Abstract
ABSTRACTThe ablation areas of debris-covered glaciers typically consist of a complex mosaic of surface features with contrasting processes and rates of mass loss. This greatly complicates glacier response to climate change, and increases the uncertainty of predictive models. In this paper we present a series of high-resolution DEMs and repeat lake bathymetric surveys on Ngozumpa Glacier, Nepal, to study processes and patterns of mass loss on a Himalayan debris-covered glacier in unprecedented detail. Most mass loss occurs by melt below supraglacial debris, and melt and calving of ice cliffs (backwasting). Although ice cliffs cover only ~5% of the area of the lower tongue, they account for 40% of the ablation. The surface debris layer is subject to frequent re-distribution by slope processes, resulting in large spatial and temporal differences in debris-layer thickness, enhancing or inhibiting local ablation rates and encouraging continuous topographic inversion. A moraine-dammed lake on the lower glacier tongue (Spillway Lake) underwent a period of rapid expansion from 2001 to 2009, but later experienced a reduction of area and volume as a result of lake level lowering and sediment redistribution. Rapid lake growth will likely resume in the near future, and may eventually become up to 7 km long.
Highlights
Glaciers in many parts of the Himalaya have lost mass in recent decades in response to warming climate (Berthier and others, 2007; Bolch and others, 2008, 2011; Quincey and others, 2009; Gardelle and others, 2012; Kääb and others, 2012)
On the basis of the visual appearance in optical imagery and the DEM difference maps, we define six morphometric categories: (1) areas where surface elevation change is influenced by glacier movement (2) ice cliffs, (3) debris-covered ice, (4) lateral moraines and ice-marginal troughs, (5) perched lakes (6) Spillway Lake
Ngozumpa Glacier crossed the threshold into the initial phase of Regime 3 in the 1990s, when Spillway Lake began to form at base level
Summary
Glaciers in many parts of the Himalaya have lost mass in recent decades in response to warming climate (Berthier and others, 2007; Bolch and others, 2008, 2011; Quincey and others, 2009; Gardelle and others, 2012; Kääb and others, 2012). Spatial patterns of mass loss have been determined, allowing the relative contribution of different processes (e.g. subdebris melting vs ice cliff retreat) to be established (Sakai and others, 2002; Han and others, 2010; Immerzeel and others, 2014; Juen and others, 2014; Reid and Brock, 2014) Such studies, require labor-intensive field campaigns, placing limits on spatial and temporal coverage (Sakai and others, 1998, 2009; Nuimura and others, 2011; Immerzeel and others, 2014). Velocities on the glacier tongue and the extent of stagnant ice were determined by feature tracking using TerraSAR-X images In combination, these data allow us to investigate ablation processes on the glacier and assess their relative contribution to mass loss. Our results provide a high-resolution snapshot of Ngozumpa Glacier at a critical stage of its evolution, and data necessary for modeling the future response of Himalayan debris-covered glaciers to climate change (Rowan and others, 2015)
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