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Glacial Response to Climate Change in the Tien Shan Mountain Range of Central Asia

  • Author(s): Smith, Taylor
  • Advisor(s): Bookhagen, Bodo
  • et al.
Abstract

High Mountain Asia (HMA) has been strongly impacted by climate change, and will continue to be impacted by diverse climate stressors in the coming years. A combination of changing weather patterns and retreating glaciers has begun to impact water resources in the region; the rate, extent, and mechanisms behind these changes are poorly quantified due to a lack of data and process understanding. As rivers sourced in HMA feed more than a billion people in the region, increased understanding of climate impacts on regional hydrology is essential for effective water management. Glaciers play a diverse role in hydrology throughout the range, with some catchments having little to no reliance on glacial meltwater, and some catchments receiving almost all of their water from glaciers. This study aims to decipher the impact of climate change on glaciers in the Tien Shan mountain range, a large and understudied region that extends roughly 2,800 km northeast from the Pamir Knot in Tajikistan through China, Kazakhstan, Kyrgyzstan, and Mongolia towards the Altai Mountains.

Previous studies in the region lack the data to decipher gradients in glacial retreat across the range. To address this data deficit, the author designed and implemented a glacial mapping algorithm which delineates both clean glacial ice -- methods which are well documented -- and glacial debris tongues, which often require extensive manual digitization. This research is the most significant part of the thesis, and improves upon methods developed to automatically delineate glacial areas using spectral, topographic, velocity, and spatial relationships. The author found that the algorithm misclassifies between 2 and 10% of glacial areas, compared to a ~700 glacier control dataset. These results show significant improvement over previously used methods, as well as large-scale glacier databases such as the Randolph Glacier Inventory (RGI) which are commonly used in mountain-belt scale studies of glaciers.

After validating the algorithm against multiple manually digitized control datasets, the author applied it to a study area encompassing six Landsat scene footprints along the strike of the Tien Shan Orogen. A statistically significant, though minor, gradient in glacier retreat was found, where those glaciers in the west of the range have retreated less than those glaciers in the east. This gradient is explained by differences in regional climate, where large winter storms tend to arrive from the west and break apart and weaken before moving towards the eastern edge of the range. This is substantiated by differences in mean annual precipitation, as derived from Tropical Rainfall Measurement Mission (TRMM) 3B43 V7 data, as well as differences in atmospheric lapse rates, where the western region of the range remains cold year-round and the more eastern edges have larger annual temperature fluctuations, as well as a lower overall lapse rate, which implies higher temperatures at higher elevations.

As glacial area extents do not present a complete picture of the impacts of climate change on glaciers, the author undertook a pilot study to examine the mass balances of glaciers throughout the range. Mass balance estimates are a proxy for changes in water equivalent stored in a glacier, and are thus essential measurements for understanding the hydrology of the range. The author presents preliminary evidence that high elevation areas in the Tien Shan are thickening, even in the context of retreating glacial tongues. This implies an overall gain of ice mass in the region, despite generally retreating glaciers. This trend was linked to recent increases in precipitation in the range, as well as increases in air temperature. There is not yet enough data to unravel changes in glacier mass balance across the range, although the author predicts that the recently coined 'Karakoram Anomaly' may in fact need to be renamed to include both the Pamir and the western Tien Shan.

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