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

==== 3.5.2.1 Overshoot and Net Negative CO 2 Emissions ====

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If near- to medium-term emissions developments deplete the remaining carbon budget, the associated warming limit will be overshot. Some pathways that return warming to 1.5°C (&gt;50%) by the end of the century show mid-century overshoots of up to 1.8°C median warming. The overshoot tends to be higher, the higher the 2030 emissions. Mitigation pathways with 2030 emissions levels in the NDCS announced prior to COP26 consistently overshoot 1.5°C by 0.15°C–0.3°C. This leads to higher risks from climate change impacts during the time of overshoot compared to pathways that limit warming to 1.5°C (&gt;50%) with no or limited overshoot ( [[#Schleussner--2016a|Schleussner et al. 2016a]] ; [[#Mengel--2018|Mengel et al. 2018]] ; [[#Hofmann--2019|Hofmann et al. 2019]] ; [[#Lenton--2019|Lenton et al. 2019]] ; [[#Tachiiri--2019|Tachiiri et al. 2019]] ; [[#Drouet--2021|Drouet et al. 2021]] ). Furthermore, even if warming is reversed by net negative emissions, other climate changes such as sea level rise would continue in their current direction for decades to millennia (AR6 WGI Sections 4.6 and 5.6).

Returning warming to lower levels requires net negative CO 2 emissions in the second half of the century (Clarke et al. 2014; [[#Fuss--2014|Fuss et al. 2014]] ; [[#Rogelj--2018|Rogelj et al. 2018]] a). The amount of net negative CO 2 emissions in pathways limiting warming to 1.5°C–2°C climate goals varies widely, with some pathways not deploying net negative CO 2 emissions at all and others deploying up to –600 to –800 GtCO 2 . The amount of net negative CO 2 emissions tends to increase with 2030 emissions levels (Figure 3.30e and Table 3.6). Studies confirmed the ability of net negative CO 2 emissions to reduce warming, but pointed to path dependencies in the storage of carbon and heat in the Earth System and the need for further research particularly for cases of high overshoot ( [[#Zickfeld--2016|Zickfeld et al. 2016]] , 2021; [[#Keller--2018a|Keller et al. 2018a]] ,b; [[#Tokarska--2019|Tokarska et al. 2019]] ). The AR6 WGI assessed the reduction in global surface temperature to be approximately linearly related to cumulative CO 2 removal and, with lower confidence, that the amount of cooling per unit CO 2 removed is approximately independent of the rate and amount of removal (AR6 WGI TS.3.3.2). Still there remains large uncertainty about a potential asymmetry between the warming response to CO 2 emissions and the cooling response to net negative CO 2 emissions ( [[#Zickfeld--2021|Zickfeld et al. 2021]] ). It was also shown that warming can adversely affect the efficacy of carbon dioxide removal measures and hence the ability to achieve net negative CO 2 emissions ( [[#Boysen--2016|Boysen et al. 2016]] ).

Obtaining net negative CO 2 emissions requires massive deployment of carbon dioxide removal (CDR) in the second half of the century, on the order of 220 (160–370) GtCO 2 for each 0.1°C degree of cooling (based on the assessment of the ''likely'' range of the transient response to cumulative CO 2 emissions in AR6 WGI [[IPCC:Wg3:Chapter:Chapter-5#5.5|Section 5.5]] in Chapter 5, not taking into account potential asymmetries in the temperature response to CO 2 emissions and removals). CDR is assessed in detail in [[IPCC:Wg3:Chapter:Chapter-12#12.3|Section 12.3]] of this report (see also Cross-Chapter Box 8 in Chapter 12). Here we only point to the finding that CDR ramp-up rates and absolute deployment levels are tightly limited by techno-economic, social, political, institutional and sustainability constraints ( [[#Smith--2016|Smith et al. 2016]] ; [[#Boysen--2017|Boysen et al. 2017]] ; [[#Fuss--2018|Fuss et al. 2018]] , 2020; [[#Nemet--2018|Nemet et al. 2018]] ; [[#Hilaire--2019|Hilaire et al. 2019]] ; [[#Jia--2019|Jia et al. 2019]] ) ( [[IPCC:Wg3:Chapter:Chapter-12#12.3|Section 12.3]] ). CDR therefore cannot be deployed arbitrarily to compensate any degree of overshoot. A fraction of models was not able to compute pathways that would follow the mitigation ambition in unconditional and conditional NDCs until 2030 and return warming to below 1.5°C by 2100 ( [[#Luderer--2018|Luderer et al. 2018]] ; [[#Roelfsema--2020|Roelfsema et al. 2020]] ; [[#Riahi--2021|Riahi et al. 2021]] ). There exists a three-way trade-off between near-term emissions developments until 2030, transitional challenges during 2030–50, and long-term CDR deployment post-2050 ( [[#Sanderson--2016|Sanderson et al. 2016]] ; [[#Holz--2018|Holz et al. 2018]] ; [[#Strefler--2018|Strefler et al. 2018]] ). For example, [[#Strefler--2018|Strefler et al. (2018)]] find that if CO 2 emission levels stay at around 40 GtCO 2 until 2030, within the range of what is projected for NDCs announced prior to COP26, rather than being halved to 20 GtCO 2 until 2030, CDR deployment in the second half of the century would have to increase by 50–100%, depending on whether the 2030–2050 CO 2 emissions reduction rate is doubled from 6% to 12% or kept at 6% yr –1 . This three-way trade-off has also been identified at the national level ( [[#Pan--2020|Pan et al. 2020]] ).

In addition to enabling a temporary budget overshoot by net negative CO 2 emissions in the second half of the century, CDR can also be used to compensate – on an annual basis – residual CO 2 emissions from sources that are difficult to eliminate and to reach net zero CO 2 emissions more rapidly if deployed before this point ( [[#Kriegler--2013b|Kriegler et al. 2013b]] ; [[#Rogelj--2018|Rogelj et al. 2018]] a). This explains its continued deployment in pathways that exclude overshoot and net negative CO 2 emissions ( [[#Riahi--2021|Riahi et al. 2021]] ). However, given the time scales that would likely be needed to ramp-up CDR to gigatonne scale ( [[#Nemet--2018|Nemet et al. 2018]] ), it can be expected to only make a limited contribution to reaching net zero CO 2 as fast as possible. In the vast majority (95%) of 1.5°C–2°C mitigation pathways assessed in this report, cumulative CDR deployment did not exceed 100 GtCO 2 until mid-century. This adds to the risk of excessively relying on CDR to compensate for weak mitigation action until 2030 by either facilitating massive net CO 2 emissions reduction rates during 2030–2050 or allowing a high temporary overshoot of 1.5°C until the end of the century. If international burden-sharing considerations are taken into account, the CDR penalty for weak action could increase further, in particular for developed countries ( [[#Fyson--2020|Fyson et al. 2020]] ). Further assessment of CDR deployment in 1.5°C–2°C mitigation pathways is found in [[#3.4.7|Section 3.4.7]] .

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