Changes in glacierisation, climate and runoff in the second half of the 20th century in the Naryn basin, Central Asia

• We determined glacier area changes in the Naryn basin, Tien Shan Mountains.
• Glacier area decreased from the 1970s to mid 2000s by 23%.
• This coincides with increasing summer temperature and number of positive degree days.
• Negative trends in winter precipitation support this process.
• However, increasing summer discharge is based on increasing annual precipitation.

Glaciers are significant fresh water storages in Central Asian high mountains and are considered to substantially contribute to the summer runoff of Central Asian Rivers. We present a comprehensive study of the glacier area changes in the Naryn catchment located in the Tien Shan Mountains. The catchment with a size of 55,944 km2 is a major tributary of the Syrdarya River which is heavily used for water supply and irrigation. We analysed the glacier retreat based on Landsat MSS, TM and ETM + imagery for the mid-1970s, late 1990s and mid-2000s and based on a SPOT scene for 2007. Our results show a decreasing glacierisation within the catchment, shrinking from 1210 ± 30 km2 (2.2% glacierisation) in the 1970s to 1019 ± 25 km2 (1.8% glacierisation) in the late 1990s and further down to 926 ± 23 km2 (1.7% glacierisation) in the mid-2000s, corresponding to an area loss of 23% in total. The analysis reveals spatially heterogeneous area loss within the catchment. This can be associated with different hypsometries, size distributions, aspects and presences of debris cover. Small glaciers (with an area < 1 km2) suffered from a strong area loss within the 30-years investigation period.Trends in air temperature, precipitation and positive degree days (PDD) at climate stations suggest that the glacier retreat is likely to be driven by the increasing summer (April–September) temperature, rather than changes in precipitation: In the period from 1960 to 2007, both summer air temperature and PDDs increased significantly at a rate of 0.19 °C/decade and 3.9 °C/decade respectively, whilst for precipitation no consistent trends were detected. However, rigorous attribution of changes is complicated by the variable glacier response times. In the two headwater sub-catchments of the Naryn basin, Small and Big Naryn, positive trends in spring and autumn discharge were detected and are likely to be associated with the enhanced snow and glacier melt driven by increasing temperatures in those seasons. However, no discharge trends in August – the month with the largest expected glacier contribution – were detected. The strong, significantly positive trends in winter and early spring runoff are associated with strongly increasing winter temperatures and number of days with maximum daily temperature above the freezing point causing snow melt. Hence, increasing glacier area reduction can be explained by the prolongation of the melting season reducing accumulation rather than by increasing annual mean temperatures. Despite the high relative changes, the absolute increase in winter discharge is very small.