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=== 2.7.3 Synthesis – Comparison with Estimates of Residual Fossil Fuel CO 2 Emissions ===

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A complementary strand of literature uses IAMs to assess the cumulative gross amount of unabated CO 2 emissions from fossil fuels across decarbonisation pathways that are not removed from the system, even under strong (short- and long-term) climate policy ambitions. Lower bound estimates for such a minimum amount of unabated residual CO 2 emissions across the 21st century that is not removed from the system, even under very ambitious climate policy assumptions, may be around 600–700 GtCO 2 ( [[#Kriegler--2018b|Kriegler et al. 2018b]] ). This range increases to 650–1800 GtCO 2 (Table 2.7) as soon as a broader set of policy assumptions are considered, including delayed action in scenarios that limit warming to 1.5°C and 2°C respectively ( [[#Luderer--2018|Luderer et al. 2018]] ).

Notably, the lower end of residual fossil fuel emissions in IAM scenarios ( [[#Luderer--2018|Luderer et al. 2018]] ) is remarkably similar to global estimates from the accounting studies of the previous section, as shown in Table 2.6. Yet, there are important conceptual and interpretative differences that are also reflected in the very different distribution of reported future CO 2 emissions attached to current and future fossil fuel infrastructures (Table 2.7). Accounting studies start from granular, plant-based data for existing fossil fuel infrastructure and make statements about their future CO 2 emissions, assuming variations of historic patterns of use and decommissioning. Expansions to the future are limited to proposals for new infrastructures that we know of today. Scenario studies quantifying residual fossil fuel emissions start from aggregate infrastructure descriptions, but dynamically update those through new investment decisions in each time step across the 21st century based on the development of energy and energy service demands, as well as technology availability, guided by defined climate policy goals (or their absence).

In accounting studies, estimates of future CO 2 emissions from current fossil fuel infrastructures are dominated by the power sector with its large fossil fuel capacities. In contrast, scenario studies highlight residual emissions from non-electric energy – particularly in the transport and industry sectors. Fossil-fuel infrastructure in the power sector can be much more easily retired than in those sectors, where there are fewer and more costly alternatives. IAMs therefore account for continued investments into fossil-based energy technologies in areas with limited decarbonisation potential, such as some areas of transportation (in particular aviation, shipping and road-based freight) or some industrial processes (such as cement production or feedstocks for chemicals). This explains the key discrepancies observable in Table 2.7. Therefore, our overall assessment of these available lines of evidence strongly emphasises the importance of decommissioning, reduced utilisation of existing power sector infrastructure, as well as continued cancellation of new power sector infrastructures in order to limit warming to well below 2°C ( ''high confidence'' ) ( [[#Kriegler--2018b|Kriegler et al. 2018b]] ; [[#Luderer--2018|Luderer et al. 2018]] ; [[#Chen--2019|Chen et al. 2019]] ; [[#Cui--2019|Cui et al. 2019]] ; [[#Fofrich--2020|Fofrich et al. 2020]] ). This is important as the power sector is comparatively easy to decarbonise (IPCC 2014a; [[#Krey--2014|Krey et al. 2014]] ; [[#Davis--2018|Davis et al. 2018]] ; [[#Méjean--2019|Méjean et al. 2019]] ) and it is crucial to make space for residual emissions from non-electric energy end uses that are more difficult to mitigate ( ''high confidence'' ). Any further delay in climate policy substantially increases carbon lock-in and mitigation challenges as well as a dependence on carbon dioxide removal technologies for meeting the Paris climate goals ( [[#Kriegler--2018b|Kriegler et al. 2018b]] ; [[#Luderer--2018|Luderer et al. 2018]] ).

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