Contrasting patterns of early twenty-first-century glacier mass change in the Himalayas

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Abstract
Glaciers are among the best indicators of terrestrial climate variability, contribute importantly to water resources in many mountainous regions(1,2) and are a major contributor to global sea level rise(3,4). In the Hindu Kush-Karakoram-Himalaya region (HKKH), a paucity of appropriate glacier data has prevented a comprehensive assessment of current regional mass balance(5). There is, however, indirect evidence of a complex pattern of glacial responses(5-8) in reaction to heterogeneous climate change signals(9). Here we use satellite laser altimetry and a global elevation model to show widespread glacier wastage in the eastern, central and southwestern parts of the HKKH during 2003-08. Maximal regional thinning rates were 0.66 +/- 0.09 metres per year in the Jammu-Kashmir region. Conversely, in the Karakoram, glaciers thinned only slightly by a few centimetres per year. Contrary to expectations, regionally averaged thinning rates under debris-mantled ice were similar to those of clean ice despite insulation by debris covers. The 2003-08 specific mass balance for our entire HKKH study region was -0.21 +/- 0.05 m yr(-1) water equivalent, significantly less negative than the estimated global average for glaciers and ice caps(4,10). This difference is mainly an effect of the balanced glacier mass budget in the Karakoram. The HKKH sea level contribution amounts to one per cent of the present-day sea level rise(11). Our 2003-08 mass budget of -12.8 +/- 3.5 gigatonnes (Gt) per year is more negative than recent satellite-gravimetry-based estimates of -5 +/- 3 Gt yr(-1) over 2003-10 (ref. 12). For the mountain catchments of the Indus and Ganges basins(13), the glacier imbalance contributed about 3.5% and about 2.0%, respectively, to the annual average river discharge(13), and up to 10% for the Upper Indus basin(14).