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

==== 9.4.2.5 Projections to 2100 ====

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The AR5 assessed the median and ''likely'' (66–100% probability) sea level contributions of the AIS in 2100 relative to 1986–2005 to be 0.06 (–0.04 to +0.16) m SLE under RCP2.6 and 0.04 (–0.08 to +0.14) m SLE under RCP8.5 (Table 9.3; no change when using the AR6 baseline). The AR5 stated that only the collapse of the marine-based sectors of the AIS, if initiated, could cause GMSL to rise substantially above the ''likely'' range during the 21st century, with ''medium confidence'' that this would not exceed several tenths of a metre during this period. The assessment of the dynamical contribution had no dependence on emissions scenarios, due to the lack of literature, so the decrease in sea level contribution in the higher-emissions scenario was solely due to increased SMB ( [[#9.4.2.3|Section 9.4.2.3]] ). The SROCC ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ) assessed the total contribution based on five new ice-sheet modelling studies that incorporated marine ice-sheet dynamics, combining their estimates and interpreting the 5–95th percentile range of the resulting distribution as the ''likely'' range (17–83% probability interval, i.e., not open-ended as in the AR5). The median and ''likely'' range contributions by 2100 were 0.04 (0.01–0.11) m under RCP2.6 and 0.12 (0.03–0.28) m under RCP8.5 (Table 9.3). The positive scenario-dependence in SROCC – where increases in dynamic losses driven by ocean warming and ice-shelf disintegration under higher emissions ( [[#9.4.2.3|Section 9.4.2.3]] ) dominate over increases in SMB – arose from a combination of physical processes and model limitations. Modelling improvements in these studies included improved representations of grounding line response to drivers, more extensive exploration of uncertainties, and inclusion of a positive feedback of meltwater on climate ( [[#Golledge--2019|Golledge et al., 2019]] ). However, two of the projections did not include SMB changes that would offset dynamic losses ( [[#Levermann--2014|Levermann et al., 2014]] ; [[#Ritz--2015|Ritz et al., 2015]] ), and the scenario dependence may have been further amplified by highly sensitive sub-shelf melt parametrizations and use of simplified SMB schemes ( [[#Golledge--2015|Golledge et al., 2015]] , 2019; [[#Bulthuis--2019|Bulthuis et al., 2019]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ).

Since SROCC, new projections have arisen from multi-model intercomparison projects ISMIP6 and LARMIP-2 (Box 9.3) and one model that includes MICI ( [[#9.4.2.4|Section 9.4.2.4]] ; Table 9.3; [[#DeConto--2021|DeConto et al., 2021]] ). Corrections are added to allow comparison: all ISMIP6-derived projections have an estimate of the historical dynamical response to pre-2015 climate forcing added, which increases contributions (Box 9.3; Figure 9.18); the LARMIP-2 dynamic projections are combined with an estimate of SMB, which decreases contributions (Sections 9.4.2.3 and 9.6.3.2); and the ISMIP6 emulated and LARMIP-2 projections were re-estimate using the global surface air temperature distributions from the two-layer energy budget emulator described in Supplementary Material 7.SM.2. The majority of the new projections indicate that, under all emissions scenarios, the AIS will lose mass overall and contribute to sea level rise. Most thinning occurs in the Amundsen Sea sector in WAIS and Totten Glacier in EAIS (Figure 9.18). The most negative contribution is –0.02 m (5th percentile of ISMIP6 combined RCP8.5 and SSP5-8.5 projections after correction) and the largest contribution is 0.57 m SLE (95th percentile; [[#Levermann--2020|Levermann et al., 2020]] ), or 0.63 m SLE with MICI (95th percentile; [[#DeConto--2021|DeConto et al., 2021]] ). ISMIP6 ensemble ranges are wider for the high scenarios (RCP8.5/SSP5-8.5) than the low (RCP2.6/SSP1-2.6), in part because more simulations were available. The ISMIP6 simulations that apply an ice-shelf collapse scenario based on exceedance of a surface meltwater threshold ( [[#Trusel--2015|Trusel et al., 2015]] ), driven by CMIP5 models, show only a small increase in mass loss (around 0–0.04 m), mostly from the Peninsula, due in part to the small number of ice shelves predicted to collapse this century ( [[#Seroussi--2020|Seroussi et al., 2020]] ). Simulations driven by the CMIP5 model HadGEM2-ES, which has unusually extreme warming in the Ross Sea ( [[#Barthel--2020|Barthel et al., 2020]] ), show a larger mass loss (up to about 0.05 m) in East Antarctica under ice-shelf collapse ( [[#Edwards--2021|Edwards et al., 2021]] ). The ISMIP6 projections do not include the efficient meltwater drainage or atmospheric feedbacks that could reduce mass loss further ( [[#Seroussi--2020|Seroussi et al., 2020]] ).

The relationship between emissions scenario and AIS response varies across the studies, with emulated ISMIP6 projections showing a slight negative scenario dependence in the median (–0.01 m) from SSP1-2.6 to SSP5-8.5, and LARMIP-2-based projections showing a slight positive scenario-dependence in the median (0.02 m; Table 9.3). A lack of clear scenario dependence in the median masks large individual variations across climate and ice-sheet models, whereby the net AIS contribution response to emissions scenario depends on the relative magnitudes of the atmosphere, ocean and ice-sheet responses ( [[#Barthel--2020|Barthel et al., 2020]] ; [[#Seroussi--2020|Seroussi et al., 2020]] ; [[#Edwards--2021|Edwards et al., 2021]] ). Climate and ice-sheet models do not project that the AIS response will be the same under high or low greenhouse gas emissions in 2100; rather, there is no consensus on the sign of the change. In contrast, strong scenario dependence is seen from RCP4.5 to RCP8.5 in projections that allow MICI ( [[#9.4.2.4|Section 9.4.2.4]] ; [[#DeConto--2021|DeConto et al., 2021]] ), though less so than earlier projections ( [[#DeConto--2016|DeConto and Pollard, 2016]] ) due to later ice-shelf disintegrations. A negative or positive scenario dependence of the AIS response this century cannot be deduced from recent observations, because there is still ''low confidence'' in attributing the causes of observed mass loss ( [[#9.4.2.1|Section 9.4.2.1]] ), and neither regional mass increases by SMB nor regional mass losses by ice flow have a linear relationship with global mean temperature (Sections 9.4.2.1, 9.4.2.2, 9.4.2.3). There is therefore ''low agreement'' on the relationship between emissions scenario and AIS response. However, in the longer term, mass loss is expected to dominate ( [[#9.4.2.6|Section 9.4.2.6]] ).

The LARMIP-2 median projections are higher than those of the ISMIP6 emulator (by 0.04–0.07 m), and the 95th percentiles are two to three times higher. Two possible reasons for the differences between the emulated ISMIP6 and LARMIP-2 projections are assessed: the set of ice-sheet models (Annex II) and the parameter values determining sub-shelf melt sensitivity to ocean temperature ( [[#9.4.2.3|Section 9.4.2.3]] ; Box 9.3). Using only the 13 ice-sheet models common to ISMIP6 and LARMIP-2 reduces the LARMIP-2 median projections by 0.02–0.03 m SLE and the 95th percentiles by 0.04–0.08 m SLE (Table 9.3). This approximately halves the difference in medians, but has a relatively small effect on the upper end. Sub-shelf melt sensitivity has a larger effect, due to the wide variation of estimates from different regions and methods. Using only the Pine Island Glacier sub-shelf melt distribution (Sections 9.4.2.2 and 9.4.2.3) in the ISMIP6 emulator gives a median Antarctic projection of about 0.08 m in 2100 in all scenarios before historical correction, compared with around 0 m using only the mean Antarctic distribution; the published projections use a joint distribution ( [[#Edwards--2021|Edwards et al., 2021]] ). [[#Reese--2020|Reese et al. (2020)]] find that using the basal melt sensitivities of LARMIP-2 yields an order of magnitude greater mass loss under RCP8.5 than with the ISMIP6 mean Antarctic values. Halving the basal melt sensitivity parameter range (i.e., in line with a continental mean estimate: [[#9.4.2.3|Section 9.4.2.3]] ) would lead to a halving of the LARMIP-2 dynamic contribution. This would reconcile the LARMIP-2 and ISMIP6 emulator median and 95th percentile projections using the common subset of models within about 0.02–0.05 m. There is therefore ''limited evidence'' that the ISMIP6 and LARMIP-2 projections could be reconciled by using common ice-sheet models and basal melt sensitivity values.

It is not possible to distinguish which of ISMIP6 and LARMIP-2 is more realistic, due to limitations in historical simulations (Box 9.3) and understanding of basal melting ( [[#9.4.2.3.2|Section 9.4.2.3.2]] ), so the projections are combined using a ‘p-box’ approach ( [[#9.6.3.2|Section 9.6.3.2]] ). The mean of the ISMIP6 emulated and LARMIP-2 medians gives the assessed median projections, and the outer edges of the 17–83% ranges give the outer edges of the assessed ''likely'' (17–83%) ranges – that is, encompassing the structural and parametric uncertainties of both methods, giving ''medium confidence'' in their combined projections. The main difference between this assessment and SROCC is to increase the medians of the lower scenarios by 0.05–0.07 m, so that all SSPs are similar to SROCC assessment of RCP8.5, and to substantially increase the upper ends of the ''likely'' ranges: by 0.14–0.16 m for RCP2.6/SSP1-2.6 and RCP4.5/SSP2-4.5, and 0.06 m for RCP8.5/SSP5-8.5. The increase relative to SROCC is partly due to the increase in LARMIP-2 projections relative to the original LARMIP study ( [[#Levermann--2014|Levermann et al., 2014]] ), arising from the larger number of participating ice-sheet models ( [[#Levermann--2020|Levermann et al., 2020]] ). The historical dynamic response to pre-2015 climate forcing applied to the ISMIP6 emulator could be overestimated, due to the assumption of a constant future rate (Box 9.3). This assessment encompasses SROCC and all projections since, except the 83rd percentiles of projections that allow MICI under RCP8.5 ( [[#DeConto--2021|DeConto et al., 2021]] ) and the Structured Expert Judgement (SEJ) under 5°C shown in SROCC ( [[#Bamber--2019|Bamber et al., 2019]] ). Both are used in further p-box estimates to give the outer limits of ''low'' ''confidence'' assessments ( [[#9.6.3.2|Section 9.6.3.2]] ).

In summary, it is ''likely'' that the AIS will continue to lose mass throughout this century under all emissions scenarios – that is, dynamic losses driven by ocean warming and ice-shelf disintegration will ''likely'' continue to outpace increasing snowfall ( ''medium confidence'' ). The upper end of projections is not well constrained, due to different assumptions about the future sensitivity of sub-shelf basal melting to ocean warming and the proposed marine ice cliff instability triggered by ice-shelf disintegration (Sections 9.4.2.3 and 9.4.2.4; Box 9.4).

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'''Table 9.3''' '''|''' '''Projected sea level contributions in metres from the Antarctic Ice Sheet in 2100 relative to 199''' '''5–2''' '''014, unless otherwise stated, for selected Representative Concentration Pathway (RCP) and Shared Socio-economic Pathways (SSP) scenarios.''' Italics denote partial contributions. The historical dynamic response omitted from ISMIP6 simulations is estimated to be 0.33 ± 0.16 mm yr <sup>–1</sup> (0.03 m ± 0.01 m in 2100 relative to 2015; Box 9.3). The climate forcing is described in Supplementary Material 7.SM.2.

{| class="wikitable"
|-
| colspan="5"| '''Representative Concentration Pathways (RCPs)'''

|-
| '''Study'''

| '''RCP2.6'''

| '''RCP4.5'''

| '''RCP8.5'''

| '''Notes'''

|-
| IPCC AR5 ( [[#Church--2013b|Church et al., 2013b]] )

| 0.06 (–0.04 to +0.16)

| 0.05 (–0.05 to +0.15)

| 0.04 (–0.08 to +0.14)

| Median and ''likely'' (≥ 66% range) contribution

|-
| IPCC SROCC ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] )

| 0.04 (0.01 to 0.11)

| 0.06 (0.01 to 0.15)

| 0.12 (0.03 to 0.28)

| Median and ''likely'' (66% range) contribution. Combination of five studies

|-
| ''ISMIP6 CMIP5-forced'' ( [[#Seroussi--2020|Seroussi et al., 2020]] ) ''; excludes historical dynamic response''

| ''–0.01 to +0.16''

| ''–''

| ''–0.08 to +0.30''

| ''Range of ISMIP6 multi-model contributions in 2100 relative to 2015 from 2 ESMs for RCP2.6 and 6 ESMs for RCP8.5''

|-
| ''LARMIP-2; excludes surface mass balance (SMB)'' ( [[#Levermann--2020|Levermann et al., 2020]] )

| ''0.13 (0.07 to 0.24)''

''[0.04 to 0.37]''

| ''0.14 (0.07 to 0.28)''

''[0.05 to 0.44]''

| ''0.17 (0.09 to 0.36)''

''[0.06 to 0.58]''

| ''Median (67% range) [90% range] LARMIP-2 multi-model dynamic contribution in 2100 relative to 1900''

|-
| MICI ( [[#DeConto--2021|DeConto et al., 2021]] )

| 0.08 (0.06 to 0.12)

[0.06 to 0.15]

| 0.09 (0.07 to 0.11)

[0.07 to 0.15]

| 0.34 (0.19 to 0.53)

[0.11 to 0.63]

| Median (66% range) [90% range]

|-
| colspan="5"|
|-
| colspan="5"| '''Shared Socio-economic Pathways (SSPs)'''

|-
| '''Study'''

| '''SSP1-2.6'''

| '''SSP2-4.5'''

| '''SSP5-8.5'''

| '''Notes'''

|-
| colspan="5"| Multi-model ensemble projections

|-
| ''ISMIP6 CMIP6-forced'' ( [[#Payne--2021|Payne et al., 2021]] ) ''; excludes historical dynamic response''

| ''–0.05 to +0.01''

| ''–''

| ''–0.09 to +0.11''

| ''Range of ISMIP6 multi-model contributions in 2100 relative to 2015 from 1 ESM for SSP1-2.6 and 4 ESMs for SSP5-8.5''

|-
| ISMIP6 all (CMIP5 and CMIP6-forced) including historical dynamic response

| –0.05 (0.04 to 0.08)

[0.03 to 0.11]

| ''–''

| 0.04 (0.00 to 0.12) [–0.02 to +0.23]

| Median (66% range) [90% range] contribution from ISMIP6 CMIP5 and CMIP5-forced multi-model ensembles, (see caption)

|-
| ''Emulated ISMIP6; excludes historical dynamic response'' ( [[#Edwards--2021|Edwards et al., 2021]] )

| ''0.04 (–0.01 to +0.10)''

''[–0.05 to +0.14]''

| ''0.04 (–0.02 to +0.10)''

''[–0.06 to +0.14]''

| ''0.04 (–0.01 to +0.09)''

''[–0.05 to +0.14]''

| ''Median (66% range) [90% range] contribution in 2100 relative to 2015 from emulator of ISMIP6 used with [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] climate forcing''

|-
| '''Emulated ISMIP6 total'''

| '''0.09 (0.03 to 0.14)'''

'''[–0.01 to +0.19]'''

| '''0.09 (0.03 to 0.14)'''

'''[–0.01 to +0.18]'''

| '''0.08 (0.03 to 0.14)'''

'''[0.00 to 0.18]'''

| '''Emulated ISMIP6, but relative to 1995–2014 and including historical dynamic response (see caption)'''

|-
| ''SMB''

| ''–0.02 (–0.03 to –0.01)''

''[–0.04 to –0.01]''

| ''–0.03 (–0.04 to –0.02)''

''[–0.06 to –0.01]''

| ''–0.05 (–0.07 to –0.03)''

''[–0.09 to –0.02]''

| ''Median (66% range) [90% range] SMB estimated for the AR5, used to correct LARMIP-2 below''

|-
| ''LARMIP-2; excludes SMB''

| ''0.15 (0.08 to 0.29)''

''[0.05 to 0.44]''

| ''0.17 (0.09 to 0.33)''

''[0.06 to 0.49]''

| ''0.20 (0.10 to 0.39)''

''[0.07 to 0.61]''

| ''Median (66% range) [90% range] dynamic contribution from LARMIP-2 multi-model method used with [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] climate forcing''

|-
| ''LARMIP-2 subset of models; excludes SMB''

| ''0.14 (0.08 to 0.26) [0.05 to 0.39]''

| ''0.15 (0.08 to 0.29) [0.05 to 0.45]''

| ''0.17 (0.10 to 0.35) [0.06 to 0.54]''

| ''As above, but using only the 13 of 16 ice-sheet models common to both ISMIP6 and LARMIP-2''

|-
| ''LARMIP-2 subset of models; includes SMB''

| ''0.11 (0.05 to 0.24) [0.03 to 0.37]''

| ''0.12 (0.05 to 0.26) [0.02 to 0.42]''

| ''0.12 (0.05 to 0.30) [0.01 to 0.49]''

| ''As above, but including the SMB estimate''

|-
| '''LARMIP-2 total'''

| '''0.13 (0.06 to 0.27)'''

'''[0.03 to 0.41]'''

| '''0.14 (0.06 to 0.29)'''

'''[0.02 to 0.46]'''

| '''0.15 (0.05 to 0.34)'''

'''[0.01 to 0.57]'''

| ''Median (66% range) [90% range] dynamic contribution from LARMIP-2 multi-model method used with [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] climate forcing, including the SMB estimate''

|-
| '''This assessment: combination of emulated ISMIP6 and LARMIP-2'''

| '''0.11 (0.03 to 0.27)'''

'''[–0.01 to +0.41]'''

| '''0.11 (0.03 to 0.29)'''

'''[–0.01 to +0.46]'''

| '''0.12 (0.03 to 0.34)'''

'''[0.00 to 0.57]'''

| '''Median (66% range) [90% range] assessment combining emulated ISMIP6 and LARMIP-2'''

|}

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