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

==== 9.5.1.3 Projections ====

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The AR5 ( [[#Vaughan--2013|Vaughan et al., 2013]] ) and SROCC ( [[#Hock--2019b|Hock et al., 2019b]] ) stated with ''high confidence'' that the world’s glaciers are presently in imbalance due to the warming of recent decades. The observed retreat of glaciers is only a partial response to the already realized warming ( [[#Christian--2018|Christian et al., 2018]] ), and they are committed to losing considerable mass in the future, even without further change in air temperature ( [[#Mernild--2013|Mernild et al., 2013]] ; [[#Trüssel--2013|Trüssel et al., 2013]] ; Zekollari and Huybrechts, 2015; [[#Huss--2016|Huss and Fischer, 2016]] ; [[#Marzeion--2018|Marzeion et al., 2018]] ; [[#Jouvet--2019|Jouvet and Huss, 2019]] ). One model estimates that 36 ± 8 % of global glacier mass is already committed to be lost due to past greenhouse gas emissions ( [[#Marzeion--2018|Marzeion et al., 2018]] ). Although accumulation and ablation instantly determine the SMB, the glacier geometries adjust to changed atmospheric conditions over a longer time ( [[#Zekollari--2020|Zekollari et al., 2020]] ). The adjustment time, often referred to as the response time, is variable from one glacier to another, depending on the glacier geometry (thickness and steepness), SMB and gradient (e.g., [[#Jóhannesson--1989|Jóhannesson et al., 1989]] ; [[#Harrison--2001|Harrison et al., 2001]] ; [[#Lüthi--2009|Lüthi, 2009]] ; [[#Zekollari--2020|Zekollari et al., 2020]] ). Response time is variable: years for smaller and steeper glaciers ( [[#Beedle--2009|Beedle et al., 2009]] ; [[#Lüthi--2010|Lüthi and Bauder, 2010]] ; [[#Rabatel--2013|Rabatel et al., 2013]] ), up to tens or hundreds of years for larger and gentle-sloped glaciers (e.g., [[#Burgess--2004|Burgess and Sharp, 2004]] ; [[#Lüthi--2010|Lüthi et al., 2010]] ; [[#Zekollari--2020|Zekollari et al., 2020]] ). The models indicate that the disequilibrium between the glaciers and present atmospheric conditions (1995 to 2014) reduces and then disappears at around year 2070 ( [[#Marzeion--2020|Marzeion et al., 2020]] ). There is therefore ''very high confidence'' that the disequilibrium of glaciers will persist as warming continues, and that glacierswill continue to lose mass for at least several decades because of their lagged response, even if global temperature is stabilized.

The SROCC assessed that global glacier mass loss by 2100, relative to 2015 will be 18 [ ''likely'' range 11 to 25] % for scenario RCP2.6 and 36 [ ''likely'' range 26 to 47] % for RCP8.5, and that many glaciers will disappear regardless of the emissions scenario ( ''very high confidence'' ). Since SROCC, new results from [[#Marzeion--2020|Marzeion et al. (2020)]] have been published (Box 9.3, Figure 9.21 and Table 9.4, including peripheral glaciers in Greenland and Antarctica). Glaciers will lose 29,000 [9000 to 49,000] Gt and 58,000 [28,000 to 88,000] Gt over the period 2015–2100 for RCP2.6 and RCP8.5, respectively ( ''medium confidence'' ), which represents 18 [5 to 31] % and 36 [16 to 56] % of their early 21st century mass, respectively (Table 9.4). Within uncertainties, these agree with SROCC estimates, although with a slightly smaller mass loss due to the inclusion of models with lower sensitivity to changing climate conditions ( [[#Marzeion--2020|Marzeion et al., 2020]] ). The greatest source of uncertainty in glacier mass loss until the middle of the 21st century is the disagreement between glacier models, with emissions scenario becoming the dominant cause of uncertainty by the end of the 21st century ( [[#Marzeion--2020|Marzeion et al., 2020]] ).

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'''Table''' '''9.4 |''' '''Projected sea level contributions from global glaciers (including peripheral glaciers in Greenland and Antarctica) by 2100 relative to 2015, for selected Representative Concentration Pathway (RCP) and Shared Socio-economic Pathway (SSP) scenarios.'''

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

|-
| Study

| RCP2.6

| RCP4.5

| RCP8.5

| Notes

|-
| ''IPCC AR5 and SROCC''

( [[#Church--2013b|Church et al., 2013b]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] )

| 0.10

(0.04–0.16) m

| 0.12

(0.06–0.19) m

| 0.17

(0.09–0.25) m

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

|-
| GlacierMIP

[[#Hock--2019a|Hock et al. (2019a)]]

| 0.094

(0.069–0.119) m

| 0.142 (107–177) m

| 0.200

(0.156–0.240) m

| Mean (±1 standard deviation range) contributions

|-
| GlacierMIP

[[#Marzeion--2020|Marzeion et al. (2020)]]

| 0.079

[0.023–0.135] m

| 0.119

[0.053–0.185] m

| 0.159

[0.073–0.245] m

| Median [90% range]

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

|-
| Study

| SSP1-2.6

| SSP2-4.5

| SSP5-8.5

| Notes

|-
| GlacierMIP experimental protocol ( [[#Marzeion--2020|Marzeion et al., 2020]] ) with CMIP6 forcing

| 0.111

(0.077–0.145)

[0.05–0.167] m

| 0.136

(0.096–0.176)

[0.07–0.201] m

| 0.190

(0.133–0.247)

[0.09–0.283] m

| Mean (66% range) [90% range] using 13 GCMs and 2 glacier models <sup>a</sup>

|-
| GlacierMIP ( [[#Marzeion--2020|Marzeion et al., 2020]] ) with AR5 parametric fit: used for rates and post-2100 projections (Supplementary Material 9.SM.4.5)

| 0.102

(0.07 6 – 0 .134) [0.05 9 – 0 .154] m

| 0.128

(0.09 5 – 0 .167) [0.07 6 – 0 .192] m

| 0.171

(0.12 4 – 0 .224) [0.09 8 – 0 .259] m

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

|-
| Emulated ( [[#Marzeion--2020|Marzeion et al., 2020]] ; [[#Edwards--2021|Edwards et al., 2021]] )

| 0.080

(0.05 9 – 0 .101) [0.04 6 – 0 .116] m

| 0.115

(0.09 3 – 0 .137) [0.07 7 – 0 .155] m

| 0.170

(0.14 4 – 0 .196) [0.12 4 – 0 .218] m

| Median (66% range) [90% range] contribution in 2100 relative to 2015 from emulator of GlacierMIP6 used with Chapter 7: Climate Forcing

|}

<sup>a</sup> OGGM ( [[#Maussion--2019|Maussion et al., 2019]] ) and GloGEM ( [[#Huss--2015|Huss and Hock, 2015]] ).

Although the GlacierMIP projections ( [[#Hock--2019a|Hock et al., 2019a]] ; [[#Marzeion--2020|Marzeion et al., 2020]] ) were forced by RCP scenarios, two global glacier models ( [[#Huss--2015|Huss and Hock, 2015]] ; [[#Maussion--2019|Maussion et al., 2019]] ) were also run with 13 GCMs and SSP scenarios (Table 9.4). These results show increased mass loss compared to the RCP forced simulations, although with fewer global glacier models. To enable the glacier contribution to future sea level rise to be estimated under the full range of SSP scenarios ( [[#9.6.3.3|Section 9.6.3.3]] ), the GlacierMIP results are emulated using a Gaussian process model (Box 9.3 and Table 9.4; [[#Edwards--2021|Edwards et al., 2021]] ). The emulated projections show a narrower range than the roughly equivalent RCP projections, which may be explained by not accounting for covariance in the regional uncertainties ( [[#Marzeion--2020|Marzeion et al., 2020]] ) and by the fact that the emulator caps sea level contribution for each region at the volume above floatation estimated by [[#Farinotti--2019|Farinotti et al. (2019)]] (Table 9.SM.2). Comparison of simulated and emulated regional sea level contributions support this explanation. Rates of change and post-2100 sea level projections are estimated with the AR5 parametric fit (Supplementary Material 9.SM.4.5; [[#Church--2013b|Church et al., 2013b]] ) applied to the GlacierMIP results ( [[#Marzeion--2020|Marzeion et al., 2020]] ), and these are also shown in Table 9.4 for comparison.

The mass loss rates vary between regions and there are distinctively different patterns between scenarios ( [[#Marzeion--2020|Marzeion et al., 2020]] ). The global models agree that regions characterized by relatively little glacier-covered area (Low Latitude, Central Europe, Caucasus, Western Canada and USA, North Asia, Scandinavia and New Zealand) will lose nearly all (&gt;80%) glacier mass by 2100 in the RCP8.5 scenario, but their corresponding contribution to sea level rise will be small. A study using detailed ice dynamics for the largest glacier of the European Alps, Great Aletsch Glacier, projects 60% of present ice volume will be lost by 2100 in RCP2.6 and an almost complete wastage of the ice in RCP8.5 ( [[#Jouvet--2019|Jouvet and Huss, 2019]] ). Due to their larger mass, the largest contribution to sea level rise comes from glaciers in the Arctic and Antarctic regions (Antarctic, Arctic Canada, Alaska, Greenland, Svalbard and Russian Arctic), in spite of having the smallest relative mass loss, and it is expected that they will continue to contribute to sea level rise beyond 2100. The regions with intermediate glacier mass (Southern Andes, High Mountain Asia and Iceland) show decreasing mass loss rates for RCP2.6 throughout the 21st century, and increasing rates for RCP8.5 that peak in the mid-to-late 21st century (Figure 9.21). The peak in mass loss rate followed by reduction is due to decreasing glacier volume and stabilizing mass balance ( [[#Marzeion--2020|Marzeion et al., 2020]] ). Vatnajökull, the largest glacier in Iceland, is projected to lose about 50% of its mass by 2300 in extended RCP4.5 and 80–100% in extended RCP8.5 scenarios ( [[#Schmidt--2019|Schmidt et al., 2019]] ). In summary, both global and regional studies agree that glacier mass loss will continue in all regions, with larger mass loss for high-emissions scenarios ( ''high confidence'' ) (see also [[IPCC:Wg1:Chapter:Chapter-8#8.4.1.7.1|Section 8.4.1.7.1]] ).

In AR5 and SROCC, glacier mass loss beyond 2100 was calculated using a parametric fit to available model simulations. In section 9.6.3.5, that same parametric fit is applied to [[#Marzeion--2020|Marzeion et al. (2020)]] projections, resulting in complete glacier mass loss at year 2300 under SSP5-8.5 and 40–100% mass loss under SSP1-2.6. [[#Clark--2016|Clark et al. (2016)]] simulate glacier mass evolution, not including glaciers peripheral to the Antarctic Ice Sheet (AIS), for different warming levels for the next 10,000 years. There is ''limited evidence'' and ''low confidence'' that, at sustained warming levels between 1.5 and 2°C, about 50–60% of glacier mass will remain, predominantly in the polar regions. At sustained warming levels between 2 and 3°C, about 50–60% of glacier mass outside Antarctica will be lost and, at sustained warming levels, between 3 and 5°C, 60–75% of glacier mass outside Antarctica will disappear. Based on [[#Marzeion--2020|Marzeion et al. (2020)]] , there is ''medium confidence'' that nearly all glacier mass in low latitudes, Central Europe, the Caucasus, western Canada and the USA, North Asia, Scandinavia and New Zealand will disappear at this high warming level.

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