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>

'''Box TS.7 | Climate and Air Quality Responses to Short-lived Climate Forcers in Shared''' '''Socio-economic''' '''Pathways'''

<div id="h2-25-siblings" class="h2-siblings"></div>

'''Future changes in emissions of short-lived climate forcers (SLCFs) are expected to cause an additional global mean warming, with a large diversity in the end-of-century response across the WGI core set of Shared Socio-economic Pathways (SSPs), depending upon the level of climate change and air pollution mitigation (Box TS.7, Figure 1). This additional warming is either due to reductions in cooling aerosols for air pollution regulation or due to increases in methane (CH <sub>4</sub> ), ozone and hydrofluorocarbons (HFCs). This additional warming is stable after 2040 in SSPs associated with lower global air pollution as long as CH <sub>4</sub> emissions are also mitigated, but the overall warming induced by SLCF changes is higher in scenarios in which air quality continues to deteriorate (induced by growing fossil fuel use and limited air pollution control) ( ''high confidence'' ).''' '''Sustained CH <sub>4</sub> mitigation reduces global surface ozone, contributing to air quality improvements, and also reduces surface temperature in the longer term, but only sustained CO <sub>2</sub> emissions reductions allow long-term climate stabilization ( ''high confidence'' ). Future changes in air quality (near-surface ozone and particulate matter, or PM) at global and local scales are predominantly driven by changes in ozone and aerosol precursor emissions rather than climate ( ''high confidence'' ). Air quality improvements driven by rapid decarbonization strategies, as in SSP1-1.9 and SSP1-2.6, are not sufficient in the near term to achieve air quality guidelines set by the World Health Organization in some highly polluted regions ( ''high confidence'' ). Additional policies (e.g., access to clean energy, waste management) envisaged to attain United Nations Sustainable Development Goals bring complementary SLCF reduction. Links to chapters 4.4.4, 6.6.3, 6.7.3, Box 6.2'''
The net effect of SLCF emissions changes on temperature will depend on how emissions of warming and cooling SLCFs will evolve in the future. The magnitude of the cooling effect of aerosols remains the largest uncertainty in the effect of SLCFs in future climate projections. Since the SLCFs have undergone large changes over the past two decades, the temperature and air pollution responses are estimated relative to the year 2019 instead of 1995–2014.

'''Temperature Response'''
In the next two decades, it is ''very likely'' that SLCF emissions changes will cause a warming relative to 2019, across the WGI core set of SSPs (see Section TS.1.3.1), in addition to the warming from long-lived GHGs. The net effect of SLCF and HFC changes in global surface temperature across the SSPs is a ''likely'' warming of 0.06°C–0.35°C in 2040 relative to 2019. This near-term global mean warming linked to SLCFs is quite similar in magnitude across the SSPs due to competing effects of warming (CH <sub>4</sub> , ozone) and cooling (aerosols) forcers (Box TS.7, Figure 1). There is greater diversity in the end-of-century response among the scenarios. SLCF changes in scenarios with no climate change mitigation (SSP3-7.0 and SSP5-8.5) will cause a warming in the ''likely'' range of 0.4°C–0.9°C in 2100 relative to 2019 due to increases in CH <sub>4</sub> , tropospheric ozone and HFC levels. For the stringent climate change and pollution mitigation scenarios (SSP1-1.9 and SSP1-2.6), the cooling from reductions in CH <sub>4</sub> , ozone and HFCs partially balances the warming from reduced aerosols, primarily sulphate, and the overall SLCF effect is a ''likely'' increase in global surface temperature of 0.0°C–0.3°C in 2100, relative to 2019. With intermediate climate change and air pollution mitigations, SLCFs in SSP2-4.5 add a ''likely'' warming of 0.2°C–0.5°C to global surface temperature change in 2100, with the largest warming resulting from reductions in aerosols. Links to chapters 4.4.4, 6.7.3

Assuming implementation and efficient enforcement of both the Kigali Amendment to the Montreal Protocol on Substances that Deplete the Ozone Layer and current national plans result in limiting emissions (as in SSP1-2.6), the effects of HFCs on global surface temperature, relative to 2019, would remain below +0. 02°C from 2050 onwards versus about +0.04°C–0.08°C in 2050 and +0.1°C–0.3°C in 2100 considering only national HFC regulations decided prior to the Kigali Amendment (as in SSP5-8.5) ( ''medium confidence'' ). Links to chapters 6.6.3, 6.7.3

'''Air Quality Responses'''
Air pollution projections range from strong reductions in global surface ozone and PM (e.g., SSP1-2.6, with stringent mitigation of both air pollution and climate change) to no improvement and even degradation (e.g., SSP3-7.0 without climate change mitigation and with only weak air pollution control) ( ''high confidence'' ). Under the SSP3-7.0 scenario, PM levels are projected to increase until 2050 over large parts of Asia, and surface ozone pollution is projected to worsen over all continental areas through 2100 ( ''high confidence'' ). In SSP5-8.5, a scenario without climate change mitigation but with stringent air pollution control, PM levels decline through 2100, but high CH <sub>4</sub> levels hamper the decline in global surface ozone at least until 2080 ( ''high confidence'' ). Links to chapters 6.7.1

[[File:c2c71d66beca10ee183027f1605c09cb IPCC_AR6_WGI_TS_Box_7_Figure_1.png]]

'''Box TS.7, Figure 1 |'''

'''Effects of short-lived climate forcers (SLCFs) on global surface temperature and air pollution across the WGI core set of Shared Socio-economic Pathways (SSPs).''' ''The intent of this figure is to show the climate and air quality (surface ozone and particulate matter smaller than 2.5 microns in diameter, or PM'' ''2.5'' '') response to SLCFs in the SSP scenarios for the near and long-term.'' Effects of net aerosols, tropospheric ozone, hydrofluorocarbons (HFCs; with lifetimes less than 50 years), and methane (CH <sub>4</sub> ) are compared with those of total anthropogenic forcing for 2040 and 2100 relative to year 2019. The global surface temperature changes are based on historical and future evolution of effective radiative forcing (ERF) as assessed in [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] of this Report. The temperature responses to the ERFs are calculated with a common impulse response function (RT) for the climate response, consistent with the metric calculations in [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] (Box 7.1). The RT has an equilibrium climate sensitivity of 3.0°C for a doubling of atmospheric CO <sub>2</sub> concentration (feedback parameter of –1.31 W m <sup>–2</sup> °C <sup>–1</sup> ). The scenario total (grey bar) includes all anthropogenic forcings (long- and short-lived climate forcers, and land-use changes). Uncertainties are 5–95% ranges. The global changes in air pollutant concentrations (ozone and PM 2.5 ) are based on multimodel Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations and represent changes in five-year mean surface continental concentrations for 2040 and 2098 relative to 2019. Uncertainty bars represent inter-model ±1 standard deviation. Links to chapters 6.7.2, 6.7.3, Figure 6.24

<div id="box-ts.8" class="h2-container box-container"></div>

'''Box TS.8 | Earth System Response to Solar Radiation Modification'''

<div id="h2-26-siblings" class="h2-siblings"></div>

'''Since AR5, further modelling work has been conducted on aerosol-based solar radiation modification (SRM) options such as stratospheric aerosol injection, marine cloud brightening, and cirrus cloud thinning <sup>[[#footnote-000|21]]</sup> and their climate and biogeochemical effects. These investigations have consistently shown that SRM could offset some of the effects of increasing greenhouse gases on global and regional climate, including the carbon and water cycles ( ''high confidence'' ). However, there would be substantial residual or overcompensating climate change at the regional scales and seasonal time scales ( ''high confidence'' ), and large uncertainties associated with aerosol–cloud–radiation interactions persist. The cooling caused by SRM would increase the global land and ocean CO <sub>2</sub> sinks ( ''medium confidence'' ), but this would not stop CO <sub>2</sub> from increasing in the atmosphere or affect the resulting ocean acidification under continued anthropogenic emissions ( ''high confidence'' ). It is ''likely'' that abrupt water cycle changes will occur if SRM techniques are implemented rapidly. A sudden and sustained termination of SRM in a high CO <sub>2</sub> emissions scenario would cause rapid climate change ( ''high confidence'' ). However, a gradual phase-out of SRM combined with emissions reduction and carbon dioxide removal (CDR) would avoid these termination effects ( ''medium confidence'' ). Links to chapters 4.6.3, 5.6.3. 6.4.6, 8.6.3 .'''
Solar radiation modification (SRM) refers to deliberate, large-scale climate intervention options that are studied as potential supplements to deep mitigation, for example, in scenarios that overshoot climate stabilization goals. SRM options aim to offset some of the warming effects of GHG emissions by modification of Earth’s shortwave radiation budget. Following SR1.5, the SRM assessed in this Report also includes some options, such as cirrus cloud thinning, that alter the longwave radiation budget.

SRM contrasts with climate change mitigation activities, such as emissions reductions and CDR, as it introduces a ‘mask’ to the climate change problem by altering Earth’s radiation budget, rather than attempting to address the root cause of the problem, which is the increase in GHGs in the atmosphere. By masking only the climate effects of GHG emissions, SRM does not address other issues related to atmospheric CO <sub>2</sub> increase, such as ocean acidification. This Report assesses physical understanding of the Earth system response to proposed SRM, and the assessment is based primarily on idealized climate model simulations. There are other important considerations, such as risk to human and natural systems, perceptions, ethics, cost, governance, and trans-boundary issues and their relationship to the United Nations Sustainable Development Goals – issues that the WGII (Chapter 16) and WGIII (Chapter 14) Reports address. Links to chapters 4.6.3

SRM options include those that increase surface albedo, brighten marine clouds by increasing the amount of cloud condensation nuclei, or reduce the optical depth of cirrus clouds by seeding them with ice nucleating particles. However, the most commonly studied approaches attempt to mimic the cooling effects of major volcanic eruptions by injecting reflective aerosols (e.g., sulphate aerosols) or their precursors (e.g., sulphur dioxide) into the stratosphere. Links to chapters 4.6.3, 5.6.3, 6.4.6

SRM could offset some effects of greenhouse gas-induced warming on global and regional climate, but there would be substantial residual and overcompensating climate change at the regional scale and seasonal time scales ( ''high confidence'' ). Since AR5, more modelling work has been conducted with more sophisticated treatment of aerosol-based SRM approaches, but the uncertainties in cloud–aerosol–radiation interactions are still large ( ''high confidence'' ). Modelling studies suggest that it is possible to stabilize multiple large-scale temperature indicators simultaneously by tailoring the deployment strategy of SRM options ( ''medium confidence'' ) but with large residual or overcompensating regional and seasonal climate changes. Links to chapters 4.6.3

SRM approaches targeting shortwave radiation are ''likely'' to reduce global mean precipitation, relative to future CO <sub>2</sub> emissions scenarios, if all global mean warming is offset. In contrast, cirrus cloud thinning, targeting longwave radiation, is expected to cause an increase in global mean precipitation ( ''medium confidence'' ). If shortwave approaches are used to offset global mean warming, the magnitude of reduction in regional precipitation minus evapotranspiration (P–E) (Box TS.5), which is more relevant to freshwater availability, is smaller than precipitation decrease because of simultaneous reductions in both precipitation and evapotranspiration ( ''medium confidence'' ). Links to chapters 4.6.3, 8.2.1, 8.6.3 .

If SRM is used to cool the planet, it would cause a reduction in plant and soil respiration and slow the reduction of ocean carbon uptake due to warming ( ''medium confidence'' ). The result would be an enhancement of the global land and ocean CO <sub>2</sub> sinks ( ''medium confidence'' ) and a slight reduction in atmospheric CO <sub>2</sub> concentration relative to unmitigated climate change. However, SRM would not stop CO <sub>2</sub> from increasing in the atmosphere or affect the resulting ocean acidification under continued anthropogenic emissions ( ''high confidence'' ). Links to chapters 5.6.3

The effect of stratospheric aerosol injection on global temperature and precipitation is projected by models to be detectable after one to two decades, which is similar to the time scale for the emergence of the benefits of emissions reductions. A sudden and sustained termination of SRM in a high GHG emissions scenario would cause rapid climate change and a reversal of the SRM effects on the carbon sinks ( ''high confidence'' ). It is also ''likely'' that a termination of strong SRM would drive abrupt changes in the water cycle globally and regionally, especially in the tropical regions by shifting the Inter-tropical Convergence Zone and Hadley cells. At the regional scale, non-linear responses cannot be excluded, due to changes in evapotranspiration. However, a gradual phase-out of SRM combined with emissions reductions and CDR would avoid larger rates of changes ( ''medium confidence'' ). Links to chapters 4.6.3, 5.6.3, 8.6.3 .

<div id="box-ts.9" class="h2-container box-container"></div>

'''Box TS.9 | Irreversibility, Tipping Points and Abrupt Changes'''

<div id="h2-27-siblings" class="h2-siblings"></div>

'''The present rates of response of many aspects of the climate system are proportionate to the rate of recent temperature change, but some aspects may respond disproportionately. Some climate system components are slow to respond, such as the deep ocean overturning circulation and the ice sheets (Box TS.4). It is ''virtually certain'' that irreversible, committed change is already underway for the slow-to-respond processes as they come into adjustment for past and present emissions.''' '''The paleoclimate record indicates that tipping elements exist in the climate system where processes undergo sudden shifts toward a different sensitivity to forcing, such as during a major deglaciation, where 1°C degree of temperature change might correspond to a large or small ice-sheet mass loss during different stages (Box TS.2). For global climate indicators, evidence for abrupt change is limited, but deep ocean warming, acidification and sea level rise are committed to ongoing change for millennia after global surface temperatures initially stabilize and are irreversible on human time scales ( ''very high confidence'' ). At the regional scale, abrupt responses, tipping points and even reversals in the direction of change cannot be excluded ( ''high confidence'' ). Some regional abrupt changes and tipping points could have severe local impacts, such as unprecedented weather, extreme temperatures and increased frequency of droughts and forest fires.''' '''Models that exhibit such tipping points are characterized by abrupt changes once the threshold is crossed, and even a return to pre-threshold surface temperatures or to atmospheric carbon dioxide concentrations does not guarantee that the tipping elements return to their pre-threshold state. Monitoring and early warning systems are being put into place to observe tipping elements in the climate system. Links to chapters 1.3, 1.4.4, 1.5, 4.3.2, Table 4.10, 5.3.4, 5.4.9, 7.5.3, 9.2.2, 9.2.4, 9.4.1, 9.4.2, 9.6.3, Cross-chapter Box 12.1'''
Understanding of multi-decadal reversibility (i.e., the system returns to the previous climate state within multiple decades after the radiative forcing is removed) has improved since AR5 for many atmospheric, land surface and sea ice climate metrics following sea surface temperature recovery. Some processes suspected of having tipping points, such as the Atlantic Meridional Overturning Circulation (AMOC), have been found to often undergo recovery after temperature stabilization with a time delay ( ''low confidence'' ). However, substantial irreversibility is further substantiated for some cryosphere changes, ocean warming, sea level rise, and ocean acidification. Links to chapters 4.7.2, 5.3.3, 5.4.9, 9.2.2, 9.2.4, 9.4.1, 9.4.2, 9.6.3

Some climate system components are slow to respond, such as the deep ocean overturning circulation and the ice sheets. It is ''likely'' that under stabilization of global warming at 1.5°C, 2.0°C or 3.0°C relative to 1850–1900, the AMOC will continue to weaken for several decades by about 15%, 20% and 30% of its strength and then recover to pre-decline values over several centuries ( ''medium confidence'' ). At sustained warming levels between 2°C and 3°C, there is ''limited evidence'' that the Greenland and West Antarctic ice sheets will be lost almost completely and irreversibly over multiple millennia; both the probability of their complete loss and the rate of mass loss increases with higher surface temperatures ( ''high confidence'' ). At sustained warming levels between 3°C and 5°C, near-complete loss of the Greenland Ice Sheet and complete loss of the West Antarctic Ice Sheet is projected to occur irreversibly over multiple millennia ( ''medium confidence'' ); with substantial parts or all of Wilkes Subglacial Basin in East Antarctica lost over multiple millennia ( ''low confidence'' ). Early-warning signals of accelerated sea level rise from Antarctica could possibly be observed within the next few decades. For other hazards (e.g., ice-sheet behaviour, glacier mass loss and global mean sea level change, coastal floods, coastal erosion, air pollution, and ocean acidification) the time and/or scenario dimensions remain critical, and a simple and robust relationship with global warming level cannot be established ( ''high confidence'' ). Links to chapters 4.3.2, 4.7.2, 5.4.3, 5.4.5, 5.4.8, 8.6, 9.2, 9.4, Box 9.3, Cross-Chapter Box 12.1

For global climate indicators, evidence for abrupt change is limited. For global warming up to 2°C above 1850–1900 levels, paleoclimate records do not indicate abrupt changes in the carbon cycle ( ''low confidence'' ). Despite the wide range of model responses, uncertainty in atmospheric CO <sub>2</sub> by 2100 is dominated by future anthropogenic emissions rather than uncertainties related to carbon–climate feedbacks ( ''high confidence'' ). There is no evidence of abrupt change in climate projections of global temperature for the next century: there is a near-linear relationship between cumulative CO <sub>2</sub> emissions and maximum global mean surface air temperature increase caused by CO <sub>2</sub> over the course of this century for global warming levels up to at least 2°C relative to 1850–1900. The increase in global ocean heat content (Section TS.2.4) will likely continue until at least 2300 even for low emissions scenarios, and global mean sea level will continue to rise for centuries to millennia following cessation of emissions (Box TS.4) due to continuing deep ocean heat uptake and mass loss of the Greenland and Antarctic ice sheets ( ''high confidence'' ). Links to chapters 2.2.3; Cross-Chapter Box 2.1; 5.1.1; 5.4; Cross-Chapter Box 5.1; Figures 5.3, 5.4, 5.25, and 5.26; 9.2.2; 9.2.4

The response of biogeochemical cycles to anthropogenic perturbations can be abrupt at regional scales and irreversible on decadal to century time scales ( ''high confidence'' ). The probability of crossing uncertain regional thresholds increases with climate change ( ''high'' '''''Box TS.9'''''

''confidence'' ). It is ''very unlikely'' that gas clathrates (mostly methane) in deeper terrestrial permafrost and subsea clathrates will lead to a detectable departure from the emissions trajectory during this century. Possible abrupt changes and tipping points in biogeochemical cycles lead to additional uncertainty in 21st century atmospheric GHG concentrations, but future anthropogenic emissions remain the dominant uncertainty ( ''high confidence'' ). There is potential for abrupt water cycle changes in some high emissions scenarios, but there is no overall consistency regarding the magnitude and timing of such changes. Positive land surface feedbacks, including vegetation, dust, and snow, can contribute to abrupt changes in aridity, but there is only ''low confidence'' that such changes will occur during the 21st century. Continued Amazon deforestation, combined with a warming climate, raises the probability that this ecosystem will cross a tipping point into a dry state during the 21st century ( ''low confidence'' ). (Section TS.3.2.2) Links to chapters 5.4.3, 5.4.5, 5.4.8, 5.4.9, 8.6.2, 8.6.3, Cross-Chapter Box 12.1

<div id="TS.4" class="h1-container"></div>

<span id="ts.4-regional-climate-change"></span>