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

===== 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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