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

=== 9.5.1 Glaciers ===

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==== 9.5.1.1 Observed and Reconstructed Glacier Extent and Mass Changes ====

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===== 9.5.1.1.1 Global glacier contribution =====

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The IPCC’s Fifth Assessment Report (AR5; [[#Vaughan--2013|Vaughan et al., 2013]] ) assessed glacier changes from studies based on the regions defined in the Randolph Glacier Inventory (RGI; RGI version 2.0): a satellite observation-based, global inventory of glacier outlines for the year 2000. Following Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC; [[#Hock--2019b|Hock et al., 2019b]] ; [[#Meredith--2019|Meredith et al., 2019]] ), we report on studies based on RGI version 6.0 ( [[#RGI%20Consortium--2017|RGI Consortium, 2017]] ). Increased volume of satellite observations and the inclusion of detailed regional glacier inventories has resulted in an improved inventory ( [[#RGI%20Consortium--2017|RGI Consortium, 2017]] ). A new consensus estimate for the ice thickness distribution of all glaciers in RGI 6.0 was obtained from an ensemble of five numerical models. However, only one out of five models covered all regions ( [[#Farinotti--2019|Farinotti et al., 2019]] ), and was, where possible, calibrated and validated with the worldwide Glacier Thickness Database (GlaThiDa 3.0: [[#GlaThiDa%20Consortium--2019|GlaThiDa Consortium, 2019]] ; [[#Welty--2020|Welty et al., 2020]] ). The updated inventory shows decreases in estimated glacier volume in the Arctic, High Mountain Asia and Southern Andes, partially compensated by increases in Antarctica. 15% of the total glacier volume is estimated to be below sea level and would not contribute to sea level rise if melted ( [[#Farinotti--2019|Farinotti et al., 2019]] ). Supplementary Material Table 9.SM.2 shows the inventory glacier area and mass for each region in the year 2000.

The SROCC found a globally coherent trend of glacier decline in the last decades, despite large annual variability and regional differences ( ''very high confidence'' ). [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.3|Section 2.3.2.3]] assesses the global glacier mass changes for the whole 20th century (see Table 9.5 for contribution to the sea level budget. Note that the peripheral glaciers in Greenland and Antarctica are added to the ice sheets for the budget). The AR6 assessment is based on [[#Marzeion--2015|Marzeion et al. (2015)]] , using glacier-length reconstructions ( [[#Leclercq--2011|Leclercq et al., 2011]] ) and a glacier model forced by gridded climate observations ( [[#Marzeion--2012|Marzeion et al., 2012]] ), and not considering the estimated mass loss of uncharted glaciers (100 ± 50 Gt yr <sup>–1</sup> ; [[#Parkes--2018|Parkes and Marzeion, 2018]] ). The time series are assumed independent, resulting in larger uncertainty than presented in SROCC (see also [[#9.6.1|Section 9.6.1]] ). The rate of global glacier mass loss (excluding the periphery of ice sheets) for the period 1901–1990 is estimated to be ''very likely'' 210 ± 90 Gt yr <sup>–1</sup> , representing 16 [28 to 7] % of the glacier mass in 1901, in agreement with SROCC within uncertainty estimates.

Since SROCC, new regional estimates for the Andes ( [[#Dussaillant--2019|Dussaillant et al., 2019]] ), High Mountain Asia ( [[#Shean--2020|Shean et al., 2020]] ), Iceland ( [[#Aðalgeirsdóttir--2020|Aðalgeirsdóttir et al., 2020]] ), the European Alps ( [[#Davaze--2020|Davaze et al., 2020]] ; Sommer et al., 2020) and Svalbard ( [[#Schuler--2020|Schuler et al., 2020]] ), two new global ( [[#Ciracì--2020|Ciracì et al., 2020]] ; [[#Hugonnet--2021|Hugonnet et al., 2021]] ) and an ad hoc estimate for the latest glaciological observations ( [[#Zemp--2020|Zemp et al., 2020]] ) have extended the glacier mass change time series up to 2018–2019 (Figure 9.21 and Supplementary Material Table 9.SM.3). A reconciled global estimate for the period 1962–2019 has been compiled by [[#Slater--2021|Slater et al. (2021)]] . However, in contrast to [[#Slater--2021|Slater et al. (2021)]] , after 2000 this assessment is based on the first globally complete and consistent estimate of 21st-century glacier mass change from differencing of digital elevation models ( [[#Hugonnet--2021|Hugonnet et al., 2021]] ) covering 94.7% of glacier area with glacier mass change for each glacier in the inventory produced with unprecedented accuracy. The estimates from [[#Hugonnet--2021|Hugonnet et al. (2021)]] agree within uncertainties with new and previous estimates at global ( [[#Hock--2019b|Hock et al., 2019b]] ; [[#Wouters--2019|Wouters et al., 2019]] ; [[#Zemp--2019|Zemp et al., 2019]] ; [[#Ciracì--2020|Ciracì et al., 2020]] ; [[#Slater--2021|Slater et al., 2021]] ) and regional scale ( [[#Dussaillant--2019|Dussaillant et al., 2019]] ; [[#Aðalgeirsdóttir--2020|Aðalgeirsdóttir et al., 2020]] ; [[#Schuler--2020|Schuler et al., 2020]] ; [[#Shean--2020|Shean et al., 2020]] ). Excluding peripheral glaciers of ice sheets (RGI regions 5 and 19), glacier mass loss rate was ''very likely'' 170 ± 80 Gt yr <sup>–1</sup> for the period 1971 to 2019 (8 [4 to 14] % of 1971 glacier mass) '','' 210 ± 50 Gt yr <sup>–1</sup> over the period 1993–2019 (6 [4 to 8] % of 1993 glacier mass) and 240 ± 40 Gt yr <sup>–1</sup> over the period 2006–2019 (3 [2 to 4] % of 2006 glacier mass; Sections 2.3.2.3 and 9.6.1, Table 9.5, <sup>[[#footnote-001|4]]</sup> and Cross-Chapter Box 9.1). Including the peripheral glaciers of the ice sheets, the global glacier mass loss rate in the period 2000–2019 is ''very likely'' 266 ± 16 Gt yr <sup>–1</sup> (4 [3 to 6] % of glacier mass in 2000) with an increase in the mass loss rate from 240 ± 9 Gt yr <sup>–1</sup> in 2000–2009 to 290 ± 10 Gt yr <sup>–1</sup> in 2010–2019 ( ''high confidence'' ). These estimates are in agreement with SROCC estimate and extend the period to 2018–2019. In summary, new evidence published since SROCC shows that, during the decade 2010–2019, glaciers lost more mass than in any other decade since the beginning of the observational record ( ''very high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-8#8.3.1.7.1|Section 8.3.1.7.1]] and Figure 9.20).

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[[File:f624bd4a7729400d28f893ddf72f1b43 IPCC_AR6_WGI_Figure_9_20.png]]

'''Figure 9.20''' '''|''' '''Global and regional glacier mass change rate between 1960 and 2019.''' The time series of annual and decadal mean mass change are based on glaciological and geodetic balances ( [[#Zemp--2019|Zemp et al., 2019]] , 2020). Superimposed are the 2002–2019 average rates by ( [[#Ciracì--2020|Ciracì et al., 2020]] ) based on the Gravity Recovery and Climate Experiment (GRACE), 2006–2015 estimated rates as assessed in Special Report on Ocean and Cryosphere in a Changing Climate (SROCC) and the new decadal averages (2000–2009 and 2010–2019) by [[#Hugonnet--2021|Hugonnet et al. (2021)]] . * New regional estimates for the Andes ( [[#Dussaillant--2019|Dussaillant et al., 2019]] ), High Mountain Asia ( [[#Shean--2020|Shean et al., 2020]] ), Iceland ( [[#Aðalgeirsdóttir--2020|Aðalgeirsdóttir et al., 2020]] ), Central Europe ( [[#Sommer--2020|Sommer et al., 2020]] ) and Svalbard ( [[#Schuler--2020|Schuler et al., 2020]] ) are also shown. The uncertainty reported in each study is shown. See Figure 9.2 for the location of each region. Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

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===== 9.5.1.1.2 Regional glacier changes =====

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A major advance since SROCC is the availability of high-accuracy mass loss estimates for individual glaciers ( [[#Hugonnet--2021|Hugonnet et al., 2021]] ). These results show that, during the last 20 years, the highest regional mass loss rates (&gt;720 kg m <sup>–2</sup> yr <sup>–1</sup> ) were observed in the Southern Andes, New Zealand, Alaska, Central Europe, and Iceland. Meanwhile, the lowest regional mass loss rates (&lt;250 kg m <sup>–2</sup> yr <sup>–1</sup> ) were observed in High Mountain Asia, the Russian Arctic, and the periphery of Antarctica. Glacier mass loss in Alaska (25% of 2000–2019 total mass loss), the periphery of Greenland (13%), Arctic Canada North (11%), Arctic Canada South (10%), the periphery of Antarctica (8%), the Southern Andes (8%) and High Mountain Asia (8%), represent the majority (83%) of the total glacier mass loss during the last 20 years (2000–2019).

The glacier mass loss rate from geodetic mass balance assessments in the Southern Andes during 2006–2015 was smaller (720 ± 70 kg m <sup>–2</sup> yr <sup>–1</sup> ; [[#Braun--2019|Braun et al., 2019]] ; [[#Dussaillant--2019|Dussaillant et al., 2019]] ; [[#Hugonnet--2021|Hugonnet et al., 2021]] ) than previously assessed in SROCC (860 ± 160 kg m <sup>–2</sup> yr <sup>–1</sup> ), though within uncertainties. In the Central and Desert regions of the Southern Andes, an increase in mass loss from 2000–2009 to 2010–2018, and a high loss rate in Patagonia for the whole period, are observed ( [[#Dussaillant--2019|Dussaillant et al., 2019]] ). Records of glacier mass loss in Peru ( [[#Seehaus--2019a|Seehaus et al., 2019a]] ) and Bolivia ( [[#Seehaus--2019b|Seehaus et al., 2019b]] ) in the period 2000–2016 show an increase in mass loss towards the end of the observation period. In western North America, outside of Alaska and western Yukon, there was a fourfold increase in mass loss for 2009–2018 (860 ± 320 kg m <sup>–2</sup> yr <sup>–1</sup> ) compared to 2000–2009 (203 ± 214 kg m <sup>–2</sup> yr <sup>–1</sup> ; [[#Menounos--2019|Menounos et al., 2019]] ), and in the Canadian Arctic there was a doubling of mass loss in the last two decades compared with pre-1996 ( [[#Noël--2018|Noël et al., 2018]] ; [[#Cook--2019|Cook et al., 2019]] ). The peripheral glaciers in NE Greenland experienced a 23% increase in mass loss in 1980–2014 compared to the period 1910 to 1978–1987 ( [[#Carrivick--2019|Carrivick et al., 2019]] ). In Iceland, 16 ± 4% of the around 1890 glacier mass has been lost; about half of that loss occurred in the period 1994–2019 ( [[#Aðalgeirsdóttir--2020|Aðalgeirsdóttir et al., 2020]] ). Glacier records starting in 1960 in Norway show that half of the observed glaciers advanced in the 1990s but all have retreated since 2000 ( [[#Andreassen--2020|Andreassen et al., 2020]] ). In Svalbard, glaciers have been losing mass since the 1960s, with a tendency towards more negative mass balance since 2000 ( [[#Deschamps-Berger--2019|Deschamps-Berger et al., 2019]] ; [[#Van%20Pelt--2019|Van Pelt et al., 2019]] ; [[#Morris--2020|Morris et al., 2020]] ; [[#Noël--2020|Noël et al., 2020]] ; [[#Schuler--2020|Schuler et al., 2020]] ). A similar increase in mass loss has been observed for Franz Josef Land in the Russian Arctic ( [[#Zheng--2018|Zheng et al., 2018]] ). Rapid retreat and downwasting throughout the European Alps in the early 21st century is reported ( [[#Sommer--2020|Sommer et al., 2020]] ) and long-term records, although limited, indicate sustained glacier mass loss in High Mountain Asia since around 1850, with increased mass loss in recent decades ( [[#Shean--2020|Shean et al., 2020]] ). In summary, although interannual variability is high in many regions, glacier mass records throughout the world show with ''very high confidence'' that the loss rate has been increasing in the last two decades (see also [[IPCC:Wg1:Chapter:Chapter-8#8.3.1.7.1|Section 8.3.1.7.1]] and 12.4 for regional glacier assessment).

[[IPCC:Wg1:Chapter:Chapter-2#2.3.2.3|Section 2.3.2.3]] assesses that the rate and global character of glacier retreat in the latter part of 20th century, and finds that the first decades of the 21st century appear to be unusual in the context of the Holocene ( ''medium confidence'' ) and the global glacier recession in the beginning of the 21st century to be unprecedented in the last 2000 years ( ''medium confidence'' ). These assessments are supported by regional evidence. New reconstructions of the Patagonian Ice Sheet suggest that 20th-century glacial recession occurred faster than at any time during the Holocene ( [[#Davies--2020|Davies et al., 2020]] ). The reconstructions of glacier variations show that the glaciers in some regions are now smaller than previously recorded: since the mid-16th century in the Mont Blanc and Grindelwald regions of the European Alps ( [[#Nussbaumer--2012|Nussbaumer and Zumbühl, 2012]] ), since the 9th century in Norway ( [[#Nesje--2012|Nesje et al., 2012]] ), and for the past 1800 years in north-west Iceland ( [[#Harning--2016|Harning et al., 2016]] , 2018). In Arctic Canada and Svalbard, many glaciers are now smaller than they have been in at least 4000 years ( [[#Lowell--2013|Lowell et al., 2013]] ; [[#Miller--2013|Miller et al., 2013]] , 2017; [[#Schweinsberg--2017|Schweinsberg et al., 2017]] , 2018) and more than 40,000 years in Baffin Island ( [[#Pendleton--2019|Pendleton et al., 2019]] ). Although the millennial glacier length variation records are incomplete and discontinuous, and glacier fluctuations depend on multiple factors (e.g., temperature, precipitation, topography, internal glacial dynamics), there is a coherent relationship between rising temperatures, negative mass balance and glacier retreat on centennial time scales across most of the world. Glaciological and geodetic observations show that the rates of early 21st-century mass loss are the highest since 1850 ( [[#Zemp--2015|Zemp et al., 2015]] ). For all regions with long-term observations, glacier mass in the decade 2010–2019 was the smallest since at least the beginning of the 20th century ( ''medium confidence'' ).

In contrast to the global glacier mass decline (Figure 9.21, Table 9.5, and Supplementary Material 9.SM.2), a few glaciers have gained mass or advanced due to internal glacier dynamics or locally restricted climatic causes. The SROCC discusses the ‘Karakoram anomaly’ (centred on the western Kunlun range (at about 80°E, 35°N), but also covering part of the Pamir and Karakoram ranges), where glaciers have been close to balance since at least the 1970s, and had a slightly positive mass balance since the 2000s. Since SROCC, new evidence suggests that this anomaly is related to a combination of low-temperature sensitivity of debris-covered glaciers, a decrease of summer air temperatures (Cross-Chapter Box 10.3), and an increase in snowfall, possibly caused by increases in evapotranspiration from irrigated agriculture ( [[#Bonekamp--2019|Bonekamp et al., 2019]] ; [[#de%20Kok--2020|de Kok et al., 2020]] ; [[#Farinotti--2020|Farinotti et al., 2020]] ; [[#Shean--2020|Shean et al., 2020]] ). However, a recent geodetic mass balance estimate suggests substantially increased thinning rates of High Mountain Asian glaciers after about 2010 ( [[#Hugonnet--2021|Hugonnet et al., 2021]] ). There is ''limited evidence'' to assess whether the Karakoram anomaly will persist in coming decades but, due to the projected increase in air temperature throughout the region, its long-term persistence is ''unlikely'' ( ''high confidence'' ) (Cross-Chapter Box 10.3; [[#Kraaijenbrink--2017|Kraaijenbrink et al., 2017]] ; [[#de%20Kok--2020|de Kok et al., 2020]] ; [[#Farinotti--2020|Farinotti et al., 2020]] ).

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===== 9.5.1.1.3 Drivers of glacier change =====

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The AR5 ( [[#Masson-Delmotte--2013|Masson-Delmotte et al., 2013]] ) noted that early-to-mid-Holocene glacier minima could be attributed to high summer insolation ( ''high confidence'' ), unlike the current situation. Since AR5, new and improved chronologies of glacier size variations from the end of the last glacial period and the Holocene (e.g., [[#Solomina--2015|Solomina et al., 2015]] , 2016; [[#Eaves--2019|Eaves et al., 2019]] ; [[#Hall--2019|Hall et al., 2019]] ; [[#Marcott--2019|Marcott et al., 2019]] ; [[#Bohleber--2020|Bohleber et al., 2020]] ; [[#Davies--2020|Davies et al., 2020]] ; [[#Palacios--2020|Palacios et al., 2020]] ) confirm the dominant role of orbital forcing for millennial-scale glacier fluctuations, but emphasize the role of other forcings – solar and volcanic activity, ocean circulation, sea ice and internal climate variability – in explaining the regional variability of glacier fluctuations at shorter time scales. [[#Shakun--2015|Shakun et al. (2015)]] demonstrated that, during the last deglacial transition (18–11 ka), the mid-to-low-latitude glacier retreat was driven by an increase in atmospheric CO <sub>2</sub> and global temperature.

In the Northern Hemisphere, where summer insolation decreased during the Holocene ( [[IPCC:Wg1:Chapter:Chapter-2#2.2.1|Section 2.2.1]] ), glaciers generally waxed ( [[#Briner--2016|Briner et al., 2016]] ; [[#Kaufman--2016|Kaufman et al., 2016]] ; [[#Lecavalier--2017|Lecavalier et al., 2017]] ; [[#Zhang--2017|Zhang et al., 2017]] ; [[#Axford--2019|Axford et al., 2019]] ; [[#Geirsdóttir--2019|Geirsdóttir et al., 2019]] ; [[#Larsen--2019|Larsen et al., 2019]] ; [[#Luckman--2020|Luckman et al., 2020]] ). Conversely, in the Southern Hemisphere, where summer insolation increased during the Holocene, glaciers generally waned ( [[#Solomina--2015|Solomina et al., 2015]] ; [[#Kaplan--2016|Kaplan et al., 2016]] ; [[#Reynhout--2019|Reynhout et al., 2019]] ). However, these general global trends were modulated by regional climate variations in temperature and precipitation ( [[#Murari--2014|Murari et al., 2014]] ; [[#Kaplan--2016|Kaplan et al., 2016]] ; [[#Batbaatar--2018|Batbaatar et al., 2018]] ; [[#Saha--2018|Saha et al., 2018]] ) and there are a number of examples of this. A precipitation increase led to a local early Holocene (7–8 ka) glacier maximum in arid Mongolia (Gichginii Range). Glacier advances at about 9 ka in south-west Greenland have been suggested to be a consequence of the freshwater pulse from the Laurentide Ice Sheet, which led to cooling in the Baffin Bay area ( [[#Schweinsberg--2018|Schweinsberg et al., 2018]] ). Lake sediments indicate that the glaciers in the region were smaller than today, or absent between 8.6 and 1.4 ka ( [[#Larocca--2020|Larocca et al., 2020]] ). Glaciers on the Antarctic Peninsula and in Patagonia during the Holocene were strongly affected by the southern westerly winds, sea ice extent, and ocean circulation ( [[#García--2020|García et al., 2020]] ). Recent studies indicate that explosive volcanism can drive glacier advances ( [[#Solomina--2015|Solomina et al., 2015]] , 2016; [[#Schweinsberg--2018|Schweinsberg et al., 2018]] ; [[#Brönnimann--2019|Brönnimann et al., 2019]] ). In summary, on millennial time scales over the Holocene, there is ''high confidence'' that orbital forcing drove hemispheric-scale glacier variations, but new studies provide a nuanced picture of responses to a variety of regional-scale forcings.

( [[IPCC:Wg1:Chapter:Chapter-3#3.4.3.1|Section 3.4.3.1]] assesses new attribution studies for glaciers and finds that human influence is ''very likely'' the main driver of the global, near-universal retreat of glaciers since the 1990s. The SROCC assessed that it is ''very likely'' that atmospheric warming is the primary driver for the global glacier recession. Since SROCC, a study of glaciers in New Zealand used event attribution to confirm a connection between extreme glacier mass loss years and anthropogenic warming ( [[#Vargo--2020|Vargo et al., 2020]] ).

The SROCC stated with ''high confidence'' that, besides temperature, other factors, such as precipitation changes or internal glacier dynamics, have modified the temperature-induced glacier response in some regions. Deposition of a thin layer (&lt;2 cm) of light-absorbing particles (e.g., black carbon, brown carbon, algae, mineral dust or volcanic ash) can exert an important control on glacier mass balance, by decreasing surface albedo and thus increasing absorbed shortwave radiation and melt (see also [[IPCC:Wg1:Chapter:Chapter-7#7.3.4.3|Section 7.3.4.3]] ). The SROCC found ''limited evidence'' and ''low agreement'' that this process has had a significant effect on observed long-term glacier changes. Several studies have shown melt increases due to the deposition of light-absorbing particles ( [[#Schmale--2017|Schmale et al., 2017]] ; [[#Wittmann--2017|Wittmann et al., 2017]] ; [[#Sigl--2018|Sigl et al., 2018]] ; [[#Di%20Mauro--2019|Di Mauro et al., 2019]] , 2020; [[#Magalhães--2019|Magalhães et al., 2019]] ; [[#Constantin--2020|Constantin et al., 2020]] ). Conversely, increasingly thick debris cover (&gt;2–5 cm) on retreating glaciers can slow down glacier melt ( [[#Pratap--2015|Pratap et al., 2015]] ; [[#Brun--2016|Brun et al., 2016]] ). Although debris covers only about 4–7% of the total glacier area globally ( [[#Scherler--2018|Scherler et al., 2018]] ; [[#Herreid--2020|Herreid and Pellicciotti, 2020]] ), many glaciers are heavily debris-covered in their lower reaches, especially in High Mountain Asia, the Caucasus, the European Alps, Southern Andes and Alaska, resulting in different responses to warming than similar clean-ice glaciers. A shift in regional meteorological conditions, driven by the location and strength of the upper level zonal wind, has been found to have forced recent high mass loss rates in Western North America ( [[#Menounos--2019|Menounos et al., 2019]] ). High geothermal heat flux areas underneath glaciers and high energy dissipation in the flow of water and ice causes additional mass loss of the glaciers in Iceland ( [[#Jóhannesson--2020|Jóhannesson et al., 2020]] ), accounting for 20% of the mass loss since 1994 (Aðalgeirsdóttir et al. 2020). Glacier lake volume in front of retreating glaciers, has increased globally by around 48% between 1990 and 2018 ( [[#Shugar--2020|Shugar et al., 2020]] ), which can increase both subaqueous melt and calving. In summary, there is ''high confidence'' that non-climatic drivers have and will continue to modulate the first-order temperature response of glaciers in some regions.

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[[File:caba43843191c2254314bf2c24b48612 IPCC_AR6_WGI_Figure_9_21.png]]

'''Figure 9.21''' '''|''' '''Global and regional glacier mass evolution between 1901 and 2100 relative to glacier mass in 2015.''' Reconstructed glacier mass change through the 20th century ( [[#Marzeion--2015|Marzeion et al., 2015]] ) and observed during 1961–2016 ( [[#Zemp--2019|Zemp et al., 2019]] ). Projected (2015–2100) glacier mass evolution is based on the median of three RCP emissions scenarios ( [[#Marzeion--2020|Marzeion et al., 2020]] ). In all cases, uncertainties are the 90% confidence interval. For a better comparison between regions, the maximum relative mass change was set to 200%, although for three regions, the volume changes between 1901 and 2015 exceeded that value. For the Low Latitude, New Zealand, and High Mountain Asia glaciers, the changes were larger than 1000%, 350%, and 250%, respectively. See Figure 9.2 for the location of each region. Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

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==== 9.5.1.2 Model Evaluation ====

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Since AR5, glacier mass projections have been coordinatedby the Glacier Model Intercomparison Project (GlacierMIP; [[#Hock--2019a|Hock et al., 2019a]] ; [[#Marzeion--2020|Marzeion et al., 2020]] ). The SROCC ( [[#Hock--2019b|Hock et al., 2019b]] ) relied on six global-scale glacier models based on previously published glacier model projections ( [[#Hock--2019a|Hock et al., 2019a]] ). It found with ''high confidence'' that glaciers will lose substantial mass by the end of the century, but assigned ''medium confidence'' to the magnitude and timing of the projected glacier mass loss, because of the simplicity of the models, the limited observations in some regions to calibrate them, and the diverging initial glacier volumes.

Since SROCC, [[#Marzeion--2020|Marzeion et al. (2020)]] projected 21st century global-scale glacier mass changes based on seven global-scale and four regional-scale glacier models (Annex II). All models used the same initial and boundary conditions, forming a more coherent ensemble of projections compared to SROCC. Nevertheless, challenges remain because of scarcity of glacier thickness, surface mass balance (SMB) and frontal ablation data for model calibration, but also due to uncertainties in glacier outlines, surface elevations and ice velocities. The global SMB models are of varying complexity, including mass balance sensitivity approaches (van de Wal and Wild, 2001), temperature-index methods ( [[#Anderson--2012|Anderson and Mackintosh, 2012]] ; [[#Marzeion--2012|Marzeion et al., 2012]] ; [[#Radić--2014|Radić et al., 2014]] ; [[#Huss--2015|Huss and Hock, 2015]] ; [[#Kraaijenbrink--2017|Kraaijenbrink et al., 2017]] ; [[#Maussion--2019|Maussion et al., 2019]] ; [[#Zekollari--2019|Zekollari et al., 2019]] ; [[#Rounce--2020|Rounce et al., 2020]] ) and simplified energy balance calculations ( [[#Sakai--2017|Sakai and Fujita, 2017]] ; [[#Shannon--2019|Shannon et al., 2019]] ). Compared to simpler, empirical parametrizations, full energy-balance models are not necessarily the most appropriate choice for simulating future glacier response to climate change, even at the local scale ( [[#Réveillet--2017|Réveillet et al., 2017]] , 2018), because of parameter and forcing uncertainties. All models account for glacier retreat and advance, but only two models ( [[#Anderson--2012|Anderson and Mackintosh, 2012]] ; [[#Huss--2015|Huss and Hock, 2015]] ) include frontal ablation.

Secondary processes such as debris-cover thickening (e.g., [[#Herreid--2020|Herreid and Pellicciotti, 2020]] ), albedo changes due to light-absorbing particles (e.g., [[#Magalhães--2019|Magalhães et al., 2019]] ; [[#Williamson--2019|Williamson et al., 2019]] ), trends of refreezing and water storage in firn (e.g., [[#Ochwat--2021|Ochwat et al., 2021]] ), dynamic instabilities such as surges (e.g., [[#Thøgersen--2019|Thøgersen et al., 2019]] ) or glacier collapse (e.g., [[#Kääb--2018|Kääb et al., 2018]] ), are not represented in global glacier models, resulting in both underestimated and overestimated sensitivity to warming that is currently not possible to quantify. Furthermore, challenges for future projections are caused by the low-resolution and high-spatial variability at sub-grid scale of the precipitation amount provided by general circulation models (GCMs), which requires downscaling to the spatial scale of a glacier ( [[#Maussion--2019|Maussion et al., 2019]] ; [[#Zekollari--2019|Zekollari et al., 2019]] ; [[#Marzeion--2020|Marzeion et al., 2020]] ). In summary, in agreement with SROCC, progress in global scale glacier modelling efforts allows ''medium confidence'' in the capability of current-generation glacier models to simulate the magnitude and timing of glacier mass changes as a response to climatic forcing.

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==== 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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