Surface velocity and ice discharge of the ice cap on King George Island, Antarctica B. OSMANOG ˘ LU, 1 M. BRAUN, 1;2 R. HOCK, 1;3 F.J. NAVARRO 4 1 Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA E-mail: batu@gi.alaska.edu 2 Department of Geography, University of Erlangen, Erlangen, Germany 3 Department of Earth Sciences, Uppsala University, Uppsala. Sweden 4 Department of Applied Mathematics, Technical University of Madrid, Madrid, Spain ABSTRACT. Glaciers on King George Island, Antarctica, have shown retreat and surface lowering in recent decades, concurrent with increasing air temperatures. A large portion of the glacier perimeter is ocean-terminating, suggesting possible large mass losses due to calving and submarine melting. Here we estimate the ice discharge into the ocean for the King George Island ice cap. L-band synthetic aperture radar images covering the time-span January 2008 to January 2011 over King George Island are processed using an intensity-tracking algorithm to obtain surface velocity measurements. Pixel offsets from 40 pairs of radar images are analysed and inverted to estimate a weighted average surface velocity field. Ice thicknesses are derived from simple principles of ice flow mechanics using the computed surface velocity fields and in situ thickness data. The maximum ice surface speeds reach >225 m a –1 , and the total ice discharge for the analysed flux gates of King George Island is estimated to be 0.720 ± 0.428 Gt a –1 , corresponding to a specific mass loss of 0.64 ± 0.38 m w.e. a –1 over the area of the entire ice cap (1127 km 2 ). INTRODUCTION The Antarctic Peninsula has shown considerable warming, with maximum surface air temperature increases of up to 2.58C in 50 years at Vernadsky (Faraday) station, corres- ponding to a trend of þ0:0538  0:02658Ca 1 . Further north, the warming is less pronounced, although still significant (at the 5% level), with warming trends of þ0:0221  0:01688Ca 1 at Bellingshausen station (1969– 2011) and þ0:0094  0:01068Ca 1 for summer (1968–2011; significant at the 10% level) (Turner and others, 2005; Marshall, 2012). Ice masses on the northern Antarctic Peninsula, the South Shetland and sub-Antarctic islands are subject to a maritime climate. Glaciers in such climates are generally considered sensitive to climate change, due to high accumulation rates and temperatures close to the melting point (Knap and others, 1996; Oerlemans, 2001; Hock and others, 2009; Jonsell and others, 2012). Significant changes have been observed in the ice masses on the Antarctic Peninsula, including widespread retreat (Cook and others, 2005), increased surface melt (Vaughan, 2006), acceleration and potential dynamic thinning of the glacier tongues (Pritchard and Vaughan, 2007) and surface lowering (Pritchard and others, 2009). Estimates of ice mass loss and the associated contribution to sea-level rise are still ambiguous and strongly driven by eastern and western Antarctic Peninsula glacier speed-up after collapse of the Larsen A and B ice shelves (Rott and others, 2011; Shuman and others, 2011). Pritchard and Vaughan (2007) give a value of 0:16  0:06 mm sea-level equivalent (SLE) a 1 (58  22 Gt a 1 ), combining surface melt estimates by Vaughan (2006) and dynamic mass losses for the entire Ant- arctic Peninsula. Chen and others (2009) and Ivins and others (2011) found mass losses of 40 Gt a 1 (0.111 mm SLE a 1 ) for the period 2003–09, based on the Gravity Recovery and Climate Experiment (GRACE) for the same region. For the western Antarctic Peninsula, Rignot and others (2008) found mass losses of 7  4 Gt a 1 (0:019  0:011 mm SLE a 1 ) in 1996, 10  5 Gt a 1 (0:028  0:014 mm SLE a 1 ) in 2000 and 13  7 Gt a 1 (0:036  0:019 mm SLE a 1 ) in 2006. Hock and others (2009) estimated 0:22  0:16 mm SLE a 1 (79  58 Gt a 1 ) as the melt contribution from all mountain glaciers and ice caps around Antarctica (most of which are located around the Antarctic Peninsula) for the period 1961–2004, based on temperature trends and mod- elled mass-balance sensitivities. These estimates are con- sidered lower bounds, since mass losses by calving or marine melting were neglected. However, some recent studies suggest that the regional trend of ice mass losses has slowed during the last decade (e.g. Davies and others, 2011; Navarro and others, 2013). The range of reported rates of mass loss for various domains of the Antarctic Peninsula and surrounding islands highlights the need for further detailed studies using the capabilities of the new generation of sensors with more detailed spatial resolution. In this paper we analyse multitemporal data from the Advanced Land Observing Satellite (ALOS) with its Phased Array-type L-band synthetic aperture radar (PALSAR). We derive surface velocity fields by intensity feature tracking and combine those results with ice thickness estimates to quantify the ice mass flux into the ocean of the ice cap on King George Island. STUDY AREA AND PREVIOUS WORK King George Island is the largest island in the South Shetland Island group, located 100 km off the tip of the Antarctic Peninsula (Fig. 1). King George Island has a total area of 1250 km 2 of which >90% is currently ice-covered (1127 km 2 ). The ice cap is 70 km long and 25 km wide, with elevations up to 700 m a.s.l. Most of the perimeter of Annals of Glaciology, Vol. 54, No. 63, 2013 doi: 10.3189/2013AoG63A517 111