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

=== Box 3.4 | Consistency of Remaining Carbon Budgets in the WGI Assessment and Cumulative CO 2 Emissions in WGIII Mitigation Pathways ===

<div id="h2-11-siblings" class="h2-siblings"></div>

Introduction

The WGI assessment has shown that the increase in global mean temperature has a near-linear relationship with cumulative CO 2 emissions (Chapter 5, [[IPCC:Wg3:Chapter:Chapter-5#5.5|Section 5.5]] , Box 5.3 of AR6 WGI report). Consistently, WGI has confirmed that net zero CO 2 emissions are required to halt CO 2 -induced warming. This permits the estimation of carbon budgets consistent with specific temperature goals. In Chapter 3, we present the temperature outcomes and cumulative CO 2 emissions associated with different warming levels for around 1200 scenarios published in the literature and which were classified according to different warming levels ( [[#3.2|Section 3.2]] and Annex III.II.3.2). In this box, we discuss the consistency of the assessments presented here and in IPCC AR6 WGI. The box summarises how the remaining carbon budgets assessed by AR6 WGI relate to the remaining cumulative CO 2 emissions until the time of net zero CO 2 emissions in mitigation pathways (Tables 3.2 and SPM.1) assessed by AR6 WGIII.

In its assessment, AR6 WGI uses a framework in which the various components of the remaining carbon budget are informed by various lines of evidence and assessed climate system characteristics. The AR6 WGIII, instead, uses around 1200 emission scenarios with estimated warming levels that cover the scenario range presented in AR6 WGI but also contain many more intermediate projections with varying emission profiles and a combination of CO 2 emissions and other greenhouse gases. In order to assess their climate outcomes, climate model emulators are used. The emulators are reduced complexity climate models that are provided by AR6 WGI, and which are calibrated to the AR6 WGI assessment of future warming for various purposes (a detailed description of the use of climate model emulators in the AR6 WGI and WGIII assessments can be found in Cross-Chapter Box 7.1 in the AR6 WGI report, with the connection of WGI and WGIII discussed in Annex III.2.5.1).

'''Remaining carbon budgets estimated by AR6 WGI'''

The AR6 WGI estimated the remaining carbon budgets from their assessment of (i) the transient climate response to cumulative emissions of carbon dioxide (TCRE), and estimates of (ii) the historical human-induced warming, (iii) the temperature change after reaching net zero CO 2 emissions, (iv) the contribution of future non-CO 2 warming (derived from the emissions scenarios assessed in the Special Report on 1.5°C Warming using WGI-calibrated emulators), and (v) the Earth System feedbacks (AR6 WGI Chapter 5.5, Box 5.2). For a given warming level, AR6 WGI assessed the remaining carbon budget from the beginning of 2020 onwards. These are 650/500/400 GtCO 2 for limiting warming to 1.5°C with 33%/50%/ 67% chance and 1350/1150 GtCO 2 for limiting warming to 2°C with 50%/67% chance. The estimates are subject to considerable uncertainty related to historical warming, future non-CO 2 forcing, and poorly quantified climate feedbacks. For instance, variation in non-CO 2 emissions across scenarios are estimated to either increase or decrease the remaining carbon budget estimates by 220 GtCO 2 . The estimates of the remaining carbon budget assume that non-CO 2 emissions are reduced consistently with the tight temperature targets for which the budgets are estimated.

Cumulative CO 2 emissions until net zero estimated by AR6 WGIII

The AR6 WGIII provides estimates of cumulative net CO 2 emissions (from 2020 inclusive) until the time of reaching net zero CO 2 emissions (henceforth called ‘peak cumulative CO 2 emissions’) and until the end of the century for eight temperature classes that span a range of warming levels. The numbers can be found in Table 3.2 (330–710 GtCO 2 for C1; 530–930 for C2; and 640–1160 for C3).

Comparing the AR6 WGI remaining carbon budgets and remaining cumulative CO 2 emissions of the AR6 WGIII scenarios

A comparison between AR6 WGI and WGIII findings requires recognising that, unlike in WGI, cumulative emissions in WGIII are not provided for a specific peak-warming threshold or level but are instead provided for a set of scenarios in a category, representing a specific range of peak-temperature outcomes (for instance the C4 category contains scenarios with a median peak warming anywhere between approximately 1.8°C and up to 2°C). When accounting for this difference, the AR6 WGI and WGIII findings are very consistent for temperature levels below 2°C. Figure 1 compares the peak temperatures and associated cumulative CO 2 emissions (i.e., peak cumulative CO 2 emissions) for the WGIII scenarios to the remaining carbon budgets assessed by WGI. This shows only minor differences between the WGI and WGIII approaches.

<div id="_idContainer034" class="_idGenObjectStyleOverride-2"></div>

[[File:3cf307f6bcda7d4aa1cb8cce69661ddd IPCC_AR6_WGIII_Box_3_4_Figure_2.png]]

'''Box 3.4, Figure 2 | (a) Differences in regressions of the relationship between peak surface temperature and associated cumulative CO 2e missions from 2020 derived from scenarios of eight integrated assessment model frameworks.''' The coloured lines show the regression at median for scenarios of the eight modelling frameworks, each with more than 20 scenarios in the database and a detailed land-use representation. The red dotted lines indicate the non-CO 2 uncertainty range of AR6 WGI [[IPCC:Wg3:Chapter:Chapter-5|Chapter 5]] (±220 GtCO 2 ), here visualised around the median of the eight model framework lines. Carbon budgets from 2020 until 1.5°C (0.43K above 2010–2019 levels) and 2.0°C (0.93K above 2010–2019 levels) are shown for minimum and maximum model estimates at the median, rounded to the nearest 10 GtCO 2 . Panel '''(b)''' shows the relationship between the estimated non-CO 2 warming in mitigation scenarios that reach net zero and the associated peak surface temperature outcomes. The coloured lines show the regression at median for scenarios of the eight modelling frameworks with more than 20 scenarios in the database and a detailed land-use representation. The black dashed line indicates the non-CO 2 relationship based on the scenarios and climate emulator setup as was assessed in AR6 WGI Chapter 5.

After correcting for the categorisation, some (small) differences between the AR6 WGI and WGIII numbers arise from remaining differences between the outcomes of the climate emulators and their set-up (IPCC AR6 WGI Cross-Chapter Box 7.1) and the differences in the underlying scenarios. Moreover, the WGI assessment estimated the non-CO 2 warming at the time of net zero CO 2 emissions based on a relationship derived from the SR1.5 scenario database with historical emission estimates as in [[#Meinshausen--2020|Meinshausen et al. (2020)]] (AR6 WGI Chapter 5). The WGIII assessment uses the same climate emulator with improved historical emissions estimates ( [[#Nicholls--2021|Nicholls et al. 2021]] ) (AR6 WGI Cross-Chapter Box 7.1). Annex III.II.2.5.1 further explores the effects of these factors on the relationship between non-CO 2 warming at peak cumulative CO 2 and peak surface temperature.

Estimates of the remaining carbon budgets thus vary with the assumed level of non-CO 2 emissions, which are a function of policies and technology development. The linear relationship used in the AR6 WGI assessment between peak temperature and the warming as a result of non-CO 2 emissions (based on the SR1.5 data) is shown in the right panel of Figure 2 (dashed line). In the AR6 WGIII approach, the non-CO 2 warming for each single scenario is based on the individual scenario characteristics. This is shown in the same figure by plotting the outcomes of scenario outcomes of a range of models (dots). The lines show the fitted data for individual models, emphasising the clear differences across models and the relationship with peak warming (policy level). In some scenarios, stringent non-CO 2 emission reductions provide an option to reach more stringent climate goals with the same carbon budget. This is especially the case for scenarios with a very low non-CO 2 warming, for instance, as a result of methane reductions through diet change. The left panel shows how these differences impact estimates of the remaining carbon budget. While the AR6 scenarios database includes a broad range of non-CO 2 emission projections the overall range is still very consistent with the WGI relationship and the estimated uncertainty with a ±220 GtCO 2 range (see also Figure 5 in Annex III.II.2.5.1).

Overall, the slight differences between the cumulative emissions in AR6 WGIII and the carbon budget in AR6 WGI are because the non-CO 2 warming in the WGIII scenarios is slightly lower than in the SR1.5 scenarios that are used for the budget estimates in WGI (Annex III.2.5.1). In addition, improved consistency with Cross-Chapter Box 7.1 in Chapter 7, AR6 WGI results in a non-CO 2 -induced temperature difference of about about 0.05K between the assessments. Recalculating the remaining carbon budget using the WGI methodology combined with the full AR6 WGIII scenario database results in a reduction of the estimated remaining 1.5°C carbon budget by about 100 GtCO 2 (–20%), and a reduction of about 40 GtCO 2 (–3%) for 2°C. Accounting also for the categorisation effect, the difference between the WGI and WGIII estimates is found to be small and well within the uncertainty range (Figure 1). This means that the cumulative CO 2 emissions presented in WGIII and the WGI carbon budgets are highly consistent.

A detailed comparison of the impact of different assessment steps (i.e., the new emulators, scenarios, and harmonisation methods), has been made and is presented in Figure 6 in Annex III.II.3.2 .

Policy implications

The concept of a finite carbon budget means that the world needs to get to net zero CO 2 , no matter whether global warming is limited to 1.5°C or well below 2°C (or any other level). Moreover, exceeding the remaining carbon budget will have consequences by overshooting temperature levels. Still, the relationship between the timing of net zero and temperature targets is a flexible one, as discussed further in Cross-Chapter Box 3 in this chapter. It should be noted that the national-level inventory as used by UNFCCC for the land use, land-use change and forestry sector is different from the overall concept of anthropogenic emissions employed by IPCC AR6 WGI. For emissions estimates based on these inventories, the remaining carbon budgets must be correspondingly reduced by approximately 15%, depending on the scenarios ( [[#Grassi--2021|Grassi et al., 2021]] ) (Chapter 7).

One of the uncertainties of the remaining carbon budget is the level of non-CO 2 emissions which is a function of policies and technology development. This represents a point of leverage for policies rather than an inherent geophysical uncertainty. Stringent non-CO 2 emission reductions hence can provide – to some degree – an option to reach more stringent climate goals with the same carbon budget.

The near-linear relationship implies that cumulative CO 2 emissions are critically important for climate outcomes ( [[#Collins--2013|Collins et al. 2013]] ). The maximum temperature increase is a direct function of the cumulative emissions until net zero CO 2 emissions is reached (the emission budget) (Figure 3.13, left side). The end-of-century temperature correlates well with cumulative emissions across the century (right panel). For long-term climate goals, positive emissions in the first half of the century can be offset by net removal of CO 2 from the atmosphere (net negative emissions) at the cost of a temporary overshoot of the target ( [[#Tokarska--2019|Tokarska et al. 2019]] ). The bottom panels of Figure 3.13 show the contribution of net negative CO 2 emissions.

Focusing on cumulative emissions, the right-hand panel of Figure 3.12b shows that for high-end scenarios (C6–C7), most emissions originate from fossil fuels, with a smaller contribution from net deforestation. For C5 and lower, there is also a negative contribution to emissions from both AFOLU emissions and energy systems. For the energy systems, these negative emissions originate from bioenergy with carbon capture and storage (BECCS), while for AFOLU, they originate from reforestation and afforestation. For C3–C5, reforestation has a larger CDR contribution than BECCS, mostly due to considerably lower costs ( [[#Rochedo--2018|Rochedo et al. 2018]] ). For C1 and C2, the tight carbon budgets imply in many scenarios more CDR use ( [[#Riahi--2021|Riahi et al. 2021]] ). Please note that net negative emissions are not so relevant for peak-temperature targets, and thus the C1 category, but CDR can still be used to offset the remaining positive emissions ( [[#Riahi--2021|Riahi et al. 2021]] ). While positive CO 2 emissions from fossil fuels are significantly reduced, inertia and hard-to-abate sectors imply that in many C1–C3 scenarios, around 800–1000 GtCO 2 of net positive cumulative CO 2 emissions remain. This is consistent with literature estimates that current infrastructure is associated with 650 GtCO 2 (best estimate) if operated until the end of its lifetime ( [[#Tong--2019|Tong et al. 2019]] ). These numbers are considerably above the estimated carbon budgets for 1.5°C estimated in AR6 WGI, hence explaining CDR reliance (either to offset emissions immediately or later in time).

Creating net negative emissions can thus be an important part of a mitigation strategy to offset remaining emissions or compensate for emissions earlier in time. As indicated above, there are different ways to potentially achieve this, including reforestation and afforestation and BECCS (as often covered in IAMs) but also soil carbon enhancement, direct air carbon capture and storage (DACCS) and ocean alkalinisation (Chapter 12). Except for reforestation, these options have not been tested at large scale and often require more R&amp;D. Moreover, the reliance on CDR in scenarios has been discussed given possible consequences of land use related to biodiversity loss and food security (BECCS and afforestation), the reliance on uncertain storage potentials (BECCS and DACCS), water use (BECCS), energy use (DACCS), the risks of possible temperature overshoot and the consequences for meeting Sustainable Development Goals (SDGs) ( [[#Anderson--2016|Anderson and Peters 2016]] ; [[#Smith--2016|Smith et al. 2016]] ; [[#Venton--2016|Venton 2016]] ; [[#Peters--2017|Peters and Geden 2017]] ; [[#van%20Vuuren--2017|van Vuuren et al. 2017]] ; [[#Honegger--2021|Honegger et al. 2021]] ). In the case of BECCS, it should be noted that bioenergy typically is associated with early-on positive CO 2 emissions and net negative effects are only achieved in time (carbon debt), and its potential is limited ( [[#Cherubini--2013|Cherubini et al. 2013]] ; [[#Hanssen--2020|Hanssen et al. 2020]] ); most IAMs have only a very limited representation of these time dynamics. Several scenarios have therefore explored how reliance on net negative CO 2 emissions can be reduced or even avoided by alternative emission strategies ( [[#Grubler--2018|Grubler et al. 2018]] ; [[#van%20Vuuren--2018|van Vuuren et al. 2018]] ) or early reductions by more stringent emission reduction in the short term ( [[#Rogelj--2019b|Rogelj et al. 2019b]] ; [[#Riahi--2021|Riahi et al. 2021]] ). A more in-depth discussion of land-based mitigation options can be found in Chapter 7. It needs to be emphasised that even in strategies with net negative CO 2 emissions, the emission reduction via more conventional mitigation measures (efficiency improvement, decarbonisation of energy supply) is much larger than the CDR contribution ( [[#Tsutsui--2020|Tsutsui et al. 2020]] ).

<div id="cross-chapter-box-3" class="h2-container box-container"></div>

<span id="cross-chapter-box-3-understanding-net-zero-co-2-and-net-zero-ghg-emissions"></span>