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

=== 1.6.3 Cumulative Carbon Dioxide Emissions ===

<div id="h2-35-siblings" class="h2-siblings"></div>

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.

<div id="_idContainer079" class="_idGenObjectStyleOverride-1"></div>

[[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).

<div id="box-1.4" class="h2-container box-container"></div>

'''Box 1.4 | The Relationships Between 'Net Zero' Emissions, Temperature Outcomes and Carbon Dioxide Removal'''

<div id="h2-36-siblings" class="h2-siblings"></div>

Article 4 of the Paris Agreement sets an objective to ‘achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases’ ( [[#1.2|Section 1.2]] ). This box addresses the relationship between such a balance and the corresponding evolution of global surface temperature, with or without the deployment of large-scale carbon dioxide removal (CDR), using the definitions of ‘net zero CO <sub>2</sub> emissions’ and ‘net zero greenhouse gas (GHG) emissions’ of the AR6 Glossary (Annex VII).

‘Net zero CO <sub>2</sub> emissions’ is defined in AR6 as the condition in which anthropogenic CO <sub>2</sub> emissions are balanced by anthropogenic CO <sub>2</sub> removals over a specified period. Similarly, ‘net zero GHG emissions’ is the condition in which metric-weighted anthropogenic GHG emissions are balanced by metric-weighted anthropogenic GHG removals over a specified period. The quantification of net zero GHG emissions thus depends on the GHG emissions metric chosen to compare emissions of different gases, as well as the time horizon chosen for that metric. (For a broader discussion of metrics, see Box 1.3 and Section 7.6, and WGIII Cross-Chapter Box 2.)

Technical notes expanding on these definitions can be found as part of their respective entries in the Glossary. The notes clarify the relation between ‘net zero’ CO <sub>2</sub> and GHG emissions and the concept of carbon and GHG neutrality, and the metric usage set out in the Paris Rulebook [Decision 18/CMA.1, annex, paragraph 37].

A global net zero level of CO <sub>2</sub> , or GHG, emissions will be achieved when the sum of anthropogenic emissions and removals across all countries, sectors, sources and sinks reaches zero. Achieving net zero CO <sub>2</sub> or GHG emissions globally, at a given time, does not imply that individual entities (i.e., countries, sectors) have to reach net zero emissions at that same point in time, or even at all (see WGIII, TS Box 4 and Chapter 3).

Net zero CO <sub>2</sub> and net zero GHG emissions differ in their implications for the subsequent evolution of global surface temperature. Net zero CO <sub>2</sub> emissions result in approximately stable CO <sub>2</sub> -induced warming, but overall warming will depend on any further warming contribution of non-CO <sub>2</sub> GHGs. The effect of net zero GHG emissions on global surface temperature depends on the GHG emissions metric chosen to aggregate emissions and removals of different gases. For GWP100 (the metric in which Parties to the Paris Agreement have decided to report their aggregated emissions and removals), net zero GHG emissions would generally imply a peak in global surface temperature, followed by a gradual decline (Section 7.6.2; see also [[IPCC:Wg1:Chapter:Chapter-4#4.7.1|Section 4.7.1]] regarding the zero emissions commitment). However, other anthropogenic factors, such as aerosol emissions or land use-induced changes in albedo, may still affect the climate.

The definitions of net zero CO <sub>2</sub> and GHG should also be seen in relation to the various CDR methods discussed in the context of climate change mitigation (see Section 5.6, which also includes an assessment of the response of natural sinks to CDR), and how it is employed in scenarios used throughout the WGI and WGIII reports ( [[#1.6.1|Section 1.6.1]] ; see also WGIII Chapters 3, 7 and 12.)

For virtually all scenarios assessed by the IPCC, CDR is necessary to reach both global net zero CO <sub>2</sub> and net zero GHG emissions, to compensate for residual anthropogenic emissions. This is in part because for some sources of CO <sub>2</sub> and non-CO <sub>2</sub> emissions, abatement options to eliminate them have not yet been identified. For a given scenario, the choice of GHG metric determines how much net CDR is necessary to compensate for residual non-CO <sub>2</sub> emissions, in order to reach net zero GHG emissions (Section 7.6.2).

If CDR is further used to go beyond net zero, to a situation with net-negative CO <sub>2</sub> emissions (i.e., where anthropogenic removals exceed anthropogenic emissions), anthropogenic CO <sub>2</sub> -induced warming will decline. A further increase of CDR, until a situation with net zero or even net-negative GHG emissions is reached, would increase the pace at which historical human-induced warming is reversed after its peak (SR1.5, [[#IPCC--2018|IPCC, 2018]] ). Net negative anthropogenic GHG emissions may become necessary to stabilize the global surface temperature in the long term, should climate feedbacks further affect natural GHG sinks and sources (Chapter 5).

CDR can be achieved through a number of measures (Section 5.6; SRCCL , [[#IPCC--2019a|IPCC, 2019a]] ) . These include additional afforestation, reforestation, soil carbon management, biochar, direct air capture and carbon capture and storage (DACCS), and bioenergy with carbon capture and storage (BECCS; [[#de%20Coninck--2018|de Coninck et al., 2018]] , SR1.5 Ch4; [[#Minx--2018|Minx et al., 2018]] ; see also WGIII Chapters 7 and 12). Differences between land use, land-use change and forestry (LULUCF) accounting rules, and scientific bookkeeping approaches for CO <sub>2</sub> emissions and removals from the terrestrial biosphere, can result in significant differences between the amount of CDR that is reported in different studies ( [[#Grassi--2017|Grassi et al., 2017]] ). Different measures to achieve CDR come with different risks, negative side effects and potential co-benefits – also in conjunction with sustainable development goals – that can inform choices around their implementation (Section 5.6; [[#Fuss--2018|Fuss et al., 2018]] ; [[#Roe--2019|Roe et al., 2019]] ). Technologies to achieve direct large-scale anthropogenic removals of non-CO <sub>2</sub> GHGs are speculative at present (Yoon et al. , 2009; Ming et al. , 2016; Kroeger et al. , 2017; Jackson et al., 2019) .

<div id="1.7" class="h1-container"></div>

<span id="final-remarks"></span>