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

==== 3.3.1.4 Components of Greenland Ice Sheet Mass Change ====

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Ongoing GIS mass loss over recent years has resulted from a combined increase in dynamic thinning and a decrease in SMB. Of these, reduced SMB due to an increase in surface melting and runoff recently came to dominate ( ''high confidence'' ) (Andersen et al., 2015 <sup>[[#fn:r1019|1019]]</sup> ; Fettweis et al., 2017 <sup>[[#fn:r1020|1020]]</sup> ; van den Broeke et al., 2017 <sup>[[#fn:r1021|1021]]</sup> ; King et al., 2018 <sup>[[#fn:r1022|1022]]</sup> ), accounting for 42% of losses for 2000–2005, 64% for 2005–2009 and 68% for 2009–2012 (Enderlin et al., 2014 <sup>[[#fn:r1023|1023]]</sup> ) (Figure 3.7).

The GIS was close to balance in the early years of the 1990s (Hanna et al., 2013 <sup>[[#fn:r1024|1024]]</sup> ; Khan et al., 2015 <sup>[[#fn:r1025|1025]]</sup> ), the interior above 2000 m altitude gained mass from 1961 to 1990 (Colgan et al., 2015 <sup>[[#fn:r1026|1026]]</sup> ) and both coastal and ice sheet sites experienced an increasing precipitation trend from 1890 to 2012 and 1890 to 2000 respectively (Mernild et al., 2015 <sup>[[#fn:r1027|1027]]</sup> ), but since the early 1990s multiple observations and modelling studies show strong warming and an increase in runoff ( ''very high confidence'' ). High-altitude GIS sites NEEM and Summit warmed by, respectively, 2.7°C ± 0.33°C over the past 30 years (Orsi et al., 2017 <sup>[[#fn:r1028|1028]]</sup> ) and by 2.7°C ± 0.3°C from 1982 to 2011 (McGrath et al., 2013 <sup>[[#fn:r1029|1029]]</sup> ), while satellite thermometry showed statistically significant widespread surface warming over northern GIS from 2000 to 2012 (Hall et al., 2013 <sup>[[#fn:r1030|1030]]</sup> ). The post-1990s period experienced the warmest GIS near-surface summer air temperatures of 1840–2010 (+1.1˚C) (statistically highly significant) (Box, 2013), and ice core analysis found the 2000–2010 decade to be the warmest for around 2000 years (Vinther et al., 2009 <sup>[[#fn:r1031|1031]]</sup> ; Masson-Delmotte et al., 2012 <sup>[[#fn:r1032|1032]]</sup> ), and possibly around 7000 years (Lecavalier et al., 2017 <sup>[[#fn:r1033|1033]]</sup> ) . This significant summer warming since the early 1990s increased GIS melt event duration (Mernild et al., 2017 <sup>[[#fn:r1034|1034]]</sup> ) and intensity to levels exceptional over at least 350 years (Trusel et al., 2018 <sup>[[#fn:r1035|1035]]</sup> ), and melt frequency to levels unprecedented for at least 470 years (Graeter et al., 2018 <sup>[[#fn:r1036|1036]]</sup> ). GIS melt intensity for 1994–2013 was two to fivefold the pre-industrial intensity ( ''medium confidence'' ) (Trusel et al., 2018 <sup>[[#fn:r1037|1037]]</sup> ). In response, GIS meltwater production and runoff increased (Hanna et al., 2012 <sup>[[#fn:r1038|1038]]</sup> ; Box, 2013; Fettweis et al., 2013 <sup>[[#fn:r1039|1039]]</sup> ; Tedstone et al., 2015 <sup>[[#fn:r1040|1040]]</sup> ; van den Broeke et al., 2016 <sup>[[#fn:r1041|1041]]</sup> ; Fettweis et al., 2017 <sup>[[#fn:r1042|1042]]</sup> ), resulting in 1994–2013 runoff being 33% higher the 20th century mean and 50% higher than the 18th century (Trusel et al., 2018 <sup>[[#fn:r1043|1043]]</sup> ), and 80% higher in a western-GIS marginal river catchment in 2003–2014 relative to 1976–2002 (Ahlstrom et al., 2017 <sup>[[#fn:r1044|1044]]</sup> ).

Only around half of the 1960–2014 surface melt ran off, most of the rest being retained in firn and snow (Steger et al., 2017 <sup>[[#fn:r1045|1045]]</sup> ), particularly in recently observed firn aquifers in south and west Greenland (Humphrey et al., 2012 <sup>[[#fn:r1046|1046]]</sup> ; Forster et al., 2013 <sup>[[#fn:r1047|1047]]</sup> ; Munneke et al., 2014 <sup>[[#fn:r1048|1048]]</sup> ; Poinar et al., 2017 <sup>[[#fn:r1049|1049]]</sup> ) that cover up to 5% of GIS (Miège et al., 2016 <sup>[[#fn:r1050|1050]]</sup> ; Steger et al., 2017 <sup>[[#fn:r1051|1051]]</sup> ) and stored around one fifth of the meltwater increase since the late 1990s (Noël et al., 2017) ( ''medium confidence'' ). While potential aquifer storage is equivalent to about a quarter of annual GIS melt production (Koenig et al., 2014 <sup>[[#fn:r1053|1053]]</sup> ; van den Broeke et al., 2016 <sup>[[#fn:r1054|1054]]</sup> ) and aquifers have spread to higher altitudes (Steger et al., 2017 <sup>[[#fn:r1055|1055]]</sup> ), their potential to buffer runoff has been reduced by firn densification (Polashenski et al., 2014 <sup>[[#fn:r1056|1056]]</sup> ), diversion of water to the bed via crevasses (Poinar et al., 2017 <sup>[[#fn:r1058|1058]]</sup> ), and the formation of ice layers that prevent drainage and promote surface ponding on the firn (Charalampidis et al., 2016) ( ''high confidence'' ). Such ponding lowers the firn albedo, promoting further melting ( ''high confidence'' ) (e.g., Charalampidis et al., 2015), but the extent of bare ice is a fivefold stronger control on melt (Ryan et al., 2019 <sup>[[#fn:r1059|1059]]</sup> ). Bare ice produced ~78% of runoff from 1960 to 2014, and its extent is expected to increase non-linearly as snow cover retreats to higher, flatter areas of ice sheet (Steger et al., 2017 <sup>[[#fn:r1060|1060]]</sup> ). This extent is not well reproduced in climate models, however, with biases of –6% to 13% (Ryan et al., 2019 <sup>[[#fn:r1061|1061]]</sup> ).

The remaining ~40% of non-SMB GIS mass loss from 1991 to 2015 has resulted from increased ice discharge due to dynamic thinning ( ''high confidence'' ) (Enderlin et al., 2014 <sup>[[#fn:r1062|1062]]</sup> ; van den Broeke et al., 2016 <sup>[[#fn:r1063|1063]]</sup> ; King et al., 2018 <sup>[[#fn:r1064|1064]]</sup> ) (Figure 3.7). From 2000 to 2016, dynamic thinning of 89% of GIS outlet glaciers accounted for –682 ± 31 Gt mass change, of which 92% came from the northwest and southeast GIS (King et al., 2018 <sup>[[#fn:r1065|1065]]</sup> ). Half came from only four glaciers (Jakobshavn Isbræ, Kangerdlugssuaq, Koge Bugt, and Ikertivaq South) (Enderlin et al., 2014 <sup>[[#fn:r1066|1066]]</sup> ). Glacier thinning has decreased glacier discharge, however, reducing the dynamic contribution to GIS mass loss (e.g., from 58% from 2000 to 2005 to 32% between 2009 and 2012; Enderlin et al., 2014). Furthermore, there is now ''high confidence'' that for most of the GIS, increased surface melt has not led to sustained increases in glacier flux on annual timescales because subglacial drainage networks have evolved to drain away the additional water inputs (e.g., Sole et al., 2013; Tedstone et al., 2015 <sup>[[#fn:r1068|1068]]</sup> ; Stevens et al., 2016 <sup>[[#fn:r1069|1069]]</sup> ; Nienow et al., 2017 <sup>[[#fn:r1070|1070]]</sup> ; King et al., 2018).

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