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

==== 3.3.1.5 Drivers of ice sheet mass change ====

<div id="section-3-3-1-5-drivers-of-ice-sheet-mass-change-block-1"></div>

<span id="ocean-drivers"></span>
===== 3.3.1.5.1 Ocean drivers =====

The reduction of ice shelf buttressing that has dominated AIS mass loss (Section 3.3.1.2) has been driven primarily by increases in sub-ice shelf melting (Khazendar et al., 2013 <sup>[[#fn:r1071|1071]]</sup> ; Pollard et al., 2015 <sup>[[#fn:r1072|1072]]</sup> ; Cook et al., 2016 <sup>[[#fn:r1073|1073]]</sup> ; Rintoul et al., 2016 <sup>[[#fn:r1074|1074]]</sup> ; Walker and Gardner, 2017 <sup>[[#fn:r1075|1075]]</sup> ; Adusumilli et al., 2018 <sup>[[#fn:r1076|1076]]</sup> ; Dow et al., 2018a <sup>[[#fn:r1077|1077]]</sup> ; Minchew et al., 2018 <sup>[[#fn:r1078|1078]]</sup> ) ( ''high confidence'' ). Shoaling of relatively warm Circumpolar Deep Water has controlled recent variability in melting in the Amundsen and Bellingshausen seas, Wilkes Land (Roberts et al., 2018 <sup>[[#fn:r1079|1079]]</sup> ) and the AP ( ''medium confidence'' ) (Jacobs et al., 2011 <sup>[[#fn:r1080|1080]]</sup> ; Pritchard et al., 2012 <sup>[[#fn:r1081|1081]]</sup> ; Depoorter et al., 2013 <sup>[[#fn:r1082|1082]]</sup> ; Rignot et al., 2013 <sup>[[#fn:r1083|1083]]</sup> ; Dutrieux et al., 2014 <sup>[[#fn:r1084|1084]]</sup> ; Paolo et al., 2015 <sup>[[#fn:r1085|1085]]</sup> ; Wouters et al., 2015 <sup>[[#fn:r1086|1086]]</sup> ; Christianson et al., 2016 <sup>[[#fn:r1087|1087]]</sup> ; Cook et al., 2016 <sup>[[#fn:r1088|1088]]</sup> ; Jenkins et al., 2018 <sup>[[#fn:r1089|1089]]</sup> ; Roberts et al., 2018 <sup>[[#fn:r1090|1090]]</sup> ). Changes in winds have driven this shoaling by affecting continental shelf edge undercurrents (Walker et al., 2013 <sup>[[#fn:r1091|1091]]</sup> ; Dutrieux et al., 2014 <sup>[[#fn:r1092|1092]]</sup> ; Kimura et al., 2017 <sup>[[#fn:r1093|1093]]</sup> ) and overturning in coastal polynyas (St ‐ Laurent et al., 2015 <sup>[[#fn:r1094|1094]]</sup> ; Webber et al., 2017 <sup>[[#fn:r1095|1095]]</sup> ) ( ''medium confidence'' ). Winds over the Amundsen Sea are highly variable, however, with complex interactions between SAM, El Niño/Southern Oscillation (ENSO), Atlantic Multidecadal Oscillation, and the Amundsen Sea Low (Uotila et al., 2013 <sup>[[#fn:r1095|1095]]</sup> ; Li et al., 2014 <sup>[[#fn:r1096|1096]]</sup> ; Turner et al., 2016 <sup>[[#fn:r1097|1097]]</sup> ) (SM3.1.3).

Through their effects on Antarctic coastal ocean circulation, ENSO or other tropical-ocean variability may have triggered changes to Pine Island Glacier in the 1940s (Smith et al., 2017c <sup>[[#fn:r1098|1098]]</sup> ) and again in the 1970s and 1990s (Jenkins et al., 2018 <sup>[[#fn:r1099|1099]]</sup> ), and recent ENSO variability is correlated with recent changes in ice shelf thickness (Paolo et al., 2018 <sup>[[#fn:r1100|1100]]</sup> ) ( ''medium confidence'' ). Such coupling between wind variability, ocean upwelling, ice shelf melt, buttressing and glacier flow rate has also been observed in EAIS, at Totten Glacier, Wilkes Land (Greene et al., 2017 <sup>[[#fn:r1101|1101]]</sup> ).

Around Greenland, an anomalous inflow of subtropical water driven by wind changes, multi-decadal natural ocean variability (Andresen et al., 2012 <sup>[[#fn:r1102|1102]]</sup> ), and a long-term increase in the North Atlantic’s upper ocean heat content since the 1950s (Cheng et al., 2017 <sup>[[#fn:r1103|1103]]</sup> ), all contributed to a warming of the subpolar North Atlantic (Häkkinen et al., 2013 <sup>[[#fn:r1104|1104]]</sup> ) ( ''medium confidence'' ). Water temperatures near the grounding zone of GIS outlet glaciers are critically important to their calving rate (O’Leary and Christoffersen, 2013) ( ''medium confidence'' ), and warm waters have been observed interacting with major GIS outlet glaciers ( ''high confidence'' ) (e.g., Holland et al., 2008; Straneo et al., 2017 <sup>[[#fn:r1105|1105]]</sup> ).

The processes behind warm-water incursions in coastal Greenland that force glacier retreat remain unclear, however (Straneo et al., 2013 <sup>[[#fn:r1106|1106]]</sup> ; Xu et al., 2013b <sup>[[#fn:r1107|1107]]</sup> ; Bendtsen et al., 2015 <sup>[[#fn:r1108|1108]]</sup> ; Murray et al., 2015 <sup>[[#fn:r1109|1109]]</sup> ; Cowton et al., 2016 <sup>[[#fn:r1110|1110]]</sup> ; Miles et al., 2016 <sup>[[#fn:r1111|1111]]</sup> ), and there is ''low confidence'' in understanding coastal GIS glacier response to ocean forcing because submarine melt rates, calving rates (Rignot et al., 2010 <sup>[[#fn:r1112|1112]]</sup> ; Todd and Christoffersen, 2014 <sup>[[#fn:r1113|1113]]</sup> ; Benn et al., 2017 <sup>[[#fn:r1114|1114]]</sup> ), bed and fjord geometry and the roles of ice melange and subglacial discharge (Enderlin et al., 2013 <sup>[[#fn:r1115|1115]]</sup> ; Gladish et al., 2015 <sup>[[#fn:r1116|1116]]</sup> ; Slater et al., 2015 <sup>[[#fn:r1117|1117]]</sup> ; Morlighem et al., 2016 <sup>[[#fn:r1118|1118]]</sup> ; Rathmann et al., 2017 <sup>[[#fn:r1119|1119]]</sup> ) are poorly understood, and extrapolation from a small sample of glaciers is impractical (Moon et al., 2012 <sup>[[#fn:r1120|1120]]</sup> ; Carr et al., 2013 <sup>[[#fn:r1121|1121]]</sup> ; Straneo et al., 2016 <sup>[[#fn:r1122|1122]]</sup> ; Cowton et al., 2018 <sup>[[#fn:r1123|1123]]</sup> ).

<div id="section-3-3-1-5-drivers-of-ice-sheet-mass-change-block-2"></div>

<span id="atmospheric-drivers"></span>
===== 3.3.1.5.2 Atmospheric drivers =====

Snow accumulation and surface melt in Antarctica are influenced by the Southern Hemisphere extratropical circulation (SM3.1.3), which has intensified and shifted poleward in austral summer from 1950 to 2012 (Arblaster et al., 2014 <sup>[[#fn:r1124|1124]]</sup> ; Swart et al., 2015a <sup>[[#fn:r1125|1125]]</sup> ) ( ''medium confidence'' ). The austral summer SAM has been in its most positive extended state for the past 600 years (Abram et al., 2014 <sup>[[#fn:r1126|1126]]</sup> ; Dätwyler et al., 2017 <sup>[[#fn:r1127|1127]]</sup> ), and from 1979 to 2013 has contributed to intensified atmospheric circulation and increasing and decreasing snowfall in the western and eastern AP respectively (Marshall et al., 2017 <sup>[[#fn:r1128|1128]]</sup> ) ( ''medium confidence'' ). WAIS accumulation trends (Section 3.3.1.2) resulted from a deepening of the Amundsen Sea Low over recent decades (Raphael et al., 2016 <sup>[[#fn:r1129|1129]]</sup> ) ( ''high confidence'' ).

During the 1990s, WAIS experienced record surface warmth relative to the past 200 years, though similar conditions occurred for 1% of the preceding 2000 years (Steig et al., 2013 <sup>[[#fn:r1130|1130]]</sup> ), and WAIS surface melting remains limited. In contrast, AP surface melting has intensified since the mid-20th century and the last three decades were unprecedented over 1000 years (Abram et al., 2013a <sup>[[#fn:r1131|1131]]</sup> ). The northeast AP began warming 600 years ago and past-century rates were unusual over 2000 years (Mulvaney et al., 2012 <sup>[[#fn:r1132|1132]]</sup> ; Stenni et al., 2017 <sup>[[#fn:r1133|1133]]</sup> ). Increased föhn winds due to the more positive SAM (Cape et al., 2015 <sup>[[#fn:r1134|1134]]</sup> ) caused increased surface melting on the Larsen ice shelves (Grosvenor et al., 2014 <sup>[[#fn:r1135|1135]]</sup> ; Luckman et al., 2014 <sup>[[#fn:r1136|1136]]</sup> ; Elvidge et al., 2015 <sup>[[#fn:r1137|1137]]</sup> ) and after 11,000 years intact, the 2002 melt-driven collapse of the Larsen B ice shelf followed strong warming between the mid–1950s and the late 1990s (Domack et al., 2005 <sup>[[#fn:r1138|1138]]</sup> ) ( ''medium confidence'' ).

In Greenland, associations between atmospheric pressure indices such as the North Atlantic Oscillation (NAO) and temperature, insolation and snowfall indicate with ''high confidence'' that, as in Antarctica, variability of large-scale atmospheric circulation is an important driver of SMB changes (Fettweis et al., 2013 <sup>[[#fn:r1139|1139]]</sup> ; Tedesco et al., 2013 <sup>[[#fn:r1140|1140]]</sup> ; Ding et al., 2014 <sup>[[#fn:r1141|1141]]</sup> ; Tedesco et al., 2016b <sup>[[#fn:r1142|1142]]</sup> ; Ding et al., 2017 <sup>[[#fn:r1143|1143]]</sup> ; Hofer et al., 2017 <sup>[[#fn:r1144|1144]]</sup> ). A post-1990s decrease in summer NAO reflects increased anticyclonic weather (e.g., Tedesco et al., 2013; Hanna et al., 2015 <sup>[[#fn:r1145|1145]]</sup> ) that advected warm air over the GIS, explaining ~70% of summer surface warming from 2003 to 2013 (Fettweis et al., 2013 <sup>[[#fn:r1146|1146]]</sup> ; Tedesco et al., 2013 <sup>[[#fn:r1147|1147]]</sup> ; Mioduszewski et al., 2016 <sup>[[#fn:r1148|1148]]</sup> ), and reduced the cloud cover, increasing shortwave insolation (Tedesco et al., 2013 <sup>[[#fn:r1149|1149]]</sup> ) that, combined with albedo feedbacks (Box et al., 2012 <sup>[[#fn:r1150|1150]]</sup> ; Charalampidis et al., 2015 <sup>[[#fn:r1151|1151]]</sup> ; Tedesco et al., 2016a <sup>[[#fn:r1152|1152]]</sup> ; Stibal et al., 2017 <sup>[[#fn:r1153|1153]]</sup> ; Ryan et al., 2018 <sup>[[#fn:r1154|1154]]</sup> ) ( ''high confidence'' ), explains most of the post-1990s melt increase (Hofer et al., 2017 <sup>[[#fn:r1155|1155]]</sup> ). These drivers culminated in July 2012 in exceptional warmth and surface melt up to the ice sheet summit (Nghiem et al., 2012 <sup>[[#fn:r1156|1156]]</sup> ; Tedesco et al., 2013 <sup>[[#fn:r1157|1157]]</sup> ; Hanna et al., 2014 <sup>[[#fn:r1158|1158]]</sup> ; Hanna et al., 2016 <sup>[[#fn:r1159|1159]]</sup> ; McLeod and Mote, 2016 <sup>[[#fn:r1160|1160]]</sup> ).

<div id="section-3-3-1-6natural-and-anthropogenic-forcing"></div>

<span id="natural-and-anthropogenic-forcing"></span>