Recent retreat at a temperate Icelandic glacier in the context of the last ~80 years of climate change in the North Atlantic region
Over recent decades, glaciers outside of Greenland and Antarctica have displayed accelerating rates of mass loss and ice-frontal retreat, and this has been associated with unequivocal climatic and oceanic warming. Icelandic glaciers are particularly sensitive to climate variations on short-term timescales owing to their maritime setting, and have shown rapid rates of retreat and mass loss during the past decade. This study uses annual moraine spacing as a proxy for ice-frontal retreat to examine variability in glacier retreat at Skálafellsjökull, SE Iceland, over the last ~80 years. Two pronounced six-year periods (1936–1941 and 1951–1956) of ice-frontal retreat are recognised in the record for comparison with the most recent phase of retreat (2006–2011), and these three retreat phases are shown to be similar in style and magnitude. Analysis of climate data indicates that these periods of glacier retreat are associated with similar summer air temperature values, which is a key control on Icelandic terminus variations. This demonstrates that both the most recent phase of ice-frontal retreat at Skálafellsjökull and the recent warming of summer temperatures are not unusual in the context of the last ~80 years. These findings demonstrate the importance of placing observations of contemporary glacier change in a broader decadal- to centennial-scale context.
KeywordsIce-frontal retreat Annual moraines Glacier-climate interactions Iceland
Glaciers are now losing mass in response to unequivocal atmospheric and oceanic warming, according to the Intergovernmental Panel on Climate Change . This mass loss has contributed to global mean sea-level rise, with mass loss from glaciers and ice-caps accounting for the majority of the recent cryospheric contribution (~56 % between 1993 and 2010 ). Recent studies of glacier mass balance and ice-frontal positions of glaciers outside of Greenland and Antarctica have demonstrated accelerating rates of mass loss and ice-frontal retreat since the 1970s (e.g. [11, 15, 25, 32, 36, 44, 60, 65]). Icelandic glaciers, in particular, are highly sensitive climate indicators owing to their maritime setting, and are particularly sensitive to short-term (annual to decadal-scale) climatic fluctuations [10, 59]. Over the past decade, studies have demonstrated that Icelandic glaciers have undergone rapid rates of ice-frontal retreat and mass loss (e.g. [7, 10, 29, 46, 59]). Moreover, a recent study at Falljökull  has suggested that retreat at this outlet is unprecedented in the context of the last ~80 years. An assessment of ongoing ice-frontal retreat at other Icelandic outlets is, therefore, of key importance to placing the present period of atmospheric warming and associated glacier retreat in a broader centennial context.
Long, continuous records of ice-front fluctuations form an integral component of monitoring glacier change at regional and global scales, but such detailed records may be absent or incomplete in many localities. Annual moraines, which represent annual ice-frontal variations, offer a valuable geomorphological proxy for examining ice-frontal retreat where such records are unavailable (e.g. [4, 9, 10, 42]). These features are a characteristic signature of active temperate glaciers in SE Iceland (e.g. [17, 19, 20, 52]), and this region is an exemplar for demonstrating the potential of annual moraine records. Previous studies of annual moraines have primarily focused on process-form observations (e.g. [18, 30, 52, 54, 55]), the climatic significance of the moraines (e.g. [4, 9, 10]), or a combination thereof (e.g. [8, 13, 39, 42]). However, there has so far been limited use of annual moraine records to undertake detailed comparisons of variations between phases of ice-frontal retreat within the same record (e.g. ), with the emphasis on drivers of the overall pattern.
Icelandic termini variations
Comparison of non-surge-type glacier termini variations since the 1930s with climate variations shows that ice-front fluctuations are in sympathy with air temperature variations, with warming and cooling trends coinciding with periods of ice-marginal retreat and advance, respectively (Fig. 2, [35, 58, 59]). Relatively high air temperatures during the period 1931–1960, particularly during 1931 and 1940, are associated with a period of rapid ice-front retreat. The reversal of this retreat trend to a period of advance after ~1965 coincides with a period of climate cooling, and the number of non-surge-type advancing glaciers reached its maximum in the 1975–1990 period following a summer air temperature minimum around 1980 [35, 59]. Icelandic glaciers returned to ice-marginal retreat during the 1990s, particularly after 1995, as temperatures began rising rapidly [57, 59]. Thus, the instrumental record appears to demonstrate that summer air temperature variations exert a dominate control on Icelandic glacier variations [35, 58, 59].
Despite the large body of data on Icelandic glacier termini variations and previous analyses of this (e.g. ), as well as a detailed examination of Icelandic climate , to date there has been limited quantitative analysis of ice-frontal retreat rates between different periods of retreat and the associated climate variations (e.g. ). As such, there are unresolved research issues: (a) Are pronounced periods of Icelandic ice-frontal retreat in the last ~80 years comparable? (b) Are current rates of ice-frontal retreat more rapid than in previous periods in the record? (c) Are prominent periods of ice-frontal retreat, including the present period, associated with comparable climate anomalies? Our contribution explores these issues through inter-comparison and quantitative analysis of ice-frontal retreat periods at Skálafellsjökull, with the intention that this will stimulate a detailed assessment of ice-frontal retreat rates at a regional scale and highlight the importance of placing ice-frontal retreat in a long-term context.
Moraine spacing as a proxy for ice-frontal retreat
Although a comprehensive database of ice-front measurements exists for Icelandic glaciers, there are extended periods in the records of many outlets where measurements are sporadic/data are incomplete, with Skálafellsjökull being a notable example (see Fig. 2). To circumvent these issues, the spacing between annual moraines can be applied as a geomorphological proxy for ice-frontal retreat rates (e.g. [4, 9, 10, 42]. Small-scale, annual ice-marginal fluctuations manifest in the form of annual moraines in front of many active temperate glaciers in Iceland and elsewhere (e.g. [8, 9, 10, 20, 30, 45, 52, 54, 55, 61, 63]). These features form as a result of short-lived seasonal re-advances during overall retreat (e.g. [1, 8, 39]). Despite an overall net negative mass balance over a number of consecutive years, much-reduced ablation in winter and early spring may result in a temporary switch to seasonally positive mass balance when the glacier snout advances . Provided recession during the ablation season is greater than advance during the accumulation season over consecutive years, a long sequence of inset, consecutively younger annual moraines may be formed [6, 8, 42]. Since the spacing between successive annual moraines equates to the net ice-front recession in a single balance year, annual moraine sequences facilitate the examination of the relationship between annual ice-frontal retreat rates and climate variations (e.g. [4, 9, 10, 42]). Consequently, annual moraines offer the opportunity to link ice-marginal moraines to specific climatic and glaciological conditions, and to increase understanding of glacier dynamics [8, 39, 42, 54]. To date, analyses of the climatological drivers of moraine spacing (ice-frontal retreat) have utilised linear regression and focused on the overall record [4, 9, 13, 42], but there is the opportunity build on this and apply annual moraine records to examine variations between different periods of retreat within the same record, along with concurrent variations in climate.
Before detailed examination of the ice-front retreat record (moraine spacing) can be undertaken, it must be demonstrated that the moraines represent successive annual ice-frontal positions. This was achieved by describing, analysing and interpreting: (a) the distribution and geomorphology of the moraines; (b) the chronology of the moraines; and (c) moraine sedimentology (cf. ). On this basis, annual ice-frontal retreat rates have been calculated for the period covered by the moraine record (1936–1964, 1969–1974 and 2006–2011) using crest-to-crest (i.e. longitudinal) spacing (cf. [9, 42]). Moraine spacing was measured in ArcMap along a number of transects through the annual moraine sequences on the central and northern parts of the foreland, as no part of the foreland contains a ‘complete’ sequence covering the entire period. The sub-metre resolution of the imagery utilised in mapping ensures that the accuracy and precision of the mapped moraines are sufficient to calculate spacing to the nearest metre: (a) high-resolution scans of 2006 colour aerial photographs [0.41 m Ground Sampled Distance (GSD)]; (b) pansharpened multispectral (8-band) WorldView-2 satellite imagery captured in June 2012 (0.5 m GSD); and (c) a Digital Elevation Model (DEM) generated from Unmanned Aerial Vehicle (UAV)-captured imagery (spatial resolution: 0.09 m). To assess the statistical significance of the differences between periods of pronounced glacier recession identified in the record of ice-frontal retreat, unpaired Wilcoxon rank-sum (or Mann–Whitney U) tests have been applied (cf. ). We also conduct time-series analysis of climate data (ambient air temperatures and sea surface temperatures) for the entire period covered by the moraine record at Skálafellsjökull, and then compare the climate anomalies associated with the prominent periods we investigated.
Ice-frontal retreat record
Comparison of three prominent periods of ice-frontal retreat at Skálafellsjökull, SE Iceland
Minimum retreat rate (m a−1)
Maximum retreat rate (m a−1)
Average retreat rate (m a−1)
Wilcoxon rank-sum test
Inter-annual variability in climate
In our previous analysis of the overall retreat record and climate drivers, we demonstrated that the Skálafellsjökull ice-margin appears to be most sensitive to summer ambient air temperature (AAT) variations , in accordance with previous comparisons (e.g. [9, 59]). Additionally, we also showed that summer sea surface temperatures (SSTs) may have an (indirect) influence on ice-front retreat at this outlet. Based on this, we present time-series analysis of summer AATs and SSTs since the 1930s in this section and then, subsequently, compare the anomalies associated with the prominent periods of ice-frontal retreat identified in the record.
Comparison with other Icelandic outlets
The calculated ice-frontal retreat rates for Skálafellsjökull are comparable to retreat rates calculated from annual moraine spacing at other Icelandic outlet glaciers [9, 10]. For the period 1936–1941, Lambatungnajökull underwent the same amount of glacier recession as Skálafellsjökull: the ice-front retreated 200 m, at an average of ~33 m a−1 . Meanwhile, the Falljökull ice-front retreated a distance of 310 m during the period 1935–1945, at an average of ~28 m a−1 . Comparable ice-frontal retreat rates were displayed by Skálafellsjökull during the 1930s and 1940s, with the ice-front retreating 277 m between 1936 and 1945 (average: ~28 m a−1). This demonstrates that the glaciers underwent similar change during the twentieth century, and is supported by ice-front measurements from all non-surge-type outlet glaciers in Iceland (cf. . More recently, Falljökull underwent ~230 m of recession during the period 2005–2011 (average: ~33 m a−1), representing a significant increase in the rate of frontal retreat . Falljökull has undergone ice-frontal retreat in every year since 1990, the longest series of net retreat on record at this glacier. Owing to this trend and the magnitude of ice-front retreat, it has been argued that this recent very rapid retreat at Falljökull is an exceptional and unusual event . This contrasts with Skálafellsjökull which has retreated by 134 m during the most recent phase of ice-frontal retreat (2006–2011), with earlier periods in the record exhibiting greater glacier recession (see above). The differences evident between Falljökull and Skálafellsjökull are likely to reflect site-specific conditions, with the more rapid ice-frontal retreat rates and recent change in dynamics at Falljökull (cf. [10, 49, 50]) being a consequence of the smaller, steeper nature of the glacier, high mass turnover and the influence of an increasingly flooded overdeepening at the base of the icefall.
Climate during the pronounced retreat periods
Comparison of the climate variations associated with the three prominent periods of ice-frontal retreat at Skálafellsjökull, SE Iceland
Wilcoxon rank-sum test
Summer AAT anomalies
Summer SST anomalies
Examination of SST anomalies shows that greater differences exist in summer SST between the phases of glacier recession. The most recent period of ice-frontal retreat at Skálafellsjökull (2006–2011) coincides with SST anomalies of +0.75 °C, whereas the earlier periods are associated with somewhat lower averages of +0.40 °C (1936–1941) and 0.20 °C (1951–1956), respectively. Additionally, three out of 6 years between 1951 and 1956 exhibit negative summer SST anomalies. During the period 1932–1941, only 1 year displays a positive summer SST anomaly >1 °C (10-year average: +0.48 °C). By comparison, the ten-year period 2002–2011 experiences positive summer SST anomalies >1 °C on three occasions (10-year average: +0.72 °C). Thus, the most recent phase of ice-frontal retreat at Skálafellsjökull is associated with a phase of somewhat warmer summer SSTs. Nevertheless, Wilcoxon rank-sum tests performed on the data show that no statistically significant differences exist between the summer SST anomalies for the three periods (Table 2). It should also be recognised that the influence of SST variations on Icelandic termini variations is still relatively unknown. Statistically significant relationships have been identified between ice-frontal retreat at Skálafellsjökull and summer SST variations , but the coefficient of determination is low (r2 = 0.1623, p = 0.0010). Additionally, no such relationships have been previously identified at other Icelandic glaciers and this, therefore, requires further exploration.
In this study, we have shown that both recent ice-frontal retreat at Skálafellsjökull and warming summer air temperatures in SE Iceland are not unusual in the context of the ~80-year records examined. Our findings are important at a time when glaciers both in Iceland (e.g. [29, 59] and elsewhere (e.g. [11, 25, 44, 60]) have shown accelerating rates of mass loss and retreat. Moreover, contemporary glacier change has been associated with unequivocal warming (e.g. ). This highlights the need for further and continued investigation of current glacier change, and the need for comparison with previous periods of ice-frontal retreat in order to provide a broader context for current glacier variations. Individual outlet glaciers have also been shown to exhibit variable behaviour at short-term timescales (e.g. Skálafellsjökull and Falljökull), reflecting site-specific conditions. Although internal mechanisms are important, this signal will nonetheless be overridden by longer-term external forcing mechanisms, as demonstrated by the coincidence of phases of ice-front retreat and advance at Icelandic outlet glaciers (cf. ). The approach employed in this study has the capability to provide decadal to centennial-scale records of ice-front retreat, complementing observations of contemporary change (sub-decadal to decadal-scale) and studies of the glacial geological record (millennial-scale).
Complexity in the moraine record
Although annual moraine spacing represents a valuable proxy for ice-frontal retreat, it should be recognised that there are a number of potential issues with utilising this approach and that there is complexity in recessional (annual) moraine records. The principal issue with using the Skálafellsjökull moraine record—and other moraine sequences—relates to establishing a robust moraine chronology. In this case, some uncertainty remains over the earliest moraines (pre-1945) in our record, as these formed before the first aerial photograph was captured (cf. ). As a result, lichenometric dating of these features was undertaken, but it is recognised that this technique is associated with a number of uncertainties (e.g. [33, 48, 64]), bringing into question the validity of ages ascribed purely on the basis of lichenometric dating. Nevertheless, we have confidence in the dates of formation ascribed to these moraines given that remote-sensing data provide strong evidence for annual moraine formation both in this sequence and on other parts of the foreland (cf. ). Indeed, a number of previous studies have assumed that moraines formed on an annual basis if the number of ridges between two moraines of ‘known’ age is equal to the time elapsed between the formation of those reference moraines (e.g. [4, 9, 39, 41, 42]). Although sub-annual formation has been identified at the southeastern sector of the ice-front, we argue that this relates to specific conditions—the presence of a reverse bedrock slope, an aquiclude and highly saturated subglacial sediments—in this area of the foreland (see , for further details).
Depositional and erosional censoring may also affect the integrity (preservation) of the moraine sequences, thus reducing their representativeness of ice-frontal fluctuations (cf. [3, 26, 37, 38]). With respect to Skálafellsjökull, both self- and external censoring processes (sensu  may have impacted the annual moraine sequences to varying degrees. Firstly, there is some evidence of localised glacier overriding and superimposition of moraines (obliterative overlap) (cf. [13, 19]). Secondly, ice-cored moraines have previously been identified at Skálafellsjökull (cf. ) and meltout of debris-covered ice in ice-cored moraines may have impacted the moraine record (e.g. [1, 40, 42, 43, 54]). Although ice-cored moraines were not identified during excavations through moraines in 2014, their presence cannot be ruled out altogether and isolated, large dead-ice bodies were found underlying other surficial deposits . Finally, glaciofluvial processes are a notable feature of active glacial landsystems (cf. [17, 20, 21], and references therein), and the moraine sequences have, in places, been partially affected by glaciofluvial activity (cf. [14, 19]. Although these processes introduce uncertainties, these were minimised by utilising multiple transects across the foreland (see “Methods”).
Aside from these issues, it is also recognised that there is complexity in the links between the moraine record and climate. The annual moraines at Skálafellsjökull—and elsewhere in Iceland—primarily reflect seasonally driven submarginal processes active in a given year (cf. , and references therein), and will therefore largely reflect short-term climate variability. This rapid short-term behaviour at the ice-front (glacier reaction time) should be distinguished from the integrated longer-term behaviour of the whole glacier (glacier response time), which is usually of the order of decades in maritime glaciers (cf. [2, 5, 10, 16, 27, 34]). Indeed, whilst the time lag between climate variation and the detection of change at the ice-front is small, it does not necessarily indicate that the ice-front has fully responded to the climate variation (cf. [5, 59]). Thus, our analyses demonstrate that Skálafellsjökull is currently reacting in a similar manner to previous periods in the 1930s, 1940s and 1950s. Nevertheless, the links between the moraine record and climate are inherently complex, and the record will integrate underlying longer-term climate variations (glacier response) in addition to short-term variations (glacier reaction), with multiple periodicities reinforcing or modulating each other (cf. [37, 38]).
Using the crest-to-crest spacing of annual moraines on the foreland of Skálafellsjökull, a temperate non-surging outlet glacier of the Vatnajökull ice-cap in SE Iceland, we calculated ice-frontal retreat rates since the 1930s. From the calculated record of ice-front retreat, we recognised two pronounced periods of glacier recession for comparison with the most recent phase of retreat (2006–2011). We undertook quantitative analysis to examine variability between these three periods of retreat, and showed that they are comparable both in style and magnitude. Analysis of climate data for SE Iceland also indicates that the three periods of ice-frontal retreat identified are associated with similar summer air temperature values, which has previously been shown to be a key control in terminus variations in Iceland. We, therefore, demonstrated that the coincidence of the most recent phase of ice-frontal retreat at Skálafellsjökull (2006–2011) and warming summer temperatures is not unusual in the context of the last ~80 years. This highlights the need to place observations of contemporary glacier change in a broader, longer-term (centennial) context. Moreover, the novel approach used in this study demonstrates the potential of annual moraine records for analysing ice-frontal variations where continuous records of glacier length change are absent or incomplete.
Oddur Sigurðsson and Trausti Jónsson (both Icelandic Meteorological Office) kindly provided ice-front measurements and meteorological data, respectively. The UK Meteorological Office and British Atmospheric Data Centre granted access to the HadSST2 dataset. This research was supported by a Van Mildert College Postgraduate Award, Van Mildert College Principal’s Award and QRA New Research Workers’ Award. Hannah Bickerdike, Jonathan Chandler and Bertie Miles are thanked for assistance and companionship in the field. Regína Hreinsdóttir kindly granted permission to undertake fieldwork within the Vatnajökull National Park. The research was conducted under RANNÍS Agreement 4/2014. This paper is based on initial research completed by BMPC while an MSc student at Durham University.
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