Editing IPCC:AR6/WGI/Chapter-8 (section)

===== 8.4.1.7.1 Glaciers =====

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Previous assessments have concluded that recent warming has led to a reduction in low-elevation snow cover ( ''high confidence'' ) (SROCC), permafrost ( ''high confidence'' ) (SROCC), and glacier mass ( ''high to very high confidence'' ) (AR5; SROCC). The SROCC noted that these declines are projected to continue almost everywhere over the 21st century ( ''high confidence'' ), with complete glacier loss expected in regions with only small glaciers ( ''very high confidence'' ). The SROCC supported the AR5 finding that glacier recession would continue even without further changes in climate. The SROCC concluded that cryosphere changes had already altered the seasonal timing and volume of runoff ( ''very high confidence'' ), which in turn had affected water resources and agriculture ( ''medium confidence'' ), and projected peak water runoff had already been reached before 2019 in some of the glacier regions considered.

( [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] provides detailed assessment of glacier observations and projections (Figures 9.20 and 9.21, and [[IPCC:Wg1:Chapter:Chapter-9#9.5.1|Section 9.5.1]] ). Here, a summary of their key findings is presented. Since SROCC, the coordinated Glacier Model Intercomparison Project (GlacierMIP; Box 9.3; [[#Marzeion--2020|Marzeion et al., 2020]] ) has advanced modelling efforts. Global glacier volumes will substantially decline in coming decades regardless of emissions scenario; under a high-emissions scenario some areas will lose nearly all of their glacier mass ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.1.3|Section 9.5.1.3]] ). The projected global glacier mass loss over 2015 – 2100 is 29,000 ± 20,000 Gt for SSP1-2.6 to 58,000 ± 30,000 Gt for SSP5-8.5 ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.1|Section 9.5.1]] ). Because of their lagged response to warming, glaciers will continue to lose mass for decades even if global temperature is stabilized ( ''very high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.1|Section 9.5.1]] ).

Global glacier mass loss projections show a scenario-dependent geographic partitioning of when peak in runoff occurs ( [[#Marzeion--2020|Marzeion et al., 2020]] ), consistent with previous studies ( [[#Radić--2014|Radić et al., 2014]] ; [[#Huss--2018|Huss and Hock, 2018]] ; [[#Hock--2019b|Hock et al., 2019b]] ). Under a low-emissions scenario ( [[#Marzeion--2020|Marzeion et al., 2020]] ) all regions exhibit runoff in the decades prior to 2050. Under a high-emissions scenario however, low- and mid-latitude regions show peak runoff before approximately 2060, whereas Arctic regions peak in later decades around 2070 – 2090. Antarctic glacier losses will not have peaked by the end of the century in the high-emissions scenario. Globally, peak runoff of 2.5 to 3 mm yr <sup>–1</sup> sea level equivalent occurs around 2090 ( [[#Marzeion--2020|Marzeion et al., 2020]] ). Regional projections are presented in detail in [[IPCC:Wg1:Chapter:Chapter-9#9.5.1%20|Section 9.5.1]] and Figure 9.21, and briefly summarized below.

'''Himalaya and Central Asia:''' Glaciers in the Himalayas feed ten of the world’s most important river systems and are critical water sources for nearly two billion people ( [[#Wester--2019|Wester et al., 2019]] ). However, they are some of the most vulnerable ‘water towers’ ( [[#Immerzeel--2020|Immerzeel et al., 2020]] ) that are projected to experience volume losses of approximately 30 to 100% by 2100 depending on global emissions scenarios ( [[#Marzeion--2020|Marzeion et al., 2020]] ). Under mid-range emissions scenarios glaciers in this region are projected to reach peak runoff during the period 2020 to 2040 ( [[#Marzeion--2020|Marzeion et al., 2020]] ).

'''Alaska, Yukon, British Columbia:''' Post-AR5 but pre-SROCC projections indicated a potential 70 ± 10% reduced volume of glacier ice in western Canada relative to 2005 (Clarke et al. , 2015) , with few glaciers remaining in the Interior and Rockies regions and maritime glaciers in north-western British Columbia surviving only in a diminished state. Recent global projections support these earlier findings, showing that glacier mass in western Canada and the USA may reduce by 50% under low-emissions scenarios and be completely lost under the highest emissions and most sensitive glacier model combinations (Figure 9.21; Marzeion et al. , 2020) . Arctic Canada and Alaskan glaciers are projected to experience more modest mass loss (0–60% depending on region, scenario, and model; Marzeion et al., 2020) .

'''Andes:''' [[#Huss--2018|Huss and Hock (2018)]] concluded that peak glacier mass was reached prior to 2019 for 82–95% of the glacier area in the tropical Andes. This is consistent with more recent global model simulations that show mass loss rates from low latitude glaciers that universally decline from the start of simulations in 2015, regardless of emissions scenario ( [[#Marzeion--2020|Marzeion et al., 2020]] ). Peak runoff in low-latitude Andean glacier-fed rivers has therefore already passed ( [[#Frans--2015|Frans et al., 2015]] ; [[#Polk--2017|Polk et al., 2017]] ) but in the Southern Andes may occur in the latter half of the century under high-emissions scenarios ( [[#Marzeion--2020|Marzeion et al., 2020]] ).

In summary, glaciers are projected to continue to lose mass under all emissions scenarios ( ''very high confidence'' ). Runoff from glaciers is projected to peak at different times in different places, with maximum rates of glacier mass loss in low latitude regions taking place in the next few decades in all scenarios ( ''high confidence'' ). While runoff from small glaciers will typically decrease because of glacier mass depletion, runoff from larger glaciers will increase with increasing global warming until glacier mass is similarly depleted, after which runoff peaks and then declines and which tends to occurs later in basins with larger glaciers and higher ice-cover fractions ( ''high confidence'' ). Glaciers in the Arctic and Antarctic will continue to lose mass through the latter half of the century and beyond ( ''high co'' ''nfidence'' ).

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