Editing IPCC:AR6/WGI/Chapter-9 (section)

==== 9.4.2.6 Projections Beyond 2100 ====

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The SROCC assessed the median and ''likely'' range of Antarctic SLE contributions at 2300 as 0.16 (0.07–0.37) m under RCP2.6 and 1.46 (0.60–2.89) m under RCP8.5, based on three studies. It was noted that ''deep uncertainty'' remained beyond 2100: while solid Earth feedbacks could reduce ice loss over multi-century time scales, MICI ( [[#9.4.2.4|Section 9.4.2.4]] ) might give contributions higher than the ''likely'' ranges. The SROCC also presented structured expert judgement (SEJ) projections for comparison ( [[#Bamber--2019|Bamber et al., 2019]] ), which give higher values. Since SROCC, three studies have made projections to 2300: (i) [[#Rodehacke--2020|Rodehacke et al. (2020)]] assessed two methods for implementing precipitation changes (based on repeating 2071–2100 forcings beyond 2100), which both gave negative projections at 2300 because the dynamic response was very small (–0.11 to –0.01 m SLE for RCP2.6; –0.25 to –0.07 m for RCP8.5 forcing); (ii) In contrast, simulations forced by 2081–2100 ocean-only projections under RCP8.5/SSP5-8.5 beyond 2100, using two implementations of the ISMIP6 ‘non-local’ basal melt parametrizations (Box 9.3 and [[#9.4.2.2|Section 9.4.2.2]] ) and two sliding laws, are all positive (0.08 m to 0.96 m SLE by 2300), though these do not include the negative contribution from SMB changes ( [[#Lipscomb--2021|Lipscomb et al., 2021]] ); (iii) Finally, [[#DeConto--2021|DeConto et al. (2021)]] update projections for the MICI hypothesis ( [[#9.4.2.4|Section 9.4.2.4]] ) using the extensions of the RCPs to 2300, and obtain far higher contributions: median (17–83%) ranges of 1.09 (0.71–1.35) m SLE under RCP2.6 and 9.60 (6.87–13.54) m SLE under RCP8.5. These are larger than previous estimates ( [[#DeConto--2016|DeConto and Pollard, 2016]] ), particularly at the upper end: 0.68 (0.29–1.13) m SLE for RCP2.6 and 8.40 (7.47–9.76) m for RCP8.5 ( [[#Edwards--2019|Edwards et al., 2019]] ), which can largely be explained by the higher maximum ice cliff calving rate. LARMIP-2 dynamic projections (Box 9.3) are also estimated under the extended SSPs and corrected with SMB (as in [[#9.4.2.5|Section 9.4.2.5]] ), giving median (17–83%) ranges of 0.40 (0.18–0.78) m SLE at 2300 under SSP1-2.6 and 1.57 (0.68–3.14) m under SSP5-8.5. The longer time scale may invalidate the linear response assumption of LARMIP-2, which neglects any self-dampening or self-amplifying processes. The ranges of projections for 2300 without MICI ( [[#Golledge--2015|Golledge et al., 2015]] ; [[#Bulthuis--2019|Bulthuis et al., 2019]] ; [[#Levermann--2020|Levermann et al., 2020]] ; [[#Rodehacke--2020|Rodehacke et al., 2020]] ; [[#Lipscomb--2021|Lipscomb et al., 2021]] ; ‘assessed ice-sheet contributions’ in [[#9.6.3.5|Section 9.6.3.5]] are –0.14 to +0.78 m SLE under RCP2.6/SSP1-2.6, and –0.27 to 3.14 m SLE under RCP8.5/SSP5-8.5). The lower bounds are the 5th percentile of [[#Bulthuis--2019|Bulthuis et al. (2019)]] and the lowest mean/median from [[#Rodehacke--2020|Rodehacke et al. (2020)]] , respectively; the upper bounds are the 83% percentiles of the LARMIP-2 estimates. These ranges are wider than SROCC ''likely'' ranges, and more consistent with the SEJ ( [[#Bamber--2019|Bamber et al., 2019]] ). However, projections in which Antarctica contributes much more than the assessed ranges under sustained very high greenhouse gas emissions – that is, around 7–14 m to GMSL by 2300 ( [[#DeConto--2021|DeConto et al., 2021]] ), cannot be ruled out, and are taken as a sensitivity case ( [[#9.6.3.5|Section 9.6.3.5]] ; Table 9.11). In summary, there is ''high confidence'' that Antarctic mass loss will be greater beyond 2100 under high greenhouse gas emissions, but the large range of projections mean we have only ''low confidence'' in the likely AIS contribution to GMSL by 2300 for a given scenario. ''Deep uncertainty'' remains in the role of AIS instabilities under very high emissions.

The West and East Antarctic ice sheets are considered to be tipping elements – that is, susceptible to critical thresholds. The SR1.5 ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ) assessed that a threshold for WAIS instability may be close to 1.5°C–2°C ( ''medium confidence'' ), as only RCP2.6 led to long-term projections of less than 1 m ( [[#Golledge--2015|Golledge et al., 2015]] ; [[#DeConto--2016|DeConto and Pollard, 2016]] ). Based on the agreement of a further study ( [[#Bulthuis--2019|Bulthuis et al., 2019]] ), SROCC confirmed that low emissions would limit Antarctic ice loss over multi-century time scales ( ''high confidence'' ), but it was not possible to determine whether this was sufficient to prevent substantial ice loss ( ''medium confidence'' ). Since SROCC, new studies have revisited this topic ( [[#Garbe--2020|Garbe et al., 2020]] ; [[#Rodehacke--2020|Rodehacke et al., 2020]] ; [[#Van%20Breedam--2020|Van Breedam et al., 2020]] ; [[#DeConto--2021|DeConto et al., 2021]] ; [[#Lipscomb--2021|Lipscomb et al., 2021]] ), allowing a more complete assessment along with other studies ( [[#Feldmann--2015|Feldmann and Levermann, 2015]] ; [[#Clark--2016|Clark et al., 2016]] ; [[#Golledge--2017a|Golledge et al., 2017a]] ; [[#Edwards--2019|Edwards et al., 2019]] ) and the extension to LARMIP-2 above. The majority project 0–1.3 m SLE on multi-century time scales under scenarios of 1°C–2°C warming. Projections can increase up to 2 m SLE under high basal melt sensitivity to ocean warming ( [[#9.4.2.3|Section 9.4.2.3]] ; [[#Lipscomb--2021|Lipscomb et al., 2021]] ) or MICI ( [[#9.4.2.4|Section 9.4.2.4]] ). On multi-millennial time scales (≥2,000 years), many projections remain below 1.6 m SLE under 1°C–2°C warming – that is, less than about half of the WAIS in SLE (see also [[#9.6.3.5|Section 9.6.3.5]] and Figure 9.30). Other studies project majority or total loss of WAIS under 1°C–2°C warming, exceeding 2 m SLE, under the higher end of the warming range (≥1.5°C), or high ocean warming (≥0.5°C) and/or high basal melting around WAIS, or MICI. All but two of these multi-millennial studies use variants of the same ice-sheet model, though different modelling choices mean they can be considered quasi-independent. Simulations of previous interglacial periods often show near or total WAIS disintegration, with mass loss exceeding 3 m SLE (e.g. Figure 9.18), although limitations of these studies or inferences that can be drawn under different forcings limit confidence in the robustness of these as quantitative analogues (Sections 9.4.2.4 and 9.6.2). Overall, increased evidence and agreement on the time scales and drivers of mass loss confirm the SR1.5 assessment that a threshold for WAIS instability may be close to 1.5°C–2°C ( ''medium confidence'' ), and that the probability of passing a threshold is larger for 2°C warming than for 1.5°C ( ''medium confidence'' ), particularly under strong ocean warming. New projections agree with previous studies that only part of WAIS would be lost on multi-century time scales if warming remains less than 2°C ( ''medium confidence'' ). There is ''limited agreement'' about whether complete disintegration would eventually occur at this level of warming, but ''medium confidence'' this would take millennia.

Under around 2°C–3°C peak warming, complete or near-complete loss of the WAIS is projected in most studies after multiple millennia ( ''low confidence'' ), with continent-wide mass losses of around 2–5 m SLE or more; this could occur on multi-century time scales under very high basal melting ( [[#Lipscomb--2021|Lipscomb et al., 2021]] ) or widespread ice-shelf loss and/or MICI ( ''low confidence'' ) ( [[#Sun--2020|Sun et al., 2020]] ; [[#DeConto--2021|DeConto et al., 2021]] ). Mass losses under around 2°C–3°C warming could be less than 2 m SLE, particularly for multi-century time scales, low basal melting, or less responsive sliding laws. If warming exceeds around 3°C above pre-industrial, part of the EAIS (typically the Wilkes Subglacial Basin) is projected to be lost on multi-millennial time scales ( ''low confidence'' ), with total AIS mass loss equivalent to around 6–12 m or more sea level rise; mass loss could be much smaller if the dynamic response is small ( [[#Bulthuis--2019|Bulthuis et al., 2019]] ; [[#Rodehacke--2020|Rodehacke et al., 2020]] ), or much faster under widespread ice-shelf loss and/or MICI ( [[#Sun--2020|Sun et al., 2020]] ; [[#DeConto--2021|DeConto et al., 2021]] ). A study by [[#Garbe--2020|Garbe et al. (2020)]] suggests that 6°C sustained warming and associated mass loss of about 12 m SLE may be a critical threshold beyond which the ice sheet reorganizes to a new state, leading to large losses from East Antarctica (including the Aurora Subglacial Basin) and leading to a further 10 m sea level contribution per degree of warming; other studies also show much higher mass loss per °C at higher levels of warming ( [[#9.6.3.5|Section 9.6.3.5]] and Figure 9.30; [[#Van%20Breedam--2020|Van Breedam et al., 2020]] ; [[#DeConto--2021|DeConto et al., 2021]] ).

The SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ) assessed that Antarctic mass losses could be irreversible over decades to millennia ( ''low confidence'' ). [[#Garbe--2020|Garbe et al. (2020)]] show that the AIS is always volumetrically smaller when regrowing under a given warming level than when it retreats under the same forcing. Even if retreat followed by regrowth results in a net zero change in volume, the spatial distribution of mass may be altered, especially in parts of West Antarctica vulnerable to MISI. Projections that start reducing CO <sub>2</sub> concentrations from 2030 onwards, reaching pre-industrial levels around 2300, show sea level contributions exceeding 1 m by 2500 when including MICI ( [[#DeConto--2021|DeConto et al., 2021]] ). New research therefore confirms SROCC assessment that mass loss from the AIS is irreversible on decadal to millennial time scales ( ''low confidence'' ) (FAQ 9.1), and suggests that reducing atmospheric CO <sub>2</sub> concentrations or temperatures to pre-industrial levels may not be sufficient to prevent or reverse substantial Antarctic mass losses ( ''low confidence'' ).

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