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

==== 3.3.2.2 The Role of Carbon Dioxide and Other Greenhouse Gases ====

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The trajectory of future CO 2 emissions plays a critical role in mitigation, given CO 2 long-term impact and dominance in total greenhouse gas forcing. As shown in Figure 3.12, CO 2 dominates total greenhouse gas emissions in the high-emissions scenarios but is also reduced most, going from scenarios in the highest to lower categories. In C4 and below, most scenarios exhibit net negative CO 2 emissions in the second half of the century compensating for some of the residual emissions of non-CO 2 gases as well as reducing overall warming from an intermediate peak. Still, early emission reductions and further reductions in non-CO 2 emissions can also lead to scenarios without net negative emissions in 2100, even in C1 and C3 (shown for the 85–95th percentile). In C1, avoidance of significant overshoot implies that immediate gross reductions are more relevant than long-term net negative emissions (explaining the lower number than in C2) but carbon dioxide removal (CDR) is still playing a role in compensating for remaining positive emissions in hard-to-abate sectors.

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'''Figure 3.12 |''' '''(a) The role of CO''' 2 '''and other greenhouse gases.''' Emission in CO 2 -eq in 2100 (using AR6 GWP-100) (other = halogenated gases) and '''(b)''' cumulative CO 2 emissions in the 2020–2100 period. Panels '''(c)''' and '''(d)''' show the development of CH 4 and N 2 O emissions over time. Energy emissions include the contribution of BECCS. For both energy and AFOLU sectors, the positive and negative values represent the cumulated annual balances. In both panels, the three bars per scenario category represent the lowest 5–15th percentile, the average value and the highest 5–15th percentile. These illustrate the range of scenarios in each category. The definition of C1–C7 can be found in Table 3.1.

CH 4 and N 2 O emissions are also reduced from C7 to C1, but this mostly occurs between C7 and C5. The main reason is the characteristics of abatement potential: technical measures can significantly reduce CH 4 and N 2 O emissions at relatively low costs to about 50% of the current levels (e.g., by reducing CH 4 leaks from fossil fuel production and transport, reducing landfill emissions gazing, land management and introducing measures related to manure management, see also [[IPCC:Wg3:Chapter:Chapter-7|Chapter 7]] and 11). However, technical potential estimates become exhausted even if the stringency of mitigation is increased ( [[#Harmsen--2019a|Harmsen et al. 2019a]] ,b; [[#Höglund-Isaksson--2020|Höglund-Isaksson et al. 2020]] ). Therefore, further reduction may come from changes in activity levels, such as switching to a less meat-intensive diet, therefore reducing livestock ( [[#Stehfest--2009|Stehfest et al. 2009]] ; [[#Willett--2019|Willett et al. 2019]] ; [[#Ivanova--2020|Ivanova et al. 2020]] ) (Chapter 7). Other non-CO 2 GHG emissions (halogenated gases) are reduced to low levels for scenarios below 2.5°C.

Short-lived climate forcers (SLCFs) also play an important role in climate change, certainly for short-term changes (AR6 WGI, Figure SPM.2) ( [[#Shindell--2012|Shindell et al. 2012]] ). These forcers consist of (i) substances contributing to warming, such as methane, black carbon and tropospheric ozone, and (ii) substances contributing to cooling (other aerosols, such as related to sulphur emissions). Most SLCFs are also air pollutants, and reducing their emissions provides additional co-benefits ( [[#Shindell--2017a|Shindell et al. 2017a]] ,b; [[#Hanaoka--2020|Hanaoka and Masui 2020]] ). In the case of the first group, emission reduction thus leads to both air pollution and climate benefits. For the second, group there is a possible trade-off ( [[#Shindell--2019|Shindell and Smith 2019]] ; [[#Lund--2020|Lund et al. 2020]] ). As aerosol emissions are mostly associated with fossil fuel combustion, the benefits of reducing CO 2 could, in the short term, be reduced as a result of lower aerosol cooling. There has been an active discussion on the exact climate contribution of SLCF-focused policies in the literature. This discussion partly emerged from different assumptions on possible reductions in the absence of ambitious climate policy and the uncertain global climate benefit from aerosol (black carbon) ( [[#Rogelj--2014|Rogelj et al. 2014]] ). The latter is now assessed to be smaller than originally thought ( [[#Takemura--2019|Takemura and Suzuki 2019]] ; [[#Smith--2020b|Smith et al. 2020b]] ) (see also AR6 WGI [[IPCC:Wg3:Chapter:Chapter-6#6.4|Section 6.4]] ). Reducing SLCF emissions is critical to meet long-term climate goals and might help reduce the rate of climate change in the short term. Deep SLCF emission reductions also increase the remaining carbon budget for a specific temperature goal ( [[#Rogelj--2015a|Rogelj et al. 2015a]] ; [[#Reisinger--2021|Reisinger et al. 2021]] ) (Box 3.4). A more detailed discussion can be found in AR6 WGI Chapters 5 and 6.

For accounting of emissions and the substitution of different gases as part of a mitigation strategy, typically, emission metrics are used to compare the climate impact of different gases. Most policies currently use Global Warming Potentials (GWPs) with a 100-year time horizon as this is also mandated for emissions reporting in the Paris Rulebook (for a wider discussion of GHG metrics, see Box 2.1 in [[IPCC:Wg3:Chapter:Chapter-2|Chapter 2]] of this report, and AR6 WGI, Chapter 7, [[IPCC:Wg3:Chapter:Chapter-7#7.6|Section 7.6]] ). Alternative metrics have also been proposed, such as those using a shorter or longer time horizon, or those that focus directly on the consequences of reaching a certain temperature target (Global Temperature Change Potential – GTP), allowing a more direct comparison with cumulative CO 2 emissions ( [[#Allen--2016|Allen et al. 2016]] ; [[#Lynch--2020|Lynch et al. 2020]] ) or focusing on damages (Global Damage Potential) (an overview is given in Chapter 2, and Cross-Chapter Box 3 in Chapter 3). Depending on the metric, the value attributed to reducing short-lived forcers such as methane can be lower in the near term (e.g., in the case of GTP) or higher (GWP with a short reference period). For most metrics, however, the impact on mitigation strategies is relatively small, among others, due to the marginal abatement cost curve of methane (low costs for low-to-medium mitigation levels; expensive for high levels). The timing of reductions across different gases impacts warming and the co-benefits ( [[#Harmsen--2016|Harmsen et al. 2016]] ; [[#Cain--2019|Cain et al. 2019]] ). Nearly all scenarios in the literature use GWP-100 in cost-optimisation, reflecting the existing policy approach; the use of GWP-100 deviates from cost-optimal mitigation pathways by at most a few percent for temperature goals that limit warming to 2°C (&gt;67%) or lower (Box 2.1).

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===== Cumulative CO 2 emissions and temperature goals =====

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The dominating role of CO 2 and its long lifetime in the atmosphere and some critical characteristics of the Earth System implies that there is a strong relationship between cumulative CO 2 emissions and temperature outcomes (Allen et al. 2009; [[#Matthews--2009|Matthews et al. 2009]] ; [[#Meinshausen--2009|Meinshausen et al. 2009]] ; [[#MacDougall--2015|MacDougall and Friedlingstein 2015]] ). This is illustrated in Figure 3.13, which plots the cumulative CO 2 emissions against the projected outcome for global mean temperature, both until peak temperature and through to end of century (or 2100). The deviations from a linear relationship in Figure 3.13 are mostly caused by different non-CO 2 emission and forcing levels (see also [[#Rogelj--2015b|Rogelj et al. 2015b]] ). This means that reducing non-CO 2 emissions can play an important role in limiting peak warming: the smaller the residual non-CO 2 warming, the larger the carbon budget. This impact on carbon budgets can be substantial for stringent warming limits. For 1.5°C pathways, variations in non-CO 2 warming across different emission scenarios have been found to vary the remaining carbon budget by approximately 220 GtCO 2 (AR6 WGI Chapter 5, [[IPCC:Wg3:Chapter:Chapter-5#5.5.2|Section 5.5.2]] .2). In addition to reaching net zero CO 2 emissions, a strong reduction in methane emissions is the most critical component in non-CO 2 mitigation to keep the Paris climate goals in reach ( [[#Collins--2018|Collins et al. 2018]] ; [[#van%20Vuuren--2018|van Vuuren et al. 2018]] ) (see also AR6 WGI, Chapters 5, 6 and 7). It should be noted that the temperature categories (C1–C7) generally aligned with the horizontal axis, except for the end-of-century values for C1 and C2 that coincide.

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'''Figure 3.13 | The near-linear relationship between cumulative CO''' 2 '''emissions and temperature.''' The left panel shows cumulative emissions until net zero emission is reached. The right panel shows cumulative emissions until the end of the century, plotted against peak and end-of-century temperature, respectively. Both are shown as a function of non-CO 2 forcing and cumulative net negative CO 2 emissions. Position temperature categories (circles) and IPs are also indicated, including two 2°C sensitivity cases for ''Neg'' (Neg-2.0) and ''Ren'' (Ren-2.0).

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