Slight mass gain of Karakoram glaciers in the early twenty-first century

Assessments of the state of health of Hindu-Kush–Karakoram– Himalaya glaciers and their contribution to regional hydrology and global sea-level rise suffer from a severe lack of observations1. The globally averaged mass balance of glaciers and ice caps is negative1–3. An anomalous gain of mass has been suggested for the Karakoram glaciers2,4–6, but was not confirmed by recent estimates of mass balance. Furthermore, numerous glacier surges in the region that lead to changes in glacier length and velocity7–11 complicate the interpretation of the available observations. Here, we calculate the regional mass balance of glaciers in the central Karakoram between 1999 and 2008, based on the difference between two digital elevation models. We find a highly heterogeneous spatial pattern of changes in glacier elevation, which shows that ice thinning and ablation at high rates can occur on debris-covered glacier tongues. The regional mass balance is just positive at +0.11±0.22 m yr−1 water equivalent and in agreement with the observed reduction of river runoff that originates in this area12. Our measurements confirm an anomalous mass balance in the Karakoram region and indicate that the contribution of Karakoram glaciers to sea-level rise was −0.01 mm yr−1 for the period from 1999 to 2008, 0.05 mm yr−1 lower than suggested before13. The Karakoram mountain range, at the west end of the Himalayan arc, is covered by ∼19,950 km2 of glaciers4. Even though glaciological studies are scarce in this region, owing to remoteness and political issues, it seems that during the past three decades Karakoram glaciers did not follow the global trend of glacial decline2. Analysis of satellite imagery over six regions spread along the Hindu-Kush–Karakoram–Himalaya (HKKH) revealed that, in contrast to the central and eastern Himalaya where most glaciers were retreating, more than 50% of Karakoram glaciers were advancing or stable between 2000 and 2008 (ref. 6). Furthermore, Fujita and Nuimura14 reported a descending trend in the modelled equilibrium-line altitude in the Karakoram during 1976–1995. The gradual acceleration of the ice flow of nonsurging Baltoro Glacier during the 2000s is another sign of a stable or growing glacier15. These results, although they indirectly indicate a possible mass gain or an equilibrium state for glaciers in the region, are difficult to interpret because of the occurrence of surges and complex glacier behaviours5,7. For example, the equilibrium-line altitude and its temporal variations may not be readily interpreted in terms of glacier health16, particularly on surge-type glaciers. Hence, the existence of a Karakoram anomaly remains controversial and deserves urgent attention4. In this context, mass-balance data are needed to assess the state of health of Karakoram glaciers and constrain their contribution to sea-level rise1,3.