Remote sensing of mountain glaciers over the Qinghai-Tibet Plateau

Abstract

The mountain glaciers over the Qinghai-Tibet Plateau (QTP) and its surrounding areas represent an important part of fresh water reserve on the earth. The melt water from these glaciers is critical for water supply for the plateau and feeds many major river systems of Asia. As a key indicator of climate change, glacier change provides an important clue to climate variations in the remote high altitude areas, and impacts various aspects of the environment. In-situ observations of glaciers over the plateau are very limited in spatial and temporal coverages due to the inaccessibility and logistical difficulties, while remote sensing techniques can bridge the gap by providing various information about the glaciers over an extensive region, from different aspects and with regular revisit time. At present, spatial variability of glacier changes and the driving forces over different regions has not been well understood, due to the highly variable topographical and climate conditions which pose challenges for remote sensing techniques. There is necessity to address many key issues in remote sensing of mountain glaciers, such as mapping debris-covered glaciers and estimating glacier mass balances, and more observations are required to improve understanding of the heterogeneity of glacier changes and the mechanisms. In this context, this thesis illustrates the use of remote sensing techniques for glacier mapping, glacier change detection and mass balance estimates in different parts of the plateau, from the monsoon-influenced southeast to the central inland area and the northwest regions mainly controlled by westerlies. First, a semi-automated method is developed for mapping the clean-ice and debris-covered glaciers over the heavily cloud-affected southeastern (SE) QTP, by integration of multi-temporal and multi-source observations from Landsat satellites and InSAR techniques. The compiled new glacier inventory is a baseline data for exploring the characteristics of glaciers in the SE QTP and for various studies related to glacier changes. In a second study, the ICESat laser altimetry measurements (2003-2008) are deployed to derive elevation changes of the mountain glaciers, with a case study on the Dongkemadi glaciers (DKMD) located in the central QTP. Then with the ICESat data, in combination with observations from the Landsat imagery, quantitative estimates of glacier elevation changes and mass balances are given over three glacierized regions (SE QTP, DKMD region, and the western Kunlun Mountain (WKM)). The results reveal a contrasting pattern of glacier behaviors over different parts of the plateau in the past decade, from the strongest mass loss of temperate glaciers (-0.72±0.20 m water equivalent per year (w.e.a⁻¹)) in the SE QTP, to moderate mass loss of semi-continent glaciers (-0.42±0.15 m w.e.a⁻¹) over the central plateau and to most significant mass gain of continental glaciers (0.17±0.08 m w.e.a⁻¹) over the WKM in the northwest. The glacier changes and glacier-climate interactions over the three sub-regions are analyzed separately. For temperate glaciers over the SE QTP, ICESat data reveal strong glacier mass loss (4.71±1.30 Gt a⁻¹ , 0.012±0.005 mm sea level rise) and remarkable variability among the sub-regions (ranging from -0.38±0.27 to -1.05±0.36 m w.e.a⁻¹), which is generally consistent with GRACE observations. Faster thinning rate is found over the debris-covered ice (-0.96 ±0.34 m w.e.a⁻¹) than that of the clean-ice parts (-0.78±0.20 m w.e.a⁻¹). Climatic data at meteorological stations show that the SE QTP experienced dramatic rise of temperature in warm seasons in the past decade (0.43°C /10a over 20032013), which is probably responsible for the widespread glacier thinning. The difference in precipitation variations and the varying altitudes contributed to the spatial variability of glacier changes. For sub-continental glaciers over the DKMD region, the study finds significant inter-annual variation of the glacier area which showed sensitive response to local temperature and precipitation variations over 1976-2013. The annual mean temperature exerts strongest impact on the regional glacier changes, despite the high level of precipitation in the recent decade. In contrary to traditional assumptions on the dominant control of the Indian summer monsoon, this study shows that the continental westerly circulations dominate local climate variations and are closely connected with glacier changes in the past decades. The glacier variation shows tele-connection to large-scale atmospheric circulations associated with NAO (North Arctic Oscillation) and ENSO (La Niña) events. The continental glaciers in the WKM region exhibited high heterogeneity in surface elevation changes, which may be associated with individual glacier parameters including altitude, aspect, glacier hypsometry, ice flow, as well as with glacier surging events which are not directly driven by climate change. The Landsat observations confirm the relatively stable terminus of most glaciers in the WKM in the past 16 years and show three surging events characterized by different pattern of advancements. The pattern of glacier changes is similar to the 'Karakorum anomaly' which was found over the Karakorum glaciers in the northwestern QTP, while the overall mass gain in the WKM (0.20±0.04 m a⁻¹ , over 2003-2008) is the highest over the plateau. The techniques presented in this dissertation promote the use of multi-mission remote sensing observations for glacier mapping and analyzing glacier changes, and the study can be extended to other parts of the plateau. The research results contribute to a better understanding of the challenges in glacier mapping, the complexity and uncertainties of glacier changes, and the effects of the changing climate on the glacier mass balances.