Spatially variable response of Himalayan glaciers to climate change affected by debris cover

Controversy about the current state and future evolution of Himalayan glaciers has been stirred up by erroneous statements in the fourth report by the Intergovernmental Panel on Climate Change1,2. Variable retreat rates3–6 and a paucity of glacial mass-balance data7,8 make it difficult to develop a coherent picture of regional climate-change impacts in the region. Here, we report remotely-sensed frontal changes and surface velocities from glaciers in the greater Himalaya between 2000 and 2008 that provide evidence for strong spatial variations in glacier behaviour which are linked to topography and climate. More than 65% of the monsoon-influenced glaciers that we observed are retreating, but heavily debris-covered glaciers with stagnant low-gradient terminus regions typically have stable fronts. Debris-covered glaciers are common in the rugged central Himalaya, but they are almost absent in subdued landscapes on the Tibetan Plateau, where retreat rates are higher. In contrast, more than 50% of observed glaciers in the westerlies-influenced Karakoram region in the northwestern Himalaya are advancing or stable. Our study shows that there is no uniform response of Himalayan glaciers to climate change and highlights the importance of debris cover for understanding glacier retreat, an effect that has so far been neglected in predictions of future water availability9,10 or global sea level11. Snow and glacial meltwaters make a important contribution to the drinking water, agriculture, and hydropower supply of densely populated regions in South and Central Asia12,13. Because global warming is expected to increase mountain–river discharge in the short term, but reduce it in the long term9, detailed and reliable data on present-day climate change in mountainous Asia and its impact on the cryosphere are essential for predicting future water supplies10. However, the remoteness of this region hampers ground-based monitoring and results in very poor data coverage7. When mass-balance data are unavailable, scientists often refer to glacier retreats and advances as indicators of their response to climate change7,14, but frontal changes are not unambiguous indicators. Supraglacial debris cover influences the terminus dynamics and can thereby modify a glacier’s response to climate change. In the centralHimalaya, recent studies found several debriscovered glaciers with stagnant, that is, non-flowing, glacier reaches that extend several kilometres upstream from their termini15,16. Although growing meltwater ponds and surface lowering indicate that such glaciers are currently shrinking, their fronts remain remarkably stable17, as also been observed in other regions18,19. So far, however, the significance of debris cover and its impact on regional differences in the frontal dynamics of Himalayan glaciers has not been established at themountain-belt scale. Here, we assess regional differences in the terminus dynamics of glaciers in the greater Himalaya from remotely-measured frontal