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Retreat of Northern Tn.Argins of George VI and Wilkins Ice Shelves, Antarctic Peninsula

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Retreat of Northern Tn.Argins of George VI and Wilkins Ice Shelves, Antarctic Peninsula Annals cifGlaciology 27 1998 © International Glaciological Society Retreat of northern tn.argins of George VI and Wilkins Ice Shelves, Antarctic Peninsula B. K. LUCCHITTA, C. E. ROSANOVA us. Geological Survey, 2255 North Gemini Drive, Flagstaff,AZ 86001, US.A. ABSTRACT. The George VI and Wilkins Ice Shelves are considered at risk of disinteg­ ration due to a regional atmospheric warming trend on the Antarctic Peninsula. Retreat of the northern margin of the George VI Ice Shelf has been observed previously, but the Wilkins Ice Shelf was thought to be stable. We investigated the positions of the northern fronts of these shelves from the literature and looked for changes on 1974 Landsat and 1992 and 1995 European remote-sensing satellite (ERS) synthetic aperture radar images. Our investigation shows that the northern George VI Ice Shelf lost a total of 906 km 2 between 1974 and 1992, and an additional 87 km2 by 1995. The northern margin of the Wilkins Ice Shelf lost 796 km2 between 1990 and 1992, and another 564 km2 between 1992 and 1995. Armadas of tabular icebergs were visible in front of this shelf in the ERS images. These two ice shelves mark the southernmost documented conspicuous retreat of ice-shelf margins. INTRODUCTION tic Peninsula has completely disappeared (Doake and Vaughan, 1991). Until now this shelf has marked the south­ Mercer (1978) predicted that global greenhouse warming ern limit of ice-shelf disintegration. Farther south, retreat would lead to disintegration of ice shelves on the Antarctic of the northern margin of the George VI Ice Shelf was Peninsula. Whether related to global or local processes, cli­ observed by Mercer (1978), Lennon and other: (1982) and matic warming on the Antarctic Peninsula has recently Potter and others (1988). By contrast, the Wilkins Ice Shelf become evident (Raper and others, 1984; Morrison, 1990; has been considered stable, though near its limit of viability Ridley, 1993; King, 1994; Thompson and others, 1994; (Vaughan and others, 1993). Here we report that the retreat Vaughan and Doake, 1996) and local ice-sheet disintegra­ of the northern margin of the George VI Ice Shelf is con­ tion has been documented (Doake and Vaughan, 1991; tinuing and that recession of the northern margin of the Skvarca, 1993, 1994; Ward 1995; Rott and others, 1996). For Wilkins Ice Shelf has begun (Fig. 1). We determined past instance, the Wordie Ice Shelf on the west side of the Antarc- frontal positions of these shelves from the literature and measured recent changes on older Landsat and more recent European remote-sensing satellite synthetic aperture radar (ERS SAR) images. METHOD We used two band 7 (near-infrared) 1974 Landsat multi­ spectral scanner (MSS; 80 m/pixel resolution) images in the form of computer-compatible tapes, archived at the Table 1. Satellite images used in study George V[ Ice Shelf Wilk£ns fee Shelf 60° Scene ID Dale Scene ID Dale B Landsat 1532/2325 6Jan 1974 Landsat 1535/2500 9 Jan 1974 ERS-I 5350/5049 24 Jul1992 ERS-I 5436/5049 30Jul1992 Landsat ••• , ERS-I 5350/5067 24 Jul1992 ERS-I 5436/5067 30Jul1992 ERS-I 7268/5067 5 1992 ERS-I 5436/5085 30Jul1992 ERS-1 Dec ERS-I 7268/5085 5 Dec 1992 ERS-I 22313/5049 21 OCl 1995 ERS-I 22227/5049 15 Oct 1995 ERS-I 22313/5067 21 Oct 1995 Fig. 1. Antarctic Peninsula. Location map cif ice shelves, ERS-I 22227/5067 15 Oct 1995 ERS-I 22313/5085 21 Oct 1995 Landsat and ERS -1 images. B, Bellingshausen Sea; D, Dyer ERS-I 22642/5067 13 Nav 1995 Plateau; F, Faraday Station; M, Marguerite Bay; R, ERS-I 22642/5085 13 Nav 1995 Rothera Station; W,Wordie Ice Shelf. 41 Lucchitta and Rosanova: Retreat ifnorthern margins ifGeorge VI and Wilkins Ice Shelves I U.S. Geological Survey, Flagstaff, AZ (Fig. I; Table 1). Addi­ was estimated to be as high as 10 m a- (Lennon and others, tionally, we used 1992 and 1995 ERS-l image mosaics (25 m 1982). resolution at 12.5 m/pixel), in the form of CD-RaMs in The British Graham Land Expedition visited Cape Universal Tr ansverse Mercator map projection using the Jeremy in 1936 and reported that shelf ice filled George VI World Geodetic System 1984 ellipsoid, and furnished by Sound (Fig. 2) and was broken by rifts which were floored by the European Space Agency through the German DLR sea ice (Fleming and others, 1938). This statement has been (Deutsche Forschungsanstalt fur Luft und Raumfahrt) interpreted to indicate that the sound was filled with shelf (Fig. I; Table I). The DLR also provided all images geometri­ ice to CapeJeremy (Doake, 1982). A 1979 sketch map (U.S. cally adjusted to reference levels of sea level (0 m elevation). Geological Survey, 1979) shows the ice-shelf front extending This procedure ensured that the coastline, where at sea from Niznik Island southwestward across the sound. The level, has no range displacement or layover errors. Thus, shelf south of the front is occupied by broken shelf segments. the coastlines are correct on map projections and match on This portrayal was based on the early but unenhanced the mosaics. However, within the SAR images, all eleva­ Landsat images, which showed the position of the ice front tions above this level are subject to terrain distortion. poorly. Doake (1982) commented that it was difficult to Because the early Landsat images have unstable internal locate the true ice-shelf front because of the fragmented geometry, we registered the Landsat images to the ERS shelf The frontal position as shown on this sketch map is re­ images. To co-register ERS images, we used the furnished produced in most subsequent articles discussing the George latitude and longitude coordinates based on orbital para­ VI Ice Shelf (Bishop and Walton, 1981; Lennon and others, meters. Previous investigations (Lucchitta and others, 1995) 1982). Our contrast-stretched Landsat image of 1974 depicts showed that images coregistered by furnished coordinates the ice-shelf front clearly and shows that the previously pub­ may have relative maximum ground location errors of lished front positions were incorrect; the front extended 50 m (Roth and others, 1993). Compressing the images to from just north of Niznik Island westward straight across 25 m/pixel by averaging squares of four pixels reduced the sound to Alexander Island (Fig. 2). By 1974, the area speckle on the radar images. Noisy data were also removed north of this front had cleared of the rifted icebergs and by applying adaptive box filters described in Eliason and shelf segments reported by the 1936 Fleming expedition. If McEwen (1990). We enhanced the Landsat images to bring the shelf in 1936 indeed extended all the way to CapeJeremy, out details in the ice-shelf fronts by applying contrast-en­ this clearing of rifted ice would mean a 40 km retreat of the hancing stretches. shelf between 1936 and 1974. After the above processing, we recorded the location of ice-shelf fronts and crevasses. For multi-temporal composites, we overlayed the mosaics taken at three different times rep­ resented by the early Landsat and two later ERS images. The colors on the composites are due to spectral reflectivity of the Landsat band and backscatter intensity on the ERS images. On Landsat images, the differences between shelf ice and sea ice are small, making it difficult to distinguish between them. On ERS images these ice types are readily distinguished. After converting the multi-temporal composites to Lambert azimuthal equal-area projections, we digitized the locations of the ice-shelf margins at different times and calculated the areas of ice lost during retreat. GEORGE VI ICE SHELF The George VIIce Shelf measures about 25 000 km2 (Potter and others, 1988). It receives most of its ice from the Antarc­ tic Peninsula, whose ice streams extend into the shelf and al­ most reach its western margin. Ice from Alexander Island extends only a few kilometers into the shelf (Bishop and Walton, 1981; Potter and others, 1984). The ice shelf varies in thickness from about 100 m near its northern ice front to a maximum of about 500 m (Bishop and Walton, 1981; Lennon 6 and others, 1982; Ta lbot, 1988). Warm-water intrusion from Fig. 2. Excerpt ifLandsat image, January 1974, showing the Circumpolar Deep Water current from the southeast George VI Sound and covering northern margin ifGeorge VI Pacific basin emerges onto the continental shelf and extends Ice Shelf.Arrows denote location ifice-shelf front. A, Alexan­ N, underneath the northern George VI Ice Shelf (Lennon and der Island; Cl, CapeJeremy; Niznik Island; p,polynya. others, 1982; Potter and others, 1984; Potter and Paren, 1985; Talbot, 1988). The warm water contributes significantly to By 1974, as shown on maps and observed by Doake basal melting of the shelf (Potter and others, 1988), with a (1982), the region behind the front was again occupied by resulting average equilibrium basal melt rate of about complexly divided large shelf segments separated by rifts, 2 m a-] (Lennon and others, 1982; Potter and others, 1984; including the fractured tongue of an unnamed glacier. Potter and Paren, 1985). The basal melt rate at the ice front Lennon and others (1982) report on ice-front retreats 42 Lucchitta and Rosanova: Retreat qfnorthern margins qfGeorgeVI and Wilkins Ice Shelves (Lennon and others, 1982; Potter and Paren, 1985; Potter and others, 1988). This polynya is now gone due to shelf retreat, and our ERS radar images show no trace of it in the sea ice. Our investigation shows that a total of 906 km2 of ice was lost between 1974 and 1992 , amounting to an aver­ age of about 50 km2 a-I (Fig.
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