Editing IPCC:AR6/WGI/Chapter-2 (section)

==== 2.3.2.3 Glacier Mass ====

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The AR5 concluded with ''high confidence'' that, during the Holocene, glaciers were at times smaller than at the end of the 20th century. The AR5 stated further with ''very high confidence'' that most glaciers had been shrinking since the mid-1800s, and the mass loss from all glaciers worldwide ''very likely'' increased from 1970 to 2009. The SROCC reported a globally coherent picture of continued glacier recession in recent decades ( ''very high confidence'' ) based on in situ and satellite observations of changes in glacier area, length and mass, although there were considerable inter-annual and regional variations. Between 2006 and 2015 the global glacier mass change assessed by SROCC was –278 ± 113 Gt yr <sup>–1</sup> .

Two recent global reviews on glaciers over the Holocene ( [[#Solomina--2015|Solomina et al., 2015]] ) and the past 2 kyr ( [[#Solomina--2016|Solomina et al., 2016]] ) summarize the chronologies of respectively 189 and 275 glaciers. The former shows that glaciers retreated during the LDT and retracted to their minimum extent between 8 ka and 6 ka. Except for some glaciers in the SH and tropics, glaciers expanded thereafter, reaching their maximum extent beyond their present-day margins during the mid-15th to late 19th centuries CE. With few exceptions, glacier margins worldwide have retreated since the 19th century, with the rate of retreat and its global character since the late 20th century being unusual in the context of the Holocene ( [[#Solomina--2016|Solomina et al., 2016]] , Figure 2.23a). However, the areal extents of modern glaciers in most places in the NH are still larger than those of the early and/or middle Holocene ( [[#Solomina--2015|Solomina et al., 2015]] ). When considering Holocene and present glaciers extents, it is important to account for the relatively long adjustment time of glaciers (often referred to as response time; Section 9.5.1.3); the majority of modern glaciers are currently out of equilibrium with current climate, even without further global warming ( [[#Mernild--2013|Mernild et al., 2013]] ; [[#Christian--2018|Christian et al., 2018]] ; [[#Marzeion--2018|Marzeion et al., 2018]] ; [[#Zekollari--2020|Zekollari et al., 2020]] ). The size of glaciers during other periods warmer than the Early to Mid-Holocene, such as the MPWP and LIG, is largely unknown because the deposits marking previous extents were in almost all cases over-ridden by later glaciations. For Arctic glaciers, different regional studies consistently indicate that in many places glaciers are now smaller than they have been in millennia ( [[#Lowell--2013|Lowell et al., 2013]] ; [[#Miller--2013|Miller et al., 2013]] , 2017; [[#Harning--2016|Harning et al., 2016]] , 2018; [[#Schweinsberg--2017|Schweinsberg et al., 2017]] , 2018; [[#Pendleton--2019|Pendleton et al., 2019]] ).

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[[File:e8a440f7a81bbc3bb2e4a294feacc564 IPCC_AR6_WGI_Figure_2_23.png]]

'''Figure 2.23''' '''|''' '''Mountain glacier advance and annual mass change. (a)''' Number of a finite selection of surveyed glaciers that advanced during the past 2000 years. '''(b)''' Annual and decadal global glacier mass change (Gt yr <sup>–1</sup> ) from 1961 until 2018. In addition, mass change mean estimates are shown. Ranges show the 90% confidence interval. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

New glacier outline ( [[#RGI%20Consortium--2017|RGI Consortium, 2017]] ) and glacier mass compilations ( [[#Zemp--2019|Zemp et al., 2019]] , 2020; [[#Ciracì--2020|Ciracì et al., 2020]] ; [[#Hugonnet--2021|Hugonnet et al., 2021]] ) improve, refine and update the quantification of glacier areal and mass changes based on observations from in situ and remote sensing data. Observations between the 1960s and 2019 indicate that mass loss has increased over recent decades (Figure 2.23b). The overall global glacier mass loss rate has increased from 240 ± 9 Gt yr <sup>–1</sup> over 2000–2009 to 290 ± 10 Gt yr <sup>–1</sup> over 2010–2019 ( [[#Hugonnet--2021|Hugonnet et al., 2021]] ), confirming that the last decade exhibits the most negative glacier mass balance since the beginning of the observational record. Observations are in general consistent with trends revealed by global glacier mass change modelling for almost the entire 20th century (1901–1990) implying an estimated mass loss (without uncharted glaciers ( [[#Parkes--2018|Parkes and Marzeion, 2018]] ) and excluding peripheral glaciers of Greenland and Antarctica) of ''very likely'' 210 ± 90 Gt yr <sup>–1</sup> and ''very likely'' 170 ± 80 Gt yr <sup>–1</sup> for the period 1971–2019 ( [[#Marzeion--2015|Marzeion et al., 2015]] ; Section 9.5.1 and Table 9.5).

In summary, there is ''very high confidence'' that, with few exceptions, glaciers worldwide have retreated since the second half of the 19th century, and continue to retreat. The current global character of glacier mass loss is highly unusual (almost all glaciers simultaneously receding) in the context of at least the last 2 kyr ( ''medium confidence'' ). Glacier mass loss rates have increased since the 1970s ( ''high confidence'' ). Although many surveyed glaciers are currently more extensive than during the MH ( ''high confidence'' ), they generally are in disequilibrium with respect to current climate conditions and hence are committed to further ice loss.

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