geosciences

Article Ice-Cliff Morphometry in Identifying the Surge Phenomenon of Tidewater (, )

Joanna Ewa Szafraniec

Faculty of Natural Sciences, Institute of Earth Sciences, University of Silesia in Katowice, B˛edzi´nska60, 41-200 Sosnowiec, Poland; [email protected]; Tel.: +48-32-3689-687

 Received: 27 May 2020; Accepted: 18 August 2020; Published: 20 August 2020 

Abstract: In this study, 110 tidewater glaciers from Spitsbergen were studied to characterize the frontal zone using morphometric indicators. In addition, their time variability was also determined based on features of the active phase of surges. Landsat satellite imagery and topographic maps were used for digitalization of the ice-cliffs line. In recent years (2014–2017) all the glaciers studied can be thus classified as: stagnant (33%), retreating and deeply recessing (33%), starting to move forward/fulfilling the frontal zone (23%), and surging (11%). Indicators of the glacier frontal zone (CfD and CfE) allow to identify the beginning and the end of the active phase through changes in their values by ca. 0.05–0.06 by the year and get even bigger for large glaciers as opposed to typical interannual differences within the limits of 0.01 to 0.02. The active phase lasted an average ± of 6–10 years. The presence of a “glacier buttress system” and the “pointed arch” structure of the ice-cliff seem to be an important factor regulating the tidewater glacier stability.

Keywords: ice-cliff morphometry; “glacier buttress system”; glacier surge; Spitsbergen; tidewater glaciers

1. Introduction Spitsbergen glaciers are in rapid recession. This is observed in both land-based glaciers [1–5] and those terminating at sea [6–8] and is regarded as a manifestation of the Arctic amplification effect, whereby changes in the net radiation balance tend to produce a larger increase in temperature near the pole than the planetary average [9,10]. This process contributes to the expansion of low-albedo, ice-free sea surfaces with positive feedback (e.g., [11–13]). In Spitsbergen (Svalbard Archipelago), surging glaciers represent a significant proportion of the glacier cover [6,14–22]. They operate within the so-called “climate envelope” [23], i.e., in optimal thermal and humidity conditions. Their behavior is mainly driven through snow mass supply in the glacier reservoir area (e.g., [24,25]) and glacier enthalpy disorder, mainly at the ice-cliff area [22,26]. The surging tidewater glaciers behave differently than the land-based glaciers. On land, the increased stress mainly propagates through the glacier surface from the accumulation area to the zone where the maximum ice flow velocities are also measured. This sometimes manifests as an ice bulge [19,27,28]. For tidewater glaciers, on the other hand, the properties of the active phase are often more obvious at the glacier ice-cliff. This phenomenon tends to be seen as an expanding crevassed zone [22], as a result of the steep slope on the glacier surface propagating due to ablation of its lowermost part—the frontal zone. These features have been observed in Aavatsmarkbreen, for example. Other glaciers also show a characteristic lowering of the frontal zone due to ablation, and consequently intensive calving and retreat into deeper water from the pinning point. This is, for example, the case of Wahlenbergbreen, Mendeleevbreen, or Paierlbreenn [6]. Meltwater and precipitation entering the glacier also enhances propagation towards the top of the glacier [22,29,30]. The presence of in the frontal

Geosciences 2020, 10, 328; doi:10.3390/geosciences10090328 www.mdpi.com/journal/geosciences Geosciences 2020, 10, 328 2 of 21 zone is important for determining tidewater glacier dynamics. In particular, crevasse formation increases the susceptibility of the glacier to mechanical ablation by calving [6,7,20,31,32], facilitates the distribution of meltwater through en- and subglacial drainage systems [22,33–35], particularly at low inclinations, crevasse allows penetration of direct radiation into deeper parts (also through multiple reflection of radiation from the crevasse ice walls [36]) and facilitates ice-atmosphere and ice-ground/sea thermal conductivity. The length of the ice-cliff [20] also influences thermal conductivity processes at the common ice–water–atmosphere boundary. As such, the ice-cliff wall also represents an additional heat and energy exchange zone. The glacier behavior is also influenced by the depth of the into which the glacier flows [37–39]. Thus, glaciers terminating in deeper parts of are more susceptible to form a dense network of crevasses, further supplying heat to the underwater sections of the ice-cliff. This effect is particularly pronounced in areas where warmer West Spitsbergen Current waters penetrate deeply into the fjord [40]. Regardless of the mechanisms determining the surging behaviour, a common sign of the active phase from all surging glaciers is the quick and constant change in geometry of the glacier frontal zone. If the ice-cliff bent towards the sea (convex), it is indicative of an advancing phase, whilst a bending towards the land (concave) is a sign of glacier retreat through intensive calving. This geometry is indeed representative of its dynamic state. The purpose of this study is to analyze Spitsbergen tidewater glaciers using selected morphometric parameters of the frontal zone. This study combines data from 110 tidewater glaciers, including surging glaciers. Here, data from 1936 to 2017, both from recessive and advancing fronts is analyzed. In particular, the study focuses on annual geometrical changes of glaciers surging in the 1985–2017 period. Frontal morphometry was thus assessed as potential indicator to determine the initiation and termination of the active phase of the glacier surge. These data was used to forecast the initiation of active phases in 2017 and later, based on frontal morphology data from the 2014–2017 period. The results suggest that the “glacier buttresses system” of the frontal zone anchored on land plays a potential role on ice-cliff stabilization.

2. Materials and Methods Landsat 7 Enhanced Thematic Mapper Plus (ETM+) satellite images with 30 m 30 m resolution × (543 bands) were collected from the summer seasons in year 2000 supplemented with a few in 2002. Landsat 8 Operational Land Imager/Thermal InfraRed Sensor (OLI/TIRS) imagery (654 bands) were collected for the 2014, 2017, 2018, and 2019 seasons (Table S1: Emblems of topographic maps and IDs and acquisition dates of Landsat satellite images used in the research). These images were downloaded using the EarthExplorer browser [41]. They were published by the United States Geological Survey (USGS) and the National Aeronautics and Space Administration (NASA) in the public domain. The selected data cover the ablation season. Topographic maps at 1:100,000 scale published by Norwegian Polar Institute (old and new editions [42]) were used to define the ice-cliff edges for the years 1936, for the 1960s and 1970s, and partly for the 1990s. The maps from the older edition were georeferenced to the UTM 33 coordinate system using the ED50 ellipsoid (EPSG:23033) and then transformed into ETRS89 (EPSG:25833). Older Landsat 5 Thematic Mapper (TM) (543 bands) images from the period 1985–1998 and data described earlier were used to track yearly changes in the morphometry of 15 selected glaciers (1985–2017) (Table S2. IDs and acquisition dates of Landsat satellite images for selected tidewater glaciers in Spitsbergen in the period 1985–2019). For a few cases Terra ASTER © NASA satellite scenes were used. Collected satellite images were also used to digitize the surface area of glaciers on available imagery in the year (max. 5 years) before the occurrence of the active phase of the glacier surge (see Table S2—data pointed by red color of font). The vector layers (polygons) were used to calculate the basic morphometric parameters of a given glacier in order to examine the relationship between them and morphometry of the frontal zone. The average glacier slope is determined as the ratio between the elevation difference (maximum and minimum altitudes of the glacier found using the TopoSvalbard portal [43]) and the length of the glacier measured along the Geosciences 2020, 10, 328 3 of 21 Geosciences 2020, 10, x FOR PEER REVIEW 3 of 21 centralmeasured line. along The the compactness central line. coe Theffi cientcompactness was calculated coefficient as thewas ratio calculated of the as glacier the ratio perimeter of the glacier to the circleperimeter perimeter to the ofcircle area perimeter equal to the of area glacier equal area. to the glacier area. The datadata were were analyzed analyzed using using QGIS QGIS software software (different (different versions). versions). The ice-cli Theff line ice-cliff was delineated line was (vectordelineated layers) (vector manually layers) inmanually a 1:20,000 in a scale. 1:20,000 The scale. composition The composition of 543 (654) of 543 spectral (654) spectral bands clearlybands highlightsclearly highlights the glacier the glacier ice area ice (blue), area (blue), the non-glaciated the non-glaciated area on area land on (brown,land (brown, red) and red) sea and water sea water area (black,area (black, dark dark navy navy blue). blue). For maps,For maps, the glacierthe glacier front front delineated delineated in the in originalthe original study study was was used used as the as clitheff cliffline. line. The length of the ice-cliff ice-cliff line may vary between seasons, particularly depending onon whether the image was takentaken atat thethe beginningbeginning oror thethe endend ofof thethe ablation season. Uniform quality images from the whole ofof SpitsbergenSpitsbergen are are not not available available from from the the same sa day,me day, due todue variable to variable cloud cloud cover, cover, different different glacier exposureglacier exposure (shading), (shading), or other or variables other variables such as thesuch presence as the ofpresence pack ice. of Thepack glacier ice. The range glacier defined range in thedefined maps in used the weremaps also used based were on also aerial based photographs on aerial and photographs satellite images and takensatellite on diimagesfferent taken days foron thedifferent same days reasons. for the same reasons. This study assumes that the glacier frontal zone reactsreacts clearest and fastest to changes in glacier dynamics than any other glacier sections. The following definition definition of the frontal zone (Figure1 1)) waswas used here as standard: the “frontal zone Ag” of tidewater glaciers is defineddefined as the section contained within thethe AcAc circlecirclewith with a a diameter diameterDc Dcequal equal to to the the distance distance between between the the parts parts of of the the glacier glacier ice-cli ice-cliffff Lc anchoredLc anchored on on land. land.

Figure 1. Frontal zone Ag of Spitsbergen tidewater glaciersglaciers on the example of Nigerbreen (based on the 654654 spectralspectral bandsbands compositioncomposition of the Landsat 8 satellite imagery ID: LC82150032015238LGN01, 26.08.2015, USGSUSGS/NASA/NASA LandsatLandsat [[41].41].

Following the definition of the glacier frontal zone, the morphometric indicators—”glacier front dynamics indicator CfD” and “ice-cliff balance indicator CfE”—were defined. Thus, CfD represents the ratio of glacier frontal zone area Ag to circle area Ac:

Ag CfD = . (1) Ac

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Following the definition of the glacier frontal zone, the morphometric indicators—”glacier front dynamics indicator CfD” and “ice-cliff balance indicator CfE”—were defined. Thus, CfD represents the ratio of glacier frontal zone area Ag to circle area Ac:

Ag C f D = . (1) Geosciences 2020, 10, x FOR PEER REVIEW Ac 4 of 21 AA CfD close toto 11 oror higherhigher represents represents a a convex convex frontal frontal zone zone typical typical of advancingof advancing glaciers glaciers (Figure (Figure2a). 2a).The The highest highest absolute absolute value value of CfD of CfD= 1.11 = 1.11 was was found found for for Negribreen Negribreen in in 1969. 1969. A A value value of ofCfD CfDabove above 1 1(extreme (extreme cases) cases) usually usually means means that that the the frontalfrontal zonezone hashas spilledspilled outout and moved far out into into the the sea, sea, beyondbeyond thethe outletoutlet of of the the valley that that limits limits it, and it, in and consequence in consequence beyond beyo the perimeternd the perimeter of the theoretical of the theoreticalcircle Ac.A circleCfD ofAc approximately. A CfD of approximately 0.5 represents 0.5 arepresents semi-circular a semi-circular frontal zone frontal with azone cliff linewith closely a cliff linefollowing closely the following diameter the of diameter the circle of and the usually circle an presentingd usually verypresenting little variability very little (Figure variability2b). Finally,(Figure 2b).a CfD Finally,close toa CfD or below close to 0 isor indicative below 0 is of indicative a clearly of concave a clearly glacier concave front glacier in deep front recession, in deep whererecession, the whereinflexion the point(s) inflexion of thepoint(s) ice-cli offf theline ice-cliff is often line at a longis often distance at a long from distance the line from between the theline anchor between points the anchor(Figure 2pointsc). A negative (Figure value2c). A appears negative when value the appears greater part when of thethe frontalgreate zoner part is of outside the frontal the circle zone area is outsideAc. Then the this circle “minus” area Ac area. Then needs this to “minus” be subtracted area needs from theto be value subtracted of the frontalfrom the zone value area of lying the frontal inside zonethe circle area (usuallylying inside lateral the part circle anchored (usually on lateral land). part The anchored lowest CfD on= land).1.15 wasThe foundlowest forCfD Samarinbreen = −1.15 was − foundin 1961. for Samarinbreen in 1961.

(a) (b) (c)

FigureFigure 2. ExamplesExamples of of the the shape shape of of th thee frontal zone expressed by CfD and CfE indicators depending onon the dynamic state of a tidewater glacier: ( (aa)) Advancing Advancing glacier; glacier; ( (bb)) Glacier Glacier in in the the momentary momentary equilibriumequilibrium state; and ( c)) Glacier undergoing recession. An An arro arroww points the glacier flow flow direction.

The ice-cliffice-cliff balance balance indicator, indicator,CfE CfE, is, the is ratiothe ratio of the of circle the circle diameter diameter length Dclengthto the Dc ice-cliff to the length ice-cliffLc: length Lc: Dc C f E = Dc . (2) CfE = Lc. (2) Lc TheThe absolute value value (a.v., (a.v., modulus), used to co comparedmpared among different different glaciers, represents the degreedegree of curving of the ice-cliff. ice-cliff. Thus, Thus, an absolute absolute of 1 indicates indicates a straight straight line; that is, is, that that the the length length ofof the ice-cliff ice-cliff Lc is equal to the circlecircle diameterdiameter Dc. This This can can be be achieved achieved by by any any glacier glacier during during the the evolutionevolution between the concave and convex shape of it itss frontal frontal zone zone (and (and vice vice versa). versa). Ice-cliff Ice-cliff bendingbending levellevel increases increases as as the the absolute values values approach approach 0, 0, with with the the number of of inflexion inflexion points increasing concomitantly.concomitantly. TheThe greatestgreatest curvaturecurvature of of the the ice-cli ice-cliffff was was found found for for Sefströmbreen Sefströmbreen in1990 in 1990 (|CfE (|| =CfE0.21).| = The symbols + and are used to distinguish between convex (+) and concave ( ) glaciers. A negative 0.21). The symbols −+ and − are used to distinguish between convex (+) and concave− (−) glaciers. A negativesign is when sign mostis when of the most measured of the measured ice-cliff length ice-cliffLc islength located Lc betweenis located the between circle diameter the circleDc diameterand the Dcpart and of thethe frontal part of zone the closerfrontal to zone theglacier closer body.to the In glacier turn, whenbody. most In turn, of the when cliff linemost exceeds of the thecliff circle line exceedsdiameter theDc circletowards diameter the sea, Dc the towards sign for theCfE sea,is the positive sign for (cf. CfE Figure is positive2). (cf. Figure 2).

3. Results

3.1. Geometric Parameters of the Glacier Ice-Cliffs in Spitsbergen (1936–2017) Ice geometry data from Spitsbergen glaciers (Table 1) were compiled for the year 1936 (28 glaciers; no data available from the NW and N parts of Spitsbergen), the period 1961–1966 (35 glaciers; no data available from the N and NE parts of Spitsbergen), 1969–1977 (17 glaciers; no data available from S Spitsbergen), 1990 (73 glaciers), 2000, 2014, and 2017 (110 glaciers each year). The different periods include different combinations of glacier systems (their complexity). For example, Strongbreen was configured as one glacier system in 1936 while it is currently shaped as several

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3. Results

3.1. Geometric Parameters of the Glacier Ice-Cliffs in Spitsbergen (1936–2017) Ice geometry data from Spitsbergen glaciers (Table1) were compiled for the year 1936 (28 glaciers; no data available from the NW and N parts of Spitsbergen), the period 1961–1966 (35 glaciers; no data available from the N and NE parts of Spitsbergen), 1969–1977 (17 glaciers; no data available from S Spitsbergen), 1990 (73 glaciers), 2000, 2014, and 2017 (110 glaciers each year). The different periods include different combinations of glacier systems (their complexity). For example, Strongbreen was configured as one glacier system in 1936 while it is currently shaped as several separate land-terminated (e.g., Karlbreen) and tidewater glaciers (e.g., the Kvalbreen–Indrebøbreen system, Moršnevbreen, the Sokkelbreen–Naglebren–Nuddbreen system Strongbreen, the Vindeggbreen–Persejbreen system), ⇒ as a result of recession. Due to the asymmetrical distribution of morphometric parameters of the glaciers in terms of their area and their frontal zones, the calculation of average values for the glaciers population from each period took into account the median and interquartile range (IQR) values. Glaciers between 19.5 km2 and 110.4 km2 analyzed here (within the IQR; based on data by Błaszczyk et al. [20]) can be considered as typical in terms of surface area.

Table 1. Median values of morphometric parameters of glacier frontal zones of Spitsbergen tidewater glaciers (the interquartile range IQR is given in brackets).

Lc Dc Ag Ac CfE Year CfD [km] [km] [km2] [km2] (a.v.) 1936 4.24 3.35 3.62 8.82 0.40 0.79 (28 glaciers) (3.43–5.74) (2.31–4.10) (1.01–6.24) (4.18–13.26) (0.35–0.45) (0.71–0.87) 1960–1966 2.47 1.85 0.91 2.67 0.37 0.75 (35) (1.93–4.71) (1.43–3.05) (0.43–1.92) (1.60–7.24) (0.30–0.43) (0.63–0.81) 1969–1977 5.58 3.03 2.71 7.21 0.38 0.61 (17) (4.38–6.61) (2.39–3.43) (–0.87–4.57) (4.47–9.19) (–0.20–0.48) (0.40–0.81) 1990 3.16 2.24 0.93 3.93 0.30 0.60 (73) (2.28–5.09) (1.44–3.12) (0.11–2.46) (1.62–7.69) (0.16–0.40) (0.30–0.80) 2000 2.71 1.71 0.74 2.30 0.39 0.79 (110) (1.17–3.73) (1.00–2.66) (0.20–1.92) (0.79–5.55) (0.27–0.47) (0.66–0.87) 2014 2.78 1.72 0.59 2.34 0.30 0.71 (110) (1.39–4.38) (1.00–2.89) (0.19–1.89) (0.78–6.59) (0.18–0.40) (0.57–0.79) 2017 2.61 1.86 0.69 2.70 0.29 0.70 (110) (1.46–4.23) (1.04–3.01) (0.08–1.95) (0.85–7.12) (0.13–0.44) (0.57–0.80)

For the following parameters—Lc, Dc, Ag, and Ac—the results show that the highest median appeared in 1936. The values subsequently decreased until the year 2000. Thus, the median ice-cliff length (Lc) decrease by 1.6-fold, the median valley width at the glacier front anchoring points (Dc) showed a 2-fold decrease, the frontal zone area (Ag) decreased by 4.9-fold, and the median value of area of the circle surrounding the glacier frontal zone (Ac) decreased by 3.8-fold. However, the period 1969–1977, when the values were slightly higher, was an exception. In addition, an increase in Lc was observed between 2000 and 2014 followed by a 1.1-fold decrease. Ag, on the other hand, showed a 1.3-fold decrease first followed by a 1.2-fold increase measured in 2017. The CfD and CfE indicators also show a general downward trend, suggesting that ice-cliffs are presenting an increasingly recessive nature. The exception was the year 2000, when these indicators increased. If the interaction between CfD and CfE for each glacier is considered, then most of the glaciers were in a state of recession throughout all the periods discussed here (negative CfE values and CfD values below 0.5) (Figure3). However, there is always a group of advancing glaciers (or shortly after the active phase of the glacier surge), representing 10.5% (1990) to 18.2% (2000) of the population of Geosciences 2020, 10, 328 6 of 21

tidewater glaciers. These glaciers are characterized by a positive CfE value and a CfD value often above

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(a) (b) (c)

(d) (e) (f)

FigureFigure 3. RelationshipsRelationships between between the the glacier glacier front front dynamics dynamics indicator indicator CfDCfD andand the the ice-cliff ice-cliff balancebalance indicatorindicator CfECfE forfor Spitsbergen Spitsbergen tidewater tidewater glaciers glaciers in in selected selected years years of of the the 1936–2017 1936–2017 period: period: ( (aa)) 1936; 1936; ( (bb)) 1961–1977;1961–1977; ( cc)) 1990; 1990; ( (d)) 2000; 2000; ( (e)) 2014; 2014; and ( f) 2017.

3.2. Frontal Zone Morphometry of the Retreating Glaciers 3.2. Frontal Zone Morphometry of the Retreating Glaciers The values of morphometric indicators of the glacier frontal zones during recession phase were The values of morphometric indicators of the glacier frontal zones during recession phase were plotted against the mean annual air temperature (MAAT) measured at the Svalbard Airport for each plotted against the mean annual air temperature (MAAT) measured at the Svalbard Airport for each of the different periods considered (Figure4). Glaciers showed a less intense recession following the of the different periods considered (Figure 4). Glaciers showed a less intense recession following the minimum temperatures recorded at the beginning of the 20th century, presenting the highest median minimum temperatures recorded at the beginning of the 20th century, presenting the highest median CfD and CfE (specifically, CfD = 0.39 and CfE = 0.81 a.v. in 1936). In contrast, the same glaciers showed CfD and CfE (specifically, CfD = 0.39 and CfE = 0.81 a.v. in 1936). In contrast, the same glaciers showed their strongest recession episode over 30–40 years later, where the median values for both indicators their strongest recession episode over 30–40 years later, where the median values for both indicators dropped to CfD = 0.24 and CfE = 0.63 a.v., (median from 1969–1977). Until the end of the 20th century, dropped to CfD = 0.24 and CfE = 0.63 a.v., (median from 1969–1977). Until the end of the 20th century, the median values increased reaching CfD = 0.36 and CfE = 0.78 a.v. in 2000. The recession process the median values increased reaching CfD = 0.36 and CfE = 0.78 a.v. in 2000. The recession process further intensified over the first 17 years of the 21st century, accompanied by a decrease in CfD and further intensified over the first 17 years of the 21st century, accompanied by a decrease in CfD and CfE, down to 0.26 and 0.69 a.v., respectively, in 2017. Both values were close to the levels observed in CfE, down to 0.26 and 0.69 a.v., respectively, in 2017. Both values were close to the levels observed in the 1990s, which was followed by the largest number of surging tidewater glaciers in Spitsbergen. the 1990s, which was followed by the largest number of surging tidewater glaciers in Spitsbergen.

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Figure 4. MedianFigure 4. Median values values of the of CfDthe CfDand andCfE CfEindicators indicators for for the the selected selected years yearsof the 1936–2017 of the 1936–2017 period period underFigure theunder background 4. Median the background values of the of mean ofthe the CfD annualmean and annual CfE air indicators temperature air temperature for the at atthe selected the Svalbard Svalbard years Airport of the 1936–2017(temperature (temperature period values values based on data made available by the Norwegian Meteorological Institute [44]. basedunder on data the madebackground available of the by mean the Norwegian annual air Meteorologicaltemperature at the Institute Svalbard [44 Airport]. (temperature values based on data made available by the Norwegian Meteorological Institute [44]. In relation to ice-cliff exposure in 1990, south-east and north-west facing glaciers showed the In relationstrongest to recessive ice-cliff natureexposure (Figure in5). These 1990, glaciers south-east showed and a median north-west CfD of 0.11 facing and a relatively glaciers low showed the In relation to ice-cliff exposure in 1990, south-east and north-west facing glaciers showed the strongest recessiveCfE (a.v.) compared nature (Figureto the other5). glacier These groups. glaciers showed a median CfD of 0.11 and a relatively low strongest recessive nature (Figure 5). These glaciers showed a median CfD of 0.11 and a relatively low CfE (a.v.) compared to the other glacier groups. CfE (a.v.) compared to the other glacier groups.

(a) (b)

Figure 5. Median values of CfD (a) and CfE (a.v.) (b) indicators relative to the exposition of the Spitsbergen tidewater glaciers during the recession phase in years: 1990, 2000, 2014, and 2017. (a) (b) In the year 2000, a clear growing trend was observed for the dynamically active of most glacier FigureFigure 5. Median 5. Median values values of ofCfD CfD( a(a)) andand CfECfE (a.v.) (b (b) )indicators indicators relative relative to the to theexposition exposition of the of the fronts, apart from those from the southern sector. This was most noticeable for ice-cliffs facing the Spitsbergen tidewater glaciers during the recession phase in years: 1990, 2000, 2014, and 2017. Spitsbergensouth-east tidewater sector, with glaciers CfD duringvalues reaching the recession over 0.44. phase The in ice-cl years:iff lengths 1990, 2000, in relation 2014, to and the 2017. circle diameter also shortened, presenting median CfE (a.v.) of approximately 0.80. In the year 2000, a clear growing trend was observed for the dynamically active of most glacier In the yearMore 2000, recessive a clear stage growing was also trendobserved was for observedthe 2014–2017 for period, the dynamically apparent as a clearly active visible of most glacier fronts,fronts, apartmelting apart from offrom the those mostthose from dynamic from the the part southern southern of the glacier sector.sector. frontal Th This zones,is was was particularly most most noticeable noticeable obvious for for forice-cliffsice-cliffs ice-cli facing facingffs facing the the south-east sector, with CfD values reaching over 0.44. The ice-cliff lengths in relation to the circle south-eastto sector, the southern with sector.CfD Atvalues this stage, reaching ice-cliffs overwithdraw 0.44. considerably The ice-cli withff thelengths CfD reaching in relation its lowest to the circle diameterrecorded also valueshortened, at −0.06. presenting This curving median is also CfEreflected (a.v.) in of the approximately lowest median CfE0.80. reaching 0.46 (a.v.). A diameter also shortened, presenting median CfE (a.v.) of approximately 0.80. Moresignificant recessive decrease stage in wasCfD andalso CfE observed was also for observed the 2014–2017 from north-facing period, apparent glaciers, andas a to clearly a lesser visible meltingMoreextent, recessive of the for most west- stage dynamic and wasnorth part also west-facing of observed the glacier fronts. for frontalThe the fluctuation 2014–2017zones, particularly in CfD period, and CfE obvious apparent was smaller for ice-cliffs as for a clearlyeast- facing visible meltingto the of andsouthern the south most west-facingsector. dynamic At thisglaciers. part stage, of the ice-cliffs glacier withdraw frontal considerably zones, particularly with the obvious CfD reaching for ice-cli its lowestffs facing to therecorded southern value sector. at −0.06. At this This stage, curving ice-cli is alsoffs reflected withdraw in the considerably lowest median with CfE the reachingCfD reaching 0.46 (a.v.). its A lowest 3.3. Frontal Zone Morphometry of the Advancing Glaciers recordedsignificant value decrease at 0.06. in ThisCfD and curving CfE was is also also reflected observed in from the north-facing lowest median glaciers,CfE reachingand to a lesser 0.46 (a.v.). Over −the years 1990–2017, advancing ice-cliffs were found in glaciers from all geographical A significantextent, for decrease west- and in northCfD west-facingand CfE was fronts. also The observed fluctuation from in north-facingCfD and CfE was glaciers, smaller and for toeast- a lesser aspects, and roughly around the same number of cases (up to 5% difference). This pattern was and south west-facing glaciers. extent, forparticularly west- and obvious north west-facingfor all north-, fronts.east-, or Thesouth fluctuation west-facing tidewater in CfD and glaciersCfE fromwas Spitsbergen, smaller for east- and south west-facingwith fewer glaciers.cases found from north-east (8% difference), south, and west. However, the proportion of 3.3. Frontalcases changedZone Morphometry over this period. of the Advancing Thus, in 1990–2000 Glaciers the most active glacier fronts were found

3.3. FrontalOver Zone the Morphometry years 1990–2017, of the advancing Advancing ice-cliffs Glaciers were found in glaciers from all geographical aspects, and roughly around the same number of cases (up to 5% difference). This pattern was Over the years 1990–2017, advancing ice-cliffs were found in glaciers from all geographical aspects, particularly obvious for all north-, east-, or south west-facing tidewater glaciers from Spitsbergen, and roughlywith fewer around cases thefound same from number north-east of cases (8% (updifference), to 5% difference). south, and west. This patternHowever, was the particularly proportion obviousof for allcases north-, changed east-, orover south this west-facing period. Thus, tidewater in 1990–2000 glaciers the from most Spitsbergen, active glacier with fronts fewer caseswere foundfound from north-east (8% difference), south, and west. However, the proportion of cases changed over this period. Thus, in 1990–2000 the most active glacier fronts were found particularly in the N–S axis (e.g., Petermannbreen, Monacobreen, Waggonwaybreen, Paierlbreen, and Storbreen) and with north-east exposure (e.g., Davisbreen, and Inglefieldbreen-Nordsysselbreen). In turn, in 2014–2017, active ice-cliffs dominate in the E–W axis Geosciences 2020, 10, x FOR PEER REVIEW 8 of 21 particularlyGeosciences 2020 in, 10 the, 328 N–S axis (e.g., Petermannbreen, Monacobreen, Waggonwaybreen, Paierlbreen,8 of 21 and Storbreen) and with north-east exposure (e.g., Davisbreen, and Inglefieldbreen- Nordsysselbreen). In turn, in 2014–2017, active ice-cliffs dominate in the E–W axis (e.g., Vindeggbreen-Persejbreen,(e.g., Vindeggbreen-Persejbreen, Sjettebreen, Sjettebreen, Arnesenbreen Arnesenbreen,, and and Strongbreen) Strongbreen) and and from from south south west-facing west-facing glaciersglaciers (e.g., Blomstrandbreen,Blomstrandbreen, Kollerbreen, Kollerbreen, and and Marstrandbreen). Marstrandbreen). AnnualAnnual variations variations in in the the frontal frontal zone zone geometrical geometrical indicators indicators were wereanalyzed analyzed for 15 forglaciers. 15 glaciers. These glaciersThese glaciers experienced experienced an active an phase active of phasethe glacie of ther surge glacier at some surge point at some during point 1985–2017 during (based 1985–2017 on: Murray(based on: et al. Murray [45]; B etłaszczyk al. [45]; et Błaszczyk al. [20]; Sund et al. et [20 al.]; Sund[19]), or/a et al.nd [19 the]), oradvance/and the could advance be directly could beobserved directly fromobserved data fromanalyzed data in analyzed this study inthis (Figure study 6, (FigureTable 2).6, As Table the2 ).glaciers As the advance, glaciers advance, their CfD their valuesCfD changedvalues fromchanged 0.33 from(0.18 0.33to 0.40 (0.18 of IQR) to 0.40 during of IQR) the during quiescent the quiescentphase to 0.61 phase (0.54 to to 0.61 0.71 (0.54 of IQR) to 0.71 during of IQR) the duringactive the active phase. The median CfE changed from 0.72 ( 0.81 to 0.56 of IQR) during the quiescent phase. The median CfE changed from −0.72 (−0.81 −to −0.56− of IQR)− during the quiescent phase to 0.78 (0.69phase to to 0.88 0.78 of (0.69 IQR) to during 0.88 of the IQR) active during phase the of active the glacier phase surge. of the glacier surge.

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

(j) (k) (l)

Figure 6. Cont.

Geosciences 2020, 10, 328 9 of 21 Geosciences 2020, 10, x FOR PEER REVIEW 9 of 21

(m) (n) (o)

FigureFigure 6. 6. CfD CfD(left Y(left axis; Y blue axis; circles) blue and circles) CfE (right and YCfE axis;(right red rhombs) Y axis; values red for rhombs) selected values tidewater for Spitsbergenselected tidewater glaciers, Spitsbergenwhich experi glaciers,enced an whichepisode experienced of the active an phase episode of the of glacier the active surge phasein 1985– of 2017the glacier(X axis–years): surge in ( 1985–2017a) the Nathorstbreen–Zawadzkibreen (X axis–years): (a) the Nathorstbreen–Zawadzkibreen system; (b) Monacobreen; system;(c) the Strongbreen–Moršnev(b) Monacobreen; (c)breen the Strongbreen–Moršnevbreen system; (d) the Osbornebreen–Vin system;tervegen (d) the Osbornebreen–Vintervegensystem; (e) Tunabreen; (f) Wahlenbergbreen;system; (e) Tunabreen; (g) (Paierlbreen;f) Wahlenbergbreen; (h) Blomstrandbreen; (g) Paierlbreen; (i ()h )Richardsbreen; Blomstrandbreen; (j) (thei) Richardsbreen; Persejbreen– Vindeggbreen(j) the Persejbreen–Vindeggbreen system; (k) Esmarkbreen; system; (l ()k Fridtjovbreen;) Esmarkbreen; (m (l)) Mendelejeevbreen; Fridtjovbreen; (m) Mendelejeevbreen;(n) Arnesenbreen; and(n) Arnesenbreen;(o) Chauveauxbreen. and (Ano) arrow Chauveauxbreen. indicates the mo Anment arrow of the indicates glacier movement the moment forward. of the glacier movement forward.

Table 2. The beginning and the end of the active phase of the glacier surge for selected Spitsbergen tidewatertidewater glaciers glaciers based based on on publications publications and morphometric indicators of the glacier frontal zone.

AccordingAccording to theto the Ice-Cli Ice-Cliffff AccordingAccording to Publishedto Published Data Data MorphometryMorphometry Surging Source of Data Source of Data EndEnd (and (and EndEnd (and (and SurgingGlaciers Glaciers (Published) StartStart of theof the Start ofStart the of (Published) Duration—inDuration—in Duration—inDuration—in ActiveActive Activethe Active Brackets)Brackets) of the of the Brackets)Brackets) of theof the PhasePhase PhasePhase ActiveActive Phase Phase ActiveActive Phase Phase NathorstbreenNathorstbreen 2003–stage2003–stage 1 1 20132013 (?) (?) 2014–20152014–2015 SundSund et et al. al. [46 [46]] 2006–20082006–2008 systemsystem 2008–stage2008–stage 2 2 (~5–10(~5–10 years) years) (6–9(6–9 years) years) MurrayMurray et et al. al. [45 [45]] 1990–19921990–1992 19971997 2001–20022001–2002 MonacobreenMonacobreen 1991–19921991–1992 MansellMansell et et al. al. [27 [27]] 1993–19951993–1995 (2–7(2–7 years) years) (9–10(9–10 years) years) DowdeswellDowdeswell et et al. al. [14 [14];]; 1990–19911990–1991 OsbornebreenOsbornebreen 1986–19871986–1987 ?? 1986–19871986–1987 RolstadRolstad et et al. al. [47 [47]] (3–5(3–5 years) years) 2004–2005 Flink et al. [48]; 2002–2003 2004–2005 2006–2010 Tunabreen Flink et al. [48]; 2002–2003 2005 2002–2004 2006–2010 Tunabreen Sevestre et al. [49] 2003 2005 2002–2004 (2–8 years) Sevestre et al. [49] 2003 (1–3 years) (2–8 years) (1–3 years) Wahlenbergbreen Sevestre at al. [22] 2009 – 2012–2013 – Wahlenbergbreen Sevestre at al. [22] 2009 – 2012–2013 – 1999 (?) 2004–2006 Paierlbreen Błaszczyk et al. [6] 1993 (?) 1999 (?) 1990–1992 2004–2006 Paierlbreen Błaszczyk et al. [6] 1993 (?) (6 years?) 1990–1992 (12–16 years) (6 years?) (12–16 years) 2010 (?) Mansell et al. [27]; 2007 2010 (?) 2016–2017 Blomstrandbreen Mansell et al. [27]; 2007 2013 2007–2010 2016–2017 Blomstrandbreen Burton et al. [50] 2009 2013 2007–2010 (6–10 years) Burton et al. [50] 2009 (1–6 years) (6–10 years) (1–6 years) Dowdeswell and 2015–2016 Persejbreen Dowdeswell and 2000–2001 ? 1999–2000 2015–2016 Persejbreen Benham [51] 2000–2001 ? 1999–2000 (5–17 years) Benham [51] (5–17 years) 1994–1995 Murray et al. [45]; 1997 1994–1995and 1998–1999 Fridtjovbreen MurrayMurray et et al. al. [52 [45];]; (2–3 years1997 1995–1996 re-advanceand re- (2–41998–1999 years) Fridtjovbreen MurrayLønne et [53 al.] [52]; or 3–5(2–3 years) years 1995–1996 in 1998–1999advance in (2–4 years) Lønne [53] or 3–5 years) between1998–1999 between 2002–2010 2006–2010 Mendeleevbreen Błaszczyk et al. [6] 1996–1997 1995–2002between (upbetween to 15 years?) 2002–2010 (9–142006–2010 years) Mendeleevbreen Błaszczyk et al. [6] 1996–1997 1995–2002 (up to 15 years?) (9–14 years)

The comparison presented in Table 2 allows assessing how the timing of the initiation and end of the active phase can be determined based on CfD and CfE. This relation differed slightly for the published data: by ±0 to 4 years for the beginning and ±1 to 6 years for the end of the active phase. The average duration of the active phase based on data published for 10 glaciers was approximately 3–10 years, while the CfD and CfE indicators showed an average active phase of 6–10 years. Some of

Geosciences 2020, 10, 328 10 of 21

The comparison presented in Table2 allows assessing how the timing of the initiation and end of the active phase can be determined based on CfD and CfE. This relation differed slightly for the published data: by 0 to 4 years for the beginning and 1 to 6 years for the end of the active phase. ± ± The average duration of the active phase based on data published for 10 glaciers was approximately 3–10 years, while the CfD and CfE indicators showed an average active phase of 6–10 years. Some of the published results, however, appeared in print during the active phase and the glacier could have continued surging afterwards. The increase in CfD and CfE was nearly concomitant to the glacier movement for larger glaciers (e.g., Nathorstbreen, Monacobreen, and Paierlbreen). However, a delay of few years was also observed (e.g., Mendeleevbreen and Wahlenbergbreen). A correlation analysis between the values of both indicators also show a delay of up to five years before the indicator values jump (Table3). The largest significant correlation of indicators at the significance level of 0.05 took place one year and five years before the active phase. In all cases, the active phase ended with an obvious decrease in CfD and CfE; thus, entering a quiescent phase characterized by small interannual changes in these values.

Table 3. The relationship between CfD and CfE up to 5 years before the active phase of the glacier surge expressed as a correlation coefficient.

Year Before Surge 1 2 3 4 5 Coeff. of corr. 0.84 0.73 0.76 0.77 * 0.80 − − − − − * Not statistically significant.

Preliminary analysis indicates that the length of the ice-cliff just before the active phase of the glacier surge is well correlated with the glacier surface slope and the glacier compactness. The gentler the slope of the glacier surface and more complex the glacier in terms of its geometry, the longer its ice-cliff. This is because the glaciers that are most complex in shape are usually large valley glacier systems with numerous tributaries. These studies will be continued.

3.4. Application of CfD and CfE Indicators in the Classification of Spitsbergen Tidewater Glaciers in Terms of Dynamics Interannual changes in indicator values were presented to describe the dynamic state of five Spitsbergen tidewater glaciers with the most complete data sets for the period 1985–2017: Monacobreen (to 2019; N part of Spitsbergen), the Vindeggbreen–Persejbreen system (E part of the island), the Osbornebreen–Vintervegen system and Blomstrandbreen (both in W part of Spitsbergen), and Paierlbreen (S part of the island) (Figure7; see Table S2). The data indicates that most of the time the ice-cliffs are characterized by slight interannual changes in indicator values. The situation changes significantly with the beginning of the active phase of the glacier surge when the increase in values appears as a clear deviation in the I quadrant of the graph (advance), and this is also recorded in Figure6. A similar clear deviation occurs in the III quadrant during the transition of the active phase to the quiescent phase. Then in Figure6 the CfE values go negative (retreat). Geosciences 2020, 10, 328 11 of 21

Geosciences 2020, 10, x FOR PEER REVIEW 11 of 21

Figure 7. FigureInterannual 7. Interannual changes changes in thein the values values ofof CfD andand CfECfE indicatorsindicators for five for selected five selected tidewaters tidewaters glaciers surgingglaciers betweensurging between 1985 1985 and and 2017. 2017.

The analysisThe analysis of the of interannual the interannual variation variation inin the CfDCfD andand CfECfE indicatorsindicators allows allowsdescribing describing the the following dynamic models for tidewater surging glaciers in Spitsbergen: following dynamic models for tidewater surging glaciers in Spitsbergen: 1. At the beginning of “the active phase of the glacier surge” both CfD and CfE indicators increase 1. At the beginningrapidly by 3–5 of “thetimes activeduring approximately phase of the 1–5 glacier years. surge”For larger both glaciers,CfD theand rateCfE of increaseindicators can increase rapidly byreach 3–5 a maximum times during of a 10× approximately increase (I quadrant 1–5 in years. Figures For 7 and larger 8). In this glaciers, phase, the frontal rate ofzone increase can protrudes (convex shape) and the glacier moves forward. Then the ice-cliff position is marked reach a maximum of a 10 increase (I quadrant in Figures7 and8). In this phase, the frontal zone by slight fluctuations× or “stagnates”, and numerous inflexion points associated with intensive protrudescalving (convex appear shape) in the cliff and line. the glacier moves forward. Then the ice-cliff position is marked by slight2. After fluctuations the active orphase, “stagnates”, the glacier first and loses numerous its frontal inflexion zone convexity, points mainly associated as a result with of intensive calving appearintensive in calving, the cli andff line. it is subject to “recession” (including the quiescent phase of the glacier surge). At this point, CfD and CfE values decrease interannually to an average of 0.05–0.06 (for 2. After theCfE active the sign phase, changes the glacierto negative; first III loses quadrant). its frontal zone convexity, mainly as a result of intensive calving,3. andSubsequently, it is subject the glacier to “recession” can enter a (including “deep recession the quiescentphase”, when phase its frontal of the zone glacier is strongly surge). At this point, CfDconcave,and CfEespeciallyvalues for the decrease largest glaciers interannually (IV quadrant). to an“Glacier average buttresses” of 0.05–0.06 are also observed, (for CfE the sign changesi.e., tonegative; parts of the III glacier quadrant). front anchored on land, from which the ice-cliff bends into an arch. During the quiescent phase, CfD and CfE values change very little interannually and are ca. 3. Subsequently,0.015–0.025. the glacierThe value can of CfD enter decreases a “deep the recessionmost (it can phase”,even be negative), when its with frontal the CfE zone value is strongly concave,slowly especially increasing for thefrom largest −1 to approximately glaciers (IV −0.5. quadrant). “Glacier buttresses” are also observed, i.e., parts4. The of theglacier glacier begins front to lose anchored its maximum on land,ice-cliff from concavity which at the end ice-cli of theff bends recession into with an “the arch. During frontal zone filling and the slow forward movement” beginning (II quadrant). At this point, the the quiescent phase, CfD and CfE values change very little interannually and are ca. 0.015–0.025. CfD value slowly increases and CfE value slowly decreases (from −0.5 to −1) until the protrusion The valueof oftheCfD glacierdecreases front and the the mostvalue changes (it can evento plus be sign. negative), with the CfE value slowly increasing from 1 to approximately 0.5. − The presented model was− adopted for Monacobreen for the 1985–2019 period (Table 4). Based 4. Theon glacier the indicators, begins the to beginning lose its of maximum the active phas ice-clie (I) ffof concavitythe glacier surge at thewas enddetermined of the in recession1991– with “the1992. frontal For zonethe first filling 2 years, and the frontal the slow zone was forward getting movement”convex and the beginningglacier advanced (II quadrant).about 1100 At this point,m thein westernCfD value part to slowly about 1500 increases m in eastern and partCfE ofvalue the cliff slowly line compared decreases to 1991. (from Then it0.5 entered to a1) until the − − protrusion of the glacier front and the value changes to plus sign. The presented model was adopted for Monacobreen for the 1985–2019 period (Table4). Based on the indicators, the beginning of the active phase (I) of the glacier surge was determined in 1991–1992. For the first 2 years, the frontal zone was getting convex and the glacier advanced about 1100 m in western part to about 1500 m in eastern part of the cliff line compared to 1991. Then it entered a period of stagnation/minor fluctuations in the position of its ice-cliff, rising slightly in the western part and about 700 m in the eastern part of the cliff. At the end of the active phase (2000–2001), retreat began with the still convex frontal zone. The active phase lasted until 2001–2002 (10 years). After this Geosciences 2020, 10, 328 12 of 21 period, the glacier entered the quiescent phase of the glacier surge (negative CfE). Slow withdrawal of theGeosciences ice-cliff 2020position, 10, x FOR was PEER accompanied REVIEW by ablation by calving and melting (concave shape).12 of 21 By 2007, theperiod glacier of withdrew stagnation/minor from about fluctuations 700 m in in the the position eastern of part its ice-cliff, of the ice-clirisingff slightlyto about in the 800–900 western m in the westernpart and part. about The 700 appearance m in the eastern of inflexion part of the points cliff. At (2005) the end indicates of the active probably phase (2000–2001), an important retreat role of the outflowbegan of with subglacial the still convex waters. frontal After zone. 2008, The the active quiescent phase phase lasted wasuntil marked2001–2002 by (10 some years). recovery After this in glacier behaviour,period, the reflected glacier entered in changes the quiescent in the indicatorsphase of the values glacier fromsurge year(negative to year CfE). up Slow to 2018.withdrawal The glacier retreatedof the ice-cliff by 500–600 position m. Thewas quiescentaccompanied phase by ablation lastedInterannual 17 by years. calving In and 2018–2019, melting (concave a new active shape). phase By (II) of Interannual Indicators InterannualChanges in the2007, glacier the surgeglacierThe began Ice-Cliffwithdrew (positive from aboutIndicatorsCfE), 700 as before,m in theInterannual fromChanges eastern the part in bending of the ofice-cliff the ice-cli to aboutff line 800–900 towards m in the sea Year The Ice-Cliff Indicators IndicatorsInterannualChanges Value in Dynamics theYear western Thepart. Ice-Cliff The appearance Indicators of inflexion pointsIndicatorsInterannualChanges (2005) Value inindicates probablyDynamics an important role of and anYear advance The ofShape Ice-Cliff about 800–900 Indicators m on the centralIndicatorsInterannualChanges line of Value in the glacier. Dynamics theYear outflow Theof subglacialShape Ice-Cliff waters.Indicators After 2008, theIndicatorsCh. quiescentInterannualChanges of Ch.Value phasin of e was markedDynamics by some recovery in Year TheShape Ice-Cliff CfDIndicators CfE IndicatorsCh.InterannualChanges of Ch.Value in of Dynamics Year TheShape Ice-Cliff CfDIndicators CfE IndicatorsCh.CfDInterannualChanges of Ch.Value CfEin of Dynamics glacierYear behaviour,TheShape Ice-Cliff reflected in CfDchangesIndicators CfEin the indiIndicatorsCh.CfDInterannualChangescators of valuesCh. Value CfEin of from year to Dynamicsyear up to 2018. The YearTable4. CharacteristicsThe Ice-Cliff of MonacobreenCfDIndicatorsIndicators CfE dynamics IndicatorsCh.CfDInterannualChanges based of on Ch.Value CfECfDin of and CfE indicatorsDynamics and their interannual glacierYear retreatedTheShape Ice-Cliffby 500–600 m. TheCfDIndicators quiescent CfE phaseIndicatorsCh.CfDInterannualChanges laof sted Ch. 17ValueCfE in years. of In 2018–2019,Dynamics a new active phase Yearchanges inThe 1985–2019.Shape Ice-Cliff CfDIndicators CfE IndicatorsCh.CfDChanges of Ch. Value CfEin of Dynamics Year TheShape Ice-Cliff CfDIndicators CfE IndicatorsCh.CfDChanges of Ch.ValueCfE in of Dynamics (II)Year1985 of the glacierTheShape Ice-Cliff surge began (positive0.367CfD − CfE0.713), as Indicatorsbefore,Ch.CfD? of from Ch.ValueCfE the ? of bending ? of theDynamics ice-cliff ? line towards Year1985 Shape 0.367CfD −0.713CfE IndicatorsCh.CfD? of Ch.ValueCfE ? of ? Dynamics ? the1985Year sea and an advanceShape of about 0.367CfD 800–900 −0.713CfE m on theIndicatorsCh.CfD ?centralInterannual of line Ch.ValueCfE ? Changesof the ? glacier. in Dynamics ? 1985 Shape 0.367CfD Indicators −CfE0.713 Ch.CfD? of Ch.CfE ? of ? ? 1985 0.367CfD −0.713CfE Ch.CfD? of Indicators Ch.CfE ? of Value ? ? Year1985 The Ice-Cliff Shape 0.367CfD −CfE0.713 CfD? CfE ? ? ? Dynamics 1985 0.367 −0.713 CfD? CfE ? ? ? 19861985Table 4. Characteristics of Monacobreen0.4280.367CfD −0.7110.713 CfE dynamics 0.061CfD Ch.? based of CfD 0.002 CfEon ? CfDCh. of?and CfE CfE indicators ? and their 19861985 0.4280.367 −0.7110.713 0.061? 0.002 ? ? ? 19851986interannual changes in 1985–2019. 0.3670.428 −0.7130.711 0.061? 0.002 ? ? ? 19861985 0.4280.367 −0.7110.713 0.061? 0.002 ? ? ? 19851986 0.3670.428 −0.7130.711 0.061? 0.002 ? ? ? 198519861985 0.4280.3670.367 −0.7110.7130.713 0.061? ???? 0.002 ? ? ? 1986 0.428 −0.711− 0.061Interannual 0.002 1986 0.428 −0.711 0.061 0.002 Frontal zone 19871986 0.4020.428Indicators −0.711 −0.0610.026Changes 0.000 0.002 in Frontal zone 19871986 0.4020.428 −0.711 −0.0610.026 0.000 0.002 filling/stagnationFrontal zone 19861987 The Ice-Cliff 0.4280.402 −0.711 −0.0610.026 0.000 0.002 filling/stagnationFrontal zone Year19871986 0.4020.428 −0.711 Indicators−0.0610.026 Value0.000 0.002 filling/stagnationDynamicsFrontal zone 198619861987 Shape 0.4280.4020.428 −0.7110.711 −0.0610.0260.061 0.0020.000 0.002 filling/stagnationFrontal zone 19871986 0.4020.428 −0.711− Ch.−0.0610.026 of Ch.0.000 0.002 of filling/stagnationFrontal zone 1987 0.402 −0.711 −0.026 0.000 filling/stagnationFrontal zone 1987 0.402CfD −CfE0.711 −0.026 0.000 filling/stagnationFrontal zone 19881987 0.4540.402 −0.7990.711 −0.052CfD0.026 −0.000CfE0.088 filling/stagnationFrontal zone 19871988 0.4020.454 −0.7110.799 −0.0520.026 −0.0000.088 filling/stagnationFrontal zoneFrontal zone 198719881987 0.4540.4020.402 −0.7990.7110.711 −0.0520.026 0.026 −0.0000.088 0.000 filling/stagnationfilling/stagnationFrontal zone 198519871988 0.3670.4020.454 −0.7130.7110.799− −0.0520.026? − −0.0000.088 ? ? filling/stagnationFrontal ? zonefilling /stagnation 19881987 0.4540.402 −0.7990.711 −0.0520.026 −0.0000.088 filling/stagnation filling/stagnation 1988 0.454 −0.799 0.052 −0.088 19881989 0.4540.437 −0.7990.729 −0.0520.017 −0.0700.088 19891988 0.4370.454 −0.7290.799 −0.0520.017 −0.0700.088 1988198619891988 0.4280.4370.4540.454 −0.7110.7290.7990.799 −0.0610.0520.0170.052 − 0.0020.0700.088 0.088 Deep recession stage— 19881989 0.4540.437 −0.7990.729− −0.0520.017 −0.0700.088 − Deep recession stage— 19891988 0.4370.454 −0.7290.799 −0.0520.017 −0.0700.088 Deep recession stage— 1989 0.437 −0.729 −0.017 0.070 stronglyDeep recession concave stage— ice-cliff stronglyDeepFrontal recession concave zone stage— ice-cliff 19871989 0.4020.437 −−0.7110.729 −−0.0260.017 0.0000.070 stronglyDeep recession concaveshape stage— ice-cliff 198919901989 0.2440.4370.437 −0.6410.7290.729 −0.1930.0170.017 0.0880.070 0.070 stronglyDeepfilling/stagnation recession concaveshape stage— ice-cliff 1989 0.437 −0.729 −0.017 0.070 stronglyDeep recession concaveshape stage— ice-cliff 19891990 0.4370.244 −0.7290.641− −0.0170.193− 0.0700.088 stronglyDeep recession concaveshape stage— ice-cliff 19901989 0.2440.437 −0.6410.729 −0.1930.017 0.0880.070 stronglyDeep recession concaveshapeDeep stage— ice-cliff recession 19891990 0.4370.244 −0.7290.641 −0.0170.193 0.0700.088 stronglyDeep recession concaveshape stage— ice-cliff 19881990 0.4540.244 −0.7990.641 −0.0520.193 −0.0880.088 stronglyDeep recession concaveshapestage—strongly stage— ice-cliff concave 1990 0.244 −0.641 −0.193 0.088 stronglyDeep recession concaveshape stage— ice-cliff stronglyDeep recession concaveshape ice-cli stage— ice-cliffff shape 19901990 0.2440.244 −0.6410.641 −0.1930.193 0.088 0.088 strongly concaveshape ice-cliff 19911990 0.4480.244 −0.7700.641− −0.2040.193− −0.0880.129 strongly concaveshape ice-cliff 198919901991 0.4370.2440.448 −−0.7290.6410.770 −0.2040.0170.193 −0.0700.0880.129 shape 19901991 0.2440.448 −0.6410.770 −0.2040.193 −0.0880.129 shape 19911990 0.4480.244 −0.7700.641 −0.2040.193 −0.0880.129 Deep recession stage— 19901991 0.2440.448 −0.6410.770 −0.2040.193 −0.0880.129 Entering the active phase 1991 0.448 −0.770 0.204 −0.129 stronglyEntering concavethe active ice-cliff phase 1991 0.448 −0.770 0.204 −0.129 Entering the active phase 19911991 0.4480.448 −0.7700.770 0.2040.204 −0.129 0.129 (I)Entering of the glacier the active phase—an phase 199019921991 0.2440.5320.448 −−0.7380.6410.770− −0.0840.2040.193 −0.088 1.5080.129 − (I)Entering of the glacier theshape active phase—an phase 19921991 0.5320.448 −0.7380.770 0.0840.204 − 1.5080.129 (I)advance,Entering of the glacier theconvex active phase—an shape phase of 19911992 0.4480.532 −0.7380.770 0.2040.084 − 1.5080.129 (I)advance,Entering of the glacier theconvex active phase—an shape phase of 19921991 0.5320.448 −0.7380.770 0.0840.204 − 1.5080.129 advance,Entering theconvexEntering active shape the phase active of phase (I) 19911992 0.4480.532 −0.7380.770 0.2040.084 − 1.5080.129 (I)advance,Entering of thethe glacier convexthe ice-cliff active phase—an shape phase of 1992 0.532 0.738 0.084 1.508 (I)advance,Entering of thethe glacier convexthe ice-cliffof active the phase—an glaciershape phase phase—anof 19921992 0.5320.532 0.7380.738 0.084 0.084 1.508 1.508 (I)advance,Entering of thethe glacier theconvex ice-cliff active phase—an shape phase of 19911992 0.4480.532 −0.7380.770 0.2040.084 − 1.5080.129 (I)advance,Entering of thethe glacier theconvex advance,ice-cliff active phase—an shape convexphase of shape of 19931992 0.6630.532 0.7580.738 0.1310.084 0.0201.508 (I)(I)Enteringadvance, ofof thethethe glacierglacier theconvex ice-cliff active phase—anphase—an shape phase of 19931992 0.6630.532 0.7580.738 0.1310.084 0.0201.508 (I)advance,Entering of thethe glacier theconvex ice-cliff active phase—an shapethe phase ice-cli of ff 19921993 0.5320.663 0.7380.758 0.0840.131 1.5080.020 (I)advance, of thethe glacier convex ice-cliff phase—an shape of 19931992 0.6630.532 0.7580.738 0.1310.084 0.0201.508 (I)(I)advance, ofof thethethe glacierglacier convex ice-cliff phase—anphase—an shape of 19921993 0.5320.663 0.7380.758 0.0840.0840.131 1.508 1.5080.020 advance,the convex ice-cliff shape of 19931993 0.6630.663 0.7580.758 0.131 0.131 0.020 0.020 advance,thethe convex ice-cliffice-cliff shape of 19941993 0.6910.663 0.7890.758 0.0280.131 0.0310.020 advance,the convex ice-cliff shape of 19931994 0.6630.691 0.7580.789 0.1310.028 0.0200.031 the ice-cliff 19941993 0.6910.663 0.7890.758 0.0280.131 0.0310.020 thethe ice-cliffice-cliff 19941993 0.6910.663 0.7890.758 0.0280.1310.131 0.0310.020 0.020 199419931994 0.6630.6910.691 0.7580.7890.789 0.1310.1310.028 0.028 0.020 0.0200.031 0.031 19931994 0.6630.691 0.7580.789 0.1310.028 0.0200.031 199319951994 0.6630.6460.691 0.7580.7830.789 −0.1310.0280.045 − 0.0200.0310.006 19941995 0.6910.646 0.7890.783 −0.0280.045 − 0.0310.006 199419951994 0.6910.6460.691 0.7890.7830.789 −0.0280.0280.045 − 0.031 0.0310.006 199519951994 0.6460.6910.646 0.7830.7890.783 −0.0280.0450.045 − 0.0310.006 0.006 19941995 0.6910.646 0.7890.783 −0.0280.045− − 0.0310.006 − 19941995 0.6910.646 0.7890.783 −0.0280.045 − 0.0310.006 1995199419961995 0.6460.6910.6700.646 0.7830.7890.7240.783 −−0.0280.0240.0450.045 −− 0.0310.0060.0590.006 19951996 0.6460.670 0.7830.724 −0.0240.045 −0.0060.059 Stagnation—convexStagnation—convex ice- 199619961995 0.6700.6460.670 0.7240.7830.724 −0.0240.045 0.024 −0.0590.006 0.059 Stagnation—convex ice- 19961995 0.6700.646 0.7240.783 −0.0240.045 −−0.0590.006 − cliff,Stagnation—convex many inflexionice-cliff, many points ice- inflexion 19961995 0.6700.646 0.7240.783 −0.0240.0240.045 −0.0590.006 cliff,Stagnation—convex many inflexion points ice- 19951996 0.6460.670 0.7830.724 −0.0240.045 −0.0060.059 cliff,Stagnation—convex many inflexionpoints points ice- 199519971996 0.6460.6750.670 0.7830.7320.724 −0.0050.0240.045 − 0.0080.0060.059 cliff,Stagnation—convex many inflexion points ice- 19951996 0.6460.670 0.7830.724 −0.0240.045 −0.0060.059 cliff,Stagnation—convex many inflexion points ice- 1997199719961997 0.6750.6700.6750.675 0.7320.7240.7320.732 0.0050.0050.024 0.005 − 0.008 0.0080.059 0.008 cliff,Stagnation—convex many inflexion points ice- 19971996 0.6750.670 0.7320.724 0.0050.024 − 0.0080.059 cliff,cliff,Stagnation—convex manymany inflexioninflexion pointspoints ice- 19961997 0.6700.675 0.7240.732 0.0240.005 − 0.0080.059 cliff,Stagnation—convex many inflexion points ice- 199819961997 0.6400.6700.675 0.7590.7240.732 −0.0240.0050.035 −0.027 0.0080.059 cliff,Stagnation—convex many inflexion points ice- 1998199619981997 0.6700.6400.6750.640 0.7240.7590.7320.759 −0.0240.0050.035 0.035 −0.027 0.0080.059 0.027 cliff,Stagnation—convex many inflexion points ice- 1996 0.670 0.724 0.024 −0.059 cliff,Stagnation—convex many inflexion points ice- 19981997 0.6400.675 0.7590.732 −0.0050.035− 0.027 0.008 cliff, many inflexion points 1998 0.640 0.759 −0.035 0.027 cliff, many inflexion points 19971998 0.6750.640 0.7320.759 −0.0050.0050.035 0.027 0.008 0.008 cliff, many inflexion points 1999199919971998 0.6410.6750.6400.641 0.7500.7320.7590.750 −0.0010.0050.0010.035 0.001 −0.027 0.0080.009 0.009 199719981999 0.6750.6400.641 0.7320.7590.750 −0.0050.0010.035 − 0.0080.0270.009 − 199719991998 0.6750.6410.640 0.7320.7500.759 −0.0050.0010.035 − 0.0080.0270.009 19991998 0.6410.640 0.7500.759 −0.0010.035 −0.0270.009 19991998 0.6410.640 0.7500.759 −0.0010.035 −0.0270.009 Deep recession by calving 199919982000 0.6410.6400.575 0.7500.7590.753 −0.0010.0350.066 −0.0270.0030.009 Deep recession by calving 199819992000 0.6400.6410.575 0.7590.7500.753 −0.0010.0350.066 −0.0270.0030.009 Deep recessionand melting by calving 199820001999 0.6400.5750.641 0.7590.7530.750 −0.0010.0350.066 −0.0270.0030.009 Deep recessionand melting by calving 200019981999 0.5750.6400.641 0.7530.7590.750 −0.0010.0660.035 −0.0030.0270.009 Deep recessionand melting by calving 20001999 0.5750.641 0.7530.750 −0.0010.066 −0.0030.009 Deep recessionand melting by calving 19992000 0.6410.575 0.7500.753 −0.0010.066 −0.0030.009 and melting 19992000 0.6410.575 0.7500.753 −0.0010.066 −0.0030.009 Deep recessionand melting by calving 19992000 0.6410.575 0.7500.753 −0.0010.066 −0.0030.009 Deep recessionand melting by calving 2000 0.575 0.753 −0.0661 0.003 Deep recessionand melting by calving 2000 0.575 0.753 −0.0661 0.003 Deep recessionand melting by calving 2000 0.575 0.753 −0.0661 0.003 Deep recessionand melting by calving 2000 0.575 0.753 −0.0661 0.003 Deep recessionand melting by calving 2000 0.575 0.753 −0.0661 0.003 and melting 1 and melting 1 1 1 1 1 1

Interannual Indicators Changes in The Ice-Cliff Year Indicators Value Dynamics Shape Ch. of Ch. of CfD CfE CfD CfE

1985 0.367 −0.713 ? ? ? ?

1986 0.428 −0.711 0.061 0.002

Frontal zone 1987 0.402 −0.711 −0.026 0.000 filling/stagnation

1988 0.454 −0.799 0.052 −0.088

1989 0.437 −0.729 −0.017 0.070

Deep recession stage— strongly concave ice-cliff shape 1990 0.244 −0.641 −0.193 0.088

1991 0.448 −0.770 0.204 −0.129

Entering the active phase (I) of the glacier phase—an 1992 0.532 0.738 0.084 1.508 advance, convex shape of

the ice-cliff

1993 0.663 0.758 0.131 0.020

1994 0.691 0.789 0.028 0.031

1995 0.646 0.783 −0.045 −0.006

1996 0.670 0.724 0.024 −0.059 Stagnation—convex ice-

Geosciences 2020, 10, 328 cliff, many inflexion points 13 of 21

2001 0.570 0.885 −0.005 0.132 19972001 0.6750.570 0.7320.885 −0.0050.005 0.0080.132 2001 0.570 0.885 −0.005 0.132 2001 0.570 0.885 Table−0.005 4. Cont. 0.132 2001 0.570 0.885 −0.005 0.132 200119982002 0.5700.6400.475 −0.8850.7590.794 −0.0050.0350.095 −0.1320.0271.679 20022001 0.4750.570 −0.8850.794 −0.0950.005Interannual −0.1321.679 Changes in 2002 0.475Indicators −0.794 −0.095 −1.679 Year 20022001 The Ice-Cliff Shape 0.4750.570 −0.8850.794 −0.0950.005Indicators −0.1321.679 Value Dynamics 19992001 0.6410.570 0.7500.885 −0.0010.005 −0.1320.009 20022001 0.4750.570 −0.8850.794 −0.0950.005 −0.1321.679 20022003 0.4750.463CfD −0.7940.837 CfE −0.0950.012 Ch. of CfD−1.6790.043 Ch. of CfE 200320012002 0.4630.5700.475 −0.8850.8370.794 −0.0120.0050.095 −0.1320.0431.679 20032001 0.4630.570 −0.8850.837 −0.0120.005 −0.1320.043 Deep recession by calving 20002000200120032002 0.5750.5700.4630.4750.575 −0.7530.8850.8370.7940.753 −0.0660.0050.0120.0950.066 −0.0030.1320.0431.679 0.003 200320022004 – 0.4630.475– −0.8370.794– −0.0120.095– − −0.0431.679 – – and melting 200420022001 – 0.4750.570– −0.8850.794– −0.0950.005– −0.1321.679 – – Deep recession by calving 200320012004 – 0.4630.570– −0.8850.837– −0.0120.005– −0.1320.043 – – 20012001200220042003 – 0.5700.4750.463– 0.570 −0.8850.7940.837–0.885 −0.0050.0950.012– 0.005 −0.1321.6790.043 – 0.132– and melting 2004200320022005 – 0.4630.4750.466– −0.8370.7940.695– −0.0120.095–? − −0.0431.679– ? –? 2004200520022003 – 0.4660.4750.463– −0.6950.7940.837– −0.0950.012–? −1.6790.043– ? –? 1 2005200320022004 – 0.4660.4630.475– −0.6950.8370.794– −0.0120.095–? −0.0431.679 –? –? 2002 2002200320042005 – 0.4750.4630.466– 0.475 −0.7940.8370.695– 0.794 −0.0950.012–? 0.095 −1.6790.043 –? 1.679–? 200220052004 – 0.4750.466– −0.7940.695–− −0.095?– − −1.679 ?– − ?– 20052003 0.4660.463 −0.6950.837 −0.012? −0.043 ? ? 200320042005 – 0.4630.466– −0.8370.695– −0.012–? −0.043 –? –? Entering the quiescent 2003200420032006 – 0.4630.375– 0.463 −0.8370.722– 0.837 −–0.0910.012– 0.012 −0.0430.027 – 0.043– Entering the quiescent 2003200620042005 – 0.4630.3750.466– −0.8370.7220.695–− −–0.0910.012–? − −0.0430.027 –? − –? phaseEntering of the the glacier quiescent stage— 2003200420062005 – 0.4630.3750.466– −0.8370.7220.695– −–0.0910.012–? −0.0430.027 –? –? phaseEntering of the the glacier quiescent stage— 200420052006 – 0.4660.375 – −0.6950.722–––– –0.091? −0.027 ? ? retreat,phaseEntering of few the theinflexion glacier quiescent stage— points 20062004 – 0.375– −0.722– –0.091– −0.027 – – retreat,phaseEntering of few the theinflexion glacier quiescent stage— points 200620042005 – 0.3750.466– −0.7220.695– –0.091–? −0.027 –? –? retreat,phaseEnteringon of fewthe the ice-clifftheinflexion glacier quiescent line stage— points 20042005 – 0.466– −0.695– –? – ? –? retreat,phaseon of fewthe the ice-cliff inflexion glacier line stage— points 2005200520062007 0.4660.3750.466 −0.6950.7220.7700.695 –0.0910.000? ???−0.0270.048 ? ? retreat,Enteringon fewthe ice-clifftheinflexion quiescent line points 2006 0.375 −0.722− –0.091 −0.027 phaseEnteringon of the the ice-cliffthe glacier quiescent line stage— 20072005 0.3750.466 −0.7700.695 0.000? −0.048 ? ? retreat,phaseEntering of few the inflexionthe glacier quiescent stage— points 200520072006 0.4660.375 −0.6950.7700.722 –0.0910.000? −0.0480.027 ? ? retreat,on fewthe ice-cliff inflexionEntering line thepoints quiescent 200520072006 0.4660.375 −0.6950.7700.722 –0.0910.000? −0.0480.027 ? ? phaseEnteringon of the the ice-cliffthe glacier quiescent line stage— 2007 0.375 −0.770 0.000 −0.048 retreat,phaseon of fewthe the ice-cliffinflexion glacierphase line stage— of points the glacier 20062008 – 0.375– −0.722– –0.091– −0.027 – – retreat,Entering few theinflexion quiescent points 2006200720082006 – 0.375– 0.375 −0.7700.722– 0.722 –0.0910.000– –0.091 −0.0480.027 – 0.027– retreat,phaseEnteringon of fewthe the ice-clifftheinflexion glacierstage—retreat, quiescent line stage— points few 200620082007 – 0.375– −0.7220.770–− –0.0910.000– −0.0270.048 – − – phaseon of the the ice-cliff glacier line stage— retreat,phaseEntering of few the theinflexion glacierinflexion quiescent stage— points on the 200620072008 – 0.375– −0.7220.770– –0.0910.000– −0.0270.048 – – Enteringon the ice-cliffthe quiescent line 2006200820072009 – 0.3750.420– −0.7220.7700.839– –0.0910.000–? −0.0270.048– ? –? retreat,phaseEntering of few the theinflexion glacier quiescentice-cli stage— pointsff line 2008200620092007 – 0.3750.420– −0.7220.8390.770– –0.0910.000–? −0.0270.048– ? –? retreat,phaseon of fewthe the ice-cliffinflexion glacier line stage— points 20092008 – 0.420– −0.839– –? –? –? retreat,phaseon of fewthe the ice-cliffinflexion glacier line stage— points 200720072009 0.3750.4200.375 −0.7700.8390.770 0.000? 0.000 −0.048 ? 0.048? retreat,on fewthe ice-cliffinflexion line points 20072008 – 0.375– −0.770–− 0.000– −0.048 – − – retreat, few inflexion points 200720092008 – 0.3750.420– −0.7700.839– 0.000?– −0.048 ?– ?– on the ice-cliff line 20092008 – 0.420– −0.839– ?– ?– ?– on the ice-cliff line 200820072009 – 0.3750.420 – −0.7700.839–––– 0.000? −0.048 ? ? on the ice-cliff line 200720082010 – 0.3750.402– −0.7700.801– −0.0000.018– −0.0380.048 – – 2007201020082009 – 0.3750.4020.420– −0.7700.8010.839– −0.0000.018–? −0.0380.048 –? –? 200820102009 – 0.4020.420– −0.8010.839– −0.018–? 0.038 –? –? 200920102009 0.4020.4200.420 −0.8010.8390.839 −0.018? ???0.038 ? ? 2008 – – –− – – – 200820092010 – 0.4200.402– −0.8390.801– −0.018–? 0.038 –? –? 201020082009 – 0.4020.420– −0.8010.839– −0.018–? 0.038– ? –? 200920102011 0.4200.4020.488 −0.8390.8010.870 −0.0860.018? −0.0380.069 ? ? 2011 0.488 −0.870 0.086 −0.069 201120092010 0.4880.4200.402 −0.8700.8390.801 −0.0860.018? −0.0380.069 ? ? 2010200920112010 0.4200.4880.4020.402 −0.8390.8700.8010.801 −0.0860.018? 0.018 −0.0380.069 ? 0.038? 200920102012 – 0.4200.402– −0.8390.801–− −0.018?– − 0.038 ?– ?– 20122011 – 0.488– −0.870– 0.086– −0.069 – – 201120122010 – 0.4880.402– −0.8700.801– −0.0860.018– −0.0380.069 – – 201220102011 – 0.4020.488– −0.8010.870– −0.0860.018– −0.0380.069 – – 20112012201020112013 – 0.4020.4880.427– 0.488 −0.8010.8700.809– 0.870 −0.0860.018–? 0.086 −0.0380.069– ? 0.069–? 20122013 – 0.427– −0.809–− –? – ? − –? 20102011 0.4020.488 −0.8010.870 −0.0860.018 −0.0380.069 2010201320112012 – 0.4020.4270.488– −0.8010.8090.870– −0.0860.018–? −0.0380.069 –? –? 2012201020122013 – 0.4020.427– – −0.8010.809– –––– −0.018–? 0.038 –? –? 20132011 0.4270.488 −0.8090.870 0.086? −0.069 ? ? Small interannual 201320112012 – 0.4270.488– −0.8090.870– 0.086?– −0.069 ?– ?– Small interannual 20132011201220132014 – 0.4880.4270.474– 0.427 −0.8700.8090.845– 0.809 0.0860.047–? ???−0.0690.036 –? –? Smallfluctuations interannual and 201420122011 – 0.4740.488– −0.8450.870–− 0.0470.086– −0.0360.069 – – Smallfluctuations interannual and 2011201420122013 – 0.4880.4740.427– −0.8700.8450.809– 0.0860.047–? −0.0690.036 –? –? stagnation—fillingSmallfluctuations interannual and the 2011201220142013 – 0.4880.4740.427– −0.8700.8450.809– 0.0860.047–? −0.0690.036 –? –? stagnation—fillingSmallfluctuations interannual and the 20142013 0.4740.427 −0.8450.809 0.047? −0.036 ? ? frontalstagnation—fillingfluctuations zone balancedSmall and interannual the by 2012 – – – – – – frontalstagnation—fillingSmall zone interannual balanced the by 2014201420122013 – 0.4740.427– 0.474 −0.8450.809– 0.845 0.047–? 0.047 −0.036 –? 0.036–? Smallfluctuations interannualfluctuations and and 2012201320142015 – 0.4270.4740.388– −0.8090.8450.804–− −0.0470.086–? −0.0410.036– ? − –? frontalstagnation—fillingthefluctuations ice-cliff zone balanced retreating and the by 20152013 0.3880.427 −0.8040.809 −0.086? 0.041 ? ? frontalstagnation—fillingtheSmall ice-cliff zone interannualstagnation—filling balanced retreating the by the 20152014 0.3880.474 −0.8040.845 −0.0470.086 −0.0410.036 frontalstagnation—fillingtheSmallfluctuations ice-cliff zone interannual balanced retreating and the by 201520132014 0.3880.4270.474 −0.8040.8090.845 −0.0470.086? −0.0410.036 ? ? frontalthefluctuations ice-cliff zonefrontal balanced retreating and zone balancedby by 20132014 0.4270.474 −0.8090.845 0.047? −0.036 ? ? stagnation—fillingtheSmallfluctuations ice-cliff interannual retreating and the 201520132015 0.4270.3880.388 −0.8090.8040.804 −0.086? 0.086 0.041 ? 0.041? frontalstagnation—fillingSmall zone interannual thebalanced ice-cli theff byretreating 201520142016 0.3880.4740.431 −0.8040.8450.835− −0.0470.0430.086− −0.0410.0360.031 frontalstagnation—fillingtheSmallfluctuations ice-cliff zone interannual balancedretreating and the by 201620142015 0.4310.4740.388 −0.8350.8450.804 −0.0430.0470.086 −0.0410.0310.036 frontalthefluctuations ice-cliff zone balanced retreating and by 20142015 0.4740.388 −0.8450.804 −0.0470.086 −0.0410.036 stagnation—fillingtheSmallfluctuations ice-cliff interannual retreating and the 2016 0.431 −0.835 0.043 −0.031 frontalstagnation—fillingSmall zone interannual balanced the by 2016201420162015 0.4740.4310.3880.431 −0.8450.8350.8040.835 −0.0470.0430.0860.043 −0.0410.0360.031 0.031 frontalstagnation—fillingtheSmallfluctuations ice-cliff zone interannual balanced retreating and the by 201420162015 0.4740.4310.388 −0.8450.8350.804− −0.0470.0430.086 −0.0410.0360.031 − frontalthefluctuations ice-cliff zone balanced retreating and by 201620142017 0.4310.4740.390 −0.8350.8450.858 −0.0430.0470.041 −0.0310.0360.023 stagnation—fillingfluctuations and the 2015 0.388 −0.804 −0.086 0.041 frontalstagnation—fillingthe ice-cliff zone balanced retreating the by 201720152016 0.3900.3880.431 −0.8580.8040.835 −0.0430.0410.086 −0.0410.0230.031 stagnation—fillingthe ice-cliff retreating the 2017201520172016 0.3880.3900.4310.390 −0.8040.8580.8350.858 −0.0430.0860.041 0.041 −0.0410.0230.031 0.023 frontalthe ice-cliff zone balanced retreating by 20172016 0.3900.431 −0.8580.835− −0.0430.041− −0.0230.031 − frontal zone balanced by 20172015 0.3900.388 −0.8580.804 −0.0410.086 −0.0410.023 frontalthe ice-cliff zone balanced retreating by 20152016 0.3880.431 −0.8040.835 −0.0430.086 −0.0410.031 Enteringthe ice-cliff the activeretreating phase 201720152018 0.3900.3880.473 −0.8580.8040.823 −0.0830.0410.086 −0.041 0.0350.023 the ice-cliff retreating 2018201820162017 0.4730.4310.3900.473 −0.8230.8350.8580.823 −0.0830.0430.0410.083 − 0.0350.0310.023 0.035 Entering theEntering active thephase active phase 20182016 0.4730.431 −0.8230.835− 0.0830.043 − 0.0350.031 (II)Entering of the glacierthe active phase—an phase 201620182017 0.4310.4730.390 −0.8350.8230.858 −0.0430.0830.041 − 0.0350.0310.023 (II)Entering of the glacier the(II) ofactive the phase—an glacier phase phase—an 2017 0.390 −0.858 −0.041 −0.023 (II)advance,Entering of the glacier theconvex active phase—an shape phase of 201820172016 0.4730.3900.431 −0.8230.8580.835 −0.0830.0430.041 − 0.0350.0230.031 advance, convex shape of 2019201820162019 0.4730.4310.6530.653 −0.7430.8230.8350.743 0.0830.0430.180 0.180 − 0.035 1.5660.031 1.566 (II)advance,Entering of the glaciertheconvex active phase—an shape phase of 2016201920172018 0.4310.6530.3900.473 −0.7430.8350.8580.823 −0.0430.1800.0830.041 − 1.5660.0350.0310.023 (II)advance,Entering of thethe glacier theconvex ice-cliff active phase—an shapethe phase ice-cli of ff 20192017 0.6530.390 −0.7430.858 −0.1800.041 − 1.5660.023 (II)advance, of thethe glacier convex ice-cliff phase—an shape of 20192018 0.6530.473 −0.7430.823 0.0830.180 0.0351.566 (II)advance,Entering of thethe glaciertheconvex ice-cliff active phase—an shape phase of 20172018Explanation: “The ice-cliff shape” 0.3900.473 column: −0.8580.823 solid − line—0.0830.041 Lc ; dashed− 0.0350.023 line— Dc;Entering the “Indicators”the the ice-cliff active column: phase pink 2019 0.653 0.743 0.180 1.566 (II)advance,Entering of the glacierconvexthe active phase—an shape phase of 201920172018colour—active phase; blue colour—quiescent 0.6530.3900.473 −0.7430.8580.823 phase;−0.1800.0830.041 colors − in 1.5660.0350.023 the “Dynamics” (II)advance, of column—seethethe glacier convex ice-cliff phase—an Figureshape 8 of; and 201720182019 0.3900.4730.653 −0.7430.8580.823 −0.0830.1800.041 − 0.0351.5660.023 Enteringthe the ice-cliff active phase 2017stagnation—values centered around0.390 0 of−0.858 interannual −0.041 changes −0.023 within the IQR:(II)advance, of from thethe glacier convex0.037 ice-cliff to phase—an shape 0.054 forof CfD 20192018 0.6530.473 −0.7430.823 0.0830.180 0.0351.566 advance,Entering theconvex active shape phase of 2018 CfE 0.473 −0.823 0.083 0.035 (II)Entering of thethe − glacierthe ice-cliff active phase—an phase 2019and from 0.038 to 0.037 for ).0.653 0.743 0.180 1.566 (II)advance, of the glacier convex phase—an shape of 20182019 − 0.4730.653 −0.7430.823 0.0830.1802 0.0351.566 advance,Enteringthe theconvex ice-cliff active shape phase of 2018 0.473 −0.823 0.0832 0.035 (II)Entering of thethe glacierthe ice-cliff active phase—an phase 2019 0.653 0.743 0.180 1.566 (II)advance,Entering of the glaciertheconvex active phase—an shape phase of 20182019 0.4730.653 −0.7430.823 0.0830.1802 0.0351.566 advance,the convex ice-cliff shape of 2019Interannual variations in 0.653CfD and0.743CfE estimated0.1802 on 1.566 the basis (II) of of the thethe data glacier ice-cliff from phase—an 2014 and 2017 2 (II)advance, of thethe glacier convex ice-cliff phase—an shape of 2019 0.653 0.743 0.1802 1.566 advance, convex shape of were 2019 also analyzed including all0.653 glaciers 0.743 (Figure 0.1802 8) to determine 1.566 whichadvance, of themthe convex ice-cliff may shape experience of an 2019 0.653 0.743 0.180 1.566 the ice-cliff active phase of glacier surge in 2017 and later according2 to the model proposed.the ice-cliff In addition, a class 2 2 2 2 2 2 2 2 Geosciences 2020, 10, 328 14 of 21 represented stagnant glaciers has been specified based on the IQR of interannual variations of CfD and CfE. In this class CfD varied from 0.02 to 0.01 per year and CfE from 0.01 to 0.02 per year during the − − analyzedGeosciences period.2020, 10, x FOR PEER REVIEW 14 of 21

FigureFigure 8. 8.Classification Classification of Spitsbergenof Spitsbergen tidewater tidewater glaciers glaciers according according to the to criterion the criterion of the glacierof the frontalglacier zonefrontal dynamics, zone dynamics, based on based the state on inthe the state years in2014 the years and 2017—the 2014 and relation 2017—the of interannual relation of changesinterannual of CfDchangesand CfE of CfDand and percentage CfE and representation percentage representation (a pie chart inset). (a pie chart inset). A total of 28% of the 110 tidewater glaciers were in deep recession. In the 2014–2017 period, A total of 28% of the 110 tidewater glaciers were in deep recession. In the 2014–2017 period, one- one-third were stagnant glaciers with an uncertain future behavior, although this might be a sign of an third were stagnant glaciers with an uncertain future behavior, although this might be a sign of an impending active phase of the glacier surge. For example, Nathorsbreen, Tunabreen, and Persejbreen impending active phase of the glacier surge. For example, Nathorsbreen, Tunabreen, and Persejbreen showed interannual differences for CfD and CfE close to 0 about 4 years before the active phase of the showed interannual differences for CfD and CfE close to 0 about 4 years before the active phase of the glacier surge. Eleven percent of glaciers should have already been experiencing an active phase of glacier surge. Eleven percent of glaciers should have already been experiencing an active phase of the surge. the surge. This classification was further compared with other information from these glaciers, particularly This classification was further compared with other information from these glaciers, particularly whether a clear advance was observed on Landsat satellite scenes in 2017 (6 glaciers), in 2018 (additional whether a clear advance was observed on Landsat satellite scenes in 2017 (6 glaciers), in 2018 7 glaciers), and in 2019 (additional 11 glaciers) (Figure9). The advance was identified as a forward (additional 7 glaciers), and in 2019 (additional 11 glaciers) (Figure 9). The advance was identified as shift of the ice-cliff in relation to previous years, and based on the presence of a crevassed zone within a forward shift of the ice-cliff in relation to previous years, and based on the presence of a crevassed the glacier frontal zone Ag, both characteristic of rapid glacier movement during the surge [7]. Eleven zone within the glacier frontal zone Ag, both characteristic of rapid glacier movement during the of the glaciers showed overlapping classifications as those clearly advancing and showing movement surge [7]. Eleven of the glaciers showed overlapping classifications as those clearly advancing and forward (Aavatsmarkbreen, Midtbreen, Vaigattbreen, Wahlenbergbreen, Arnesenbreen, Strongbreen showing movement forward (Aavatsmarkbreen, Midtbreen, Vaigattbreen, Wahlenbergbreen, Moršneevbreen, Svalisbreen, Emmabreen, Tunabreen, Allfarvegen, and Moltkebreen), 7 as →Arnesenbreen, Strongbreen → Moršneevbreen, Svalisbreen, Emmabreen, Tunabreen, Allfarvegen, stagnant (Marstrandbreen, Nordenskiöldbreen, Kvalbreen, Sonklarbreen, Crollbreen, Markhambreen, and Moltkebreen), 7 as stagnant (Marstrandbreen, Nordenskiöldbreen, Kvalbreen, Sonklarbreen, and Recherchebreen), 5 as deeply withdrawn, ready for advance (Negribreen, Thomsonbreen, ve Crollbreen, Markhambreen, and Recherchebreen), 5 as deeply withdrawn, ready for advance Osbornebreen, Fjortende Julibreen, and Lilliehöökbreen Bjørlykkebreen), and 1—Monacobreen—as (Negribreen, Thomsonbreen, ve Osbornebreen, Fjortende→ Julibreen, and Lilliehöökbreen → retreating. Five glaciers, classified as advancing, were still (2019) in recession/quiescent Bjørlykkebreen), and 1—Monacobreen—as retreating. Five glaciers, classified as advancing, were still phase (Fjerdebreen, Chauveaubreen, the Heuglinbreen–Hayesbreen–Königsbergbreen system, and (2019) in recession/quiescent phase (Fjerdebreen, Chauveaubreen, the Heuglinbreen–Hayesbreen– Nansenbreen) or stagnant (Kollerbreen). In the case of Johansenbreen, classified in Figures8 and9 Königsbergbreen system, and Nansenbreen) or stagnant (Kollerbreen). In the case of Johansenbreen, as a stagnant glacier, an ice bulge is observed on satellite imagery (Landsat 8—2019) moving down classified in Figures 8 and 9 as a stagnant glacier, an ice bulge is observed on satellite imagery glacier, while the medial has been bent, both of which are typical first characteristics of glacier (Landsat 8—2019) moving down glacier, while the medial moraine has been bent, both of which are surge [19]. typical first characteristics of glacier surge [19].

Geosciences 2020, 10, 328 15 of 21 Geosciences 2020, 10, x FOR PEER REVIEW 15 of 21

FigureFigure 9.9. ClassificationClassification ofof tidewatertidewater glaciersglaciers basedbased onon interannualinterannual changeschanges ofof CfDCfD andand CfECfE betweenbetween 20142014 andand 2017. 2017. Circles Circles indicate indicate glaciers glaciers with with the characteristicsthe characteristics of the of active the phaseactive observedphase observed on satellite on imagerysatellite imagery in 2017, 2018,in 2017, and 2018, 2019. and 2019.

4.4. Discussion 4.1. The Role of the Ice-Cliffs Morphometry in the Glacier Surge Triggering 4.1. The Role of the Ice-Cliffs Morphometry in the Glacier Surge Triggering The recession of Spitsbergen tidewater glaciers is reflected in the morphometry of their ice-cliffs. The recession of Spitsbergen tidewater glaciers is reflected in the morphometry of their ice-cliffs. The vast majority of them showed a retreating nature for the 1936–2017 period. This was reflected in The vast majority of them showed a retreating nature for the 1936–2017 period. This was reflected in the concave shape of the ice-cliff, which increases with the size of the glacier, as demonstrated by the the concave shape of the ice-cliff, which increases with the size of the glacier, as demonstrated by the CfD and CfE indicators. Approximately 10–18% of the tidewater glaciers have been experiencing the CfD and CfE indicators. Approximately 10–18% of the tidewater glaciers have been experiencing the active phase at any given time, as manifested by the pronounced glacier front curving in the direction active phase at any given time, as manifested by the pronounced glacier front curving in the direction of flow (convex shape). of flow (convex shape). The most pressing question is at which critical point the forward glacier movement turns from The most pressing question is at which critical point the forward glacier movement turns from normal flow to glacier surge. The preliminary analysis suggests that larger, complex glaciers (with many normal flow to glacier surge. The preliminary analysis suggests that larger, complex glaciers (with tributaries), with low average surface slopes and long ice-cliffs have a bigger tendency to surge. These many tributaries), with low average surface slopes and long ice-cliffs have a bigger tendency to surge. These results also coincide with previous observations (e.g., [17,23,54–56]). The analysis also indicates

Geosciences 2020, 10, 328 16 of 21 results also coincide with previous observations (e.g., [17,23,54–56]). The analysis also indicates that the bending of the ice-cliff will rapidly follow the initiation of the glacier surge in the case of large glaciers. However, this response tends to be delayed for smaller glaciers, up to five years, as the initial phase of the surge is characterized by a forward movement followed by a slow filling of the glacier frontal zone making a protrusion. The frontal zone Ag of tidewater glaciers during the active phase usually fits into the Ac circle and the CfD indicator ranges from 0.5 to 1.0. The closer the ice-cliff is anchored at the valley mouth and the wider the valley, the more the CfD approaches 1. In extreme cases CfD even exceeds this value and the frontal zone “spills out” into the open sea (e.g., Negribreen in 1969). The median CfD in 1936 was the highest (cf. Table1). At that time, tidewater glaciers had a greater range [ 4] and greater complexity of their glacial system. The high median CfD in 2000, in turn, was mainly the result of there being the largest number of cases of surging glaciers (18.2%). The part of the glacier front that anchors the ice-cliff on land in an ice-wedge-like form can play an important role in the dynamics of tidewater glaciers. Their significance can be compared to buttresses like those found in Gothic cathedrals, designed to spread the force and support the weight. For glaciers, not normal forces (perpendicular to the glacier front line) are of particular importance for the stability of the strongly bending ice-cliff, but the bending moment and a cutting force follows a transverse to the glacier axis. Thicker and wider glacier buttress systems give the support and stabilization for the concave part of the glacier frontal zone (analogous to a parabolic arch, especially pointed arches in building structures [57]). Thus, larger glaciers also show a larger linear recession and greater ice-cliff bending, compared to smaller ones, before reaching the inflexion point (node) when, according to the catastrophe theory [58], the moment of the internal stresses is destabilized and the node (inflexion point) jumps. Therefore, larger glaciers are characterized by a rapid curving of the ice-cliff line following the glacier movement at the beginning of the active phase. Many glaciers, e.g., Nathorstbreen, Strongbreen, Storbreen, Mendelejeevbreen, Negribreen (cf. Figure1), Selfströmbreen, Monacobreen (cf. Table4), and Olsokbreen, showed a characteristic cliff indentation point(s) during deep recession, which could give rise to the stability node jump (e.g., by earthquakes, tides, waves or by the distribution of inner stresses etc.), initiating a hysteresis loop. The role of the “buttresses” in stabilizing ice-cliffs will increase as glaciers retreat into a shallower, sediment-covered valley bottoms, which may also contribute to extending the glacier surge cycle. Smaller glaciers without buttresses only achieve a reduced front curving during recession, a so-called flat arch in terms of the ice-cliff shape. This arc is “structurally” less stable because it is more susceptible to normal forces [57]. Consequently, glacier dynamics appear to adapt faster to the interaction of the combined factors affecting the glacier–atmosphere–sea boundaries. The movement of the ice mass from the glacier top to the cliff [24,25] could be the prevailing force initiating the active phase in the case of smaller glaciers. This seems to be applicable to the approaching glacier surge of Johansenbreen, whose signs were seen on the Landsat 8 satellite image from 2019. The model of glacier behavior before the glacier surge (mechanics of the structure) can be associated with the results of observations of the propagation of crevasses up the glacier from the ice-cliff [22,29,30]. The node jump of the ice-cliff line will also be favored by an asymmetry in the mass balance distribution, especially noticeable in meridionally located glaciers [59]. A disproportionate snow accumulation caused by prevailing winds and an uneven supply of direct radiation in summer (e.g., fog and shading) will also be reflected indirectly in the stress distribution throughout the entire glacier profile. Tidal amplitude [60] and fjord depth in front of the ice-cliff [37–39] also play important roles, especially in cases of anchoring the ice-cliff closer to the valley mouth. Thus, the greater the depth (hydrostatic support), the greater the impact on the cliff destabilization. The glacier buttress system loses its function following surface ablation of land-based ice. The glacier surface altitude has been decreasing over the recent years, especially in the ablation area [61,62]. In turn, a thick layer of mineral deposits can work as an isolating layer protecting the ice beneath [63]. Geosciences 2020, 10, 328 17 of 21

4.2. Duration of the Active Phase During the Glacier Surge In the light of the observed Arctic amplification, the number of surging tidewater glaciers is expected to increase. Both, glacier coverage studies (e.g., [4–8]) and their CfD and CfE coefficients, indicate a deepening recession, including especially large glacial systems with the glacier buttresses and an extensive network of tributaries with a relatively narrow and flat receiving zone. One third of the glaciers are stagnant, and therefore potentially ready for an advance. The situation for 2017 morphometrically was similar to the situation in 1990 (cf. Table1, Figure4), which was followed by the culmination of cases of the active phase of the surge. The beginning and end of the active phases of the glacier surge are the most difficult ones to determine through observation. This analysis shows that morphometric indicators can be a simple and accurate method to measure the duration of the active phase, as proven using published observational data. The active phase is estimated to last on average 3–10 years in Svalbard [14], based on published data, while the results presented here from 10 glaciers suggest an average duration of 6–10 years (see Table2). The beginning and end of the active phase remains to be defined. A clear yearly jump in the value of the CfD and CfE coefficients and the positive sign in the CfE value are directly related to a sudden glacier forward movement, whose acceleration characterizes the active phase of the glacier surge. Therefore, this rapid increase and decrease in CfD and CfE coefficients (cf. Figures6 and7, Table4) can be taken as a determinant of the beginning and end of the glacier surge. The Monacobreen case study (cf. Table4) shows that the proposed classification based on both indicators can be a useful tool for finding dynamic patterns of surging tidewater glaciers. These data refer to the results of Murray et al. [45], where the authors indicated three phases in the glacier surge cycle: acceleration, deceleration, and quiescence. Classification according to CfD and CfE indicates that both the active phase and the quiescent phase can show more variation in terms of the dynamic reaction of the glacier. The method requires further analysis to exclude overlapping of mid-seasonal fluctuations on interannual glacial dynamics. In this analysis, due to availability and quality, the data came from different parts of the ablation season.

5. Conclusions The morphometric indicators used here, the frontal zone dynamics indicator CfD and the ice-cliff balance indicator CfE, allow determining of the dynamic state of tidewater glaciers and classifying them as: deeply receding glaciers, glaciers fulfilling the frontal zone/showing a forward movement, glaciers clearly advancing (active phase of the glacier surge), those showing signs of recession (quiescent phase), and stagnant glaciers. In addition, they allow forecasting future glacier behavior using remote sensing methods (e.g., satellite imagery, aerial photographs, laser scanning data, etc.). The value of these indicators just before the surge is related to the size of the glacier and the complexity of the glacier system, the length of the ice-cliff and the average slope of its surface. The beginning and end of the active phase of the glacier surge can be identified as a sudden change in the ice-cliff morphometric indicators, as exceeding by several times the median of their interannual fluctuations in the order of approximately 0.05–0.06. This, in turn, gives the opportunity to estimate the duration of the active phase, which in the case of the 10 most-studied Spitsbergen tidewater glaciers appears to last on average 6–10 years. The analysis shows a clear difference in morphometry and the behavior of the surging glaciers between smaller and large tidewater glaciers. One of the elements affecting the rate of the dynamic processes and their scale is the presence of glacier buttresses, which drives the distribution of the internal stresses in glaciers to the land anchored part, ensuring the stabilization of the ice-cliff. Buttresses are especially characteristic of large glaciers which can reach larger linear recession values, but also react more violently to the jump of the ice-cliff stability node. This also triggers the propagation of crevasses from the ice-cliff line up the glacier, especially in cases of anchoring the ice-cliff closer to the valley mouth. The presence of the glacier buttress system is probably another variable regulating Geosciences 2020, 10, 328 18 of 21 the distribution of forces inside the glacier, which should be included in the modelling of the calving processes of tidewater glaciers. The morphometric model proposed here is of limited use for glaciers with very little contact with the sea and in the case of ice-cliff shading. Glaciers pinning on islands/capes are also a problem. In this case, each part of the ice-cliff between the island(s) and the land should be treated as a separate section. Glaciers that have not lost their complexity are best suited for long-term analyzes. Currently, Spitsbergen tidewater glaciers show a strong recession pattern, as expected following Arctic amplification models. Thus, the morphometric indicators presented here also suggest a likely intensification of the phenomenon of the glacier surge in the coming years.

Supplementary Materials: The following are available online at http://www.mdpi.com/2076-3263/10/9/328/s1, Table S1: Emblems of topographic maps and IDs and acquisition dates of Landsat satellite images used in the research; Table S2: IDs and acquisition dates of Landsat satellite images for selected tidewater glaciers in Spitsbergen in the period 1985–2019. Funding: This paper has been partly created thanks to funds of the Leading National Research Center (KNOW) No. 03/KNOW2/2014 received by the Centre for Polar Studies in the University of Silesia in Katowice for the period 2014–2018 and supported with in statutory activities No 3841/E-41/S/2019 of the Ministry of Science and Higher Education of Poland. Acknowledgments: The author is very grateful to Leszek Kolondra (University of Silesia in Katowice) for sharing his collection of topographic maps. The author thanks very much the Reviewers and the Academic Editor, Kristian Kjellerup Kjeldsen, for all corrections and remarks improving the final version of the text. Conflicts of Interest: The author declare no conflict of interest.

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