Takayuki Nuimura

Spatially heterogeneous wastage of Himalayan glaciers

We describe volumetric changes in three benchmark glaciers in the Nepal Himalayas on which observations have been made since the 1970s. Compared with the global mean of glacier mass balance, the Himalayan glaciers showed rapid wastage in the 1970s–1990s, but similar wastage in the last decade. In the last decade, a glacier in an arid climate showed negative but suppressed mass balance compared with the period 1970s–1990s, whereas two glaciers in a humid climate showed accelerated wastage. A mass balance model with downscaled gridded datasets depicts the fate of the observed glaciers. We also show a spatially heterogeneous distribution of glacier wastage in the Asian highlands, even under the present-day climate warming.

Performance of ASTER and SRTM DEMs, and their potential for assessing glacial lakes in the Lunana region, Bhutan Himalaya

To assess the potential volume of a glacial lake outburst flood (GLOF) more precisely than in previous studies, we analyze ground survey data and remote-sensing digital elevation models (DEMs) around glacial lakes in the Lunana region, Bhutan. Based on a DEM generated from differential GPS ground surveys, we first evaluate the relative accuracies of DEMs produced by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and the Shuttle Radar Topography Mission (SRTM). Root-mean-square errors of the altitudinal difference between these DEMs and ground survey data were 11.0 m for ASTER and 11.3 m for SRTM. These errors are similar to those of previous studies. We show that a topographical classification allows a better estimate of elevation on lakes/ ponds, riverbeds and glaciers due to their flat surfaces, while the relative accuracy is worse over moraines and hill slopes due to their narrow ridges and steep slopes. Using the optical satellite images and the ground survey data, we re-evaluate the GLOF volume in 1994 as (17.2 5.3) 10 6 m 3 . We show GLOF-related information such as distance, altitudinal difference and gradient at possible outburst points where the lake level is higher than the neighboring riverbed and/or glacial lake.

Recent changes in Imja Glacial Lake and its damming moraine in the Nepal Himalaya revealed by in situ surveys and multi-temporal ASTER imagery.

Changes in the area and bathymetry of Imja Glacial Lake and in the elevation of its damming moraine, Khumbu region, Nepal Himalaya are investigated. Previously reported changes in the lake area have been updated by multi-temporal ASTER images, which revealed a decreased expansion rate after 2000. A provisional expansion of the lake observed in 2004, from which some studies concluded an accelerated lake expansion due to global warming, has, from 2005, subsided to the glacier surface. Bathymetric changes for the period 1992–2002 that were first obtained for Himalayan glacial lakes suggest that the melting of debris-covered ice beneath the lake is insignificant in terms of the increase in lake volume, and that the retreat of a glacier in contact with the lake by calving is essential for the lake’s expansion. Changes in the height of a damming moraine for the period 2001–2007 suggest a continuous surface lowering near the lake, though the lowering rates are smaller than those for the period 1989–1994.

Distribution of debris thickness and its effect on ice melt at Hailuogou glacier, southeastern Tibetan Plateau, using in situ surveys and ASTER imagery

Debris cover is widely present in glacier ablation areas of the Tibetan Plateau, and its spatial distribution greatly affects glacier melt rates. High-resolution in situ measurements of debris thickness on Hailuogou glacier, Mount Gongga, southeastern Tibetan Plateau, show pronounced inhomogeneous debris distribution. An analysis of transverse and longitudinal profiles indicates that the ground-surveyed debris thicknesses and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)-derived thermal resistances of debris layers correlate strongly over the entire ablation area. Across- and along-glacier patterns of ASTER-derived thermal resistance correspond well with spatial patterns of debris thickness, which may reflect large-scale variations in the extent and thickness of the debris cover. The ice melt rate variability over the ablation area simulated by a surface energy-balance model that considered thermal resistance of the debris layer indicates clearly the crucial role of debris and its spatial continuity in modifying the spatial characteristics of melt rates. Because of the inhomogeneous distribution of debris thickness, about 67% of the ablation area on Hailuogou glacier has undergone accelerated melting, whereas about 19% of the ablation area has experienced inhibited melting, and the sub-debris melt rate equals the bare-ice melt rate in only 14% of the ablation area.

Temporal Changes in Elevation of the Debris-Covered Ablation Area of Khumbu Glacier in the Nepal Himalaya since 1978

Abstract We evaluated elevation changes at four sites on debris-covered ablation area of Khumbu Glacier, Nepal Himalaya, since 1978. In 2004, we carried out a ground survey by differential GPS in the upper- and lowermost areas of the ablation area. The amount of surface lowering was calculated by comparing digital elevation models (DEMs) with 30-m grid size, as generated from survey data corrected in 1978, 1995, and in the present study. Because we could not access the middle parts of the debris-covered area due to surface roughness, for this area we used an ASTER-DEM calibrated by the ground survey data. The amount of surface lowering during the period 1978–2004 was insignificant near the terminus. A remarkable acceleration of surface lowering was found in the middle part of the debris-covered ablation area, where the glacier surface is highly undulating. In the uppermost area, surface lowering has continued at a steady rate. Surface flow speeds have decreased since 1956, revealing that the recent decrease in ice flux from the upper accumulation area would have accelerated the rate of surface lowering of the debris-covered area of Khumbu Glacier during the period 1995–2004.

Contrasting thinning patterns between lake- and land-terminatingglaciers in the Bhutan Himalaya

Despite the importance of glacial lake development in ice dynamics and glacier thinning, in situ and satellite-based measurements from lake-terminating glaciers are sparse in the Bhutanese Himalaya, where a number of proglacial lakes exist. We acquired in situ and satellitebased observations across lakeand land-terminating debriscovered glaciers in the Lunana region, Bhutanese Himalaya. A repeated differential global positioning system survey reveals that thickness change of the debris-covered ablation area of the lake-terminating Lugge Glacier (− 4.67± 0.07 m a−1) is more than 3 times more negative than that of the land-terminating Thorthormi Glacier (−1.40± 0.07 m a−1) for the 2004–2011 period. The surface flow velocities decrease down-glacier along Thorthormi Glacier, whereas they increase from the upper part of the ablation area to the terminus of Lugge Glacier. Numerical experiments using a two-dimensional ice flow model demonstrate that the rapid thinning of Lugge Glacier is driven by both a negative surface mass balance and dynamically induced ice thinning. However, the thinning of Thorthormi Glacier is minimised by a longitudinally compressive flow regime. Multiple supraglacial ponds on Thorthormi Glacier have been expanding since 2000 and have merged into a single proglacial lake, with the glacier terminus detaching from its terminal moraine in 2011. Numerical experiments suggest that the thinning of Thorthormi Glacier will accelerate with continued proglacial lake development.