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

===== 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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