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

===== 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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