Editing IPCC:AR6/WGI/Chapter-9 (section)

==== 9.4.1.1 Recent Observed Changes ====

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In this section we present regional mass change time series for the Greenland Ice Sheet and assess the different processes that are causing the increase in mass loss. The vast increase in observational products from various platforms (e.g, GRACE, PROMICE, ESA-CCI, NASA MEaSUREs) provide a consistent and clear picture of a shrinking Greenland Ice Sheet ( [[#Colgan--2019|Colgan et al., 2019]] ; Mottram et al.,2019; [[#Mouginot--2019|Mouginot et al., 2019]] ; [[#King--2020|King et al., 2020]] ; [[#Mankoff--2020|Mankoff et al., 2020]] ; [[#Moon--2020|Moon et al., 2020]] ; [[#Sasgen--2020|Sasgen et al., 2020]] ; [[#Velicogna--2020|Velicogna et al., 2020]] ; [[#The%20IMBIE%20Team--2020|The IMBIE Team, 2020]] ). [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.4.1|Section 2.3.2.4.1]] provides an updated estimate of the total Greenland Ice Sheet mass change in a global context (Figure 2.24). The estimated ice-sheet extent at different times is shown in Figure 9.17, and the paleo perspective on Greenland Ice Sheet evolution is presented in [[#9.6.2|Section 9.6.2]] .

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[[File:1d111adb36b628cfd8a0d493ecd135b1 IPCC_AR6_WGI_Figure_9_16.png]]

'''Figure 9.16''' '''|''' '''Mass changes and mass change rates for Greenland and Antarctic ice sheet regions. (a)''' Time series of mass changes in Greenland for each of the major drainage basins shown in the inset figure ( [[#Bamber--2018b|Bamber et al., 2018b]] ; [[#Mouginot--2019|Mouginot et al., 2019]] ; [[#The%20IMBIE%20Team--2021|The IMBIE Team, 2021]] ) for the periods 1972–2016, 1992–2018, and 1992–2020. '''(b)''' Time series of mass changes for three portions of Antarctica ( [[#Bamber--2018b|Bamber et al., 2018b]] ; [[#The%20IMBIE%20Team--2021|The IMBIE Team, 2021]] ) for the period 1992–2016 and 1992–2020. Estimates of mass change rates of surface mass balance, discharge and mass balance in '''(g)''' all of Greenland and '''(c–f, h–j)''' in seven Greenland regions ( [[#Bamber--2018b|Bamber et al., 2018b]] ; [[#Mankoff--2019|Mankoff et al., 2019]] ; [[#Mouginot--2019|Mouginot et al., 2019]] ; [[#King--2020|King et al., 2020]] ). Estimates of mass change rates of surface mass balance, discharge and mass balance for '''(k)''' all of Antarctica and '''(l–n)''' for three regions of Antarctica ( [[#Bamber--2018b|Bamber et al., 2018b]] ; [[#The%20IMBIE%20Team--2018|The IMBIE Team, 2018]] ; [[#Rignot--2019|Rignot et al., 2019]] ). Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

For the 20th century, SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) presented one reconstruction for 1900–1983 and estimated mass change for the Greenland Ice Sheet and its peripheral glaciers for the period 1901–1990. Since SROCC, a comprehensive new study has extended the satellite record back to 1972 (Figure 9.16; [[#Mouginot--2019|Mouginot et al., 2019]] ). The rate of ice-sheet mass change was positive (i.e., it gained mass) in 1972–1980 (47 ± 21 Gt yr <sup>–1</sup> ) and then negative (i.e., it lost mass; –51 ± 17 Gt yr <sup>–1</sup> and –41 ± 17 Gt yr <sup>–1</sup> ) in 1980–1990 and 1990–2000, respectively. Other ice discharge time series starting in 1985 ( [[#King--2018|King et al., 2018]] , 2020; [[#Mankoff--2019|Mankoff et al., 2019]] , 2020) agree with [[#Mouginot--2019|Mouginot et al. (2019)]] (see also Figure 9.16). There is ''limited evidence'' of temporally and spatially heterogeneous Greenland outlet glacier evolution during the 20th century ( [[#Lea--2014|Lea et al., 2014]] ; [[#Lüthi--2016|Lüthi et al., 2016]] ; [[#Andresen--2017|Andresen et al., 2017]] ; [[#Khan--2020|Khan et al., 2020]] ; [[#Vermassen--2020|Vermassen et al., 2020]] ). Historical photographs ( [[#Khan--2020|Khan et al., 2020]] ) show large mass losses of Jakobshavn and Kangerlussuaq Glaciers in West Greenland from 1880 until the 1940s, exceeding their 21st-century mass loss, whereas the Helheim Glacier in East Greenland remained stable, gained mass in the 1990s, then rapidly lost mass after 2000. Together, these three large outlet glaciers, draining about 12% of the ice sheet surface area, have lost 22 ± 3 Gt yr <sup>–1</sup> in the period 1880–2012 ( [[#Khan--2020|Khan et al., 2020]] ). Overall, these studies provide a variable picture of the Greenland Ice Sheet mass change in the 20th century. The updated mass loss of Greenland Ice Sheet, including peripheral glaciers for the period 1901–1990, is 120 [70–170] Gt yr <sup>–1</sup> (see Table 9.5 and Figures 9.16 and 9.17).

Post-1992, SROCC stated that it is ''extremely likely'' that the rate of mass change of Greenland Ice Sheet was more negative during 2012–2016 than during 1992–2001, with ''very high confidence'' that summer melting has increased since the 1990s to a level unprecedented over at least the last 350 years. Since SROCC, the updated synthesis of satellite observations by the Ice Sheet Mass Balance Intercomparison Exercise ( [[#The%20IMBIE%20Team--2020|The IMBIE Team, 2020]] ) and the GRACE Follow-On (GRACE-FO) Mission ( [[#Abich--2019|Abich et al., 2019]] ; [[#Kornfeld--2019|Kornfeld et al., 2019]] ), have confirmed the mass change record, and the record has been extended to 2020 ( [[#The%20IMBIE%20Team--2021|The IMBIE Team, 2021]] ) as presented in 2.3.2.4. The Greenland Ice Sheet lost 4890 [4140–5640] Gt of ice between 1992 and 2020, causing sea level to rise by 13.5 [11.4 to 15.6] mm ( [[#The%20IMBIE%20Team--2021|The IMBIE Team, 2021]] ; see also [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.4.1|Section 2.3.2.4.1]] , Figure 9.16 and Table 9.5). The IMBIE Team’s (2020) estimates are consistent with other post-AR5 reviews (Figure 9.17, Table 9.SM.1; [[#Bamber--2018a|Bamber et al., 2018a]] ; [[#Cazenave--2018|Cazenave et al., 2018]] ; [[#Mouginot--2019|Mouginot et al., 2019]] ; [[#Slater--2021|Slater et al., 2021]] ). Recent GRACE-FO data ( [[#Sasgen--2020|Sasgen et al., 2020]] ; [[#Velicogna--2020|Velicogna et al., 2020]] ) show that, after two cold summers in 2017 and 2018, with relatively moderate mass change of about –100 Gt yr <sup>–1</sup> , the 2019 mass change (–532 ± 58 Gt yr <sup>–1</sup> ) was the largest annual mass loss in the record. The ''high agreement'' across a variety of methods confirms SROCC and [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] assessments. The mass-loss rate was, on average, 39 [–3 to 80] Gt yr <sup>–1</sup> over the period 1992–1999, 175 [131 to 220] Gt yr <sup>–1</sup> over the period 2000–2009 and 243 [197 to 290] Gt yr <sup>–1</sup> over the period 2010–2019 (see Table 9.SM.1).

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'''Figure 9.17 |''' '''Greenland Ice Sheet cumulative mass change and equivalent sea level contribution. (a)''' A p-box ( [[#9.6.3.2|Section 9.6.3.2]] ) based estimate of the range of values of paleo Greenland Ice Sheet mass and sea level equivalents relative to present day and the median over all central estimates ( [[#Simpson--2009|Simpson et al., 2009]] ; [[#Argus--2010|Argus and Peltier, 2010]] ; [[#Colville--2011|Colville et al., 2011]] ; [[#Dolan--2011|Dolan et al., 2011]] ; [[#Fyke--2011|Fyke et al., 2011]] ; [[#Robinson--2011|Robinson et al., 2011]] ; [[#Born--2012|Born and Nisancioglu, 2012]] ; K.G. [[#Miller--2012|]] [[#Miller--2012|Miller et al., 2012]] ; [[#Dahl-Jensen--2013|Dahl-Jensen et al., 2013]] ; [[#Helsen--2013|Helsen et al., 2013]] ; [[#Nick--2013|Nick et al., 2013]] ; [[#Quiquet--2013|Quiquet et al., 2013]] ; [[#Stone--2013|Stone et al., 2013]] ; [[#Colleoni--2014|Colleoni et al., 2014]] ; [[#Lecavalier--2014|Lecavalier et al., 2014]] ; [[#Robinson--2014|Robinson and Goelzer, 2014]] ; [[#Calov--2015|Calov et al., 2015]] , 2018; [[#Dutton--2015|Dutton et al., 2015]] ; [[#Koenig--2015|Koenig et al., 2015]] ; [[#Peltier--2015|Peltier et al., 2015]] ; [[#Stuhne--2015|Stuhne and Peltier, 2015]] ; [[#Vizcaino--2015|Vizcaino et al., 2015]] ; [[#Goelzer--2016|Goelzer et al., 2016]] ; [[#Khan--2016|Khan et al., 2016]] ; [[#Yau--2016|Yau et al., 2016]] ; [[#de%20Boer--2017|de Boer et al., 2017]] ; [[#Simms--2019|Simms et al., 2019]] ); '''(b, left)''' cumulative mass loss (and sea level equivalent) since 2015 from 1972 ( [[#Mouginot--2019|Mouginot et al., 2019]] ) and 1992 ( [[#Bamber--2018b|Bamber et al., 2018b]] ; [[#The%20IMBIE%20Team--2020|The IMBIE Team, 2020]] ), the estimated mass loss from 1840 ( [[#Box--2013|Box and Colgan, 2013]] ; [[#Kjeldsen--2015|Kjeldsen et al., 2015]] ) indicated with a shaded box, and projections from Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) to 2100 under RCP8.5/SSP5-8.5 and RCP2.6/SSP1-2.6 scenarios (thin lines from [[#Goelzer--2020|Goelzer et al. (2020)]] ; [[#Edwards--2021|Edwards et al. (2021)]] ; [[#Payne--2021|Payne et al. (2021)]] ) and ISMIP6 emulator under SSP5-8.5 and SSP1-2.6 to 2100 (shades and bold line; [[#Edwards--2021|Edwards et al., 2021]] ); (b, right) 17th – 83rd and 5th – 95th percentile ranges for ISMIP6 and ISMIP6 emulator at 2100. Schematic interpretations of individual reconstructions ( [[#Lecavalier--2014|Lecavalier et al., 2014]] ; [[#Goelzer--2016|Goelzer et al., 2016]] ; [[#Berends--2019|Berends et al., 2019]] ) of the spatial extent of the Greenland Ice Sheet are shown for the: '''(c)''' mid-Pliocene Warm Period; '''(d)''' the Last Interglacial; and '''(e)''' the Last Glacial Maximum: grey shading shows extent of grounded ice. Maps of mean elevation changes '''(f)''' 2010–2017 derived from CryoSat 2 radar altimetry ( [[#Bamber--2018b|Bamber et al., 2018b]] ) and '''(g)''' ISMIP6 model mean (2093–2100) projected changes for the MIROC5 climate model under the RCP8.5 scenario ( [[#Goelzer--2020|Goelzer et al., 2020]] ). Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

The SROCC assessed with ''high confidence'' that surface mass balance (SMB),rather than discharge, has started to dominate the mass loss of the Greenland Ice Sheet (due to increased surface melting and runoff), increasing from 42% of the total mass loss for 2000–2005 to 68% for 2009–2012. While these estimates have been confirmed since SROCC ( [[#Mouginot--2019|Mouginot et al., 2019]] ), the new longer record, as well as further comprehensive studies ( [[#Khan--2015|Khan et al., 2015]] ; [[#Colgan--2019|Colgan et al., 2019]] ; [[#Mottram--2019|Mottram et al., 2019]] ; [[#The%20IMBIE%20Team--2020|The IMBIE Team, 2020]] ) and detailed discharge records ( [[#King--2020|King et al., 2020]] ; [[#Mankoff--2020|Mankoff et al., 2020]] ) reveal a more complex picture than the continuous trajectory this statement may have implied. Discharge was relatively constant from 1972–1999, varying by around 6% for the whole ice sheet, while SMB varied by a factor of over two interannually, leading to either mass gain or loss in a given year (Figure 9.16). During 2000–2005, the rate of discharge increased by 18%, then remained fairly constant again (increasing by 6% from 2006–2018). After 2000, SMB decreased more rapidly than discharge increased. In summary, the consistent temporal pattern in these longer datasets leads to ''high confidence'' that the Greenland Ice Sheet mass losses are increasingly dominated by SMB, but there is ''high confidence'' that mass loss varies strongly, due to large interannual variability in SMB.

On a regional scale, the surface elevation is lowering in all regions, and widespread terminus and calving front retreats have been observed (with no glaciers advancing; [[#Mottram--2019|Mottram et al., 2019]] ; [[#Moon--2020|Moon et al., 2020]] ). The largest mass losses have occurred along the west coast and in south-east Greenland (Figure 9.16), concentrated at a few major outlet glaciers ( [[#Mouginot--2019|Mouginot et al., 2019]] ; [[#Khan--2020|Khan et al., 2020]] ). This regional pattern is consistent with independent Global Navigation Satellite System (GNSS) observations from the Greenland Global Positioning System (GPS) network which show elastic bedrock uplift of tens of centimetres between 2007–2019 as a result of ongoing ice mass loss ( [[#Bevis--2019|Bevis et al., 2019]] ). The regional time series (Figures 9.16; Atlas.30) show that SMB has been gradually decreasing in all regions, while the increase in discharge in the south-east, central east, north-west and central west has been linked to retreating tidewater glaciers (Figure 9.16). In summary, the detailed regional records show an increase in mass loss in all regions after the 1980s, caused by both increases in discharge and decreases in SMB ( ''high confidence'' ), although the timing and patterns vary between regions. The largest mass loss occurred in the north-west and the south-east of Greenland ( ''high confidence'' ).

The SROCC stated with ''high confidence'' that variability in large-scale atmospheric circulation is an important driver of short-term SMB changes for the Greenland Ice Sheet. This effect of atmospheric circulation variability on both precipitation and melt rates (and SROCC assessment) is confirmed by more recent publications ( [[#Välisuo--2018|Välisuo et al., 2018]] ; [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|B. Zhang et al., 2019]] ; [[#Velicogna--2020|Velicogna et al., 2020]] ). The strong mass loss in 2019 ( [[#Cullather--2020|Cullather et al., 2020]] ; [[#Hanna--2020|Hanna et al., 2020]] ; [[#Tedesco--2020|Tedesco and Fettweis, 2020]] ) was driven by highly anomalous atmospheric circulation patterns, both on daily ( [[#Cullather--2020|Cullather et al., 2020]] ) and seasonal time scales ( [[#Tedesco--2020|Tedesco and Fettweis, 2020]] ). Although surface melt is anticorrelated with the summer North Atlantic Oscillation Index ( [[#Välisuo--2018|Välisuo et al., 2018]] ; [[#Ruan--2019|Ruan et al., 2019]] ; [[#Sherman--2020|Sherman et al., 2020]] ), especially in West Greenland ( [[#Bevis--2019|Bevis et al., 2019]] ), Greenland Ice Sheet melt is more strongly correlated with the Greenland Blocking Index ( [[#Hanna--2016|Hanna et al., 2016]] , 2018) than with the summer North Atlantic Oscillation index ( [[#Huai--2020|Huai et al., 2020]] ).

The SROCC did not assess the role of cloud changes in detail. Studies since AR5 have shown that higher incident shortwave radiation in conjunction with reduced cloud cover leads to increased melt rates, particularly over the low-albedo ablation zone in the southern part of the Greenland Ice Sheet ( [[#Hofer--2017|Hofer et al., 2017]] ; [[#Niwano--2019|Niwano et al., 2019]] ; [[#Ruan--2019|Ruan et al., 2019]] ). Conversely, an increase in cloud cover over the high-albedo central parts of the ice sheet, leading to higher downwelling longwave radiation, was shown to lead either to increased melt ( [[#Bennartz--2013|Bennartz et al., 2013]] ) or reduced refreezing of meltwater ( [[#van%20Tricht--2016|van Tricht et al., 2016]] ). The elevation dependence of the cloud radiative effect and its control on surface meltwater generation and refreezing (W. [[#Wang--2019|]] [[#Wang--2019|]] [[#Wang--2019|Wang et al., 2019]] ; [[#Hahn--2020|Hahn et al., 2020]] ) can induce a spatially consistent response of the integrated Greenland Ice Sheet melt to dominant patterns of cloud and atmospheric variability. The shortwave and longwave radiation effects on surface melt by clouds have been shown to compensate for each other during strong atmospheric river events, and the increase in melt is caused by increased sensible heat fluxes during such events ( [[#Mattingly--2020|Mattingly et al., 2020]] ). In summary, there is ''medium confidence'' that cloud cover changes are an important driver of the increasing melt rates in the southern and western part of the Greenland Ice Sheet.

The SROCC stated with ''high confidence'' that positive albedo feedbacks contributed substantially to the post-1990s Greenland Ice Sheet melt increase. Several (mostly positive) feedbacks involving surface albedo operate on ice sheets (e.g., [[#Fyke--2018|Fyke et al., 2018]] ). Melt amplification by the observed increase of bare ice exposure through snowline migration to higher parts of the ice sheet since 2000 ( [[#Shimada--2016|Shimada et al., 2016]] ; [[#Ryan--2019|Ryan et al., 2019]] ) was five times stronger than the effect of hydrological and biological processes that lead to reduced bare ice albedo ( [[#Ryan--2019|Ryan et al., 2019]] ). Impurities, in part biologically active ( [[#Ryan--2018|Ryan et al., 2018]] ), have been observed to lead to albedo reduction ( [[#Stibal--2017|Stibal et al., 2017]] ) and are estimated to have increased runoff from bare ice in the southwestern sector of the Greenland Ice Sheet by about 10% ( [[#Cook--2020|Cook et al., 2020]] ). In summary, new studies confirm that there is ''high confidence'' that the Greenland Ice Sheet melt increase since about 2000 has been amplified by positive albedo feedbacks, with the expansion of bare ice extent being the dominant factor, and albedo in the bare ice zone being primarily controlled by distributed biologically active impurities (see also [[IPCC:Wg1:Chapter:Chapter-7#7.3.4.3|Section 7.3.4.3]] ).

The SROCC reported with ''medium confidence'' that around half of the 1960–2014 Greenland Ice Sheet surface meltwater ran off, while most of the remainder infiltrated firn and snow, where it either refroze or accumulated in firn aquifers. Studies since SROCC show a decrease of firn air content between 1998–2008 and 2010–2017 ( [[#Vandecrux--2019|Vandecrux et al., 2019]] ) in the low-accumulation percolation area of western Greenland, reducing meltwater retention capacity. Moreover, meltwater infiltration into firn can be strongly limited by low-permeability ice slabs created by refreezing of infiltrated meltwater ( [[#Machguth--2016|Machguth et al., 2016]] ). Recent observations and modelling efforts indicate that rapidly expanding low-permeability layers have led to an increase in runoff area since 2001 ( [[#MacFerrin--2019|MacFerrin et al., 2019]] ). In summary, there is ''medium confidence'' that meltwater storage and refreezing can temporarily buffer a large-scale melt increase, but limiting factors have been identified.

The SROCC reported that there was ''medium confidence'' that ocean temperatures near the grounding zone of tidewater glaciers are critically important to their calving rate, but there was ''low confidence'' in understanding their response to ocean forcing. The increase in ice discharge in the late 1990s and early 2000s ( [[#Mouginot--2019|Mouginot et al., 2019]] ; [[#King--2020|King et al., 2020]] ; [[#Mankoff--2020|Mankoff et al., 2020]] ) has been associated with a period of widespread tidewater glacier retreat ( [[#Murray--2015|Murray et al., 2015]] ; [[#Wood--2021|Wood et al., 2021]] ) and speed up ( [[#Moon--2020|Moon et al., 2020]] ). Since SROCC, new studies provide strong evidence for rapid submarine melting at tidewater glaciers ( [[#Sutherland--2019|Sutherland et al., 2019]] ; [[#Wagner--2019|Wagner et al., 2019]] ; [[#Bunce--2020|Bunce et al., 2020]] ; R.H. [[#Jackson--2020|]] [[#Jackson--2020|Jackson et al., 2020]] ). Changes in submarine melting and subglacial meltwater discharge can trigger increased ice discharge by reducing the buttressing to ice flow and promoting calving ( [[#Benn--2017|Benn et al., 2017]] ; [[#Todd--2018|Todd et al., 2018]] ; [[#Ma--2019|Ma and Bassis, 2019]] ; [[#Mercenier--2020|Mercenier et al., 2020]] ); through undercutting ( [[#Rignot--2015|Rignot et al., 2015]] ; [[#Slater--2017|]] [[#Slater--2017|D.A. Slater et al., 2017]] ; [[#Wood--2018|Wood et al., 2018]] ; [[#Fried--2019|Fried et al., 2019]] ) and frontal incision ( [[#Cowton--2019|Cowton et al., 2019]] ). Warming ocean waters have been implicated in the recent thinning and breakup of floating ice tongues in north-eastern and north-western Greenland ( [[#Mouginot--2015|Mouginot et al., 2015]] ; [[#Wilson--2017|Wilson et al., 2017]] ; [[#Mayer--2018|Mayer et al., 2018]] ; [[#Washam--2018|Washam et al., 2018]] ; [[#An--2021|An et al., 2021]] ; [[#Wood--2021|Wood et al., 2021]] ). On decadal time scales, tidewater glacier terminus position correlates with submarine melting ( [[#Slater--2019|Slater et al., 2019]] ). Over shorter time scales, individual glaciers or clusters of glaciers can behave differently and asynchronously ( [[#Bunce--2018|Bunce et al., 2018]] ; [[#Vijay--2019|Vijay et al., 2019]] ; [[#An--2021|An et al., 2021]] ), and there are not always clear associations between water temperature and glacier calving rates ( [[#Motyka--2017|Motyka et al., 2017]] ), retreat or speed-up ( [[#Joughin--2020|Joughin et al., 2020]] ; [[#Solgaard--2020|Solgaard et al., 2020]] ). Variations in ice mélange at the front of a glacier, associated with changes in ocean and air temperature, have also emerged as a plausible control on calving ( [[#Burton--2018|Burton et al., 2018]] ; [[#Xie--2019|Xie et al., 2019]] ; [[#Joughin--2020|Joughin et al., 2020]] ). In summary, there is ''high confidence'' that warmer ocean waters and increased subglacial discharge of surface melt at the margins of marine-terminating glaciers increase submarine melt, which leads to increased ice discharge. There is ''medium confidence'' that this contributed to the increased rate of mass loss from Greenland, particularly in the period 2000–2010 when increased discharge was observed in the south-east and north-west.

The SROCC reported that accurate bedrock topography is required for understanding and projecting the glacier response to ocean forcing. Accurate bathymetry is essential for establishing which water masses enter glacial fjords, and for reliable estimates of the submarine melt rates experienced by tidewater glaciers ( [[#Schaffer--2020|Schaffer et al., 2020]] ; T. [[#Slater--2020|]] [[#Slater--2020|Slater et al., 2020]] ; [[#Wood--2021|Wood et al., 2021]] ). Subglacial and lateral topography is known to strongly modulate tidewater glacier dynamics and the sensitivity of tidewater glaciers to climatic forcing ( [[#Enderlin--2013|Enderlin et al., 2013]] ; [[#Catania--2018|Catania et al., 2018]] ). Bathymetric mapping around the ice sheet has greatly improved with direct and gravimetric surveys ( [[#Millan--2018|Millan et al., 2018]] ; [[#An--2019a|An et al., 2019a]] , b; [[#Jakobsson--2020|Jakobsson et al., 2020]] ) leading to the improvement of Greenland-wide bathymetric and topographic mapping (e.g., [[#Morlighem--2017|Morlighem et al., 2017]] ). However, large uncertainties in ice thickness remain for around half of the outlet glaciers ( [[#Mouginot--2019|Mouginot et al., 2019]] ; [[#Wood--2021|Wood et al., 2021]] ) and sea ice covered and iceberg-packed regions remain poorly sampled near glacier termini ( [[#Morlighem--2017|Morlighem et al., 2017]] ). There is ''high confidence'' that bathymetry (governing the water masses that flow into fjord cavities) and fjord geometry and bedrock topography (controlling ice dynamics) modulate the response of individual glaciers to climate forcing.

The AR5 assessed that it is ''likely'' that anthropogenic forcing has contributed to the surface melting of Greenland since 1993 ( [[#Bindoff--2013|Bindoff et al., 2013]] ). [[IPCC:Wg1:Chapter:Chapter-3#3.4.3.2|Section 3.4.3.2]] assesses that it is ''very likely'' that human influence has contributed to the observed surface melting of the Greenland Ice Sheet over the past two decades. There is ''medium confidence'' of an anthropogenic contribution to recent mass loss from Greenland.

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