Editing IPCC:AR6/SROCC/Chapter-3 (section)

=== 3.3.2 Polar Glacier Changes ===

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==== 3.3.2.1 Observations, Components of Change, and Drivers ====

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Chapter 3 assesses changes in polar glaciers in the Canadian and Russian Arctic, Svalbard, Greenland and Antarctica, independent of the Greenland and Antarctic ice sheets (Figure 3.8). Glaciers in all other regions including Alaska, Scandinavia and Iceland are assessed in Chapter 2.

Changes in the mass of Arctic glaciers for the ‘present day’ (2006–2015) are assessed using a combination of satellite observations and direct measurements (Figure 3.8; Appendix 2.A, Table 1). During this period,  glacier mass loss was largest in the periphery of Greenland (–47 ± 16 Gt yr –1 ), followed by Arctic Canada North (-39 ± 8 Gt yr –1 ), Arctic Canada South (–33 ± 9 Gt yr –1 ), the Russian Arctic (–15 ± 12 Gt yr –1 ) and Svalbard and Jan Mayen (–9 ± 5 Gt yr –1 ). When combined with the Arctic regions covered in Chapter 2 (Alaska, the Yukon territory of Canada, Iceland and Scandinavia), Arctic glaciers as a whole lost mass at a rate of –213 ± 29 Gt yr –1 , a sea level contribution of 0.59 ± 0.08 mm yr –1 ( ''high confidence'' ). Overall during this period, Arctic glaciers caused a similar amount of sea level rise to the GIS (Section 3.3.1.3), but their rate of mass loss per unit area was larger (Bolch et al., 2013 <sup>[[#fn:r1187|1187]]</sup> ).

There is ''limited evidence'' ( ''high agreement)'' that the current rate of glacier mass loss is larger than at any time during the past 4000 years (Fisher et al., 2012 <sup>[[#fn:r1188|1188]]</sup> ; Zdanowicz et al., 2012 <sup>[[#fn:r1189|1189]]</sup> ). Further back in time during the early to mid- Holocene, pre-historic glacial deposits, ice core records, and numerical modelling evidence shows that many Arctic glaciers were at various stages similar to or smaller than present (Gilbert et al., 2017 <sup>[[#fn:r1190|1190]]</sup> ; Zekollari et al., 2017 <sup>[[#fn:r1191|1191]]</sup> ), experienced greater melt rates (Lecavalier et al., 2017 <sup>[[#fn:r1192|1192]]</sup> ), or may have disappeared altogether (Solomina et al., 2015 <sup>[[#fn:r1193|1193]]</sup> ) ( ''medium confidence'' ). This evidence, however, does not provide a complete assessment of the rates and magnitudes of past glacier mass loss.

Atmospheric circulation changes (Box et al., 2018 <sup>[[#fn:r1194|1194]]</sup> ) have led to pan-Arctic variability in glacier mass balance ( ''high confidence'' ), including different rates of retreat between eastern and western glaciers in Greenland’s periphery (Bjørk et al., 2018 <sup>[[#fn:r1195|1195]]</sup> ), and a high rate of surface melt in the Canadian Arctic (Gardner et al., 2013 <sup>[[#fn:r1196|1196]]</sup> ; Van Wychen et al., 2016; Millan et al., 2017 <sup>[[#fn:r1197|1197]]</sup> ) through persistently high summer air temperatures (Bezeau et al., 2014 <sup>[[#fn:r1198|1198]]</sup> ; McLeod and Mote, 2016 <sup>[[#fn:r1199|1199]]</sup> ). Atmospheric circulation anomalies from 2007 to 2012 associated with glacier mass loss are also linked to enhanced GIS melt (Section 3.3.1.4) and Arctic sea ice loss (Section 3.2.1.1), and exceed by a factor of two the interannual variability in daily mean pressure (sea level and 500 hPa) of the Arctic region over the 1871–2014 period (Belleflamme et al., 2015 <sup>[[#fn:r1203|1203]]</sup> ) (Section 3.3.1.6).

Increased surface melt on Arctic glaciers has led to a positive feedback from lowered surface albedo, causing further melt (Box et al., 2012 <sup>[[#fn:r1204|1204]]</sup> ), and in Svalbard, mean glacier albedo has reduced between 1979 and 2015 (Möller and Möller, 2017 <sup>[[#fn:r1205|1205]]</sup> ). Across the Arctic, increased surface melt and subsequent ice-layer formation via refreezing within snow and firn also reduces the ability of glaciers to store meltwater, increasing runoff (Zdanowicz et al., 2012 <sup>[[#fn:r1206|1206]]</sup> ; Gascon et al., 2013a <sup>[[#fn:r1207|1207]]</sup> ; Gascon et al., 2013b <sup>[[#fn:r1208|1208]]</sup> ; Noël et al., 2017 <sup>[[#fn:r1209|1209]]</sup> ; Noël et al., 2018 <sup>[[#fn:r1210|1210]]</sup> ).

Between the 1990s and 2017, tidewater glaciers have exhibited regional patterns in glacier dynamics; glaciers in Arctic Canada have largely decelerated, while glaciers in Svalbard and the Russian Arctic have accelerated (Van Wychen et al., 2016; Strozzi et al., 2017). Annual retreat rates of tidewater glaciers in Svalbard and the Russian Arctic for 2000–2010, have increased by a factor 2 and 2.5 respectively, between 1992 and 2000 (Carr et al., 2017). Acceleration due to surging (an internal dynamic instability) of a few key glaciers has dominated dynamic ice discharge on time-scales of years to decades (Van Wychen et al., 2014; Dunse et al., 2015).

The recent acceleration and surge behaviour of polythermal glaciers in Svalbard and the Russian Arctic is caused by destabilisation of the marine termini due to increased surface melt, and changes in basal temperature, lubrication and weakening of subglacial sediments (Dunse et al., 2015; Sevestre et al., 2018; Willis et al., 2018) or terminus thinning and response to warmer ocean temperatures (McMillan et al., 2014a) ( ''low confidence'' ). Iceberg calving rates in Svalbard are linked to ocean temperatures which control rates of submarine melt (Luckman et al., 2015; Vallot et al., 2018) ( ''medium confidence'' ). Rapid disintegration of ice shelves in the Canadian and Russian Arctic continues and has led to acceleration and thinning in tributary-glacier basins ( ''high confidence'' ) (Willis et al., 2015; Copland and Mueller, 2017).

Little information is available on Holocene and historic changes in glaciers in Antarctica (separate from the ice sheet), and on sub-Antarctic islands (Hodgson et al., 2014). Mass changes of glaciers in these regions between 2006 and 2015 (–90 ± 860 Gt yr –1 ) have ''low confidence'' as they are based on a single data compilation with large uncertainties in the Antarctic region (Zemp et al., 2019) (Figure 3.8). ''Limited evidence'' with ''high agreement'' from individual glaciers suggests that regional variability in glacier mass changes may be linked to changes in the large-scale Southern Hemisphere atmospheric circulation (Section 3.3.1.5.2). On islands adjacent to the AP, glaciers experienced retreat and mass loss during the mid to late 20th century, but since around 2009 there has been a reduction in mass loss rate or a return to slightly positive balance (Navarro et al., 2017; Oliva et al., 2017). Reduced mass loss has been linked to increased winter snow accumulation and decreased summer melt at these locations, associated with recent deepening of the circumpolar pressure trough (Oliva et al., 2017). Conversely, on the sub-Antarctic Kerguelen Islands, increased glacier mass loss (Verfaillie et al., 2015) may be due to reduced snow accumulation rather than increased air temperature as a result of southward migration of storm tracks (Favier et al., 2016).

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'''Figure 3.8'''

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'''Glacier mass budgets for the six polar regions assessed in Chapter 3. Glacier mass budgets for all other regions (including Iceland, Scandinavia and Alaska) are shown in Chapter 2, Figure 2.4. 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 […]'''
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[[File:417d2dbe37809ab2795a78c966102cf8 IPCC-SROCC-CH_3_8.jpg]]

Glacier mass budgets for the six polar regions assessed in Chapter 3. Glacier mass budgets for all other regions (including Iceland, Scandinavia and Alaska) are shown in Chapter 2, Figure 2.4. Regional time series of annual mass change are based on glaciological and geodetic balances (Zemp et al., 2019 <sup>[[#fn:r1200|1200]]</sup> ). Superimposed are multi-year averages by Wouters et al. (2019) and Gardner et al. (2013) from the Gravity Recovery and Climate Experiment (GRACE). Estimates by Gardner et al. (2013) were used in the IPCC 5th Assessment Report (AR5). Additional regional estimates in some regions are listed in Appendix 2.1, Table 1. Annual and time-averaged mass-budget estimates include the errors reported in each study. Glacier outlines and areas are based on RGI Consortium (2017).

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==== 3.3.2.2 Projections ====

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Projections of all glaciers, including those in polar regions, are covered in Cross-Chapter Box 6 in Chapter 2.

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