Editing IPCC:AR6/WGI/TS (section)

=== TS.3.3 Temperature Stabilization, Net Zero Emissions and Mitigation ===

<div id="h2-24-siblings" class="h2-siblings"></div>

<div id="TS.3.3.1 " class="h3-container"></div>

<span id="ts.3.3.1-remaining-carbon-budgets-and-temperature-stabilization"></span>
==== TS.3.3.1 Remaining Carbon Budgets and Temperature Stabilization ====

<div id="h3-9-siblings" class="h3-siblings"></div>

'''The near-linear relationship between cumulative CO <sub>2</sub> emissions and maximum global surface temperature increase caused by CO <sub>2</sub> implies that stabilizing human-induced global temperature increase at any level requires net anthropogenic CO <sub>2</sub> emissions to become zero. This near-linear relationship further implies that mitigation requirements for limiting warming to specific levels can be quantified in terms of a carbon budget (''high confidence''). Remaining carbon budget estimates have been updated since AR5 with methodological improvements, resulting in larger estimates that are consistent with SR1.5. Several factors, including estimates of historical warming, future emissions from thawing permafrost, variations in projected non-CO <sub>2</sub> warming, and the global surface temperature change after cessation of CO <sub>2</sub> emissions, affect the exact value of carbon budgets (''high confidence''). Links to chapters 1.3.5, Box 1.2, 4.7.1, 5.5'''

Limiting further climate change would require substantial and sustained reductions of GHG emissions. Without net zero CO <sub>2</sub> emissions, and a decrease in the net non-CO <sub>2</sub> forcing (or sufficient net negative CO <sub>2</sub> emissions to offset any further warming from net non-CO <sub>2</sub> forcing), the climate system will continue to warm. There is ''high confidence'' that mitigation requirements for limiting warming to specific levels over this century can be estimated using a carbon budget that relates cumulative CO <sub>2</sub> emissions to global mean temperature increase (Figure TS.18, Table TS.3). For the period 1850–2019, a total of 2390 ± 240 GtCO <sub>2</sub> of anthropogenic CO <sub>2</sub> has been emitted. Remaining carbon budgets (starting from 1 January 2020) for limiting warming to 1.5°C, 1.7°C and 2.0°C are estimated at 500 GtCO <sub>2</sub> , 850 GtCO <sub>2</sub> and 1350 GtCO <sub>2</sub> , respectively, based on the 50th percentile of TCRE. For the 67th percentile, the respective values are 400 GtCO <sub>2</sub> , 700 GtCO <sub>2</sub> and 1150 GtCO <sub>2</sub> . The remaining carbon budget estimates for different temperature limits assume that non-CO <sub>2</sub> emissions are mitigated consistent with the median reductions found in scenarios in the literature as assessed in SR1.5, but they may vary by an estimated ±220 GtCO <sub>2</sub> depending on how deeply future non-CO <sub>2</sub> emissions are assumed to be reduced (Table TS.3). Links to chapters 5.5.2, 5.6, Box 5.2, 7.6

<div id="_idContainer116" class="_idGenObjectLayout-1 _idGenObjectStyleOverride-1"></div>

<div id="_idContainer114"></div>

[[File:33f1d196f83ba934a6dfadd271795bdd IPCC_AR6_WGI_TS_Figure_18.png]]
<div id="_idContainer115"></div>

'''Figure TS.18 |''' '''Illustration of (a) relationship between cumulative emissions of carbon dioxide (CO''' 2 ''') and global mean surface air temperature increase and (b) the assessment of the remaining carbon budget from its constituting components based on multiple lines of evidence.''' ''The intent of this figure is to show (i) the proportionality between cumulative CO'' 2 ''emissions and global surface air temperature in observations and models as well as the assessed range of the transient climate response to cumulative CO'' 2 ''emissions (TCRE), and (ii) how information is combined to derive remaining carbon budgets consistent with limiting warming to a specific level.'' Carbon budgets consistent with various levels of additional warming are provided in Table 5.8 and should not be read from the illustrations in either panel. In panel (a) thin black line shows historical CO <sub>2</sub> emissions together with the assessed global surface temperature increase from 1850–1900 as assessed in [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] (Box 2.3). The orange-brown range with its central line shows the estimated human-induced share of historical warming. The vertical orange-brown line shows the assessed range of historical human-induced warming for the 2010–2019 period relative to 1850–1900 (Chapter 3). The grey cone shows the assessed ''likely'' range for the TCRE ([[IPCC:Wg1:Chapter:Chapter-5#5.5.1.4|Section 5.5.1.4]]), starting from 2015. Thin coloured lines show Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations for the five scenarios of the WGI core set (SSP1-1.9, light blue; SSP1-2.6, blue; SSP2-4.5, yellow; SSP3-7.0, red; SSP5-8.5, maroon), starting from 2015 and until 2100. Diagnosed carbon emissions are complemented with estimated land-use change emissions for each respective scenario. Coloured areas show the [[IPCC:Wg1:Chapter:Chapter-4|Chapter 4]] assessed ''very likely'' range of global surface temperature projections and thick coloured central lines show the median estimate, for each respective scenario. These temperature projections are expressed relative to cumulative CO <sub>2</sub> emissions that are available for emissions-driven CMIP6 ScenarioMIP experiments for each respective scenario. For panel (b), the remaining allowable warming is estimated by combining the global warming limit of interest with the assessed historical human-induced warming ([[IPCC:Wg1:Chapter:Chapter-5#5.5.2.2.2|Section 5.5.2.2.2]]), the assessed future potential non-CO <sub>2</sub> warming contribution ([[IPCC:Wg1:Chapter:Chapter-5#5.5.2.2.3|Section 5.5.2.2.3]]) and the zero emissions commitment (ZEC; [[IPCC:Wg1:Chapter:Chapter-5#5.5.2.2.4|Section 5.5.2.2.4]]). The remaining allowable warming (vertical blue bar) is subsequently combined with the assessed TCRE (Sections 5.5.1.4 and 5.5.2.2.1) and contribution of unrepresented Earth system feedbacks ([[IPCC:Wg1:Chapter:Chapter-5#5.5.2.2.5|Section 5.5.2.2.5]]) to provide an assessed estimate of the remaining carbon budget (horizontal blue bar, Table 5.8). Note that contributions in panel (b) are illustrative and are not to scale. For example, the central ZEC estimate was assessed to be zero. Links to chapters Box 2.3, 5.2.1, 5.2.2, Figure 5.31

There is ''high confidence'' that several factors, including estimates of historical warming, future emissions from thawing permafrost, and variations in projected non-CO <sub>2</sub> warming, affect the value of carbon budgets but do not change the conclusion that global CO <sub>2</sub> emissions would need to decline to net zero to halt global warming. Estimates may vary by ±220 GtCO <sub>2</sub> depending on the level of non-CO <sub>2</sub> emissions at the time global anthropogenic CO <sub>2</sub> emissions reach net zero levels. This variation is referred to as non-CO <sub>2</sub> scenario uncertainty and will be further assessed in the AR6 Working Group III Contribution. Geophysical uncertainties surrounding the climate response to these non-CO <sub>2</sub> emissions result in an additional uncertainty of at least ±220 GtCO <sub>2</sub> , and uncertainties in the level of historical warming result in a ±550 GtCO <sub>2</sub> uncertainty. Links to chapters 5.4, 5.5.2

<div id="_idContainer033" class="_idGenObjectStyleOverride-2"></div>

'''Table TS.3 |''' '''Estimates of remaining carbon budgets and their uncertainties.''' Assessed estimates are provided for additional human-induced warming, expressed as global surface temperature, since the recent past (2010–2019), ''likely'' amounted to 0.8° to 1.3°C with a best estimate of 1.07°C relative to 1850–1900. Historical CO <sub>2</sub> emissions between 1850 and 2014 have been estimated at about 2180 ± 240 GtCO <sub>2</sub> (1-sigma range), while since 1 January 2015, an additional 210 GtCO <sub>2</sub> has been emitted until the end of 2019. GtCO <sub>2</sub> values to the nearest 50. Links to chapters Table 3.1, 5.5.1, 5.5.2, Box 5.2, Table 5.1, Table 5.7, Table 5.8

[[File:2bb994613e2250873e90c5a7cf00a459 IPCC_AR6_WGI_TS_Table_TS_3.png]]

<sup>a</sup> Human-induced global surface temperature increase between 1850–1900 and 2010–2019 is assessed at 0.8–1.3°C (''likely'' range; Cross-Section Box TS.1) with a best estimate of 1.07°C. Combined with a central estimate of TCRE (1.65°C per 1000 PgC) this uncertainty in isolation results in a potential variation of remaining carbon budgets of ±550 GtCO <sub>2</sub> , which, however, is not independent of the assessed uncertainty of TCRE and thus not fully additional.

<sup>b</sup> TCRE: transient climate response to cumulative emissions of carbon dioxide, assessed to fall ''likely'' between 1.0–2.3°C per 1000 PgC with a normal distribution, from which the percentiles are taken. Additional Earth system feedbacks are included in the remaining carbon budget estimates as discussed in [[IPCC:Wg1:Chapter:Chapter-5#5.5.2.2.5|Section 5.5.2.2.5]] .

<sup>c</sup> Estimates assume that non-CO <sub>2</sub> emissions are mitigated consistent with the median reductions found in scenarios in the literature as assessed in SR1.5. Non-CO <sub>2</sub> scenario variations indicate how much remaining carbon budget estimates vary due to different scenario assumptions related to the future evolution of non-CO <sub>2</sub> emissions in mitigation scenarios from SR1.5 that reach net zero CO <sub>2</sub> emissions. This variation is additional to the uncertainty in TCRE. The Working Group III Contribution to AR6 will reassess the potential for non-CO <sub>2</sub> mitigation based on literature since SR1.5.

<sup>d</sup> Geophysical uncertainties reported in these columns and TCRE uncertainty are not statistically independent, as uncertainty in TCRE depends on uncertainty in the assessment of historical temperature, non-CO <sub>2</sub> versus CO <sub>2</sub> forcing, and uncertainty in emissions estimates. These estimates cannot be formally combined, and these uncertainty variations are not directly additional to the spread of remaining carbon budgets due to TCRE uncertainty reported in columns three to seven.

<sup>e</sup> Recent emissions uncertainty reflects the ±10% uncertainty in the historical CO <sub>2</sub> emissions estimate since 1 January 2015.

Methodological improvements and new evidence result in updated remaining carbon budget estimates. The assessment in AR6 applies the same methodological improvements as in SR1.5, which uses a recent observed baseline for historic temperature change and cumulative emissions. Changes compared to SR1.5 are therefore small: the assessment of new evidence results in updated median remaining carbon budget estimates for limiting warming to 1.5°C and 2°C being the same and about 60 GtCO <sub>2</sub> smaller, respectively, after accounting for emissions since SR1.5. Meanwhile, remaining carbon budgets for limiting warming to 1.5°C would be about 300–350 GtCO <sub>2</sub> larger if evidence and methods available at the time of AR5 would be used. If a specific remaining carbon budget is exceeded, this results in a lower probability of keeping warming below a specified temperature level and higher irreversible global warming over decades to centuries, or alternatively a need for net negative CO <sub>2</sub> emissions or further reductions in non-CO <sub>2</sub> greenhouse gases after net zero CO <sub>2</sub> is achieved to return warming to lower levels in the long term. Links to chapters 5.5.2, 5.6, Box 5.2

Based on idealized model simulations that explore the climate response once CO <sub>2</sub> emissions have been brought to zero, the magnitude of the zero CO <sub>2</sub> emissions commitment (ZEC, see Glossary) is assessed to be ''likely'' smaller than 0.3°C for time scales of about half a century and cumulative CO <sub>2</sub> emissions broadly consistent with global warming of 2°C. However, there is ''low confidence'' about its sign on time scales of about half a century. For lower cumulative CO <sub>2</sub> emissions, the range would be smaller yet with equal uncertainty about the sign. If the ZEC is positive on decadal time scales, additional warming leads to a reduction in the estimates of remaining carbon budgets, and vice versa if it is negative. Links to chapters 4.7.1, 5.5.2

Permafrost thaw is included in estimates together with other feedbacks that are often not captured by models. Limitations in modelling studies combined with weak observational constraints only allow ''low confidence'' in the magnitude of these estimates (Section TS.3.2.2). Despite the large uncertainties surrounding the quantification of the effect of additional Earth system feedback processes, such as emissions from wetlands and permafrost thaw, these feedbacks represent identified additional risk factors that scale with additional warming and mostly increase the challenge of limiting warming to specific temperature levels. These uncertainties do not change the basic conclusion that global CO <sub>2</sub> emissions would need to decline to net zero to halt global warming. Links to chapters 5.4.8, 5.5.2, Box 5.1

<div id="TS.3.3.2" class="h3-container"></div>

<span id="ts.3.3.2-carbon-dioxide-removal"></span>

==== TS.3.3.2 Carbon Dioxide Removal ====

<div id="h3-10-siblings" class="h3-siblings"></div>

'''Deliberate carbon dioxide removal (CDR) from the atmosphere has the potential to compensate for residual CO <sub>2</sub> emissions to reach net zero CO <sub>2</sub> emissions or to generate net negative CO <sub>2</sub> emissions. In the same way that part of current anthropogenic net CO <sub>2</sub> emissions are taken up by land and ocean carbon stores, net CO <sub>2</sub> removal will be partially counteracted by CO <sub>2</sub> release from these stores (''very high confidence''). Asymmetry in the carbon cycle response to simultaneous CO <sub>2</sub> emissions and removals implies that a larger amount of CO <sub>2</sub> would need to be removed to compensate for an emission of a given magnitude to attain the same change in atmospheric CO <sub>2</sub> (''medium confidence''). CDR methods have wide-ranging side-effects that can either weaken or strengthen the carbon sequestration and cooling potential of these methods and affect the achievement of sustainable development goals (''high confidence''). Links to chapters 4.6.3, 5.6'''

Carbon dioxide removal (CDR) refers to anthropogenic activities that deliberately remove CO <sub>2</sub> from the atmosphere and durably store it in geological, terrestrial or ocean reservoirs, or in products. Carbon dioxide is removed from the atmosphere by enhancing biological or geochemical carbon sinks or by direct capture of CO <sub>2</sub> from air. Emissions pathways that limit global warming to 1.5°C or 2°C typically assume the use of CDR approaches in combination with GHG emissions reductions. CDR approaches could be used to compensate for residual emissions from sectors that are difficult or costly to decarbonize. CDR could also be implemented at a large scale to generate global net negative CO <sub>2</sub> emissions (i.e., anthropogenic CO <sub>2</sub> removals exceeding anthropogenic emissions), which could compensate for earlier emissions as a way to meet long-term climate stabilization goals after a temperature overshoot. This Report assesses the effects of CDR on the carbon cycle and climate. Co-benefits and trade-offs for biodiversity, water and food production are briefly discussed for completeness, but a comprehensive assessment of the ecological and socio-economic dimensions of CDR options is left to the WGII and WGIII reports. Links to chapters 4.6.3, 5.6

CDR methods have the potential to sequester CO <sub>2</sub> from the atmosphere (''high confidence''). In the same way part of current anthropogenic net CO <sub>2</sub> emissions are taken up by land and ocean carbon stores, net CO <sub>2</sub> removal will be partially counteracted by CO <sub>2</sub> release from these stores, such that the amount of CO <sub>2</sub> sequestered by CDR will not result in an equivalent drop in atmospheric CO <sub>2</sub> (''very'' ''high confidence''). The fraction of CO <sub>2</sub> removed from the atmosphere that is not replaced by CO <sub>2</sub> released from carbon stores – a measure of CDR effectiveness – decreases slightly with increasing amounts of removal (''medium confidence'') and decreases strongly if CDR is applied at lower atmospheric CO <sub>2</sub> concentrations (''medium confidence''). The reduction in global surface temperature is approximately linearly related to cumulative CO <sub>2</sub> removal (''high confidence''). Because of this near-linear relationship, the amount of cooling per unit CO <sub>2</sub> removed is approximately independent of the rate and amount of removal (''medium confidence''). Links to chapters 4.6.3, 5.6.2.1, Figure 5.32, Figure 5.34

Due to non-linearities in the climate system, the century-scale climate–carbon cycle response to a CO <sub>2</sub> removal from the atmosphere is not always equal and opposite to its response to a simultaneous CO <sub>2</sub> emission (''medium'' confidence). For CO <sub>2</sub> emissions of 100 PgC released from a state in equilibrium with pre-industrial atmospheric CO <sub>2</sub> levels, CMIP6 models simulate that 27± 6% (mean ± 1 standard deviation) of emissions remain in the atmosphere 80–100 years after the emissions, whereas for removals of 100 PgC only 23 ± 6% of removals remain out of the atmosphere. This asymmetry implies that an extra amount of CDR is required to compensate for a positive emission of a given magnitude to attain the same change in atmospheric CO <sub>2</sub> . Due to ''low agreement'' between models, there is ''low confidence'' in the sign of the asymmetry of the temperature response to CO <sub>2</sub> emissions and removals. Links to chapters 4.6.3, 5.6.2.1, Figure 5.35

Simulations with ESMs indicate that under scenarios where CO <sub>2</sub> emissions gradually decline, reach net zero and become net negative during the 21st century (e.g., SSP1-2.6), land and ocean carbon sinks begin to weaken in response to declining atmospheric CO <sub>2</sub> concentrations, and the land sink eventually turns into a source (Figure TS.19). This sink-to-source transition occurs decades to a few centuries after CO <sub>2</sub> emissions become net negative. The ocean remains a sink of CO <sub>2</sub> for centuries after emissions become net negative. Under scenarios with large net negative CO <sub>2</sub> emissions (e.g., SSP5-3.4-OS) and rapidly declining CO <sub>2</sub> concentrations, the land source is larger than for SSP1-2.6 and the ocean also switches to a source. While the general response is robust across models, there is ''low confidence'' in the timing of the sink-to-source transition and the magnitude of the CO <sub>2</sub> source in scenarios with net negative CO <sub>2</sub> emissions. Carbon dioxide removal could reverse some aspects climate change if CO <sub>2</sub> emissions become net negative, but some changes would continue in their current direction for decades to millennia. For instance, sea level rise due to ocean thermal expansion would not reverse for several centuries to millennia (''high confidence'') (Box TS.4). Links to chapters 4.6.3, 5.4.10, 5.6.2.1, Figure 5.30, Figure 5.33

<div id="_idContainer036"></div>

[[File:043b2bc26a352be878ed9b2160b57429 IPCC_AR6_WGI_TS_Figure_19.png]]
<div id="_idContainer035" class="Basic-Text-Frame"></div>

'''Figure TS.19 | Carbon sink response in a scenario with net carbon dioxide (CO''' 2 ''') removal from the atmosphere.''' ''The intent of this figure is to show how atmospheric CO'' 2 ''evolves under negative emissions and its dependence on the negative emissions technologies. It also shows the evolution of the ocean and land sinks.'' Shown are CO 2 flux components from concentration-driven Earth system model (ESM) simulations during different emissions stages of SSP1–2.6 and its long-term extension. (a) Large net positive CO <sub>2</sub> emissions, (b) small net positive CO <sub>2</sub> emissions, (c–d) net negative CO <sub>2</sub> emissions, and (e) net zero CO <sub>2</sub> emissions. Positive flux components act to raise the atmospheric CO <sub>2</sub> concentration, whereas negative components act to lower the CO <sub>2</sub> concentration. Net CO <sub>2</sub> emissions and land and ocean CO <sub>2</sub> fluxes represent the multi-model mean and standard deviation (error bar) of four ESMs (CanESM5, UKESM1, CESM2-WACCM, IPSL-CM6a-LR) and one Earth system model of intermediate complexity (Uvic ESCM). Net CO <sub>2</sub> emissions are calculated from concentration-driven ESM simulations as the residual from the rate of increase in atmospheric CO <sub>2</sub> and land and ocean CO <sub>2</sub> fluxes. Fluxes are accumulated over each 50-year period and converted to concentration units (parts per million, or ppm). Links to chapters 5.6.2.1, Figure 5.33

Carbon dioxide removal methods have a range of side effects that can either weaken or strengthen the carbon sequestration and cooling potential of these methods and affect the achievement of sustainable development goals (''high confidence''). Biophysical and biogeochemical side-effects of CDR methods are associated with changes in surface albedo, the water cycle, emissions of CH <sub>4</sub> and N <sub>2</sub> O, ocean acidification and marine ecosystem productivity (''high confidence''). These side-effects and associated Earth system feedbacks can decrease carbon uptake and/or change local and regional climate and in turn limit the CO <sub>2</sub> sequestration and cooling potential of specific CDR methods (''medium confidence''). Deployment of CDR, particularly on land, can also affect water quality and quantity, food production and biodiversity (''high confidence''). These effects are often highly dependent on local context, management regime, prior land use, and scale (''high confidence''). The largest co-benefits are obtained with methods that seek to restore natural ecosystems or improve soil carbon sequestration (''medium confidence''). The climate and biogeochemical effects of terminating CDR are expected to be small for most CDR methods (''medium confidence''). Links to chapters 4.6.3, 5.6.2.2, Figure 5.36, 8.4.3, 8.6.3

<div id="TS.3.3.3" class="h3-container"></div>

<span id="ts.3.3.3-relating-different-forcing-agents"></span>

==== TS.3.3.3 Relating Different Forcing Agents ====

<div id="h3-11-siblings" class="h3-siblings"></div>

'''When including other GHGs, the choice of emissions metric affects the quantification of net zero GHG emissions and their resulting temperature outcome (''high confidence''). Reaching and sustaining net zero GHG emissions typically leads to a peak and decline in temperatures when quantified with the global warming potential over a 100-year period (GWP-100). Carbon-cycle responses are more robustly accounted for in emissions metrics compared to AR5 (''high confidence''). New emissions metric approaches can be used to generate equivalent cumulative emissions of CO <sub>2</sub> for short-lived greenhouse gases based on their rate of emissions. Links to chapters 7.6.2''' '''Over 10- to 20-year time scales, the temperature response to a single year’s worth of current emissions of short-lived climate forcers (SLCFs) is at least as large as that of CO <sub>2</sub> , but because the effects of SLCFs decay rapidly over the first few decades after emission, the net long-term temperature response to a single year’s worth of emissions is predominantly determined by cumulative CO <sub>2</sub> emissions.'''

'''Emissions reductions in 2020 associated with COVID-19 containment led to small and positive global ERF; however, global and regional climate responses to the forcing are undetectable above internal variability due to the temporary nature of emissions reductions. Links to chapters 6.6, Cross-Chapter Box 6.1'''

The relative climate effects of different forcing agents are typically quantified using emissions metrics that compare the effects of an idealised pulse of 1 kg of some climate forcing agent against a reference climate forcing agent, almost always CO <sub>2</sub> . The two most prominent pulse emissions metrics are the global warming potential (GWP) and global temperature change potential (GTP) (see Glossary). The climate responses to CO <sub>2</sub> emissions by convention include the effects of warming on the carbon cycle, so for consistency these also need to be determined for non-CO <sub>2</sub> emissions. The methodology for doing this has been placed on a more robust scientific footing compared to AR5 (''high confidence''). Methane from fossil fuel sources has slightly higher emissions metric values than those from biogenic sources since it leads to additional fossil CO <sub>2</sub> in the atmosphere (''high confidence''). Updates to the chemical adjustments for CH <sub>4</sub> and N <sub>2</sub> O emissions (Section TS.3.1) and revisions in their lifetimes result in emissions metrics for GWP and GTP that are slightly lower than in AR5 (''medium confidence''). Emissions metrics for the entire suite of GHGs assessed in the AR6 have been calculated for various time horizons. Links to chapters 7.6.1, Table 7.15, Table 7.SM.7

New emissions metric approaches, such as GWP* and Combined-GTP (CGTP), relate changes in the emissions rate of short-lived greenhouse gases to equivalent cumulative emissions of CO <sub>2</sub> (CO <sub>2</sub> -e). Global surface temperature response from aggregated emissions of short-lived greenhouse gases over time is determined by multiplying these cumulative CO <sub>2</sub> -e by TCRE (see Section TS.3.2.1). When GHGs are aggregated using standard metrics such as GWP or GTP, cumulative CO <sub>2</sub> -e emissions are not necessarily proportional to future global surface temperature outcomes (''high confidence'') Links to chapters 7.6.1, Box 7.3

Emissions metrics are needed to aggregate baskets of gases to determine net zero GHG emissions. Generally, achieving net zero CO <sub>2</sub> emissions and declining non-CO <sub>2</sub> radiative forcing would halt human-induced warming. Reaching net zero GHG emissions quantified by GWP-100 typically leads to declining temperatures after net zero GHGs emissions are achieved if the basket includes short-lived gases, such as CH <sub>4</sub> . Net zero GHG emissions defined by CGTP or GWP* imply net zero CO <sub>2</sub> and other long-lived GHG emissions and constant (CGTP) or gradually declining (GWP*) emissions of short-lived gases. The warming evolution resulting from net zero GHG emissions defined in this way corresponds approximately to reaching net zero CO <sub>2</sub> emissions, and would thus not lead to declining temperatures after net zero GHG emissions are achieved but to an approximate temperature stabilization (''high confidence''). The choice of emissions metric hence affects the quantification of net zero GHG emissions, and therefore the resulting temperature outcome of reaching and sustaining net zero GHG emissions levels (''high confidence''). Links to chapters 7.6.1.4, 7.6.2, 7.6.3

As pointed out in AR5, ultimately, it is a matter for policymakers to decide which emissions metric is most applicable to their needs. This Report does not recommend the use of any specific emissions metric, as the most appropriate metric depends on the policy goal and context (see Chapter 7, [[IPCC:Wg1:Chapter:Chapter-7#7.6|Section 7.6]]). A detailed assessment of GHG metrics to support climate change mitigation and associated policy contexts is provided in the WGIII contribution to the AR6.

The global surface temperature response following a climate change mitigation measure that affects emissions of both short- and long-lived climate forcers depends on their lifetimes, their ERFs, how fast and for how long the emissions are reduced, and the thermal inertia in the climate system. Mitigation, relying on emissions reductions and implemented through new legislation or technology standards, implies that emissions reductions occur year after year. Global temperature response to a year’s worth of current emissions from different sectors informs about the mitigation potential (Figure TS.20). Over 10- to 20-year time scales, the influence of SLCFs is at least as large as that of CO <sub>2</sub> , with sectors producing the largest warming being fossil fuel production and distribution, agriculture, and waste management. Because the effects of the SLCFs decay rapidly over the first few decades after emission, the net long-term temperature effect from a single year’s worth of current emissions is predominantly determined by CO <sub>2</sub> . Fossil fuel combustion for energy, industry and land transportation are the largest contributing sectors on a 100-year time scale (''high confidence''). Current emissions of CO <sub>2</sub> , N <sub>2</sub> O and SLCFs from East Asia and North America are the largest regional contributors to additional net future warming on both short (''medium confidence'') and long time scales (10 and 100 years, respectively) (''high confidence''). Links to chapters 6.6.1, 6.6.2, Figure 6.16

<div id="_idContainer119"></div>

<div id="_idContainer117" class="_idGenObjectLayout-1 _idGenObjectStyleOverride-1"></div>

[[File:6a385ceb00c06dcd089b06e67b0ac68a IPCC_AR6_WGI_TS_Figure_20.png]]
<div id="_idContainer118"></div>

'''Figure TS.20 |''' '''Global surface temperature change 10 and 100 years after a one-year pulse of present-day emissions.''' ''The intent of this figure is to show the sectoral contribution to present-day climate change by specific climate forcers, including carbon dioxide (CO'' 2 '') as well as short-lived climate forcers (SLCFs).'' The temperature response is broken down by individual species and shown for total anthropogenic emissions (top) , and sectoral emissions on 10-year (left) and 100-year time scales (right) . Sectors are sorted by (high-to-low) net temperature effect on the 10-year time scale. Error bars in the top panel show the 5–95% range in net temperature effect due to uncertainty in radiative forcing only (calculated using a Monte Carlo approach and best estimate uncertainties from the literature). Emissions for 2014 are from the Coupled Model Intercomparison Project Phase 6 (CMIP6) emissions dataset, except for hydrofluorocarbons (HFCs) and aviation H 2 O, which rely on other datasets (see Section 6.6.2 for more details). CO <sub>2</sub> emissions are excluded from open biomass burning and residential biofuel use. Links to chapters 6.6.2, Figure 6.16

COVID-19 restrictions led to detectable reductions in global anthropogenic emissions of nitrogen oxides (NO x) (about 35% in April 2020) and fossil CO <sub>2</sub> (7%, with estimates ranging from 5.8% to 13.0%), driven largely by reduced emissions from the transportation sector (''medium confidence''). There is ''high confidence'' that, with the exception of surface ozone, reductions in pollutant precursors contributed to temporarily improved air quality in most regions of the world. However, these reductions were lower than what would be expected from sustained implementation of policies addressing air quality and climate change (''medium confidence''). Overall, the net global ERF from COVID-19 containment was likely small and positive for 2020 (with a temporary peak value less than 0.2 W m <sup>–2</sup>), thus temporarily adding to the total anthropogenic climate influence, with positive forcing (warming influence) from aerosol changes dominating over negative forcings (cooling influence) from CO <sub>2</sub> , NO ''x'' and contrail cirrus changes. Consistent with this small net radiative forcing, and against a large component of internal variability, Earth system models show no detectable effect on global or regional surface temperature or precipitation (''high confidence''). Links to chapters Cross Chapter Box 6.1

<div id="box-ts.7" class="h2-container box-container"></div>
<div class="container-box col-regular">