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

==== 3.3.1.6 Natural and Anthropogenic Forcing ====

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There is ''medium agreement'' but ''limited evidence'' of anthropogenic forcing of AIS mass balance through both SMB and glacier dynamics ( ''low confidence'' ). Partitioning between natural and human drivers of atmospheric and ocean circulation changes remains very uncertain. Partitioning is challenging because, along with the effects of greenhouse gas increases and stratospheric ozone depletion (Waugh et al., 2015 <sup>[[#fn:r1161|1161]]</sup> ; England et al., 2016 <sup>[[#fn:r1162|1162]]</sup> ; Li et al., 2016a <sup>[[#fn:r1163|1163]]</sup> ), atmospheric and ocean variability in the areas of greatest AIS mass change are affected by a complex chain of processes (e.g., Fyke et al., 2018; Zhang et al., 2018a <sup>[[#fn:r1164|1164]]</sup> ) that exhibit considerable natural variability and have multiple interacting links to sea surface conditions in the Pacific (Schneider et al., 2015 <sup>[[#fn:r1165|1165]]</sup> ; England et al., 2016 <sup>[[#fn:r1166|1166]]</sup> ; Raphael et al., 2016 <sup>[[#fn:r1167|1167]]</sup> ; Clem et al., 2017 <sup>[[#fn:r1168|1168]]</sup> ; Steig et al., 2017 <sup>[[#fn:r1169|1169]]</sup> ; Paolo et al., 2018 <sup>[[#fn:r1170|1170]]</sup> ) and Atlantic (Li et al., 2014 <sup>[[#fn:r1171|1171]]</sup> ), with additional local feedbacks (e.g., Stammerjohn et al., 2012; Goosse and Zunz, 2014 <sup>[[#fn:r1172|1172]]</sup> ). Recent AP warming and consequent ice shelf collapses have evidence of a link to anthropogenic ozone and greenhouse gas forcing via the SAM (e.g., Marshall, 2004; Shindell, 2004 <sup>[[#fn:r1173|1173]]</sup> ; Arblaster and Meehl, 2006 <sup>[[#fn:r1174|1174]]</sup> ; Marshall et al., 2006 <sup>[[#fn:r1175|1175]]</sup> ; Abram et al., 2014 <sup>[[#fn:r1176|1176]]</sup> ) and to anthropogenic Atlantic sea surface warming via the Atlantic Multidecadal Oscillation (e.g., Li et al., 2014). This warming was highly unusual over the last 1000 years but not unprecedented, and along with subsequent cooling is within the bounds of the large natural decadal-scale climate variability in this region (Mulvaney et al., 2012 <sup>[[#fn:r1177|1177]]</sup> ; Turner et al., 2016 <sup>[[#fn:r1178|1178]]</sup> ). More broadly over the AP and coastal WAIS where dynamic mass losses are concentrated, natural variability in atmospheric and ocean forcing appear to dominate observed mass balance ( ''medium confidence'' ) (Smith and Polvani, 2017 <sup>[[#fn:r1179|1179]]</sup> ; Jenkins et al., 2018 <sup>[[#fn:r1180|1180]]</sup> ).

Evidence exists for an anthropogenic role in the atmospheric circulation (NAO) changes that have driven GIS mass loss (Section 3.3.1.5.2) ( ''medium confidence'' ), although this awaits formal attribution testing (e.g., Easterling et al., 2016). Arctic amplification of anthropogenic warming (e.g., Serreze et al., 2009) affects atmospheric circulation (Francis and Vavrus, 2015 <sup>[[#fn:r1181|1181]]</sup> ; Mann et al., 2017 <sup>[[#fn:r1182|1182]]</sup> ) and has reduced sea ice extent (Section 3.2.1.1.1), feeding back to exacerbate both warming and NAO changes (Screen and Simmonds, 2010 <sup>[[#fn:r1183|1183]]</sup> ) that impact GIS mass balance. Negative-NAO wind patterns increased GIS melt observed in a 40-year runoff signal (Ahlstrom et al., 2017 <sup>[[#fn:r1184|1184]]</sup> ), and an increase in melting beginning in the mid-1800s closely followed the onset of industrial era Arctic warming and emerged beyond the range of natural variability in the last few decades (Graeter et al., 2018 <sup>[[#fn:r1185|1185]]</sup> ; Trusel et al., 2018 <sup>[[#fn:r1186|1186]]</sup> ) (Section 3.3.1.4).

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