• Energy Research

Knowledge about the long-term response of High Mountain Asia (HMA) glaciers to climatic variations is paramount because of their important role in sustaining Asian river flow. Here. a satellite-based time series of glacier mass balance for seven climatically different regions across HMA since the 1960s were estimated by DEM differencing of multi-temporal optical data. The DEMs were corrected for planimetric and altimetric shifts using SRTM as a reference. Elevation dependent biases, present due to the tilt between two DEMs, were also estimated for each DEM using two-dimensional first order polynomial trend surfaces relative to the SRTM DEM. To remove outliers, we analyzed individual glacier elevation differences for each 100 m altitude bin. Considering the heterogeneity of the thickness change in glacierized terrain, outliers were removed by using an elevation dependent sigmoid function. Our study reveals a constant mass loss in all regions even in regions where glaciers were previously in balance with climate.

This study is an assessment of the changes in seasonal and monthly flow in seven catchments draining the northern Tien Shan Mountains in Central Asia over a period from the 1950s to the present day. The purpose is to provide a first assessment of the flow response to climate change in regionally important catchments given their contribution to the water resource. All the catchments have a natural flow regime, and are therefore sensitive to climate change, but differ in area, elevation and glacial extent. Trends in flow were characterised using the Mann-Kendall test for standard meteorological seasons and individual months for mean flow, five flow quantiles and peak-over-threshold series for the period 1974–2013 at all sites and from the 1950s where data were available. The results were related to trends in seasonal temperature and precipitation from the regional high elevation meteorological stations and glacier mass balance, equilibrium line altitude (ELA) and accumulation\ud area ratio (AAR) records from the Tuyuksu glacier. The results show no reduction in streamflow in any catchment or season in the northern Tien Shan since the 1950s. Positive trends in all flow indicators, including peak over- threshold frequency, were observed in catchments with higher glacierization of over 10% and extensive presence of rock glaciers and permafrost indicating increased melt over the period which is characterised by a long-term increase in temperature. These trends were most evident in autumn and winter. In catchments with low glacierization, variability in summer flow was controlled primarily by precipitation of the preceding cold season. Correlation with glacier mass balance was weak but changes in ELA and AAR indicate that production of liquid runoff at higher elevations contributes to increased streamflow partly compensating for the declining glacier area. The observed changes in streamflow do not suggest any immediate problems with water availability\ud in the northern Tien Shan. On the contrary, increased autumn and winter flows point at a more prolonged recharge of reservoirs and aquifers though eventually this water source will be exhausted.

Continuous measurements of glaciological mass balance have been conducted at the Central Tuyuksu glacier, Tuyuksu group of glaciers, Ile Alatau, northern Tien Shan since 1957 showing that cumulative mass balance was negative since the 1970s. Geodetic mass balance was calculated for the 1958-1998 and 1998-2016 periods using multi-temporal digital elevation models derived from the historic photogrammetric surveys from 1958 and 1998, and the high-resolution Pléiades satellite stereo imagery from 2016. Geodetic measurements revealed a mean surface lowering of 23.2±2.2 m (0.40 ± 0.04 m a-1) and reduction in volume of (67.7±6.7)x106 m3 in 1958-2016 at the Central Tuyuksu glacier yielding a geodetic mass balance of -21.8±2.6 m w.e. Similar trends were observed at other glaciers of the Tuyuksu group which lost in total 83.4x106 m3 of ice. Annual rates of mass balance have not changed significantly from 1958-1998 (-0.39±0.05 m w.e.) to 1998-2016 (-0.35±0.18 m w.e.) at the Central Tuyuksu and at other glaciers of the Tuyuksu group whose maximum elevations exceed 4000 m a.s.l. While glacier thinning intensified in the ablation zone and affected larger area in 1998-2016 extending to 3600-3700 m a.s.l., accumulation increased at higher elevations in 1998-2016. Geodetic mass balance was more negative in 1998-2016 than in 1958-1998 at the smaller glaciers with lower maximum elevations. At the Central Tuyuksu, geodetic mass balance was in close agreement with glaciological mass balance particularly in 1958-1998 when the difference between the geodetic and cumulative glaciological mass balance values did not exceed 5 %. During 1998-2016, this difference increased to 14 % with glaciological method producing more negative mass balance. This discrepancy was attributed to a systematic bias introduced by the lack of stakes in the accumulation zone of Central Tuyuksu whose contribution to uncertainty increased in 1998-2016 in line with an increase in accumulation. Negative mass balance of the Tuyuksu group of glaciers was attributed to continuing increase in summer temperatures and low accumulation observed in the 1970s-1980s and at the turn of the century.

Impacts of projected climate and glacier change on river discharge in five glacierized catchments in the northern Tien Shan, Kazakhstan are investigated using a conceptual hydrological model HBV-ETH. Regional climate model PRECIS driven by four different GCM-scenario combinations (HadGEM2.6, HadGEM8.5, A1B using HadCM3Q0 and ECHAM5) is used to develop climate projections. Future changes in glaciation are assessed using the Blatter–Pattyn type higher-order 3D coupled ice flow and mass balance model. All climate scenarios show statistically significant warming in the 21st Century. Neither projects statistically significant change in annual precipitation although HadGEM and HadCM3Q0-driven scenarios show 20–37% reduction in July–August precipitation in 2076–2095 in comparison with 1980–2005. Glaciers are projected to retreat rapidly until the 2050s and stabilize afterwards except under the HadGEM8.5 scenario where retreat continues. Glaciers are projected to lose 38–50% of their volume and 34–39% of their area. Total river discharge in July–August, is projected to decline in catchments with low (2–4%) glacierization by 20–37%. In catchments with high glacierization (16% and over), no significant changes in summer discharge are expected while spring discharge is projected to increase. In catchments with medium glacierization (10–12%), summer discharge is expected to decline under the less aggressive scenarios while flow is sustained under the most aggressive HadGEM8.5 scenario, which generates stronger melt.