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

==== 9.4.1.3 Projections to 2100 ====

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The AR5 and SROCC projected that changes in Greenland SMB will contribute to sea level in 2100 by 0.03 (0.01 to 0.07) m sea level equivalent (SLE) under RCP2.6, and 0.07 (0.03 to 0.16) m SLE under RCP8.5. New since SROCC are the projections of SMB obtained by an ESM, two regional climate models, and reconstructions based on temperature from the CMIP5 and CMIP6 ensembles ( [[#Hofer--2020|Hofer et al., 2020]] ; [[#Noël--2021|Noël et al., 2021]] ). The range of sea level contribution from Greenland SMB in [[#Noël--2021|Noël et al. (2021)]] is comparable to the AR5 assessment when either CMIP5 or CMIP6 models are used, while [[#Hofer--2020|Hofer et al. (2020)]] find a greater mass loss across all CMIP6 emissions scenarios when compared to CMIP5 scenarios. Using SSP5-8.5 instead of RCP8.5 increases the mean projected sea level from 2005–2100 by up to 0.06 m in the regional climate model simulations of [[#Hofer--2020|Hofer et al. (2020)]] who attribute the difference mainly to a greater Arctic amplification and associated cloud and sea ice feedbacks in the CMIP6 SSP5-8.5 simulations. In summary, these new projections with fixed ice-sheet topography do not provide sufficient evidence to change the AR5 and SROCC assessments.

Reviewing modelling studies since AR5 ( [[#Church--2013b|Church et al., 2013b]] ), SROCC ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ) assessed Greenland’s contribution to future sea level to be relatively similar to AR5 (Table 9.2). The baseline for projections has shifted from 1986–2005 in SROCC, to 1995–2014 in this Report. Adjusted to the new 1995–2014 baseline by subtracting 0.01 m, SROCC projected a ''likely'' contribution of 0.07 (0.0–0.11) m SLE under RCP2.6, and 0.14 (0.08–0.27) m SLE under RCP8.5 by 2100. Since SROCC, new projections for the 21st century have included dynamic ice sheets coupled to ESMs ( [[#Muntjewerf--2020a|Muntjewerf et al., 2020a]] ; [[#Van%20Breedam--2020|Van Breedam et al., 2020]] ) or regional atmospheric models (Table 9.2; [[#Le%20clec’h--2019|Le clec’h et al., 2019]] ). The coupled ESM–ice-sheet model CESM2–CISM2 (Community Earth System Model Version 2 and Community Ice Sheet Model 2) projects a sea level rise of 0.109 m in 2100 relative to 2015 under SSP5-8.5 ( [[#Muntjewerf--2020a|Muntjewerf et al., 2020a]] ) and a similar contribution under the idealized 1% yr <sup>–1</sup> increase in CO <sub>2</sub> scenario ( [[#Muntjewerf--2020b|Muntjewerf et al., 2020b]] ). The CESM2–CISM2 simulations include ice-sheet–atmosphere interactions and ice-sheet surface meltwater routed to the ocean. The coupled regional atmospheric model and ice-sheet model MAR-GRISLI (Modèle Atmosphérique Régional and Grenoble ice sheet and land ice model) projects a sea level rise of 0.079 m in 2100 relative to 2000 under RCP8.5 (Le Clec’h et al., 2019). An ESM of lower complexity coupled to an ice-sheet model gives a sea level contribution of 0.025 to 0.064 m under RCP2.6 and 0.056 to 0.12 m under RCP8.5 (the range is due to four simulations with different parameter sets for the atmosphere model) ( [[#Van%20Breedam--2020|Van Breedam et al., 2020]] ). [[#Van%20Breedam--2020|Van Breedam et al. (2020)]] identify a simulation with a preferred parameter set that projects 0.034 m for RCP2.6 and 0.073 m for RCP8.5. Although the ocean does not directly force the ice-sheet models in these simulations, the new coupled models allow for interactions between ice-sheet dynamics, SMB and local climate. The coupled projections fall within the lower bounds of AR5 and SROCC and, as these studies do not prescribe ocean forcing directly, it is possible that the dynamic response is underestimated.

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'''Table''' '''9.2 |''' '''Projected sea level contributions in metres from the Greenland Ice Sheet by 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. Historical dynamic response omitted from ISMIP6 simulations is estimated to be 0.19 ± 0.10 mm yr <sup>–1</sup> (0.02 m ± 0.01 m in 2100 relative to 2015). The climate forcing is described in Appendix 7.SM.2.

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

|-
| '''Study'''

| '''RCP2.6'''

| '''RCP4.5'''

| '''RCP8.5'''

| '''Notes'''

|-
| IPCC AR5 and SROCC ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] )

| 0.07 (0.03 to 0.11)

| 0.08 (0.04 to 0.15)

| 0.14 (0.08 to 0.27)

| Median and ''likely'' (66% range) contributions in 2100 relative to 1995–2014. Median of multiple studies

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

| ''0.01 to 0.05''

| n/a

| ''0.04 to 0.14''

| ''Range of multi-model contributions in 2100 relative to 2015 from 1 ESM for RCP2.6 and 6 ESMs for RCP8.5 (see caption)''

|-
| Coupled regional atmosphere–ice sheet model ( [[#Le%20clec’h--2019|Le clec’h et al., 2019]] )

| n/a

| n/a

| 0.079

| Contribution in 2100 relative to 2000 from AR-GRISLI model

|-
| Coupled Earth system model (ESM) of lower complexity-ice-sheet model ( [[#Van%20Breedam--2020|Van Breedam et al., 2020]] )

| 0.034 (0.025 to 0.064)

| n/a

| 0.073 (0.056 to 0.12)

| Contribution in 2100 relative to 2000 from LOVECLIM-AGISM model; preferred parameter set and range from four simulations with different parameters for atmosphereodel

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

|-
| '''Study'''

| '''SSP1-2.6'''

| '''SSP2-4.5'''

| '''SSP5-8.5'''

| '''Notes'''

|-
| Coupled ESM–ice sheet model ( [[#Muntjewerf--2020a|Muntjewerf et al., 2020a]] )

| n/a

| n/a

| 0.109

| Contribution in 2100 relative to 2015 from coupled CESM2–CISM2

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

| ''0.02 to 0.06''

| n/a

| ''0.08 to 0.25''

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

|-
| ISMIP6 CMIP5 and CMIP6 forced ensemble including historical dynamic response

| 0.06 (0.05 to 0.07)

[0.04 to 0.08]

| n/a

| 0.11 (0.09 to 0.14)

[0.07 to 0.17]

| Median (66% range) [90% range] contribution from ISMIP6 CMIP5- and CMIP6-forced multi-model ensembles

|-
| ISMIP6 with AR5 parametric fit: used to estimate rates (Supplementary Material 9.SM.4.4) including historical dynamic response

| 0.08 (0.06 to 0.10)

[0.05 to 0.12]

| 0.10 (0.08 to 0.13) [0.07 to 0.15]

| 0.14 (0.11 to 0.18)

[0.10 to 0.22]

| Median (66% range) [90% range] contribution from AR5 parametric fit to ISMIP6 ensemble, relative to 1995–2014

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

| ''0.03 (–0.01 to 0.08)''

''[–0.04 to 0.12]''

| ''0.06 (0.01 to 0.10)''

''[–0.02 to 0.15]''

| ''0.11 (0.06 to 0.16)''

''[0.03 to 0.21]''

| ''Median (66% range) [90% range] contribution in 2100 relative to 2015 from emulator of ISMIP6 used with Chapter 7: Climate Forcing''

|-
| '''This assessment: emulated ISMIP6 total'''

| '''0.06 (0.01 to 0.10)'''

'''[–0.02 to 0.15]'''

| '''0.08 (0.04 to 0.13)'''

'''[0.01 to 0.18]'''

| '''0.13 (0.09 to 0.18)'''

'''[0.05 to 0.23]'''

| '''As above, but relative to 1995–2014 and including historical dynamic response'''

|}

Since SROCC, projections of the Greenland Ice Sheet are also available from The Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) (Box 9.3; Annex II; Figure 9.17; [[#Nowicki--2016|Nowicki et al., 2016]] , 2020a). ISMIP6 multi-model projections are corrected with an assessment of the historical dynamical response to pre-2015 climate forcing (Box 9.3). For the period 2015–2100, the ISMIP6 uncorrected multi-model ensemble projects sea level contributions ranging from 0.01 to 0.05 m under RCP2.6, 0.04 to 0.14 m under RCP8.5 ( [[#Goelzer--2020|Goelzer et al., 2020]] ), 0.02 to 0.06 m under SSP1-2.6, and 0.08 to 0.25 m under SSP5-8.5 (Table 9.2; [[#Payne--2021|Payne et al., 2021]] ). The higher mass loss in the SSPs is attributed to a larger decrease in SMB due to the high climate sensitivity of the models used ( [[#Payne--2021|Payne et al., 2021]] ). This finding is confirmed by [[#Choi--2021|Choi et al. (2021)]] , where CMIP6 SSP5-8.5 SMB leads to larger ice loss than CMIP5 RCP8.5, while ice discharge is similar. As the ISMIP6 framework considers a subset of the RCPs/SSPs and CMIP models, SSP-based projections have been inferred from multiple approaches. First, the ISMIP6 CMIP5-forced ( [[#Goelzer--2020|Goelzer et al., 2020]] ) and CMIP6-forced ( [[#Payne--2021|Payne et al., 2021]] ) combined ensemble projections were corrected with the historical trend (Box 9.3) using bootstrapping. Second, an emulator of the ISMIP6 projections (Box 9.3; [[#Edwards--2021|Edwards et al., 2021]] ) is forced by distributions of global surface air temperature for each SSP from a two-layer energy budget emulator (Supplementary Material 7.SM.2) and then corrected with the historical trend in the same way. These two approaches result in projections that are similar in their median values to AR5 and SROCC projections (Table 9.2), but differ in their range. Similar results are obtained when the AR5 parametric fit is applied to the ISMIP6 models (Table 9.2, Supplementary Material 9.SM.4.4), which is used to estimate rates of change and post-2100 projections (Sections 9.4.1.4 and 9.6.3.2).

The SROCC noted that the study by [[#Aschwanden--2019|Aschwanden et al. (2019)]] projects a significantly higher Greenland contribution to sea level than the assessed ''likely'' range in AR5 and SROCC. Under RCP8.5, [[#Aschwanden--2019|Aschwanden et al. (2019)]] found that Greenland could contribute up to 0.33 m to sea level by 2100 relative to 2000 (the ensemble member that best reproduces the 2000–2015 mean SMB from a regional climate model projects Greenland mass losses of 0.08 m SLE under RCP2.6 and 0.18 m SLE under RCP8.5). The SROCC noted that the potentially high sea level contribution in this study could be due to the assumption of spatially uniform warming, which can overestimate surface melt rates. However, it also reflects the ''deep uncertainty'' surrounding atmospheric forcing, surface processes, submarine melt, calving and ice dynamics. [[#Goelzer--2020|Goelzer et al. (2020)]] ascribe 40% of the ISMIP6 multi-model ensemble spread to ice-sheet model uncertainty, 40% to climate model uncertainty and 20% to ocean forcing uncertainty. We note that this finding reflects the current challenges associated with the representation of ice–ocean interactions in models, and the uncertainty in basal conditions ( [[#9.4.1.2|Section 9.4.1.2]] ). However, this finding is consistent with the work of [[#Aschwanden--2019|Aschwanden et al. (2019)]] and thus, there is ''medium confidence'' that uncertainty in mass loss from the Greenland Ice Sheet is dominated by uncertainty in climate scenario and surface processes, whereas uncertainty in calving and frontal melt play a minor role.

The SROCC stated that surface processes, rather than ice discharged into the ocean, will dominate Greenland ice loss over the 21st century, regardless of the emissions scenario ( ''high confidence'' ). This is confirmed by the ISMIP6 projections ( [[#Goelzer--2020|Goelzer et al., 2020]] ; [[#Payne--2021|Payne et al., 2021]] ). The projected mass loss of Greenland is predominantly due to increased surface meltwater and loss in refreezing capacity resulting in decreasing SMB ( ''high confidence'' ), concurrent with rising temperatures and darkening of the ice-sheet surface ( [[#Fettweis--2013|Fettweis et al., 2013]] ; [[#Vizcaino--2015|Vizcaino et al., 2015]] ; Le Clec’h et al., 2019; [[#Muntjewerf--2020a|Muntjewerf et al., 2020a]] , b; [[#Sellevold--2020|Sellevold and Vizcaíno, 2020]] ). Mass changes due to SMB and outlet glacier dynamics are linked ( [[#Goelzer--2013|Goelzer et al., 2013]] ; [[#Fürst--2015|Fürst et al., 2015]] ; [[#Rückamp--2020|Rückamp et al., 2020]] ), as mass loss by one process decreases mass loss by the other – for example, SMB removes ice before it can reach the marine glacier terminus. There is ''medium confidence'' that the mass loss through ice discharge will decrease in the future ( [[#Fürst--2015|Fürst et al., 2015]] ; [[#Aschwanden--2019|Aschwanden et al., 2019]] ; [[#Golledge--2019|Golledge et al., 2019]] ), because an increase in mass loss (via increased discharge or surface runoff) leads, in most areas, to a retreat of the glacier margin onto land above sea level, isolating the ice sheet from marine influence.

In summary, it is ''virtually certain'' that the Greenland Ice Sheet will continue to lose mass this century under all emissions scenarios, and ''high confidence'' that total mass loss by 2100 will increase with cumulative emissions. The sea level assessment ( [[#9.6.3.3|Section 9.6.3.3]] ) is based on the emulated ISMIP6 projections, allowing a more consistent approach to a wider range of climate and ocean forcings. The Greenland Ice Sheet is ''likely'' to contribute 0.06 (0.01 to 0.10) m under SSP1-2.6 and 0.13 (0.09 to 0.18) m under SSP5-8.5 by 2100 relative to 1995–2014. These projections (as well as those of AR5 and SROCC) are lower than the study of [[#Aschwanden--2019|Aschwanden et al. (2019)]] or the range of possible sea level changes resulting from Structured Expert Judgement (SEJ; [[#9.6.3.2|Section 9.6.3.2]] ; [[#Bamber--2019|Bamber et al., 2019]] ), contributing to the ''deep uncertainty'' in projected sea level (Box 9.4). There is, however, ''high confidence'' that the loss from Greenland will become increasingly dominated by SMB and surface melt, as the ocean-forced dynamic response of glaciers will diminish as marine margins retreat to higher grounds.

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