Slight mass gain of Karakorum glaciers in the early 21st century

International audienceThere is a severe lack of observations to assess the state of health of Hindu Kush-Karakoram-Himalaya glaciers and constrain their contribution to regional hydrology and global sea-level rise1. In the context of a negative globally-average mass balance of glaciers and ice caps1,2,3, an anomalous behaviour of Karakoram glaciers has been suggested2,4,5,6, but not confirmed by recent mass balance measurements. Furthermore, the presence of numerous glacier surges in the region7,8,9,10,11 complicates the interpretation of the few observations available (e.g., length and velocity changes). Here, we observe the regional mass balance of glaciers in the central Karakoram between 1999 and 2008 by differencing two digital elevation models. The spatial pattern of glacier elevation changes is highly heterogeneous, confirms that glacier surges are widespread and shows that high ice thinning and ablation rates can occur on debris-covered glacier tongues. The regional mass balance is slightly positive (+0.11 ± 0.22 m a-1 water equivalent) and in agreement with a decreasing runoff from rivers originating in this area12. We thereby confirm the so-called "Karakoram anomaly" and show that the sea-level rise contribution of glaciers in this mountain range was -0.01 mm a-1 during 1999-2008, a reduction by 0.05 mm a-1 compared to a recent estimate1