Recent glacier changes and climate trends on South Georgia

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Global and Planetary Change 60 (2008) 72 – 84 www.elsevier.com/locate/gloplacha

Recent glacier changes and climate trends on South Georgia John E. Gordon a,b,⁎, Valerie M. Haynes c , Alun Hubbard d a Scottish Natural Heritage, 2 Anderson Place, Edinburgh EH6 5NP, Scotland, UK School of Geography and Geosciences, University of St Andrews, St Andrews KY16 9AL, Scotland, UK c School of Biological and Environmental Sciences, University of Stirling, Stirling FK9 4LA, Scotland, UK d Institute of Geography, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP, Scotland, UK b

Received in revised form 10 June 2006; accepted 25 July 2006 Available online 20 February 2007

Abstract Frontal positions for a sample of 36 out of a potential ca. 160 glaciers on the subantarctic island of South Georgia have been mapped, georeferenced in ArcGIS and analysed for 20th century fluctuations from a variety of satellite, aerial and oblique photographs, ground surveys and historical sources. Of these glaciers, 2 are currently advancing, 28 are retreating and 6 are stable or show a complex, ambiguous response. Most glaciers on the north-east coast of the island attained more advanced positions during the late 19th century. Since then, smaller mountain and valley glaciers have progressively receded. Although showing more variable behaviour, larger tidewater and sea-calving valley and outlet glaciers generally remained in relatively advanced positions until the 1980s. Since then, however, most glaciers have receded; some of these retreats have been dramatic and a number of small mountain glaciers will soon disappear. The response of these glaciers can be related to the direct effects of synoptic-scale warming on glacier mass balance, particularly since the 1950s. However, individual long-profile geometry also appears to be a significant influence on the response and sensitivity characteristics of these glaciers. Thus the delayed and varied behaviour of the larger glaciers may in part reflect their longer response time compared to small glaciers, but the combination of both larger and higherelevation basins, potentially exposed to enhanced orographic-driven accumulation, is a critical factor that cannot be discounted, especially for the few calving glaciers that have recently advanced. Our observations indicate that glacier recession on the windward south-west coast, where precipitation is significantly higher, is less widespread. © 2007 Elsevier B.V. All rights reserved. Keywords: South Georgia; subantarctic; glaciers; climate change

1. Introduction Fluctuations of glaciers are widely recognised as important indicators of climate change (IPCC, 2001; Haeberli, 2005). Synoptic climate change expresses itself as micro-scale perturbations in meteorological variables, such as incoming radiation, temperature, cloudiness and ⁎ Corresponding author. Scottish Natural Heritage, 2 Anderson Place, Edinburgh EH6 5NP, Scotland, UK. E-mail address: [email protected] (J.E. Gordon). 0921-8181/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2006.07.037

precipitation, which are translated through glacier surface mass balance into changes in glacier geometry. Thus, although delayed and dampened by the response characteristics of the glacier, changes in glacier length can be a sensitive indicator of climate change over decadal to century timescales (Oerlemans, 2005). Globally, glacier mass balances have generally decreased overall since ca. 1900, although there is considerable regional variability (Hoelzle et al., 2003; IUGG(CCS)/UNEP/UNESCO, 2005). Glacier recession and volume loss accelerated during the 1970s and particularly after 1988, possibly

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reflecting an anthropogenic contribution to increased global temperatures (Dyurgerov and Meier, 2000; Dyurgerov, 2001; Meier et al., 2003; IUGG(CCS)/UNEP/UNESCO/ WMO, 2005). As well as providing a climate signal, fluctuations of glaciers worldwide are of significantly wider interest because of their contribution to water resources (Beniston, 2003; Hock et al., 2005) and to sea-level rise over the last century and in the future (Church et al., 2001). South Georgia is a small, mountainous and heavily glacierised island in the maritime subantarctic (Fig. 1). The glaciers range from small mountain glaciers to large seacalving valley and outlet glaciers. As one of the few landmasses in the Southern Ocean, South Georgia provides a crucial geographical data point for glacier responses to climate change over different timescales (Clapperton and Sugden, 1988; Clapperton et al., 1989a). In addition, the island is located close to key global climate boundaries, just south of the Polar Frontal Zone and north of the average Antarctic winter sea-ice limit (Hansom and Gordon, 1998). Of particular interest is the response of the glaciers to contemporary climate change in the context of the wider regional warming and glacier recession affecting the Antarctic Peninsula and southern South America (e.g.

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Aniya, 1999; Vaughan et al., 2001; Rignot et al., 2003; Cook et al., 2005). There are partial documentary and geomorphological records of glacier changes at South Georgia from the late 19th century to the present day (Smith, 1960; Hayward, 1983; Gordon, Haynes and Hubbard, unpublished data). In addition, baseline field surveys and repeat measurements of glacier changes were conducted in the 1970s and 1980s, mainly on small landbased glaciers. These have been linked with climate trends obtained from instrumental records at King Edward Point (KEP), near the former whaling station at Grytviken, beginning in 1906 and continued by the British Antarctic Survey (Timmis, 1986; Gordon and Timmis, 1992). In this paper, we extend and update the trends in glacier changes reported by Gordon and Timmis (1992) and include a wider sample of large tidewater and calving glaciers mainly, but not exclusively, on the north-east coast of the island (Fig. 1). 2. Climate and glaciers of South Georgia The dominant topographic feature of South Georgia is an axial mountain chain comprising the Salvesen and

Fig. 1. South Georgia, showing the locations of the glaciers studied. The numbers refer to the glaciers listed in Table 1. The meteorological station at King Edward Point is adjacent to Grytviken.

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Allardyce Ranges which rise to 2960 m a.s.l. and provide a barrier to the prevailing south-westerly winds (Fig. 1). The climate is maritime — cool, moist and windy, reflecting the location of the island some 350 km south of the Polar Frontal Zone and its exposure to depressions tracking eastwards across the Scotia Sea throughout the year (Craig and Gordon, 1990). At the meteorological station at KEP on the more sheltered north-east coast, for the period 1951–1980, the mean annual temperature was + 2.0 °C; mean summer (December–February) temperature, + 4.8 °C; and mean winter (June–August) temperature, − 1.2 °C (Headland, 1984). Over the same period, mean annual precipitation was 1602 mm. Orographic and associated föhn effects, however, have a strong influence on the climate, and the records from KEP are generally thought to be benign compared to the rest of the island. The only systematic comparison reveals that the record at KEP is considerably drier, less windy and on average 1.8 °C warmer than at Bird Island at the western end of South Georgia, which is considered more representative of prevailing conditions (Richards and Tickell, 1968). The mountains and exposed south-west coast are generally colder, wetter, cloudier and windier than the leeward north-east coast. Föhn winds are a feature of the northeast coast, and temperatures can rise to over 20 °C during such conditions (Richards and Tickell, 1968; Headland, 1984). Today just over 50% of South Georgia is covered in permanent snow and ice, a function of the prevailing climate and the position of the island south of the Polar Frontal Zone. There is a marked contrast in ice cover between the heavily glaciated, windward south-west coast and the leeward north-east coast. The latter is more heavily indented, and a number of large, predominantly ice-free peninsulas are separated by major valleys and fjords occupied by tidewater and calving valley and outlet glaciers with sources in the Salvesen and Allardyce Ranges. The intervening peninsulas lie in a precipitation shadow and support only small mountain glaciers and icefields. Reflecting the pattern of orographic precipitation, the permanent snowline lies generally at about 300 m above sea level on the southwest coast, but rises to about 450–600 m on the northeast coast (Smith, 1960; Clapperton et al., 1989b). Broadly, there are three main categories of glacier on South Georgia: calving glaciers that enter the sea; glaciers that terminate at the coast; and glaciers that terminate on land (Table 1). We treat these separately since the glaciers vary in size and in the factors which control their frontal positions, and hence they may be expected to show variable responses to climate trends of

different duration. Sea-calving glaciers are the largest on the island (Fig. 2). They include Ross/Hindle (25), Nordenskjöld (21) and Neumayer (13) Glaciers, respectively 11.6 km, 17.6 km and 14.4 km long. They have sources either in large valley-head cirques or icefields in the Allardyce and Salvesen Ranges. A number of glaciers terminate in cliffed icefronts at the coast, usually with a narrow beach in front of them exposed at Table 1 Glacier characteristics No a Glacier

Category b Area (km2)

1. 2. 3. 4. 5. 6. 7. 8.

1 2 2→3 2→3 1 3 2→3 1

21.6 16.0 15.2 20.8 14.4 1.4 6.8 27.6

7.0 4.2 5.8 6.4 3.8 1.9 4.2 4.5

0–800 0–700 25–600 25–800 0–800 100–500 25–500 0–600

1/2 2 3 3

62.0 59.6 19.6 ~0.6

8.2 13.6 9.4 ~0.6

0–1000 0–1000 50–800 540–600

1 1 1 3 3 1 1 3 1 3 2→3 3 1 1 2→3 3

86.4 14.4 7.2 4.8 25.6 8.1 0.4 0.8 0.008 0.15 7.6 6.2 19.6 8.4 (inc Tyrrell) 2.0 2.3 135.5 17.6 15.4 9.6 27.6 8.4 1.6 3.6 100.7 11.6 16.2 6.8 13.2 5.6 2.2 3.8

180–800 0–1500 25–1000 10–1200 150–760 0–1000 0–700 0–700 210–800

1 1 2 3 2 1→2 1 3

27.0 57.8 2.7 1.0 22.4 8.7 105.0 1.2

0–1500 0–1500 100–500 90–500 0–1200 0–500 0–1400 50–600

9 10. 11. 12. 13. 14. 15. 16 17 18 19 20 21 22 23 24 25 26 27 28

29 30 31 32 33 34 35 36 a

Ryan Brunonia Grace Lucas Morris Austin Purvis Murray Snowfield Crean Fortuna König Un-named (Husvik) Neumayer Geikie Lyell Glacier Col Hodges Hamberg Harker De Geer Nordenskjöld Heaney Cook Nachtigal Ross/Hindle Weddell Bertrab Un-named (south Gold Harbour) Herz Twitcher Lewald Quensel Salomon Graae Novosilski Rocky Bay

Length Altitude (km) range (m a.s.l.)

10.6 12.0 1.6 2.2 7.6 2.6 15.0 1.3

0–1000 0–1200 0–1100 360–500 420–550 0–2000 0–2000

Numbers refer to locations in Fig. 1. Category: 1. sea-calving glaciers; 2. glaciers terminating at the coast; 3. glaciers terminating on land. Arrows indicate recent changes. b

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low tide or a low rock cliff or bar. They have similar sources and include Fortuna (10) and Crean (9) Glaciers. The former is the only glacier on the north-east coast which reaches the open sea at the mouth of its valley; others terminate back in the fjords. Glaciers which terminate on land include valley glaciers such as Heaney (22) and König (11), which are smaller and also have sources in the Salvesen and Allardyce Ranges. Smaller mountain glaciers, such as Hodges (17), have local ice sources in the mountainous peninsulas of the north-east coast. Most glaciers attained more advanced Little Ice Age (LIA) positions marked by end and lateral moraines probably during the late 19th century or earlier, although the actual dating evidence is limited (Clapperton, 1971; Clapperton and Sugden, 1988). In the last few decades, several glaciers which formerly terminated at the coast have receded inland (e.g. Grace (3), Lucas (4) and Cook (23) Glaciers). As well as being an indicator of recent climate trends, glacier changes are of wider environmental concern on South Georgia. Significant glacier recession, especially where former calving glaciers become land-based, may allow a range expansion of introduced species, notably reindeer and brown rats, with consequent increased degradation of vegetation and predation of important breeding populations of ground- and burrow-nesting birds (McIntosh and Walton, 2000). 3. Data sources and methods During the austral summers of 2002–03, 2004–05 and 2005–06, we derived past and present positions of a

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number of land-based and calving glaciers mainly on the north-east coast by mapping the present snouts and associated forefield moraines using hand-held GPS and DGPS, air photos and topographic maps. Additional historical icefront positions for these and other glaciers were obtained from earlier expedition maps, ground photographs and air photographs taken at various times during the 20th century (Hayward, 1983; Gordon, Haynes and Hubbard, unpublished data). This sample of 36 glaciers out of a potential total of ca. 160 (Hayward, 1983) is broadly representative of the glacier categories, sizes and climatological locations on the island (Fig. 1). The first topographic map of the whole island, at 1:200,000 scale, was published in 1958 (Directorate of Overseas Surveys, 1958) and based on field surveys between 1951 and 1957 by the South Georgia Survey Expeditions (Carse, 1959). A modern baseline for the whole island is provided by a Landsat7 ETM+ image (USGS: l71206098_09820030207 used as the basis of a British Antarctic Survey (2004) 1:200,000 scale map) acquired on 7 February, 2003. This image is projected in ArcGIS in UTM Zone 24S coordinates at 25 m pixel resolution and has been ground-truthed and georeferenced by DGPS positions. Icefront positions from survey data, maps and air photographs were compiled and digitised onto the common UTM reference in ArcGIS and georeferenced at numerous coastal and other key recognisable features across the island. Systematic errors are much dependent on the nature of the source material, oblique aerial and historical photographs being the most problematic. However, absolute positional accuracy of the Landsat7 ETM+ image is determined by the 25 m pixel

Fig. 2. Harker (19) (left) and Hamberg (18) (right) Glaciers at the head of Moraine Fjord photographed in 1974. Since then, the Harker icefront has remained in a similar position, but the Hamberg has retreated by as much as 1 km. The peak on the right is Mount Sugartop (2323 m a.s.l.) (photo: J.D. Hansom).

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size, but ground-truthing against recognisable coastal features around the island indicates exceptional correspondence, with positions falling within 2 pixels. 4. Glacier changes Some examples from the three categories illustrate the glacier changes summarised in Fig. 3. A few have not changed significantly during the last 50 years. They include Ryan (1), Salomon (33) and Graae (34) Glaciers which have their catchments on the south-west side of the island. The only glaciers on the north-east coast in this category are Fortuna (10) and Lyell (15). 4.1. Calving glaciers which enter the sea The calving glaciers show variable behaviour. Some of the largest (e.g. Nordenskjöld (21)) have changed comparatively little overall until recently. During the 1970s and early 1980s, they remained in relatively advanced positions close to their LIA limits. Others, such as Weddell (26), Twitcher (30) and Herz (29), began to retreat earlier (Fig. 3). Nordenskjöld Glacier comprises three main tributaries, fed from separate accumulation basins, which reveal variable responses. The centre of the calving front shows little change, only fluctuating over a distance of about 50 m between the 1950s and 2003. Over the same period, the western half of the calving front has fluctuated over about 400 m, and the eastern margin by only about half this amount. Advances occurred in the 1950s, 1970s and possibly in the 1930s and late 1980s, with retreats in between and in recent years (Fig. 3). In the early 1970s, the western land-based margin lay close to the 20th century maximum position inside the outer LIA limits. The different behaviour of the three sectors is probably due to differences in their accumulation areas. The central and western parts have the highest accumulation areas, with a large proportion N 1000 m a.s.l. The calving front of the Lyell Glacier has changed little since the early 20th century, and its land-based margin has receded by about 70 m, from a 20th century advance position just before 1955. By contrast, the adjacent Geikie and Neumayer Glaciers, although changing little between the mid-1950s and 1970s, retreated significantly between 1973/4 and 2003, the former by about 1.6 km and the latter by approximately 2.5 km. Two southern tributaries of the Neumayer, which used to contribute to the calving icefront, became land-based and separated from it. Contrasting responses of adjacent calving glaciers are also evident in Moraine Fjord (Fig. 2). In 2003,

Harker Glacier (19) was more than 1 km further advanced than in 1914 when photographed by Frank Hurley when he visited the island with Sir Ernest Shackleton. It has changed little since the 1970s and in 2003 appeared more advanced than at any time during the 20th century. The front of the adjacent Hamberg Glacier (18) was also in a relatively advanced position in the mid-1970s. However, in contrast with the Harker, the Hamberg has since receded by as much as 1 km. The middle part of the Hamberg spills over a col on the ridge to the north of the glacier. This overspill glacier terminates in an icefall above a rock cliff with ice talus cones and a reconstituted glacier below. Photographs taken between 1902 and 1928 show that the upper and lower glaciers remained narrowly connected at their eastern edge. In 1968, this connection was severed, but there were no apparent changes in the extent of the upper icefall and the lower ice talus cones. In 2003, the ice cones below the rock step had lowered significantly, but the position and overall form of upper icefall were similar to 1968. This suggests that the middle part of Hamberg Glacier that feeds the overspill glacier has not thinned, although the calving front has receded. The much larger Novosilski Glacier (35), on the south-west coast, has also advanced significantly in the 20th century. Between 1956 and 1974, it advanced some 250 m, to within 100 m of Jonassen Rocks. By 2003, its calving front was just off Jonassen Rocks and by 2005 it had reached them. The combined Ross/Hindle Glacier (25) has been documented since 1882, when it was first surveyed by the German First International Polar Year Expedition (FIPYE) (Neumayer and Borgen, 1886). The records, also reviewed by Stone (1974) and Hayward (1983), show that the calving front has alternately advanced and retreated by distances of up to 1.5 km. In 1976, the northern land-based margin and adjacent calving edge lay at, or close to, a moraine ridge marking its 20th century maximum position, which in turn lay just inside the inferred LIA limit (Gordon and Timmis, 1992). Since then, this part of the icefront has receded about 1 km, so that in 2003 it was at its farthest back since 1882. 4.2. Glaciers which terminate at the coast In the 1970s and early 1980s, larger tidewater glaciers were generally in relatively advanced positions close to their LIA limits, but had started to thin and recede (Gordon and Timmis, 1992). This pattern has continued and accelerated, as illustrated by the following examples. In 1947, Brunonia Glacier (2) terminated

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Fig. 3. Variations in glacier length relative to the first year of observation. The numbers refer to the glaciers in Table 1. The glaciers are grouped broadly according to their patterns of behaviour. In the case of Nordenskjöld Glacier, C, W and E refer to the centre of the calving front, the western land-based margin, and the eastern land-based margin, respectively. In the case of Lyell Glacier, E refers to the eastern land-based margin. The vertical scale is in units of 250 m.

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in an ice cliff on a beach, very similar to its appearance and position in a photograph taken in 1927. However, between the 1970s and 2003, the glacier receded 2.1 km. Similarly, Grace (3) and Lucas (4) Glaciers terminated in ice cliffs at the coast in the mid-1970s, but by 2003 had lowered and retreated inland by about 0.4 km and 0.75 km, respectively. At St Andrews Bay, in 1975, Cook Glacier was at or near its most advanced position since the LIA (Clapperton and Sugden, 1980) and it terminated in 30 m high ice cliffs over a 1 km long icefront at high water mark, a similar position to that recorded by the German FIPYE in 1882 (Steinen, 1890). Between 1975 and 1982, the icefront changed dramatically. It receded 25 m and degraded to a low-angled, debris-covered ice ramp 5 m high. In 2003, the snout was some 0.4 km inland from the sea with a lagoon in between. Bertrab Glacier (27) was photographed by Frank Hurley in 1914 when it terminated in an ice cliff resting on a beach at high water mark. Air photographs in 1960 and 1974 show little appreciable change, and in 1985 the glacier still terminated on a gravel beach (Poncet and Crosbie, 2005). Since then, it has retreated over 0.8 km and an open bay has appeared in front. Most of the lower section of the glacier has disappeared entirely, exposing a large rock cliff which 30 years ago was covered by a spectacular icefall. 4.3. Glaciers which terminate on land The land-terminating glaciers have generally been retreating overall during the course of the 20th century (e.g. Austin (6), Husvik (12), Hodges (17), Heaney (22) and Nachtigal (24)). There is geomorphological evidence in the form of moraine ridges and other historical evidence that this retreat was interrupted by a small readvance or stillstand during the 1930s (Smith, 1960; Clapperton, 1971). Some small mountain glaciers have reduced drastically since the 1970s and are now close to disappearing. For example, by the 1970s, Hodges Glacier had diminished to about half the area and length of its late 19th century extent in a period of about 100 years (Timmis, 1986). During the last 30 years it has shrunk to two steep ice patches, the larger upper one about 150 m long. During their recession, several such glaciers have formed sequences of ‘annual’ moraines spanning time periods of varying length. The most extensive series of such moraines occurs in the foreland of a small glacier at the head of the valley south-west of Husvik (Clapperton, 1971; Timmis, 1986). These have formed as the glacier has retreated upslope from its 19th century limit. Timmis (1986) showed that the mean rate

of horizontal retreat increased from 1.98 ma− 1 between 1906/07 and 1954/55 (a period including the advances mentioned above) to 9.17 ma− 1 between 1954/55 and 1976/77. Between 1977 and 2003, the glacier continued to recede upslope by a horizontal distance of 340 m, equivalent to a retreat rate of 13.05 ma− 1. In part, these differences in retreat rates reflect variations in slope angles and the varying lengths of the time periods. Some land-terminating glaciers, such as König (11), Heaney (22), Lewald (31) and Quensel (32), remained in relatively advanced positions until the 1970s, but since then their rate of retreat has increased. In 1916, the König did not quite reach the sea at Fortuna Bay, as Sir Ernest Shackleton walked along the beach in front of it during his epic crossing of South Georgia. Hayward (1983) noted that the glacier retreated about 0.15 km between 1956 and 1965, and that by 1974 it had readvanced to its 1956 position. Air photos from 1976 show the glacier in contact with a prominent ice-cored moraine, which is about 0.6 km inside what appears to be the LIA maximum. In 2003, the glacier had retreated 1.5 km from this position and a lake had formed in front of it. At St Andrews Bay, the front of Heaney Glacier was located about 0.4 km inland from the beach in 1928 (Kohl-Larsen, 1930). In the apparent absence of significant coastal progradation or erosion, the glacier had retreated approximately 160 m overall by the mid1970s and was located in contact with a moraine ridge. In 2003, it was 700–800 m behind this moraine. To summarise, since the late 19th century, the mountain and smaller valley glaciers have progressively thinned and receded, although interrupted by a small readvance in the 1930s; some also advanced about 1910 and 1956 (Hayward, 1983). In the 1970s and early 1980s, larger tidewater and calving valley and outlet glaciers generally remained in relatively advanced positions close to their 20th century advance positions and LIA limits, although some had started to thin and recede (Gordon and Timmis, 1992). Most glaciers on the north-east coast of the island are now receding and this recession has accelerated since the 1980s. 5. Climate trends and glacier responses 5.1. Climate trends The pattern of glacier changes outlined in the previous section is broadly reflected in the climate records from KEP (Fig. 4). They reveal a period of relatively high summer temperatures on the north-east coast during the first decade of the 20th century; a period of lower summer temperatures from the 1920s to

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the 1940s; and a period of relatively higher temperatures during the 1950s to the early 1980s. Unfortunately a gap in the records during the military occupation of the

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station coincides with the period of more recent glacier retreat. However, recent resumption of temperature recording shows a succession of very warm summers.

Fig. 4. Climate data for King Edward Point. (A) Mean summer (glacier ablation) season (November–March) temperature. (B) Mean winter (glacier accumulation) season (April–October) precipitation. (C) Mean summer (glacier ablation) season (November–March) precipitation. 5-year running means are also plotted. There are gaps in the data after 1981 during the period of military occupation of the island.

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From a detailed study of the mass balance of Hodges Glacier (17), Timmis (1986) emphasised the importance of summer temperatures for smaller glaciers on the north-east coast. The mass balance of larger glaciers with source catchments high in the Salvesen and Allardyce Ranges may be more strongly moderated by winter accumulation patterns and this may in part explain why they have remained in relatively more advanced positions until recently. Winter accumulation has been variable and shows no clear long-term trends, although values were somewhat higher in the 1960s (Fig. 4B). Summer precipitation, which falls as snow in the mountains, shows a general, though strongly fluctuating, upwards trend over the period of the available records (Fig. 4C). The period of climate warming during the second half of the 20th century coincided with the early onset of retreat of small land-based glaciers from their 20th century maximum moraines, followed by a more delayed response of some of the larger tidewater glaciers in the 1970s. The calving glaciers are beginning to respond, although their behaviour is more complicated because they are, at least in the short term, decoupled from climate with their frontal positions potentially quasi-stabilised at topographic pinning points controlled by the interplay between mass supply, basal decoupling, and calving dynamics (e.g. Van der Veen, 2002). However, if the late 20th century warming trend continues, then they may well respond more dramatically, retreating to new topographic pinning points inland, as illustrated by the relatively recent but swift retreat of Hamberg Glacier (18). The recent accelerated surface deflation observed across the lower tongue of Nordenskjöld Glacier (21), coupled with its significant reverse bed slopes and over-deepening to some 100 m below sea level up-glacier of the present terminus, is likely lead to an imminent collapse and repositioning of its calving front up-valley. 5.2. Catchment geometry and glacier response From the sample of 36 glaciers presented, there is clearly a varied individual response to the climate change experienced across the island. Catchment position with respect to the main mountain ranges is a major factor in ‘filtering’ the synoptic signal, with glaciers on the orographically sheltered and warmer north-east coast generally more responsive to the warming trend during the latter half of the 20th century. However, individual glacier response can be complicated due to numerous controls and feedbacks. In addition to the first-order influence of climatic forcing, these

include glacier dynamics (in particular the role of basal hydro-dynamics in decoupling surge or calving-type glaciers) and the role of catchment hypsometry, both in the elevation-mass balance feedback, and in the integrated down-glacier flux response to any given mass balance change (Oerlemans, 1989). Response time, broadly defined as the time taken for a glacier to complete its adjustment to climate change, is central to any discussion on the link between glacier fluctuations and climate change. Although response time is one of the most important physical variables characterising a glacier, it is difficult to define analytically. Jóhannesson et al. (1989) provided a simple and robust estimate of the response time (Tm) ∼ h / (− bT), where h is the characteristic maximum thickness scale for the glacier and − bT is the mass balance rate at the terminus. The majority of South Georgia's larger glaciers terminating at or near sea level have a characteristic h of ∼ 100 to 500 m and a terminus balance (bT) of ∼ − 5 to − 10 m, which yields a response time in the order of 10 to 100 years. At the opposite end of the spectrum, Hodges Glacier (17), which in the 1970s had a characteristic maximum thickness (h) of ∼ 30 to ∼ 50 m and a terminus balance of ∼ − 2 to − 5 m, had a response time of less than a decade (Timmis, 1986). Oerlemans (1989) investigated the role of valley geometry on glacier dynamics and emphasised the importance of the longitudinal bed profile on glacier sensitivity and response. He introduced a simple analytical solution for the steady state length (L) of a hypothetical glacier with uniform slope (γ): L¼

2ðK=g þ b0 −EÞ g

where Λ is a mass balance constant, b0 is the elevation of the glacier divide and E is the ELA. It is evident that the elevation-mass balance feedback, reflected in the term Λ/γ, is increasingly effective when gradients are low. Furthermore, the sensitivity of glacier length to ELA change is inversely proportional to the bed slope. Hence, glaciers with a gentle slope are more sensitive in terms of terminus position to a given mass balance perturbation but generally have a longer response time (Tm) since they are characterised by greater ice thickness (h). Analysis of the Shuttle Radar Topography Mission (NASA and NIMA) 90 m resolution (7 m mean vertical precision across the study area) digital terrain model of South Georgia acquired in February 2000 enables us to examine the longitudinal surface profiles of some examples of representative glaciers and investigate the role of topography in influencing their individual and

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wide-ranging responses to climate change (Fig. 5). Most significantly, there is a great variety of contrasting glacier long profiles within the sample, from Hodges Glacier (17), well under 1 km in length and spanning 300 to just over 500 m elevation a.s.l. (in the 1970s), at one end of the spectrum to Nordenskjöld Glacier (21), 19 km in length and with an elevation range from sea level to the summit of Mount Paget at 2960 m a.s.l. Hence one would likely expect a contrasting response from this wide-ranging sample of glaciers. All of the lower lying glaciers represented in the sample (Hodges (17), Rocky Bay (36), Grace (3), König (11), Heaney (22) and Neumayer (13)) with accumulation zones which fall below the 1000 m contour are currently retreating. Hodges Glacier at one extreme has almost completely wasted away, indicating that it is responsive to the fact that the localised ELA has virtually risen above the headwall (500 m a.s.l.) of this mountain glacier. Neumayer Glacier, with an elevation rising to almost 1000 m a.s.l., but with the majority of its 20 km length falling within the ablation zone, is also undergoing accelerating retreat. However, with a longer characteristic response time due shallow slopes (∼1:30) and thus greater ice thickness (h), it did not respond in earnest to the warming until the 1970s. Likewise, Heaney Glacier, with the top of its accumulation zone just falling just below the 1000 m contour, is in significant retreat, but its shorter length (10 km) and hence steeper gradient (∼1:10) make it more sensitive to warming (thereby responding earlier), but the actual response in terms of overall length change is, for this same reason, likely to be more moderated than that of the Neumayer. The next group of glaciers that rise to elevations above 1000 m a.s.l. all show a varied and more complex response. The Ross (25) started to retreat significantly after the 1980s, while the Fortuna (10) and

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Nordenskjöld (21), both also on the climatically benign north-east coast, have been stable or at least characterised by only small frontal oscillations. At first sight it seems surprising that the Nordenskjöld is showing signs of retreat, given the size of its accumulation area rising to well over 2000 m a.s.l. However, the fluctuations are small and mainly confined to the ice along the lateral tributaries, which flow from different parts of the upper catchment compared with the flowline portrayed in Fig. 5. Additionally, with one of the longest characteristic response times of any glacier on the island (∼100s years), then the recent accelerated ablation occurring across its lower tongue (leading to significant surface deflation) may not as yet be balanced by the small increase in accumulation at higher elevations due to increased precipitation witnessed in the 1970s. This scenario of rapid thinning and surface down-wasting due to accelerated melt is, though, not immediately accounted for in the above diagnostic equations of Jóhannesson et al. (1989) and Oerlemans (1989) and thus represents a significant instantaneous consequence of rapid warming which must be considered. The Harker (19) and Novosilski (35) Glaciers are both exceptional in that they have recently been advancing. However, they are also both actively calving, so the role of subglacial hydrology on basal dynamics cannot be discounted as a mechanism for any short-term advance. However, the longevity of the advances observed suggests that they are not significantly climatically decoupled and that their present anomalous response is potentially due to their location and geometry. Novosilski Glacier has a long profile which is not significantly different from that of the Nordenskjöld (Fig. 5). There are, however, two critical differences which may account for its positive mass balance: first,

Fig. 5. Examples of glacier surface profiles measured along central flowlines extracted from the SRMT 90 m digital terrain model (see text).

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the majority of its lower tongue area below the 500 m contour is, on average, 150 m higher in elevation than that of the Nordenskjöld, which will moderate melt rates; and, second, it is critically located on the southwest coast which experiences a harsher microclimatology. Harker Glacier is also anomalous in that it is the only persistently advancing glacier on the benign northeast coast. However, it has a combination of a very steep and elevated long-profile rising to almost 2000 m a.s.l., but compared to all other glaciers shown, it has a relatively small ablation area lying below 500 m a.s.l. (Fig. 5). Hence, the glacier can still maintain a positive mass balance due to the small increases in precipitation at high elevations compared to enhanced melt occurring over a relatively small ablation zone. 5.3. Wider regional patterns At a regional level, the overall pattern of climate warming and glacier recession on South Georgia is broadly matched at other subantarctic islands, on the Antarctic Peninsula and in southern South America. At Heard Island, rapid glacier recession over the last few decades reflects a temperature rise of about 1.3 °C during the last 50 years (Allison and Keage, 1986; Kiernan and McConnell, 1999, 2002; Budd, 2000). Similarly at Kerguelen, glacier recession has accelerated since the early 1970s (Frenot et al., 1993, 1997). In the South Shetland Islands, landbased and calving glaciers have retreated significantly since the 1950s (Braun et al., 2001; Simões et al., 2004). Further south along the Antarctic Peninsula, the regional climate has warmed by about 2 °C since the 1950s, accompanied by increased duration of melting conditions (Vaughan, 2006) and recession of ice shelves (Skvarca et al., 1999) and glaciers (Rau et al., 2004; Cook et al., 2005). In Patagonia, in southern South America, glaciers are also shrinking and retreating (Aniya et al., 1997; Aniya, 1999; Rignot et al., 2003; Rivera and Casassa, 2004). 6. Conclusions During the first half of the 20th century the glaciers on South Georgia receded overall by varying amounts from more advanced 19th century positions, following inferred post-LIA climate warming. Most glaciers on the northeast coast of the island are now receding in response to a sustained climate warming trend that began in the 1950s. The larger glaciers with higher accumulation areas in the Salvesen and Allardyce Ranges generally remained in relatively advanced positions until the late 1970s, their delayed responses possibly reflecting the locations of their accumulation basins in areas of higher orographic

precipitation. However, a response threshold appears to have been crossed on the north-east side of the island, to the extent that most of these larger glaciers are now receding. Glaciers on the south-west coast, experiencing a harsher, cooler and wetter climate, display a more complex response, with smaller, lower elevation glaciers still also retreating but the higher elevation glaciers stabilised or even advancing slightly. From a processoriented view, glacier hypsometry, surface slope and altitude range appear to have had a significant influence on glacier response and sensitivity to the climate warming at South Georgia. The data and sources used in this study provide a baseline for monitoring future glacier changes on South Georgia and work is in progress to develop a full glacier inventory for the island following the type of scheme published by Haeberli and Hoelzle (1995). Acknowledgements We acknowledge funding for the 2003 Scotia Centenary Expedition to South Georgia from the Royal Scottish Geographical Society, the Carnegie Trust for the Universities of Scotland, the National Geographic Society, the Binks Trust and the Brownington Foundation. We are also grateful to the Government of South Georgia and the South Sandwich Islands for permission to work on the island; Scotia Expedition colleagues; Charlie Porter and crew of Yacht Ocean Tramp; Pat Lurcock, Tim and Pauline Carr, Alasdair Reid, Irene Valenkamp, and British Antarctic Survey staff at King Edward Point; Kim Crosbie and crew and staff of Explorer; Brigadier David Nicholls; Chris Hill for preparing the SRTM DTM, and John MacArthur and Bill Jamieson for other technical and cartographic assistance. AH also thanks the Royal Society of Edinburgh, the Royal Geographical Society, the Gino Watkins Trust, Sally Poncet and Jean-Louis Etienne and the crew of Yacht Tara V. We are grateful to Roger Timmis for his support and to Joanna Rae and Rachel Cox for assistance in locating material in the British Antarctic Survey archives. We also thank Michael Zemp and an anonymous reviewer for helpful comments and Wilfried Haeberli as editor. References Allison, I.F., Keage, P.L., 1986. Recent changes in the glaciers of Heard Island. Polar Record 23, 255–271. Aniya, M., 1999. Recent glacier variations of the Hielos Patagónicos, South America, and their contribution to sea-level change. Arctic, Antarctic, and Alpine Research 31, 165–173. Aniya, M., Sato, H., Naruse, R., Skvarca, P., Casassa, G., 1997. Recent glacier variations in the Southern Patagonian icefield, South America. Arctic and Alpine Research 29, 1–12.

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