Editing IPCC:AR6/WGI/Chapter-1 (section)

=== 1.6.3 Cumulative Carbon Dioxide Emissions ===

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The AR5 WGI ([[#IPCC--2013a|IPCC, 2013a]]) and SR1.5 ([[#IPCC--2018|IPCC, 2018]]) highlighted the near-linear relationship between cumulative carbon emissions and global mean warming (Sections 1.3 and 5.5). This implies that continued CO <sub>2</sub> emissions will cause further warming and changes in all components of the climate system, independent of any specific scenario or pathway. This is captured in the TCRE concept, which relates CO <sub>2</sub> -induced global mean warming to cumulative carbon emissions (Chapter 5). This Report thus uses cumulative CO <sub>2</sub> emissions to compare the climate response across scenarios, and to categorize emissions scenarios (Figure 1.29). The advantage of using cumulative CO <sub>2</sub> emissions is that it is an inherent emissions scenario characteristic rather than an outcome of the scenario-based projections, where uncertainties in the cause–effect chain – from emissions to atmospheric concentrations to temperature change – are important.

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[[File:c5a535ca6cc859a7b3d41a472cb68a08 IPCC_AR6_WGI_Figure_1_29.png]]

'''Figure 1.29 |''' '''The role of CO2 in driving future climate change in comparison to other greenhouse gases (GHGs)''' . The GHGs included here are CH <sub>4</sub> , N <sub>2</sub> O, and 40 other long-lived, well-mixed GHGs. The blue shaded area indicates the approximate forcing exerted by CO <sub>2</sub> in Shared Socio-economic Pathways (SSP) scenarios, ranging from very low SSP1-1.9 to very high SSP5-8.5 (Chapter 7). The CO <sub>2</sub> concentrations under the SSP1-1.9 scenarios reach approximately 350 ppm after 2150, while those of SSP5-8.5 exceed 2000 ppm CO <sub>2</sub> in the longer term (up to year 2300). Similar to the dominant radiative forcing share at each point in time (lower area plots), cumulative GWP-100-weighted GHG emissions happen to be closely correlated with cumulative CO <sub>2</sub> emissions, allowing policymakers to make use of the carbon budget concept in a policy context with multi-gas GHG baskets as it exhibits relatively low variation across scenarios with similar cumulative emissions until 2050 '''(inset panel)''' . Further details on data sources and processing are available in the chapter data table (Table 1.SM.1).

There is also a close relationship between cumulative total GHG emissions and cumulative CO <sub>2</sub> emissions for scenarios in the SR1.5 scenario database (Figure 1.29; [[#IPCC--2018|IPCC, 2018]]). The dominance of CO <sub>2</sub> compared to other well-mixed GHGs (Figure 1.29 and Section 5.2.4) allows policymakers to make use of the carbon budget concept (Section 5.5) in a policy context, in which GWP-weighted combinations of multiple GHGs are used to define emissions targets. A caveat is that cumulative GWP-weighted CO <sub>2</sub> equivalent emissions over the next decades do not yield exactly the same temperature outcomes as the same amount of cumulative CO <sub>2</sub> emissions, because atmospheric perturbation lifetimes of the various GHGs differ. While carbon budgets are not derived using GWP-weighted emissions baskets but rather by explicit modelling of non-CO <sub>2</sub> -induced warming (Section 5.5 and Cross-Chapter Box 7.1), the policy frameworks based on GWP-weighted emissions baskets can still make use of the insights from remaining cumulative carbon emissions for different warming levels.

Thesame cumulative CO <sub>2</sub> emissions could lead to a slightly different level of warming over time (Box 1.4). Rapid emissions followed by steep cuts and potentially net negative emissions would be characterized by a higher maximum warming and faster warming rate, compared with the same cumulative CO <sub>2</sub> emissions spread over a longer period. As further explored in the WGIII assessment, one potential limitation when presenting emissions pathway characteristics in cumulative emissions budget categories is that path dependencies and lock-in effects (e.g. today’s decisions regarding fossil fuel-related infrastructure) play an important role in long-term mitigation strategies ([[#Davis--2010|Davis et al., 2010]]; [[#Luderer--2018|Luderer et al., 2018]]). Similarly, high emissions early on might imply strongly net negative emissions ([[#Minx--2018|Minx et al., 2018]]) later on to reach the same target envelope for cumulative emissions and temperature by the end of the century (Box 1.4). This report explores options to address some of those potential issues from a WGI perspective (Sections 5.5.2 and 5.6.2).

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