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

=== 2.2.3 Glaciers ===

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The high mountain areas considered in this chapter (Figure 2.1), including all glacier regions in the world except those in Antarctica, Greenland, the Canadian and Russian Arctic, and Svalbard (which are covered in Chapter 3) include ~170,000 glaciers covering an area of ~250,000 km 2 (RGI Consortium, 2017) with a total ice volume of 87 ± 15 mm sea level equivalent (Farinotti et al., 2019 <sup>[[#fn:r70|70]]</sup> ). These glaciers span an elevation range from sea level, for example in south-east Alaska, to &gt;8,000 m a.s.l. in the Himalaya and Karakoram, and occupy diverse climatic regions. Their mass budget is determined largely by the balance between snow accumulation and melt at the glacier surface, driven primarily by atmospheric conditions. Rapid changes in mountain glaciers have multiple impacts for social-ecological systems, affecting not only biophysical properties such as runoff volume and sediment fluxes in glacier-fed rivers, glacier related hazards, and global sea level (Chapter 4) but also ecosystems and human livelihoods, socioeconomic activities and sectors such as agriculture and tourism, as well as other intrinsic assets such as cultural values. While glaciers worldwide have experienced considerable fluctuations throughout the Holocene driven by multidecadal variations of solar and volcanic activity, and changes in atmospheric circulation (Solomina et al., 2016 <sup>[[#fn:r71|71]]</sup> ), this section focuses on observed glacier changes during recent decades and changes projected for the 21st century (Cross-Chapter Box 6 in Chapter 2).

Satellite and ''in situ'' observations of changes in glacier area, length and mass show a globally largely coherent picture of mountain glacier recession in the last decades (Zemp et al., 2015 <sup>[[#fn:r72|72]]</sup> ), although annual variability and regional differences are large (Figure 2.4; ''very high confidence'' ). The global trend is statistically significant despite considerable interannual and regional variations (Medwedeff and Roe, 2017 <sup>[[#fn:r73|73]]</sup> ). Since AR5’s global 2003 – 2009 estimate based on Gardner et al. (2013) <sup>[[#fn:r74|74]]</sup> , several new estimates of global-scale glacier mass budgets have emerged using largely improved data coverage and methods (Bamber et al., 2018 <sup>[[#fn:r75|75]]</sup> ; Wouters et al., 2019 <sup>[[#fn:r76|76]]</sup> ; Zemp et al., 2019 <sup>[[#fn:r77|77]]</sup> ).

These estimates combined with available regional estimates (Table 2.A.1) that the glacier mass budget of all mountain regions (excluding Antarctica, Greenland, the Canadian and Russian Arctic, and Svalbard) was ''very'' ''likely'' -490 ± 100 kg m -2 yr -1 (-123 ± 24 Gt yr -1 ) during the period 2006 – 2015 with most negative averages (less than -850 kg m -2 yr -1 ) in the Southern Andes, Caucasus/Middle East, European Alps and Pyrenees. High Mountain Asia shows the least negative mass budget (-150 ± 110 kg m -2 yr -1 , Figure 2.4), but variations within the region are large with most negative regional balance estimates in Nyainqentanglha, Tibet (-620 ± 230 kg m -2 yr -1 ) and slightly positive balances in the Kunlun Mountains for the period 2000 – 2016 (Brun et al., 2017 <sup>[[#fn:r78|78]]</sup> ). Due to large ice extent, the total mass loss and corresponding contribution to sea level 2006 – 2015 is largest in Alaska, followed by the Southern Andes and High Mountain Asia (Table 2.A.1). Zemp et al. (2019) estimated an increase in mean global-scale glacier mass loss by ~30% between 1986–2005 and 2006–2015.

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'''Figure 2.4'''

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'''Figure 2.4 | Glacier mass budgets for the eleven mountain regions assessed in this Chapter (Figure 2.1) and these regions combined. Mass budgets for the remaining polar regions are shown in Chapter 3, Figure 3.8. Regional time series of annual mass change are based on glaciological and geodetic balances (Zemp et al., 2019). Superimposed are […]'''
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Figure 2.4 | Glacier mass budgets for the eleven mountain regions assessed in this Chapter (Figure 2.1) and these regions combined. Mass budgets for the remaining polar regions are shown in Chapter 3, Figure 3.8. Regional time series of annual mass change are based on glaciological and geodetic balances (Zemp et al., 2019). Superimposed are multi-year averages by Wouters et al. (2019) based on the Gravity Recovery and Climate Experiment (GRACE), only shown for the regions with glacier area &gt;3,000 km2. Estimates by Gardner et al. (2013) were used in the IPCC 5th Assessment Report (AR5). Additional regional estimates available in some regions and shown here are listed in Table 2.A.1. Annual and time-averaged mass-budget estimates include the errors reported in each study. Glacier areas (A) and volumes (V) are based on RGI Consortium (2017) and Farinotti et al. (2019), respectively. Red and blue bars on map refer to regional budgets averaged over the period 2006–2015 in units of kg m-2 yr-1 and mm sea level equivalent (SLE) yr-1, respectively, and are derived from each region’s available mass-balance estimates (Appendix 2.A).

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It is ''very likely'' that atmospheric warming is the primary driver for the global glacier recession (Marzeion et al., 2014 <sup>[[#fn:r79|79]]</sup> ; Vuille et al., 2018 <sup>[[#fn:r80|80]]</sup> ). There is ''limited evidence'' ( ''high agreement'' ) that human-induced increases in greenhouse gases have contributed to the observed mass changes (Hirabayashi et al., 2016 <sup>[[#fn:r81|81]]</sup> ). It was estimated that the anthropogenic fraction of mass loss of all glaciers outside Greenland and Antarctica increased from 25 ± 35% during 1851–2010 to 69 ± 24% during 1991–2010 (Marzeion et al., 2014 <sup>[[#fn:r82|82]]</sup> ).

Other factors, such as changes in meteorological variables other than air temperature or internal glacier dynamics, have modified the temperature-induced glacier response in some regions ( ''high confidence'' ). For example, glacier mass loss over the last seven decades on a glacier in the European Alps was intensified by higher air moisture leading to increased longwave irradiance and reduced sublimation (Thibert et al., 2018 <sup>[[#fn:r83|83]]</sup> ). Changes in air moisture have also been found to play a significant role in past glacier mass changes in eastern Africa (Prinz et al., 2016 <sup>[[#fn:r84|84]]</sup> ), while an increase in shortwave radiation due to reduced cloud cover contributed to an acceleration in glacier recession in the Caucasus (Toropov et al., 2019 <sup>[[#fn:r85|85]]</sup> ). In the Tien Shan mountains changes in atmospheric circulation in the North Atlantic and North Pacific in the 1970s resulted in an abrupt reduction in precipitation and thus snow accumulation, amplifying temperature-induced glacier mass loss (Duethmann et al., 2015 <sup>[[#fn:r86|86]]</sup> ). Deposition of light absorbing particles, growth of algae and bacteria and local amplification phenomena such as the enhancement of particles concentration due to surface snow and ice melt, and cryoconite holes, have been shown to enhance ice melt (e.g., Ginot et al., 2014; Zhang et al., 2017 <sup>[[#fn:r87|87]]</sup> ; Williamson et al., 2019 <sup>[[#fn:r88|88]]</sup> ) but there is ''limited evidence'' and ''low agreement'' that long-term changes in glacier mass are linked to light absorbing particles (Painter et al., 2013 <sup>[[#fn:r89|89]]</sup> ; Sigl et al., 2018 <sup>[[#fn:r90|90]]</sup> ). Debris cover can  modulate glacier melt but there is ''limited evidence'' on its role in recent glacier changes (Gardelle et al., 2012 <sup>[[#fn:r91|91]]</sup> ; Pellicciotti et al., 2015 <sup>[[#fn:r92|92]]</sup> ). Rapid retreat of calving outlet glaciers in Patagonia was attributed to changes in glacier dynamics (Sakakibara and Sugiyama, 2014 <sup>[[#fn:r93|93]]</sup> ).

Departing from this global trend of glacier recession, a small fraction of glaciers have gained mass or advanced in some regions mostly due to internal glacier dynamics or, in some cases, locally restricted climatic causes. For example, in Alaska 36 marine-terminating glaciers exhibited a complex pattern of periods of significant retreat and advance during 1948–2012, highly variable in time and lacking coherent regional behaviour (McNabb and Hock, 2014 <sup>[[#fn:r94|94]]</sup> ). These fluctuations can be explained by internal retreat-advance cycles typical of tidewater glaciers that are largely independent of climate (Brinkerhoff et al., 2017 <sup>[[#fn:r95|95]]</sup> ). Irregular and spatially inconsistent glacier advances, for example, in Alaska, Iceland and Karakoram, have been associated with surge-type flow instabilities largely independent of changes in climate (Sevestre and Benn, 2015 <sup>[[#fn:r96|96]]</sup> ; Bhambri et al., 2017 <sup>[[#fn:r97|97]]</sup> ; Section 2.3.2). Regional scale glacier mass gain and advances in Norway in the 1990s and in New Zealand between 1983–2008 have been linked to local increases in snow precipitation (Andreassen et al., 2005 <sup>[[#fn:r98|98]]</sup> ) and lower air temperatures (Mackintosh et al., 2017 <sup>[[#fn:r99|99]]</sup> ), respectively, caused by changes in atmospheric circulation. Advances of some glaciers in Alaska, the Andes, Kamchatka and the Caucasus were attributed to volcanic activity causing flow acceleration through enhanced melt water at the ice-bed interface (Barr et al., 2018 <sup>[[#fn:r100|100]]</sup> ).

Region averaged glacier mass budgets have been nearly balanced in the Karakoram since at least the 1970s (Bolch et al., 2017 <sup>[[#fn:r101|101]]</sup> ; Zhou et al., 2017 <sup>[[#fn:r102|102]]</sup> ; Azam et al., 2018 <sup>[[#fn:r103|103]]</sup> ), while slightly positive balances since 2000 have been reported in the western Kunlun Shan, eastern Pamir, and the central and northern Karakoram mountains (Gardelle et al., 2013 <sup>[[#fn:r104|104]]</sup> ; Brun et al., 2017 <sup>[[#fn:r105|105]]</sup> ; Lin et al., 2017 <sup>[[#fn:r106|106]]</sup> ; Berthier and Brun, 2019 <sup>[[#fn:r107|107]]</sup> ). This anomalous behavior has been related to specific mechanisms countering the effects of atmospheric warming, for example, an increase in cloudiness (Bashir et al., 2017 <sup>[[#fn:r108|108]]</sup> ) and snowfall (Kapnick et al., 2014 <sup>[[#fn:r109|109]]</sup> ) spatially heterogeneous glacier mass balance sensitivity (Sakai and Fujita, 2017 <sup>[[#fn:r110|110]]</sup> ), feedbacks due to intensified lowland irrigation (de Kok et al., 2018), and changes in summer atmospheric circulation (Forsythe et al., 2017 <sup>[[#fn:r111|111]]</sup> ).

There is ''medium evidence (high agreement'' ) that recent glacier mass changes have modified glacier flow. A study covering all glaciers in High Mountain Asia showed glacier slowdown for regions with negative mass budgets since the 1970s and slightly accelerated glacier flow for Karakoram and West Kunlun regions where mass budgets were close to balance (Dehecq et al., 2019 <sup>[[#fn:r112|112]]</sup> ). Waechter et al. (2015) <sup>[[#fn:r113|113]]</sup> report reduced flow velocities in the St. Elias Mountains in North America, especially in areas of rapid ice thinning near glacier termini. In contrast Mouginot and Rignot (2015) found complex ice flow patterns with simultaneous acceleration and deceleration for glaciers of the Patagonian Icefield as well as large interannual variability during the last three decades concurrent with general thinning of the ice field.

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