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

==== 3.3.2.3 The Timing of Net Zero Emissions ====

<div id="h3-5-siblings" class="h3-siblings"></div>

In addition to the constraints on change in global mean temperature, the Paris Agreement also calls for reaching a balance of sources and sinks of GHG emissions (Art. 4). Different interpretations of the concept related to balance have been published ( [[#Rogelj--2015c|Rogelj et al. 2015c]] ; [[#Fuglestvedt--2018|Fuglestvedt et al. 2018]] ). Key concepts include that of net zero CO 2 emissions (anthropogenic CO 2 sources and sinks equal zero) and net zero greenhouse gas emissions (see Annex I: Glossary, and Box 3.3). The same notion can be used for all GHG emissions, but here ranges also depend on the use of equivalence metrics (Box 2.1). Moreover, it should be noted that while reaching net zero CO 2 emissions typically coincides with the peak in temperature increase; net zero GHG emissions (based on GWP-100) imply a decrease in global temperature ( [[#Riahi--2021|Riahi et al. 2021]] ) and net zero GHG emissions typically require negative CO 2 emissions to compensate for the remaining emissions from other GHGs. Many countries have started to formulate climate policy in the year that net zero emissions (either CO 2 or all greenhouse gases) are reached – although, at the moment, formulations are often still vague ( [[#Rogelj--2021|Rogelj et al. 2021]] ). There has been increased attention on the timing of net zero emissions in the scientific literature and ways to achieve it.

Figure 3.14 shows that there is a relationship between the temperature target, the cumulative CO 2 emissions budget, and the net zero year for CO 2 emissions (panel a) and the sum of greenhouse gases (panel b) for the scenarios published in the literature. In other words, the temperature targets from the Paris Agreement can, to some degree, be translated into a net-zero emission year (Tanaka and O’Neill 2018). There is, however, a considerable spread. In addition to the factors influencing the emission budget (AR6 WGI and [[#3.3.2.2|Section 3.3.2.2]] ), this is influenced by the emission trajectory until net zero is reached, decisions related to temperature overshoot and non-CO 2 emissions (especially for the moment CO 2 reaches net zero emissions). Scenarios with limited or no net negative emissions and rapid near-term emission reductions can allow small positive emissions (e.g., in hard-to-abate-sectors). They may therefore have a later year that net zero CO 2 emissions are achieved. High emissions in the short term, in contrast, require an early net zero year.

<div id="_idContainer018" class="Basic-Text-Frame"></div>

[[File:2b9f3422963bd2eb571a5eb82c56550b IPCC_AR6_WGIII_Figure_3_14.png]]

'''Figure 3.14 | Net zero year for CO''' 2 '''and all GHGs (based on AR6''' '''GWP100''' ''') as a function of remaining carbon budget and temperature outcomes (note that scenarios that stabilise (near) zero are also included in determining the net zero year).'''

For the scenarios in the C1 category (limit warming to 1.5°C (&gt;50% with no or limited overshoot, the net zero year for CO 2 emissions is typically around 2035–2070. For scenarios in C3 (limiting warming to 2°C (&gt;67%)), CO 2 emissions reach net zero around after 2050. Similarly, also the years for net zero GHG emissions can be calculated (see Fig 3.14b. The GHG net zero emissions year is typically around 10–40 years later than the carbon neutrality. Residual non-CO 2 emissions at the time of reaching net zero CO 2 range between 5–11 GtCO 2 -eq in pathways that limit warming to 2°C (&gt;67%) or lower. In pathways limiting warming to 2°C (&gt;67%), methane is reduced by around 19% (3–46%) in 2030 and 46% (29–64%) in 2050, and in pathways limiting warming to 1.5°C (&gt;50%) with no or limited overshoot by around 34% (21–57%) in 2030 and a similar 51% (35–70%) in 2050. Emissions-reduction potentials assumed in the pathways become largely exhausted when limiting warming to 2°C (&gt;50%). N 2 O emissions are reduced too, but similar to CH 4 , emission reductions saturate for stringent climate goals. In the mitigation pathways, the emissions of cooling aerosols are reduced due to reduced use of fossil fuels. The overall impact on non-CO 2 -related warming combines these factors.

In cost-optimal scenarios, regions will mostly achieve net zero emissions as a function of options for emission reduction, CDR, and expected baseline emission growth ( [[#van%20Soest--2021b|van Soest et al. 2021b]] ). This typically implies relatively early net zero emission years in scenarios for the Latin America region and relatively late net zero years for Asia and Africa (and average values for OECD countries). However, an allocation based on equity principles (such as responsibility, capability and equality) might result in different net zero years, based on the principles applied – with often earlier net zero years for the OECD ( [[#Fyson--2020|Fyson et al. 2020]] ; [[#van%20Soest--2021b|van Soest et al. 2021b]] ). Therefore, the emission trajectory until net zero emissions is a critical determinant of future warming ( [[#3.5|Section 3.5]] ). The more CO 2 is emitted until 2030, the less CO 2 can be emitted after that to stay below a warming limit ( [[#Riahi--2015|Riahi et al. 2015]] ). As discussed before, also non-CO 2 forcing plays a key role in the short term.

<div id="3.3.2.4" class="h3-container"></div>

<span id="mitigation-strategies"></span>