Monitoring glacial thickness changes in the Tibetan Plateau derived from ICESat data

Monitoring glacier changes is essential for estimating the water mass balance of the Tibetan Plateau. Recent research indicates that glaciers at individual regions on the Tibetan Plateau and surroundings are shrinking and thinning during the last decades. Studies considering large regions often ignored however the impact of locally varying weather conditions and terrain characteristics on glacial evolution, i.e. the impact of orographic precipitation and variation in solar radiation. Our hypothesis is therefore that adjacent glaciers of opposite orientation change in a different way. In this study, we exploit Ice Cloud and land Elevation Satellite (ICESat)/ Geoscience Laser Altimetry System (GLAS) data in combination with the NASA Shuttle Radar Topographic Mission (SRTM) digital elevation model (DEM) and the Global Land Ice Measurements from Space (GLIMS) glacier mask to estimate glacial thickness change trends between 2003 and 2009 on the whole Tibetan Plateau. The results show that 90 glacial areas could be distinguished. Most of observed glacial areas on the Tibetan Plateau are thinning, except for some glaciers in the Northwest. In general, glacial elevations on the whole Tibetan Plateau decreased at an average rate of -0.17 ± 0.47 meters per year (m a-1) between 2003 and 2009, taking together glaciers of any size, distribution, and location of the observed glacial area. Moreover, the results show that glacial elevation changes indeed strongly depend on the relative position in a mountain range.


ABSTRACT
Monitoring glacier changes is essential for estimating the water mass balance of the Tibetan Plateau.Recent research indicates that glaciers at individual regions on the Tibetan Plateau and surroundings are shrinking and thinning during the last decades.Studies considering large regions often ignored however the impact of locally varying weather conditions and terrain characteristics on glacial evolution, i.e. the impact of orographic precipitation and variation in solar radiation.Our hypothesis is therefore that adjacent glaciers of opposite orientation change in a different way.In this study, we exploit Ice Cloud and land Elevation Satellite (ICESat)/ Geoscience Laser Altimetry System (GLAS) data in combination with the NASA Shuttle Radar Topographic Mission (SRTM) digital elevation model (DEM) and the Global Land Ice Measurements from Space (GLIMS) glacier mask to estimate glacial thickness change trends between 2003 and 2009 on the whole Tibetan Plateau.The results show that 90 glacial areas could be distinguished.Most of observed glacial areas on the Tibetan Plateau are thinning, except for some glaciers in the Northwest.In general, glacial elevations on the whole Tibetan Plateau decreased at an average rate of -0.17 ± 0.47 meters per year (m a-1) between 2003 and 2009, taking together glaciers of any size, distribution, and location of the observed glacial area.Moreover, the results show that glacial elevation changes indeed strongly depend on the relative position in a mountain range.

INTRODUCTION
The Tibetan Plateau has steep and rough terrain and contains ~37,000 glaciers, occupying an area of ~56,560 km 2 (Li, 2003).Recent studies report that the glaciers have been retreating significantly in the last decades.These studies were in different parts of the Tibetan Plateau, such as the Himalayas (excluding the Karakoram) (Yao et al., 2012), the Tien Shan Mountains (Sorg et al., 2012), the Middle Qilian
Additionally, based on the ICESat/GLAS data and a DEM, Kaab et al. (2012)  The results indicated that most of the glacial sub-regions had a negative trend in glacial thickness change, excluding one subregion in the western Mt.Kunlun in the Northwest of the Tibetan Plateau.However, sampled glacial sub-regions were relative large.As a consequence, the glacial conditions were not homogeneous, due to e.g.orographic precipitation and variation in solar radiation.The significant influence of climatic parameters (Bolch et al., 2010) and spatial variability (Quincey et al., 2009) on glacial change rates has already been demonstrated for several individual glaciers on the Tibetan Plateau.In addition, the quality of ICESat elevations is known to be strongly dependent on terrain characteristics.Therefore, this study exploits ICESat/GLAS data for monitoring glacial thickness changes on the whole Tibetan Plateau, identifying sampled glacial areas based on ICESat footprints and glacier orientation.In addition, we explore the ICESat/GLAS data by applying criteria impacting the quality of footprints including acquisition condition and terrain surface characteristics.

Input data
The input data sources consist of the ICESat GLA14 land surface elevation data (Zwally et al., 2011), the SRTM DEM (Jarvis et al., 2008), and the GLIMS glacier mask (Li, 2003).Figure 1 illustrates the SRTM elevations, GLIMS glacier outlines and ICESat L2D campaign tracks on the Tibetan Plateau.The geo-location of each ICESat footprint is referenced to WGS84 in horizontal and to EMG2008 in vertical.Each GLIMS glacier is represented by a polygonal vector and is referenced to the WGS84 datum.The SRTM DEM has a resolution of 90 m at the equator corresponding to 3-arc seconds and is projected in a Geographic (latitude / longitude) projection, with the WGS84 horizontal datum and the EGM96 vertical datum.The vertical error of the SRTM DEM's is reported to be less than 5 m on relative flat areas and 16 m on steep and rough areas (Zandbergen, 2008).In addition, based on the SRTM DEM, the terrain surface parameters slope S and roughness R are estimated, using a 3x3 kernel scanning over all pixels of the grid (Verdin et al., 2007) and (Lay, 2003), where the width and the height of a grid cell in meters are computed, following to Sinnott (1984).

Methods
To estimate a glacial thickness change trend, we consider differences between glacial surface elevations derived from 2003 -2009 ICESat laser altimetry and a digital elevation model.Here the digital elevation model is used as a reference surface.In addition, a glacier mask is used to identify ICESat elevations that are likely to sample glaciers.Teunissen (2003).Note that n is required to be at least six epochs.
Subsequently, the rate v of a linear glacial thickness change and the propagated standard deviation vv of the estimated velocity v are obtained.Additionally, the root mean square error (RMSE), as standard deviation of residuals, is also computed.This value consists of a combination of possible data errors and mainly the non-validity of the linear regression model.
Continuing to the example of Figure 3, glacial area A has an elevation decrease of -1.66 ± 0.42 m a -1 and a RMSE of 3.46 m while glacial area B has an elevation increase of 0.50 ± 0.31 m a -1 and a RMSE of 3.37 m between 2003 and 2009.

RESULTS
The result indicates that 90 glacial areas on the whole Tibetan Plateau are sampled by enough ICESat footprints to estimate thickness change.For each glacial area, a temporal trend in glacial thickness is estimated.In Figure 4, a glacial thickness change rate is symbolized by a red or blue disk at a representative location in each observed glacial area.Most of the observed glacial areas in the Himalaya, the Hengduan Mountains and the Tanggula Mountains experienced a serious decrease in glacial thickness.However, in most of the observed glacial areas in the western Kunlun Mountains in the north-west of the Tibetan Plateau, glaciers oriented toward the North were thickening while those oriented toward the South were thinning.In general, glacial thickness on the whole Tibetan Plateau decreased between 2003 and 2009 at a mean rate of -0.17 ± 0.47 m a -1 .This number is obtained by averaging all estimated rates v and their propagated standard deviations vv, but note that the size, distribution and representativeness of the observed glacial areas are not taken into account.Table 1.Mean glacial thickness change rates per mountain region on the Tibetan Plateau, compared to the results of Gardner et al. (2013).

Figure 1 .
Figure 1.GLIMS glacier outlines and ICESat L2D-campaign tracks superimposed on the SRTM DEM over the Tibetan Plateau

Figure 2 .
Figure 2. ICESat footprints superimposed over the GLIMS glacier mask.The ICESat-sampled glaciers having similar orientation were grouped into glacial areas A and B

Figure 3 .
Figure 3. Distributions of the mean elevation differences and temporal glacial thickness change trends between 2003 and 2009 at the glacial areas A and B

Figure 4 :
Figure 4: Glacial thickness change rates on the Tibetan Plateau between 2003 and 2009

Hien Vu 1  Roderik Lindenbergh 2  Massimo Menenti 2 1
Monitoring glacial thickness changes in the Tibetan Plateau derived from ICESat data Ho Chi Minh city University of Technology,VNU-HCM, Vietnam 2 Delft University of Technology, The Netherlands (Manuscript Received on June 28th , 2016, Manuscript Revised August 18 rd , 2016)  Phan

Table 1 .
The strongest glacier-thinning occurs in the Himalaya range and in the Hengduan mountains.The glacial thickness change rate in the western and inner plateau is near balanced or nearly equals zero.Inversely glaciers in the western Mt.Kunlun are thickening.