Large surface velocity fluctuations of Biafo Glacier, central Karakoram, at high spatial and temporal resolution from optical satellite images Dirk SCHERLER, Manfred R. STRECKER Institute of Earth and Environmental Science, University of Potsdam, Potsdam, Germany E-mail: dirk@geo.uni-potsdam.de ABSTRACT. Despite global warming and unlike their Himalayan neighbours, glaciers in the Karakoram mountains do not show signs of significant retreat. Here we report high velocity variations of Biafo Glacier, central Karakoram, which occurred between 2001 and 2009 and which indicate considerable dynamics in its flow behaviour. We have generated a dense time series of glacier surface velocities, based on cross-correlation of optical satellite images, which clearly shows seasonal and interannual velocity variations, reaching 50% in some places. The interannual velocity variations resemble the passing of a broad wave of high velocities, with peak velocities during 2005 and some diffusion down-glacier over a period of at least 4 years. High interannual velocity variations are also observed at other glaciers in the vicinity, suggesting a common cause, although these appear to partly comprise longer acceleration phases. Analysis of weather station data provides some indications of meteorological conditions that could have promoted sustained sliding events during this period, but this does not explain the wave-like nature of the acceleration at Biafo Glacier, and the regular, protracted velocity changes. INTRODUCTION In high-altitude areas of the Hindu Kush–Himalayan (HKH) region, where meteorological observations are limited, glacier changes are valuable indicators of the effects of climate change (e.g. Oerlemans, 2005). Furthermore, chan- ging glaciers in this area influence the river discharge from one of the largest continental ice reservoirs outside the polar regions, which sustains the livelihood of several hundred million people (Barnett and others, 2005; Dyurgerov and Meier, 2005; Immerzeel and others, 2010). There has been recent debate on the speed of glacier changes in the HKH region (Cruz and others, 2007; Raina, 2009; Cogley and others, 2010), but most studies have documented widespread glacier retreat and thinning during the last century in the Himalaya and Tibet (e.g. Mayewski and Jeschke, 1979; Fujita and others, 1997; Ageta and others, 2000; Cogley, 2011). Some studies even suggest 21st-century acceleration of glacier thinning (Berthier and others, 2007; Bolch and others, 2008), but conclusive evidence is limited. In the Karakoram, however, several glaciers have been reported to thicken and advance (Hewitt, 2005; Scherler and others, 2011a). The reason for the different glacier behaviour in the Karakoram is not known, but there are hints that climatic changes associated with the mid-latitude westerlies could account for positive specific mass balances. For example, meteorological station data (Archer and Fowler, 2004; Fowler and Archer, 2006) and tree-ring records from the Karakoram (Treydte and others, 2006) and the western Himalaya (Yadav and others, 2004) document decreasing summer tempera- tures and increasing precipitation during the second half of the 20th century. Concurrently, climate reanalysis data suggest strengthening and an altitudinal descent of the westerly jet stream over Central Asia (Archer and Caldeira, 2008), which is the principal engine for transporting westerly moisture from the Mediterranean, Black and Caspian seas to the Karakoram. However, mass-balance measurements on glaciers are scarce and preclude a thorough assessment of trends in specific mass balances. In addition, the current political situation in northern Pakistan complicates extended field campaigns, and the installation and maintenance of dense monitoring networks on large and frequently debris- covered glaciers is extremely difficult. Alternatively, remote-sensing based glacier surface vel- ocities may be used as indicators of glacier conditions (e.g. Scherler and others, 2011a). For example, Quincey and others (2009) studied the surface velocity of Baltoro Glacier, central Karakoram, between 1993 and 2008 and linked velocity variations during this period to climatic changes. In particular, gradually increasing annual and winter velocities have been explained by a positive mass balance due to long-term increases in precipitation and thus accumulation in this region (Quincey and others, 2009). However, the velocity maximum of Baltoro Glacier in summer 2005 was attributed by the authors to high meltwater fluxes and increased basal sliding, following heavy winter snowfall. Such links between weather and short-term glacier sliding are well known from other temperate glaciers (e.g. Willis, 1995), but complicate the distinction between short-term anomalies and long-term trends. Another complicating fact is that surging glaciers are particularly common in the Karakoram (Hewitt, 1998, 2007; Barrand and Murray, 2006), which obey quasi-regular cyclic velocity variations, with a wide range of surge periods and magnitudes (Hewitt, 1998; Mayer and others, 2011; Quincey and others, 2011). It thus appears that differentiating velocity increases due to mass-balance effects from periods of increased sliding or surging phenomena are important, but may require long and detailed velocity records. In this study, we use a dense time series of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) optical satellite images to conduct a detailed investigation of the recent (2000–09) velocity history of the 63km long Biafo Glacier, central Karakoram (Fig. 1). This dataset includes information from 2005, when relatively Journal of Glaciology, Vol. 58, No. 209, 2012 doi: 10.3189/2012JoG11J096 569 Downloaded from https://www.cambridge.org/core. 23 Nov 2021 at 23:11:16, subject to the Cambridge Core terms of use.