The Cryosphere, 4, 77–98, 2010 www.the-cryosphere.net/4/77/2010/ The Cryosphere © Author(s) 2010. This work is distributed under the Creative Commons Attribution 3.0 License. Overview of areal changes of the ice shelves on the Antarctic Peninsula over the past 50 years A. J. Cook and D. G. Vaughan British Antarctic Survey, Cambridge, UK Received: 5 August 2009 – Published in The Cryosphere Discuss.: 14 August 2009 Revised: 18 December 2009 – Accepted: 12 January 2010 – Published: 2 February 2010 Abstract. In recent decades, seven out of twelve ice shelves and other ice fronts and is published as hardcopy maps with around the Antarctic Peninsula (AP) have either retreated detailed accompanying reports (Ferrigno et al., 2006; Fer- significantly or have been almost entirely lost. At least some rigno et al., 2008) and digital data (Scientific Committee of these retreats have been shown to be unusual within the on Antarctic Research, 2005). The main trends observed in context of the Holocene and have been widely attributed to the fronts of marine and tidewater glaciers of the Antarctic recent atmospheric and oceanic changes. To date, measure- Peninsula have already been discussed elsewhere (Cook et ments of the area of ice shelves on the AP have either been al., 2005), but the changes in ice shelf fronts were excluded approximated, or calculated for individual shelves over dis- from that study and are brought up to date and described here. similar time intervals. Here we present a new dataset contain- The retreat of ice shelves on the Antarctic Peninsula over ing up-to-date and consistent area calculations for each of the the past century has been widely documented and attributed twelve ice shelves on the AP over the past five decades. The to atmospheric warming (e.g., Vaughan and Doake, 1996; results reveal an overall reduction in total ice-shelf area by Doake and Vaughan, 1991a; Mercer, 1978; Rott et al., 1998; over 28 000 km2 since the beginning of the period. Individ- Skvarca et al., 1999) and several specific mechanisms have ual ice shelves show different rates of retreat, ranging from been suggested to explain particular phases of retreat and col- slow but progressive retreat to abrupt collapse. We discuss lapse (Scambos et al., 2000; Doake et al., 1998; MacAyeal the pertinent features of each ice shelf and also broad spa- et al., 2003; Vieli et al., 2006). The role of atmospheric tial and temporal patterns in the timing and rate of retreat. warming as the primary driver of ice-shelf retreat is not, how- We believe that an understanding of this diversity and what it ever, a view that is universally accepted, with some evidence implies about the underlying dynamics and control will pro- that ice-shelf thinning may, in part, be the result of oceano- vide the best foundation for developing a reliable predictive graphic change which may “pre-condition” ice shelves to re- skill for ice-shelf change. treat (Shepherd et al., 2003). Thus the question remains as to whether future ice-shelf change, not just around the Antarc- tic Peninsula, will be dominated by oceanographic or atmo- 1 Introduction spheric drivers of change. Greater clarity has, however, been achieved with regard The changing position of the margin of the Antarctic ice to another important question regarding ice-shelf retreat. It sheet, both floating and grounded, is currently being mapped has now been demonstrated quite clearly, that the loss of as part of the USGS Coastal-change and Glaciological Maps floating ice shelves and ice in fjords can cause profound of Antarctica programme (Williams and Ferrigno, 1998). As acceleration and thinning of the tributary glaciers flowing part of this programme, a comprehensive time-series of ice- from the Antarctic Peninsula plateau (Rignot et al., 2004, front changes around the Antarctic Peninsula was compiled 2005; Rott et al., 2002; Scambos et al., 2003; Pritchard and from sources dating from 1940 to 2002 (Cook et al., 2005). Vaughan, 2007; De Angelis and Skvarca, 2003; Hulbe et al., The time-series further reveal changes in glacier, ice shelf 2008; Pritchard et al., 2009). Although rates of acceleration will probably not persist indefinitely, it implies, and will al- most certainly continue to imply, a contribution to sea-level Correspondence to: A. J. Cook rise. Although the effects of ice-shelf retreat on the tributary ([email protected]) glaciers will not be extensively discussed in this paper, this is Published by Copernicus Publications on behalf of the European Geosciences Union. 78 A. J. Cook and D. G. Vaughan: Overview of areal changes of the ice shelves Table 1. Summary of area changes found in published material for ten ice shelves located on the Antarctic Peninsula. The figures were obtained from references that recorded area changes of a particular ice shelf between the earliest and most recent dates available. There is other published material not listed here that gives areas of ice shelves on specific dates (e.g. Reynolds, 1988; Rott et al., 1998); calculations of ice loss (e.g. Braun et al., 2009) or area calculations based on different methods (Suyetova, 1986). Ice Shelf First Last Area Area Change % of Reference recorded recorded on first on last (Km2) original date date recorded date recorded date area (Km2) (Km2) remaining Muller¨ 1947 1993 51 49 −2 96 (Ward, 1995) Jones 1947 2003 25 0 −25 0 (Fox and Vaughan, 2005) Wordie 1966 1989 2000 700 −1300 35 (Doake and Vaughan, 1991a) George VI 1974 1995 ∼ 26000 ∼ 25000 −993 96 (Luchitta and Rosanova, 1998) Wilkins 1990 1995 ∼ 17400 ∼ 16000 −1360 92 (Luchitta and Rosanova, 1998) 1995 1998 −1098 85 (Scambos et al., 2000) Prince Gustav 1945 1995 2100 ∼ 100 −2000 5 (Cooper, 1997) 2000 47 2 (Rott et al., 2002) Larsen Inlet 1986 1989 407 0 −407 0 (Rott et al., 2002) Larsen A 1986 1995 2488 320 −2168 13 (Rott et al., 1996) Larsen B 1986 2000 11 500 6831 −4669 59 (Rott et al., 2002) 2002 3631 −3200 32 (Scambos et al., 2004) Larsen C 1976 1986 ∼ 60000 ∼ 50000 −9200 82 (Skvarca, 1994; Vaughan and Doake, 1996) an important underlying reason for attempting to understand also varied, so for this reason, each coastline digitised was the processes that cause ice-shelf retreat, which is the subject assigned a reliability rating (Ferrigno et al., 2006). For the of the current study. purpose of measuring how the ice shelves have changed in Individual ice shelves have been monitored and docu- area over the last 50 years, one ice front position was chosen mented elsewhere (Table 1; see Fig. 1a for locations), but for each decade. This coastline was chosen according to the until now there has been no consistent approach to measur- following scheme: ing changes for all the ice shelves on a regular time-series. We present a discussion of ice-shelf areas measured based – As close to mid-decade as possible. on the USGS coastal-change dataset, which has been updated – Where this was not possible, a coastline at either the for this study using Envisat ASAR images from 2008/9 (ac- start or end of the decade, depending on which has the 1 quired for the IPY Polar View programme ). These data have most complete, reliable ice front or most representative been collected using a consistent methodology to give reli- line. able areas of each ice shelf over the past five decades. We present an analysis of the changes and discuss the trends ob- – Where there was no coastline available within the served. decade, a coastline at the end of the previous decade as an approximate position. 2 Method – Where none of the above information was available, the decade remained blank. The methods used in compiling the coastal-change database are described in detail elsewhere (Cook et al., 2005). In sum- It should be noted that the coastline chosen could have oc- mary, historical data sources, including early maps, aerial curred prior to or following a major ice-shelf collapse event photographs and satellite images, were used to map ice fronts and it has not been selected specifically to reflect these onto a common reference, using ArcGIS. The quantity of events. source material available varied considerably across the AP. The resulting areas therefore represent the area of the ice In some regions, photography coverage and satellite im- shelf on a particular year, and do not represent an average po- ages allowed more than one coastline position to be mapped sition for the decade. Accuracies cannot be assigned to each within each decade, whilst in other regions there was little or area value as the variation within each decade is not quan- no coverage in some decades, and this implied large gaps be- tifiable using our data. The reliability rating attached to each tween recorded positions. The quality of the source material coastline indicates the likely errors in the positioning of the ice front and should be taken into account when assessing the 1http://www.polarview.org/ accuracy of the areas (Table 2). Since 87% of all coastlines The Cryosphere, 4, 77–98, 2010 www.the-cryosphere.net/4/77/2010/ A. J. Cook and D. G. Vaughan: Overview of areal changes of the ice shelves 79 (a) (b) Fig. 1. (a) Location of ice shelves on the Antarctic Peninsula (b) Contours of interpolated mean annual temperature in 2000 A.D., as compiled by Morris and Vaughan (2003). are reliable to within 150 m, and because area accuracies de- pend not only on variations in the width but also the length of the coastline, the areas are rounded to 1 km2 precision.