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

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

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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) .
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