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

=== 3.5.1 Between Long-term Climate Goals and Near- to Medium-term Emissions Reductions ===

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The close link between cumulative CO 2 emissions and warming has strong implications for the relationship between near-, medium-, and long-term climate action to limit global warming. The AR6 WGI Assessment has estimated a remaining carbon budget of 500 (400) GtCO 2 from the beginning of 2020 onwards for staying below 1.5°C with 50% (67%) likelihood, subject to additional uncertainties about historic warming and the climate response, and variations in warming from non-CO 2 climate forcers ( [[#Canadell--2019|Canadell and Monteiro 2019]] ) (AR6 WGI Chapter 5, [[IPCC:Wg3:Chapter:Chapter-5#5.5|Section 5.5]] ). For comparison, if current CO 2 emissions of more than 40 GtCO 2 are keeping up until 2030, more than 400 GtCO 2 will be emitted during 2021–2030, already exhausting the remaining carbon budget for 1.5°C by 2030.

The relationship between warming limits and near-term action is illustrated in Figure 3.29, using a set of 1.5°C–2°C scenarios with different levels of near-term action, overshoot and non-CO 2 warming contribution from a recent study ( [[#Riahi--2021|Riahi et al. 2021]] ). In general, the more CO 2 is emitted until 2030, the less CO 2 can be emitted thereafter to stay within a remaining carbon budget and below a warming limit. Scenarios with immediate action to observe the warming limit give the longest time to exhaust the associated remaining carbon budget and reach net zero CO 2 emissions (see light blue lines in Figure 3.29 and Cross-Chapter Box 3 in this chapter). In comparison, following projected NDC emissions until 2030 would imply a more pronounced drop in emissions from 2030 levels to net zero to make up for the additional near-term emissions (see orange lines in Figure 3.29). If such a drop does not occur, the remaining carbon budget is exceeded and net negative CO 2 emissions are required to return global mean temperature below the warming limit (see black lines in Figure 3.29) (Clarke et al. 2014; [[#Fuss--2014|Fuss et al. 2014]] ; [[#Rogelj--2018|Rogelj et al. 2018]] a).

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[[File:17ae025f991eef9c87fc8471f037e387 IPCC_AR6_WGIII_Figure_3_29.png]]

'''Figure 3.29''' '''| Illustration of emissions and climate response in four mitigation pathways with different assumptions about near-term policy developments, global warming limit and non-CO''' 2 '''warming contribution drawn from Riahi''' '''et al.''' '''(2021).''' Shown are '''(a)''' CO 2 emissions trajectories, '''(b)''' cumulative CO 2 emissions, '''(c)''' effective non-CO 2 radiative forcing, and '''(d)''' the resulting estimate of the 67th percentile of global mean temperature response relative to 1850–1900. Light blue lines show a scenario that acts immediately on a remaining carbon budget of 900 GtCO 2 from 2020 without allowing net negative CO 2 emissions (i.e., temporary budget overshoot) (COFFEE 1.1, Scenario EN_NPi2020_900). Orange and black lines show scenarios drawn from the same model that follow the NDCs until 2030 and thereafter introduce action to stay within the same budget – in one case excluding net negative CO 2 emissions like before (orange lines; COFFEE 1.1., Scenario EN-INDCi2030_900) and in the other allowing for a temporary overshoot of the carbon budget until 2100 (black lines; COFFEE 1.1., Scenario EN-INDCi2030_900f). Light blue lines describe a scenario following the NDCs until 2030, and then aiming for a higher budget of 2300 GtCO 2 without overshoot (AIM/CGE 2.2, Scenario EN-INDCi2030_1200). It is drawn from another model which projects a lower anthropogenic non-CO 2 forcing contribution and therefore achieves about the same temperature outcome as the other two non-overshoot scenarios despite the higher CO 2 budget. Grey funnels include the trajectories from all scenarios that limit warming to 2°C (&gt;67%) (category C3). Historical CO 2 emissions until 2019 are from Chapter SM.2.1 EDGAR v6.0.

The relationship between warming limits and near-term action is also affected by the warming contribution of non-CO 2 greenhouse gases and other short-lived climate forcers ( [[#3.3|Section 3.3]] ; AR6 WGI [[IPCC:Wg3:Chapter:Chapter-6#6.7|Section 6.7]] ). The estimated budget values for limiting warming to 1.5°C–2°C already assume stringent reductions in non-CO 2 greenhouse gases and non-CO 2 climate forcing as found in 1.5°C–2°C pathways ( [[#3.3|Section 3.3]] and Cross-Working Group Box 1 in this chapter; AR6 WGI [[IPCC:Wg3:Chapter:Chapter-5#5.5|Section 5.5]] and Box 5.2 in Chapter 5). Further variations in non-CO 2 warming observed across 1.5°C–2°C pathways can vary the median estimate for the remaining carbon budget by 220 GtCO 2 (AR6 WGI [[IPCC:Wg3:Chapter:Chapter-5#5.5|Section 5.5]] ). In 1.5°C–2°C pathways, the non-CO 2 warming contribution differs strongly between the near, medium and long term. Changes to the atmospheric composition of short-lived climate forcers (SLCFs) dominate the warming response in the near term (AR6 WGI [[IPCC:Wg3:Chapter:Chapter-6#6.7|Section 6.7]] ). CO 2 reductions are combined with strong reductions in air pollutant emissions due to rapid reduction in fossil fuel combustion and in some cases the assumption of stringent air quality policies ( [[#Rao--2017b|Rao et al. 2017b]] ; [[#Smith--2020c|Smith et al. 2020c]] ). As air pollutants exert a net-cooling effect, their reduction drives up non-CO 2 warming in the near term, which can be attenuated by the simultaneous reduction of methane and black carbon ( [[#Shindell--2019|Shindell and Smith 2019]] ; [[#Smith--2020b|Smith et al. 2020b]] ) (AR6 WGI [[IPCC:Wg3:Chapter:Chapter-6#6.7|Section 6.7]] ). After 2030, the reduction in methane concentrations and associated reductions in tropospheric ozone levels tend to dominate so that a peak and decline in non-CO 2 forcing and non-CO 2 -induced warming can occur before net zero CO 2 is reached (Figure 3.29) ( [[#Rogelj--2018|Rogelj et al. 2018]] a). The more stringent the reductions in methane and other short-lived warming agents such as black carbon, the lower this peak and the earlier the decline of non-CO 2 warming, leading to a reduction of warming rates and overall warming in the near to medium term ( [[#Harmsen--2020|Harmsen et al. 2020]] ; [[#Smith--2020b|Smith et al. 2020b]] ). This is important for keeping warming below a tight warming limit that is already reached around mid-century as is the case in 1.5°C pathways ( [[#Xu--2017|Xu and Ramanathan 2017]] ). Early and deep reductions of methane emissions, and other short-lived warming agents such as black carbon, provide space for residual CO 2 -induced warming until the point of net zero CO 2 emissions is reached (see purple lines in Figure 3.29). Such emissions reductions have also been advocated due to co-benefits for, for example, reducing air pollution ( [[#Rao--2016|Rao et al. 2016]] ; [[#Shindell--2017a|Shindell et al. 2017a]] , 2018; [[#Shindell--2019|Shindell and Smith 2019]] ; [[#Rauner--2020a|Rauner et al. 2020a]] ; [[#Vandyck--2020|Vandyck et al. 2020]] ).

The relationship between long-term climate goals and near-term action is further constrained by social, technological, economic and political factors ( [[#Cherp--2018|Cherp et al. 2018]] ; [[#van%20Sluisveld--2018b|van Sluisveld et al. 2018b]] ; Aghion et al. 2019; [[#Mercure--2019|Mercure et al. 2019]] ; [[#Trutnevyte--2019b|Trutnevyte et al. 2019b]] ; [[#Jewell--2020|Jewell and Cherp 2020]] ). These factors influence path dependency and transition speed ( [[#Pahle--2018|Pahle et al. 2018]] ; [[#Vogt-Schilb--2018|Vogt-Schilb et al. 2018]] ). While detailed integrated assessment modelling of global mitigation pathways accounts for technology inertia ( [[#Bertram--2015a|Bertram et al. 2015a]] ; [[#Mercure--2018|Mercure et al. 2018]] ) and technology innovation and diffusion ( [[#Wilson--2013|Wilson et al. 2013]] ; [[#van%20Sluisveld--2018a|van Sluisveld et al. 2018a]] ; Luderer et al. 2021), there are limitations in capturing socio-technical and political drivers of innovation, diffusion and transition processes ( [[#Gambhir--2019|Gambhir et al. 2019]] ; [[#Köhler--2019|Köhler et al. 2019]] ; [[#Hirt--2020|Hirt et al. 2020]] ; [[#Keppo--2021|Keppo et al. 2021]] ). Mitigation pathways show a wide range of transition speeds that have been interrogated in the context of socio-technical inertia ( [[#Gambhir--2017|Gambhir et al. 2017]] ; [[#Kefford--2018|Kefford et al. 2018]] ; [[#Kriegler--2018a|Kriegler et al. 2018a]] ; [[#Brutschin--2021|Brutschin et al. 2021]] ) vs accelerating technological change and self-enforcing socio-economic developments ( [[#Creutzig--2017|Creutzig et al. 2017]] ; [[#Zenghelis--2019|Zenghelis 2019]] ) ( [[#3.8|Section 3.8]] ). Diagnostic analysis of detailed IAMs found a lag of 8–20 years between the convergence of emissions pricing and the convergence of emissions response after a period of differentiated emission prices ( [[#Harmsen--2021|Harmsen et al. 2021]] ). This provides a measure of the inertia to changing policy signals in the model response. It is about half the time scale of 20–40 years observed for major energy transitions ( [[#Grubb--2021|Grubb et al. 2021]] ). Hence, the mitigation pathways assessed here capture socio-technical inertia in reducing emissions, but the limited modelling of socio-political factors may alter the extent and persistence of this inertia.

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