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

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Median global warming estimated by MAGICC (panel a) and peak cumulative  CO <sub>2 </sub> emissions (panel b) in 1.5°C-consistent pathways in the SR1.5 scenario database, as a function of  CO <sub>2</sub> -equivalent emissions (based on AR4 GWP-100) of Kyoto-GHGs in 2030.

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Pathways that were forced to go through the NDCs or a similarly high emissions point in 2030 by design are highlighted by yellow marker edges (see caption of Figure 2.13 and text for further details on the design of these pathways). The combined range of global Kyoto-GHG emissions in 2030 for the conditional and unconditional NDCs assessed in Cross-Chapter Box 11 is shown by the grey shaded area (adjusted to AR4 GWPs for comparison). As a second line of evidence, peak cumulative CO <sub>2</sub> emissions derived from a 1.5°C pathway sensitivity analysis (Kriegler et al., 2018b) <sup>[[#fn:r351|351]]</sup> are shown by grey circles in the right-hand panel. Circles show gross fossil-fuel and industry emissions of the sensitivity cases, increased by assumptions about the contributions from AFOLU (5 GtCO <sub>2</sub> yr−1 until 2020, followed by a linear phase out until 2040) and non-CO <sub>2</sub> Kyoto-GHGs (median non-CO <sub>2</sub> contribution from 1.5°C-consistent pathways available in the database: 10 GtCO <sub>2</sub> e yr <sup>−1</sup> in 2030), and reduced by assumptions about CDR deployment until the time of net zero CO <sub>2</sub> emissions (limiting case for CDR deployment assumed in (Kriegler et al., 2018b) <sup>[[#fn:r352|352]]</sup> (logistic growth to 1, 4, 10 GtCO <sub>2</sub> yr <sup>−1</sup> in 2030, 2040, and 2050, respectively, leading to approximately 100 GtCO <sub>2</sub> of CDR by mid-century).

Original Creation for this Report using IAMC 1.5°C Scenario Data hosted by IIASA and Sensitivity cases from Kriegler et al. (2018), doi: 10.1098/rsta.2016.0457

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It is unclear whether following NDCs until 2030 would still allow global mean temperature to return to 1.5°C by 2100 after a temporary overshoot, due to the uncertainty associated with the Earth system response to net negative emissions after a peak (Section 2.2). Available IAM studies are working with reduced-form carbon cycle–climate models like MAGICC, which assume a largely symmetric Earth-system response to positive and net negative CO <sub>2</sub> emissions. The IAM findings on returning warming to 1.5°C from NDCs after a temporary temperature overshoot are hence all conditional on this assumption. Two types of pathways with 1.5°C-consistent action starting in 2030 have been considered in the literature (Luderer et al., 2018) <sup>[[#fn:r353|353]]</sup> (Figure 2.13): pathways aiming to obtain the same end-of-century carbon budget as 1.5°C-consistent pathways starting in 2020 despite higher emissions until 2030, and pathways assuming the same mitigation stringency after 2030 as in 1.5°C-consistent pathways starting in 2020 (approximated by using the same global price of emissions as found in least-cost pathways starting from 2020). An IAM comparison study found increasing challenges to implementing pathways with the same end-of-century carbon budgets after following NDCs until 2030 (Luderer et al., 2018) <sup>[[#fn:r354|354]]</sup> . The majority of model experiments (four out of seven) failed to produce NDC pathways that would return cumulative CO <sub>2</sub> emissions over the 2016–2100 period to 200 GtCO <sub>2</sub> , indicating limitations to the availability and timing of CDR. The few such pathways that were identified show highly disruptive features in 2030 (including abrupt transitions from moderate to very large emissions reduction and low carbon energy deployment rates) indicating a high risk that the required post-2030 transformations are too steep and abrupt to be achieved by the mitigation measures in the models ( ''high confidence'' ). NDC pathways aiming for a cumulative 2016–2100 CO <sub>2</sub> emissions budget of 800 GtCO <sub>2</sub> were more readily obtained (Luderer et al., 2018) <sup>[[#fn:r355|355]]</sup> , and some were classified as 1.5°C-high-OS pathways in this assessment (Section 2.1).

NDC pathways that apply a post-2030 price of emissions as found in least-cost pathways starting from 2020 show infrastructural carbon lock-in as a result of following NDCs instead of least-cost action until 2030. A key finding is that carbon lock-ins persist long after 2030, with the majority of additional CO <sub>2</sub> emissions occurring during the 2030–2050 period. Luderer et al. (2018) <sup>[[#fn:r356|356]]</sup> find 90 (80–120) GtCO <sub>2</sub> additional emissions until 2030, growing to 240 (190–260) GtCO <sub>2</sub> by 2050 and 290 (200–200) GtCO <sub>2</sub> by 2100. As a result, peak warming is about 0.2°C higher and not all of the modelled pathways return warming to 1.5°C by the end of the century. There is a four sided trade-off between (i) near-term ambition, (ii) degree of overshoot, (iii) transitional challenges during the 2030–2050 period, and (iv) the amount of CDR deployment required during the century (Figure 2.13) (Holz et al., 2018b; Strefler et al., 2018b) <sup>[[#fn:r357|357]]</sup> . Transition challenges, overshoot, and CDR requirements can be significantly reduced if global emissions peak before 2030 and fall below levels in line with current NDCs by 2030. For example, Strefler et al. (2018b) <sup>[[#fn:r358|358]]</sup> find that CDR deployment levels in the second half of the century can be halved in 1.5°C-consistent pathways with similar CO <sub>2</sub> emissions reductions rates during the 2030–2050 period if CO <sub>2</sub> emissions by 2030 are reduced by an additional 30% compared to NDC levels. Kriegler et al. (2018a) <sup>[[#fn:r359|359]]</sup> investigate a global rollout of selected regulatory policies and moderate carbon pricing policies. They show that additional reductions of about 10 GtCO <sub>2</sub> e yr <sup>−1</sup> can be achieved in 2030 compared to the current NDCs. Such a 20% reduction of year-2030 emissions compared to current NDCs would effectively lower the disruptiveness of post-2030 action. The strengthening of short-term policies in deep mitigation pathways has hence been identified as a way of bridging options to keep the Paris climate goals within reach (Bertram et al., 2015b; IEA, 2015a; Spencer et al., 2015; Kriegler et al., 2018a) <sup>[[#fn:r360|360]]</sup> .

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====== Figure 2.13 ======

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