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

==== 3.3.2.1 Overall Mitigation Profiles and Temperature Consequences ====

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Figure 3.10 shows the GHG and CO 2 emission trajectories for different temperature categories as defined in [[#3.2|Section 3.2]] (the temperature levels are calculated using simple climate models, consistent with the outcomes of the recent WGI assessment, Cross-Chapter Box 7.1). It should be noted that most scenarios currently in the literature do not account for the impact of COVID-19 (Box 3.2). The higher categories (C6 and C7) mostly included scenarios with no or modest climate policy. Because of the progression of climate policy, it is becoming more common that reference scenarios incorporate implemented climate policies. Modelling studies typically implement current or pledged policies up until 2030 ( [[#Vrontisi--2018|Vrontisi et al. 2018]] ; [[#Roelfsema--2020|Roelfsema et al. 2020]] ; [[#Sognnaes--2021|Sognnaes et al. 2021]] ) with some studies focusing also on the policy development in the long term ( [[#Höhne--2021|Höhne et al. 2021]] ; [[#IEA--2021a|]] [[#IEA--2021|IEA 2021]] a ; [[#Jeffery--2018|Jeffery et al. 2018]] ; [[#Gütschow--2018|Gütschow et al. 2018]] ). Based on the assessment in Chapter 4, reference pathways consistent with the implementation and trend from implemented policies until the end of 2020 are associated with increased GHG emissions from 59 (53–65) GtCO 2 -eq yr –1 in 2019 to 54–60 GtCO 2 -eq yr –1 by 2030 and to 47–67 GtCO 2 -eq yr –1 by 2050 (Figure 3.6). Pathways with these near-term emissions characteristics lead to a median global warming of 2.2°C to 3.5°C by 2100 (see also further in this section). These pathways consider policies at the time that they were developed. A recent model comparison that harmonised socio-economic, technological, and policy assumptions ( [[#Giarola--2021|Giarola et al. 2021]] ) found a 2.2°C–2.9°C median temperature rise in 2100 for current and stated policies, with the results sensitive to the model used and the method of implementing policies ( [[#Sognnaes--2021|Sognnaes et al. 2021]] ). Scenario inference and construction methods using similar policy assumptions lead to a median range of 2.9°C–3.2°C in 2100 for current policies and 2.4°C–2.9°C in 2100 for 2030 pledges ( [[#Höhne--2021|Höhne et al. 2021]] ). The median spread of 1°C across these studies (2.2°C–3.2°C) indicates the deep uncertainties involved with modelling temperature outcomes of 2030 policies through to 2100 ( [[#Höhne--2021|Höhne et al. 2021]] ).

The lower categories include increasingly stringent assumed climate policies. For all scenario categories, except the highest category, emissions peak in the 21st century. For the lowest categories, the emissions peak is mostly before 2030. In fact, for scenarios in the category that avoids temperature overshoot for the 1.5°C scenario (C1 category), GHG emissions are reduced already to almost zero around the middle of the century. Typically, CO 2 emissions reach net zero about 10 to 40 years before total GHG emissions reach net zero. The main reason is that scenarios reduce non-CO 2 greenhouse gas emissions less than CO 2 due to a limited mitigation potential ( [[#3.3.2.2|Section 3.3.2.2]] ). Figure 3.10 also shows that many scenarios in the literature with a temperature outcome below 2°C show net negative emissions. There are, however, also exceptions in which more immediate emission reductions limits the need for CDR. The IMPs illustrate alternative pathways to reach the C1–C3 temperature levels.

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[[File:8eb0e7468597cfb14004b5f3f327efeb IPCC_AR6_WGIII_Figure_3_10.png]]

'''Figure 3.10 | Total emissions profiles in the scenarios based on climate category for GHGs (AR6''' '''GWP-100''' ''') and CO''' 2. The Illustrative mitigation pathways (IMPs) are also indicated.

Figure 3.11 shows the possible consequences of the different scenario categories for global mean temperature calculated using a reduced complexity model (RCM) calibrated to the IPCC AR6 WGI assessment (see Annex III.II.2.5 of this report and Cross-Chapter Box 7.1 in AR6 WGI report). For the C5–C7 categories (containing most of the reference and current policy scenarios), the global mean temperature is expected to increase throughout the century (and further increase will happen after 2100 for C6 and C7). While warming would ''more likely than not'' be in the range from 2.2°C to 3.5°C, warming up to 5°C cannot be excluded. The highest emissions scenarios in the literature combine assumptions about rapid long-term economic growth and pervasive climate policy failures, leading to a reversal of some recent trends (Box 3.3). For the categories C1–C4, a peak in global mean temperature is reached mid-century for most scenarios in the database, followed by a small (C3/C4) or more considerable decline (C1/C2). There is a clear distinction between the scenarios with no or limited overshoot (typically &lt;0.1°C, C1) compared to those with high overshoot (C2): in emissions, the C1 category is characterised by steep early reductions and a relatively small contribution of net negative emissions (like ''IMP-LD'' and ''IMP-Ren'' ) (Figure 3.10). In addition to the temperature caused by the range of scenarios in each category (main panel), climate uncertainties also contribute to a range of temperature outcomes (including uncertainties regarding the carbon cycle, climate sensitivity, and the rate of change, see AR6 WGI). The bars on the right of Figure 3.11 show the uncertainty range for each category (combining scenario and climate uncertainty). While the C1 category ''more likely than not'' limits warming to 1.5°C (&gt;50%) by the end of the century, even with such a scenario, warming above 2°C cannot be excluded (95th percentile). The uncertainty range for the highest emission categories (C7) implies that these scenarios could lead to a warming above 6°C.

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[[File:ce7bf6b11a1edb5ff7bd2a93970cc700 IPCC_AR6_WGIII_Figure_3_11.png]]

'''Figure 3.11 | Global mean temperature outcome of the ensemble of scenarios included in the climate categories C1–C8 (based on a reduced complexity model – RCM – calibrated to the WGI assessment, both in terms of future and historic warming).''' The left panel shows the ranges of scenario uncertainty (shaded area) with the P50 RCM probability (line). The right panel shows the P5 to P95 range of combined RCM climate uncertainty (C1–C8 is explained in Table 3.1) and scenario uncertainty, and the P50 (line).

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