Editing IPCC:AR6/WGI/Chapter-4 (section)

== 4.7 Climate Change Beyond 2100 ==

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This section assesses changes in climate beyond 2100. An advance since AR5 is the availability of ESM results for scenarios beyond 2100 and for much longer stabilisation simulations compared with analysis predominantly based on Earth system models of intermediate complexity (EMICs) at the time of AR5 (e.g., [[#Eby--2013|Eby et al., 2013]] ; [[#Zickfeld--2013|Zickfeld et al., 2013]] ). Long-term commitment of sea level rise due to thermal expansion and ice-sheet loss is assessed in [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] (Section 9.6.3.5 and Figure 9.30). Here we assess projections of GSAT, global precipitation, and Arctic sea ice. Uncertainties relating to potential long-term changes in AMOC are treated in Section 9.2.3.1.

On multi-century time scales it is common to explore changes that are due to long-term commitment. Here we differentiate between:

* '''Committed emissions due to infrastructure.''' Infrastructure that causes greenhouse gas emissions cannot be changed straight away leading to a commitment from existing infrastructure that some emissions will continue for a number of years into the future ( [[#Davis--2014|Davis and Socolow, 2014]] ; C.J. [[#Smith--2019|]] [[#Smith--2019|Smith et al., 2019]] ). Further consideration of this aspect of commitment will be assessed by WGIII.
* '''Climate response to constant emissions.''' Some of the scenario extensions beyond 2100 make assumptions about constant emissions (either positive or negative). [[#4.7.1|Section 4.7.1]] will assess changes in climate under scenario extensions beyond 2100.
* '''Committed climate change to constant atmospheric composition.''' There is widespread literature on how the climate continues to change after stabilisation of radiative forcing. This includes diagnosing the long-term climate response to a doubling of CO <sub>2</sub> (ECS, Chapter 7). Since AR5, more GCMs have run stabilized forcing simulations for many centuries allowing new insights into their very long-term behaviour (Section 7.4.3).
* '''Committed response to zero emissions.''' How climate would continue to evolve if all emissions ceased. The SR1.5 assessed changes in climate if emissions of all greenhouse gases and aerosols ceased. [[#4.7.2|Section 4.7.2]] assesses new results considering cessation of CO <sub>2</sub> -only emissions which forms a significant term in calculating remaining carbon budgets.
* '''Irreversibility.''' Some changes do not revert if the forcing is removed, leaving a committed change to the system. [[#4.7.2|Section 4.7.2]] assesses changes in the Earth system which may be irreversible.
* '''Abrupt changes.''' If a tipping point in the climate system is passed, then some elements may continue to respond if the forcing which caused them is removed. [[#4.7.2|Section 4.7.2]] assesses the potential for abrupt changes in the Earth system.

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=== 4.7.1 Commitment and ClimateChange Beyond 2100 ===

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==== 4.7.1.1 Climate Change Following Zero Emissions ====

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The zero emissions commitment (ZEC) is the climate change commitment that would result, in terms of projected GSAT, from setting carbon dioxide (CO <sub>2</sub> ) emissions to zero. It is determined by both inertia in physical climate system components (ocean, cryosphere, land surface) and carbon cycle inertia (see Annex VII). In its widest sense it refers to emissions of all compounds including greenhouses gases, aerosols and their pre-cursors. A specific sub-category of zero emissions commitment is the zero CO <sub>2</sub> emissions commitment, which refers to the climate system response to a cessation of anthropogenic CO <sub>2</sub> emissions excluding the impact of non-CO <sub>2</sub> forcers. Assessment of remaining carbon budgets requires an assessment of zero CO <sub>2</sub> emissions commitment as well as of the transient climate response to cumulative carbon emissions (TCRE; Section 5.5.2).

There is an offset of continued warming following cessation of emissions by continued CO <sub>2</sub> removal by natural sinks ( ''high confidence'' ) (e.g., Matthews and Caldeira, 2008; [[#Solomon--2009|Solomon et al., 2009]] ; [[#Joos--2013|Joos et al., 2013]] ; [[#Ricke--2014|Ricke and Caldeira, 2014]] ). Some models continue warming by up to 0.5°C after emissions cease at 2°C of warming ( [[#Frölicher--2014|Frölicher et al., 2014]] ; [[#Frölicher--2015|Frölicher and Paynter, 2015]] ; [[#Williams--2017|Williams et al., 2017]] ), while others simulate little to no additional warming ( [[#Nohara--2015|Nohara et al., 2015]] ). In SR1.5, the available evidence indicated that past CO <sub>2</sub> emissions do not commit to substantial further warming ( [[#Allen--2018|Allen et al., 2018]] ). A ZEC close to zero was thus applied for the computation of the remaining carbon budget ( [[#Rogelj--2018b|Rogelj et al., 2018b]] ). However, the available literature consisted of simulations from a small number of models using a variety of experimental designs, with some simulations showing a complex evolution of temperature following cessation of emissions (e.g., [[#Frölicher--2014|Frölicher et al., 2014]] ; [[#Frölicher--2015|Frölicher and Paynter, 2015]] ; [[#Williams--2017|Williams et al., 2017]] ).

Here we draw on new simulations to provide an assessment of ZEC using multiple ESMs ( [[#Jones--2019|Jones et al., 2019]] ) and EMICs ( [[#MacDougall--2020|MacDougall et al., 2020]] ). Figure 4.39 shows results from 20 models that simulate the evolution of CO <sub>2</sub> and the GSAT response following cessation of CO <sub>2</sub> emissions for an experiment where 1000 PgC is emitted during a 1% per year CO <sub>2</sub> increase. All simulations show a strong reduction in atmospheric CO <sub>2</sub> concentration following cessation of CO <sub>2</sub> emissions in agreement with previous studies and basic theory that natural carbon sinks will persist. Therefore, there is ''very high confidence'' that atmospheric CO <sub>2</sub> concentrations would decline for decades if CO <sub>2</sub> emissions cease. Temperature evolution in the 100 years following cessation of emissions varies by model and across time scales, with some models showing declining temperature, others having ZEC close to zero, and others showing continued warming following cessation of emissions (Figure 4.39). The GSAT response depends on the balance of carbon sinks and ocean heat uptake ( [[#MacDougall--2020|MacDougall et al., 2020]] ). The 20-year average GSAT change 50 years after the cessation of emissions (ZEC <sub>50</sub> ) is summarized in Table 4.8. The mean value of ZEC <sub>50</sub> is –0.079°C, with 5–95% range –0.34°C–0.28°C. There is no strong relationship between ZEC <sub>50</sub> and modelled climate sensitivity (neither ECS nor TCR; [[#MacDougall--2020|MacDougall et al., 2020]] ). It is therefore ''likely'' that the absolute magnitude of ZEC <sub>50</sub> is less than 0.3°C, but we assess ''low'' ''confidence'' in the sign of ZEC on 50-year time scales. This is small compared with natural variability in GSAT.

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[[File:db2b2f90b934309946649a02c9646f1f IPCC_AR6_WGI_Figure_4_39.png]]

'''Figure 4.39''' '''|''' '''Zero emissions commitment (ZEC).''' Changes in '''(a)''' atmospheric CO <sub>2</sub> concentration and '''(b)''' evolution of global surface air temperature (GSAT) following cessation of CO <sub>2</sub> emissions branched from the 1% per year experiment after emissions of 1000 Pg C ( [[#Jones--2019|Jones et al., 2019]] ; [[#MacDougall--2020|MacDougall et al., 2020]] ). ZEC is the temperature anomaly relative to the estimated temperature at the year of cessation. ZEC <sub>50</sub> is the 20-year mean GSAT change centred on 50 years after the time of cessation (see Table 4.8) – this period is marked with the vertical dotted lines. Multi-model mean is shown as thick black line, individual model simulations are in grey. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

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'''Table 4.8''' '''|''' '''The 20-year average GSAT change 50 years after the cessation of emissions (ZEC50).''' Displayed are ZEC50 estimated from eleven ESMs (top) and nine EMICs (bottom).

{| class="wikitable"
|-
| Model

| ZEC <sub>50</sub> (°C)

|-
| ACCESS-ESM1.5

| 0.01

|-
| CanESM5

| –0.14

|-
| CESM2

| –0.31

|-
| CNRM-ESM2-1

| 0.06

|-
| GFDL-ESM2M

| –0.27

|-
| GFDL-ESM4

| –0.21

|-
| GISS-E2-1-G

| –0.15

|-
| MIROC-ES2L

| –0.08

|-
| MPI-ESM1.2-LR

| –0.27

|-
| NorESM2-LM

| –0.33

|-
| UKESM1-0-LL

| 0.28

|-
| Bern3D-LPX

| 0.01

|-
| DCESS1.0

| 0.06

|-
| CLIMBER-2

| –0.07

|-
| IAPRAS

| 0.28

|-
| LOVECLIM 1.2

| –0.04

|-
| MESM

| 0.01

|-
| MIROC-lite

| –0.06

|-
| PLASM-GENIE

| –0.36

|-
| UVic ESCM 2.10

| 0.03

|}

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<span id="change-in-global-climate-indices-beyond-2100"></span>
==== 4.7.1.2 Change in Global Climate Indices Beyond 2100 ====

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This subsection assesses changes in global climate indices out to 2300 using extensions of the SSP scenarios ( [[#Meinshausen--2020|Meinshausen et al., 2020]] ) and literature based on extensions to the RCP scenarios from CMIP5 ( [[#Meinshausen--2011|Meinshausen et al., 2011]] ), which differ from the SSPs despite similar labelling of global radiative forcing levels ( [[#4.6.2|Section 4.6.2]] ). [[#Meinshausen--2020|Meinshausen et al. (2020)]] describe the extensions to the SSP scenarios, which differ slightly from the ScenarioMIP documentation ( [[#O’Neill--2016|O’Neill et al., 2016]] ). A simplified approach across scenarios reduces emissions such that after 2100, land-use CO <sub>2</sub> emissions are reduced to zero by 2150; any net negative fossil CO <sub>2</sub> emissions are reduced to zero by 2200, and positive fossil CO <sub>2</sub> emissions are reduced to zero by 2250. Non-CO <sub>2</sub> fossil fuel emissions are also reduced to zero by 2250 while land-use-related non-CO <sub>2</sub> emissions are held constant at 2100 levels. The extensions are created up to the year 2500, but ESM simulations have only been requested, as part of the CMIP6 protocol, to run to 2300. As a result, unlike the RCP8.5 extension, SSP5-8.5 sees a decline in CO <sub>2</sub> concentration after 2250, but the radiative forcing level is similar, reaching approximately 12 W m <sup>–2</sup> during most of the extension. Both SSP1-2.6 and SSP5-3.4-OS decrease radiative forcing after 2100. SSP5-3.4-OS is designed to return to the same level of forcing as SSP1-2.6 during the first half of the 22nd century. Because relatively few CMIP6 ESMs have submitted results beyond 2100, GSAT projections using the MAGICC7 emulator (see [[#cross-chapter-box-7.1|Cross-Chapter Box 7.1]] ) are also shown here.

Changes in climate at 2300 have impacts and commitments beyond this timeframe ( ''high confidence'' ). Sea level rise may exceed 2 m on millennial time scales even when warming is limited to 1.5°C–2°C, and tens of metres for higher warming levels (Table 9.10). [[#Randerson--2015|Randerson et al. (2015)]] showed increasing importance on carbon cycle feedbacks of slow ocean processes, [[#Mahowald--2017|Mahowald et al. (2017)]] showed the long-lasting legacy of land-use effects and J.K. [[#Moore--2018|]] [[#Moore--2018|Moore et al. (2018)]] show how changes in Southern Ocean winds affect nutrients and marine productivity well beyond 2300. [[#Clark--2016|Clark et al. (2016)]] show that physical and biogeochemical impacts of 21st century emissions have a potential committed legacy of at least 10,000 years.

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<span id="global-surface-air-temperature"></span>
===== 4.7.1.2.1 Global surface air temperature =====

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Both CMIP6 and CMIP5 results show that global temperature beyond 2100 is strongly dependent on scenario, and the difference in GSAT projections between high- and low-emissions scenarios continues to increase ( ''high confidence'' ). Under the extended RCP2.6 ( [[#Caesar--2013|Caesar et al., 2013]] ) and SSP1-2.6 scenarios, where CO <sub>2</sub> concentration and radiative forcing continue to decline beyond 2100, GSAT stabilizes during the 21st century before decreasing and remaining below 2°C until 2300, except in some of the very high climate-sensitivity ESMs, which project GSAT to stay above 2°C by 2300 (Figure 4.40). Under RCP8.5, regional temperature changes above 20°C have been reported in multiple models over high-latitude land areas ( [[#Caesar--2013|Caesar et al., 2013]] ; [[#Randerson--2015|Randerson et al., 2015]] ). Non-CO <sub>2</sub> forcing and feedbacks remain important by 2300 ( ''high confidence'' ). [[#Randerson--2015|Randerson et al. (2015)]] found that 1.6°C of warming by 2300 came from non-CO <sub>2</sub> forcing alone in RCP8.5, and [[#Rind--2018|Rind et al. (2018)]] show that regional forcing from aerosols can have notable effects on ocean circulation on centennial time scales. High latitude warming led to longer growing seasons and increased vegetation growth in the CESM1 model ( [[#Liptak--2017|Liptak et al., 2017]] ), and [[#Burke--2017|Burke et al. (2017)]] found that carbon release from permafrost areas susceptible to this warming may amplify future climate change by up to 17% by 2300.

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[[File:1f36a4626186c03d88c5908762289143 IPCC_AR6_WGI_Figure_4_40.png]]

'''Figure 4.40''' '''|''' '''Simulated climate changes up to 2300 under the extended SSP scenarios.''' Displayed are '''(a)''' projected global surface air temperature (GSAT) change, relative to 1850–1900, from CMIP6 models (individual lines) and MAGICC7 (shaded plumes); '''(b)''' as (a) but zoomed in to show low-emissions scenarios; '''(c)''' global land precipitation change; and '''(d)''' September Arctic sea ice area. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

Too few CMIP6 models performed the extension simulations to allow a robust assessment of GSAT projection, and some of those which did had higher than average climate sensitivity values. Therefore, we base our assessment of GSAT projections (Table 4.9) on the MAGICC7 emulator calibrated against assessed GSAT to 2100 ( [[#4.3.4|Section 4.3.4]] , [[#cross-chapter-box-7.1|Cross-Chapter Box 7.1]] ). Because the emulator approach has not been evaluated in depth up to 2300 in the same way as it has up to 2100 ( [[#cross-chapter-box-7.1|Cross-Chapter Box 7.1]] ) we account for possible additional uncertainty by assessing the 5–95% range from MAGICC as ''likely'' instead of ''very likely'' . It is therefore ''likely'' that GSAT will exceed 2°C above that of the period 1850–1900 at the year 2300 in the extended SSP scenarios SSP2-4.5, SSP3-7.0 and SSP5-8.5 (Figure 4.40). For SSP1-2.6 and SSP1-1.9, mean warming at 2300 is 1.5°C and 0.9°C respectively. GSAT differences between SSP5-3.4-overshoot and SSP1-2.6 peak during the 21st century but decline to less than about 0.25°C after 2150 ( ''medium confidence'' ).

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'''Table 4.9''' '''|''' '''Change of global surface air temperature at 2300.''' Displayed are the median and 5–95% range of GSAT change at 2300 relative to 1850–1900 for the six scenarios used with MAGICC7.

{| class="wikitable"
|-
| Scenario

| Median (°C)

| 5–95% Range (°C)

|-
| SSP5-8.5

| 9.6

| 6.6–14.1

|-
| SSP3-7.0

| 8.2

| 5.7–11.8

|-
| SSP2-4.5

| 3.3

| 2.3–4.6

|-
| SSP5-3.4-OS

| 1.6

| 1.1–2.2

|-
| SSP1-2.6

| 1.5

| 1.0–2.2

|-
| SSP1-1.9

| 0.9

| 0.6–1.4

|}

To place the temperature projections for the end of the 23rd century into the context of paleo temperatures, GSAT under SSP2-4.5 ( ''likely'' 2.3°C–4.6°C higher than over the period 1850–1900) has not been experienced since the Mid Pliocene, about three million years ago. GSAT projected for the end of the 23rd century under SSP5-8.5 ( ''likely'' 6.6°C–14.1°C higher than over the period 1850–1900) overlaps with the range estimated for the Miocene Climatic Optimum (5°C–10°C higher) and Early Eocene Climatic Optimum (10°C–18°C higher), about 15 and 50 million years ago, respectively ( ''medium confidence'' ) (Chapter 2).

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===== 4.7.1.2.2 Global land precipitation =====

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Global land precipitation will continue to increase in line with GSAT under high emissions scenarios ( ''medium confidence'' ). Precipitation changes over land show larger variability and a less clear signal than global total precipitation. [[#Caesar--2013|Caesar et al. (2013)]] showed that under the CMIP5 extension simulations, HadGEM2-ES projected global land precipitation to remain roughly the same in RCP2.6, to increase by about 4% in RCP4.5 and to increase by about 7% in RCP8.5. Their results showed global precipitation increasing linearly with temperature while radiative forcing increases, but then more quickly if forcing is stabilized or reduced. This backs up findings of an intensification of the hydrological cycle following CO <sub>2</sub> decrease which has been attributed to a build-up of ocean heat ( [[#Wu--2010|Wu et al., 2010]] ), and to a fast atmospheric adjustment to CO <sub>2</sub> radiative forcing ( [[#Cao--2011|Cao et al., 2011]] ). Figure 4.40 shows that global land precipitation increases in CMIP6 models until 2300 for SSP5-8.5 but stabilizes in SSP1-2.6 and SSP5-3.4-OS. SSP1-2.6 and SSP5-3.4-OS are not distinguishable in behaviour of projected global land precipitation after 2100.

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===== 4.7.1.2.3 Arctic sea ice =====

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[[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] assesses with ''high confidence'' that on decadal and longer time scales, Arctic summer sea ice area will remain highly correlated with global mean temperature until the summer sea ice has vanished (Section 9.3.1.1). This means that Arctic sea ice will continue to decline in scenarios of continued warming but will begin to recover in scenarios where GSAT begins to decrease. Under the CMIP5 extension simulations, minimum (September) Arctic sea ice area began to recover for most models under RCP2.6 out to 2300, while RCP4.5 and RCP8.5 extensions became ice-free in September ( [[#Hezel--2014|Hezel et al., 2014]] ; [[#Bathiany--2016|Bathiany et al., 2016]] ). They also found increasingly strong winter responses under continued warming such that under the RCP8.5 extension, the Arctic became ice-free nearly year-round by 2300. Consistent with the assessment in Section 9.3.1.1 that Arctic sea ice area is correlated with GSAT, CMIP6 projections to 2300 show partial sea ice recovery by 2300 in SSP1-2.6 in line with GSAT (Figure 4.40), with one model (MRI-ESM2-0) showing near complete recovery to present-day values. SSP1-2.6 and SSP5-3.4-OS are not distinguishable in behaviour of Arctic sea ice in these models after 2100. SSP5-8.5 remains ice-free in September up to 2300.

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<span id="potential-for-abrupt-and-irreversible-climate-change"></span>
=== 4.7.2 Potential for Abrupt and Irreversible Climate Change ===

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Similar to AR5 and SROCC, AR6 defines an abrupt climate change as a large-scale abrupt change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades and causes substantial impacts in human and/or natural systems (Glossary). Further, AR6 considers such a perturbed state of a dynamical system as irreversible on a given time scale, if the recovery time scale from this state due to natural processes takes substantially longer than the time scale of interest (Glossary). The AR6 adopts the related definition of a tipping point as a critical threshold beyond which a system reorganizes, often abruptly and/or irreversibly, and a tipping element as a component of the Earth system that is susceptible to a tipping point (Glossary). Tipping points may involve global or regional climate changes from one stable state to another stable state or to changes that occur faster than the rate of change of forcing ( [[#Alley--2003|Alley et al., 2003]] ) and include shifts from one equilibrium state to another and other responses of the climate system to external forcing ( [[IPCC:Wg1:Chapter:Chapter-1#1.2.4.2|Section 1.2.4.2]] ). While reversibility has been defined alternatively in the literature with respect to the response specifically to idealized CO <sub>2</sub> forcing and generally GSAT change, AR6 considers both definitions synonymous, because it has been widely demonstrated that the GSAT change is reversible in models with respect to CO <sub>2</sub> with a several-year lag ( [[#Boucher--2012|Boucher et al., 2012]] ).

Abrupt and irreversible changes in the climate system are assessed across multiple chapters in AR6. This section provides a cross-chapter synthesis of these assessments as an update to Table 12.4 in AR5 and Table 6.1 in SROCC. Understanding of abrupt climate change and irreversibility has advanced considerably since AR5 with many of the projected changes in proposed Tipping Elements having grown more confident (Table 4.10). Many aspects of the physical climate changes induced by GHG warming previously demonstrated to be reversible in a single model have been confirmed in multiple models ( [[#Boucher--2012|Boucher et al., 2012]] ; [[#Tokarska--2015|Tokarska and Zickfeld, 2015]] ) with others such as sea level rise or terrestrial ecosystems confirmed to continue to respond on long time scales ( [[#Clark--2016|Clark et al., 2016]] ; [[#Zickfeld--2017|Zickfeld et al., 2017]] ; [[#Pugh--2018|Pugh et al., 2018]] ).

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'''Table 4.10''' '''|''' '''Cross-chapter assessment updating AR5 and SROCC of components in the Earth system that have been proposed as susceptible to tipping points/abrupt change, irreversibility, projected 21st century change, and overall change in assessment from previous IPCC reports.''' Also provided are confidence levels and, in parentheses, the main section(s) of this Report in which proposed tipping elements are assessed.

{| class="wikitable"
|-
| Earth System Component/Tipping Element

| Potential Abrupt Climate Change?

| Irreversibility if Forcing Reversed (Time Scales Indicated)

| Projected 21st Century Change Under Continued Warming

| Change in Assessment

|-
| Global Monsoon (4.5.1.5; 8.6)

| Yes, under AMOC collapse, ''medium confidence''

| Reversible within years to decades, ''medium confidence''

| ''Medium confidence'' in global monsoon increase; ''medium confidence'' in Asian-African strengthening and North American weakening

| More lines of evidence than AR5

|-
| Tropical Forest (5.4.8; 8.6.2)

| Yes, ''low confidence''

| Irreversible for multi-decades, ''medium confidence''

| ''Medium confidence'' of increasing vegetation carbon storage depending on human disturbance

| More confident rates than AR5

|-
| Boreal Forest (5.4.8)

| Yes, ''low confidence''

| Irreversible for multi-decades, ''medium confidence''

| ''Medium confidence'' in offsetting lower latitude dieback and poleward extension depending on human disturbance

| More confident rates than AR5

|-
| Permafrost Carbon (5.4.8)

| Yes, ''high confidence''

| Irreversible for centuries, ''high confidence''

| ''Virtually certain'' decline in frozen carbon; ''low confidence'' in net carbon change

| More confident rates than SROCC

|-
| Arctic Summer Sea Ice (4.3.2; 4.6.2.1; 9.3.1)

| No, ''high confidence''

| Reversible within years to decades, ''high confidence''

| ''Likely complete loss''

| More specificity than SROCC

|-
| Arctic Winter Sea Ice (4.3.2; 9.3.1)

| Yes, ''high confidence''

| Reversible within years to decades, ''high confidence''

| ''High confidence'' in moderate winter declines

| More specificity than SROCC

|-
| Antarctic Sea Ice (9.3.2)

| ''Yes, low confidence''

| Unknown, ''low confidence''

| ''Low confidence'' in moderate winter and summer declines

| Improved CMIP6 simulation

|-
| Greenland Ice Sheet (9.4.1)

| No, ''high confidence''

| Irreversible for millennia, ''high confidence''

| ''Virtually certain'' mass loss under all scenarios

| More lines of evidence than SROCC

|-
| West Antarctic Ice Sheet and Shelves (9.4.2; Box 9.4)

| ''Yes, high confidence''

| Irreversible for decades to millennia, ''high confidence''

| ''Likely'' mass loss under all scenarios; ''deep uncertainty'' in projections for above 3°C

| Added deep uncertainty at GWL &gt;3°C

|-
| Global Ocean Heat Content (4.5.2.1; 4.6.2.1; 9.2.2; CCBox 7.1)

| No, ''high confidence''

| Irreversible for centuries, ''very high confidence''

| ''Very high confidence'' oceans will continue to warm

| Better consistency with ECS/TCR

|-
| Global Sea-Level Rise (4.6.2.1; 4.6.3.2; 9.6.3.5; Box 9.4)

| Yes, ''high confidence''

| Irreversible for centuries, ''very high confidence''

| ''Very high confidence'' in continued rise; ''deep uncertainty'' in projections above 3°C

| Added deep uncertainty at GWL &gt;3°C

|-
| AMOC (4.6.3.2; 8.6.1; 9.2.3.1)

| Yes, ''medium confidence''

| Reversible within centuries, ''high confidence''

| ''Very likely decline; medium confidence of no collapse''

| More lines of evidence than SROCC

|-
| Southern MOC (9.2.3.2)

| Yes, ''medium confidence''

| Reversible within decades to centuries, ''low confidence''

| ''Medium confidence'' in decrease in strength

| More lines of evidence than SROCC

|-
| Ocean Acidification (4.3.2.5; 5.4.2; 5.4.4)

| Yes, ''high confidence''

| Reversible at surface; irreversible for centuries to millennia at depth, ''very high confidence''

| ''Virtually certain'' to continue with increasing CO <sub>2</sub> ; likely polar aragonite undersaturation

| More lines of evidence than SROCC

|-
| Ocean Deoxygenation (5.3.3.2)

| Yes, ''high confidence''

| Reversible at surface; irreversible for centuries to millennia at depth, ''medium confidence''

| ''Medium confidence'' in deoxygenation rates and increased hypoxia

| Improved CMIP6 simulation

|}

The Carbon Dioxide Removal Model Intercomparison Project (CDR-MIP; [[#Keller--2018|Keller et al., 2018]] ) comprises a set of 1% ramp-up, ramp-down simulations aimed at establishing a multi-model assessment of reversibility of Earth system components. Preliminary results from CDRMIP are presented in [[#4.6.3|Section 4.6.3]] . Results from the SSP5-3.4-Overshoot scenario and other quantities of climate change at the same CO <sub>2</sub> level before and after overshoot are assessed in [[#4.6.2|Section 4.6.2]] . Forcing reversal is followed by reversal of ocean surface and land temperature along with land and ocean precipitation, snow cover, and Arctic sea ice with a lag of a few years to decades (Table 4.10). Other tipping elements have much longer time scales of reversibility from decades to millennia. [[#Drijfhout--2015|Drijfhout et al. (2015)]] provided an assessment of 13 regional mechanisms of abrupt change, finding abrupt changes in sea ice, oceanic flows, land ice, and terrestrial ecosystem response, although with little consistency among the models. The potential for abrupt changes in ice sheets, the AMOC, tropical forests, and ecosystem responses to ocean acidification were also recently reviewed by ( [[#Good--2018|Good et al., 2018]] ). They found that some degree of irreversible loss of the West Antarctic Ice Sheet (WAIS) may have already begun, that tropical forests are adversely affected by drought, and rapid development of aragonite undersaturation at high latitudes affecting calcifying organisms.

New since AR5 is the fundamental recognition in SRCCL and in this Report (Chapter 5) that projected changes in forests strongly depend on the human disturbance and that tropical forest dieback in the absence of disturbance is largely driven by the increased potential for drought, while that in boreal forests includes both thermal and hydrological factors ( [[#Drijfhout--2015|Drijfhout et al., 2015]] ). For some proposed tipping elements, the role of seasonal change has become better understood. For example, the lack of a tipping point in the reduction of summer Arctic sea ice area (Stroeve and [[#Notz--2015|Notz, 2015]] ) has been further substantiated. The role of abrupt change at the edges ( [[#Bathiany--2020|Bathiany et al., 2020]] ) has also been clarified, as has been the importance of distinguishing summer from winter mechanisms and associated abruptness, because ice area reduces gradually in summer, but not necessarily in winter ( [[#Bathiany--2016|Bathiany et al., 2016]] ). For other tipping elements including AMOC ( [[IPCC:Wg1:Chapter:Chapter-19#9.2.3.1|Section 9.2.3.1]] ), mixed layer depth ( [[IPCC:Wg1:Chapter:Chapter-19#9.2.1.3|Section 9.2.1.3]] ), and sea level rise ( [[IPCC:Wg1:Chapter:Chapter-19#9.6.3.5|]] ), an increase in the diversity of model structure and sensitivity to multiple factors has led to a better understanding of the complexity of the problem, with some increase in assessed uncertainty and an assessed deep uncertainty (Glossary) related to projected sea level rise with global warming levels above 3°C ( [[IPCC:Wg1:Chapter:Chapter-19#9.6.3.5|Section 9.6.3.5]] ). In still other cases such as Antarctic sea ice ( [[IPCC:Wg1:Chapter:Chapter-19#9.3.2|Section 9.3.2]] ) and Southern Ocean Meridional Overturning Circulation (MOC; [[IPCC:Wg1:Chapter:Chapter-19#9.2.3.1|Section 9.2.3.1]] ), uncertainty remains high. Finally, it has also been postulated that models may be prone to being too stable ( [[#Valdes--2011|Valdes, 2011]] ) based on the limitations of models as well as other lines of evidence such paleo-evidence of abrupt events ( [[#Dakos--2008|Dakos et al., 2008]] ; [[#Klus--2018|Klus et al., 2018]] ; [[#Sime--2019|Sime et al., 2019]] ).

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