitate comparison with AR5, the range of the key carbon cycle and climate parameters for MAGICC and its setup are identical to those used in AR5 WGIII (Clarke et al., 2014) <sup>[[#fn:r12|12]]</sup> . For each mitigation pathway, MAGICC and FAIR simulations provide probabilistic estimates of atmospheric concentrations, radiative forcing and global temperature outcomes until 2100. However, the classification uses MAGICC probabilities directly for traceability with AR5 and because this model is more established in the literature. Nevertheless, the overall uncertainty assessment is based on results from both models, which are considered in the context of the latest radiative forcing estimates and observed temperatures (Etminan et al., 2016; Smith et al., 2018) <sup>[[#fn:r13|13]]</sup> (Section 2.2 and Supplementary Material 2.SM.1.1). The comparison of these lines of evidence shows ''high agreement'' in the relative temperature response of pathways, with ''medium agreement'' on the precise absolute magnitude of warming, introducing a level of imprecision in these attributes. Consideration of the combined evidence here leads to ''medium confidence'' in the overall geophysical characteristics of the pathways reported here.

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'''Table 2.1'''

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'''Classification of pathways that this chapter draws upon, along with the number of available pathways in each class'''

The definition of each class is based on probabilities derived from the MAGICC model in a setup identical to AR5 WGIII (Clarke et al., 2014) <sup>[[#fn:r14|14]]</sup> , as detailed in Supplementary Material  2.SM.1.4.
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{| class="wikitable"
|-
! Pathway Group
! Pathway Class
! Pathway Selection Criteria and Description
! Number of Scenarios
|-
| rowspan="3"| 1.5°C or<br />
1.5°C-consistent**
| Below-1.5°C
| Pathways limiting peak warming to below 1.5°C during the entire 21st century with 50–66% likelihood*
| 9
| rowspan="3"| 90
|-
| 1.5°C-low-OS
| Pathways limiting median warming to below 1.5°C in 2100 and with a 50–67% probability of temporarily overshooting that level earlier, generally implying less than 0.1°C higher peak warming than Below-1.5°C pathways
| 44
|-
| 1.5°C-high-OS
| Pathways limiting median warming to below 1.5°C in 2100 and with a greater than 67% probability of temporarily overshooting that level earlier, generally implying 0.1–0.4°C higher peak warming than Below-1.5°C pathways
| 37
|-
| rowspan="2"| 2°C or<br />
2°C-consistent
| Lower-2°C
| Pathways limiting peak warming to below 2°C during the entire 21st century with greater than 66% likelihood
| 74
| rowspan="2"| 132
|-
| Higher-2°C
| Pathways assessed to keep peak warming to below 2°C during the entire 21st century with 50–66% likelihood
| 58
|}
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\* No pathways were available that achieve a greater than 66% probability of limiting warming below 1.5°C during the entire 21st century based on the MAGICC model projections.

\** This chapter uses the term 1.5°C-consistent pathways to refer to pathways with no overshoot, with limited (low) overshoot, and with high overshoot. However, the Summary for Policymakers focusses on pathways with no or limited (low) overshoot.

In addition to the characteristics of the above-mentioned classes, four illustrative pathway archetypes have been selected and are used throughout this chapter to highlight specific features of and variations across 1.5°C pathways. These are chosen in particular to illustrate the spectrum of CO <sub>2</sub> emissions reduction patterns consistent with 1.5°C, ranging from very rapid and deep near-term decreases, facilitated by efficiency and demand-side measures that lead to limited CDR requirements, to relatively slower but still rapid emissions reductions that lead to a temperature overshoot and necessitate large CDR deployment later in the century (Section 2.3).

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