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

===== 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>