Contrasting patterns of early twenty-first-century glacier mass change in the Himalayas.

Most glaciers around the world have been losing mass. The loss of glacial mass could have serious and negative implications on water resources (ie. hydropower, irrigation, runoff) of the surround communities and is a major contributor to rising sea levels. Kaab et al. 2012 studied glacier thickness changes and estimated mass changes over the Hindu Kush-Karakoram-Himalaya (HKKH) region during 2003 – 2008. To do this, they combined two elevation data sets from the Ice Cloud and land Elevation Satellite (ICESat) and the Digital Elevation Model (DEM) from the Shuttle Radar Topography Mission (SRTM). The authors found that debris-covered ice thins at a similar rate to that of exposed ice, suggesting a reassessment of the role of debris mantles in glacier mass balance. This study can be considered as the first quantitative evaluation of the contribution of glacier imbalance to river runoff. —Paloma Medina

Kaab, A., Berthier, E., Nuth, C., Gardelle, J., Arnaud, Y., 2012. Contrasting patterns of early twenty-first-century glacier mass change in the Himalayas. Nature 488, 495-498

The HKKH is a conglomeration of mountain ranges in Asia spanning 2,000 km and separates the plains of the Indian subcontinent from the Tibetan Plateau. These mountains contain about 60,000 km2 of glaciers, glacierets and perennial surface ice in varying climatic regions. The HKKH accumulates or ablates ice mass in varying areas and in varying times of the year. In the east, glaciers receive most of their accumulation during summer month from the Indian monsoon, whereas in the west they accumulate snow in the winter through westerly atmospheric circulations. Therefore, variability among the glacier geography is large and could lead to different levels of impact of sea-level rise, water resources, and natural hazards. Kaab et al. looks to assess the changes in glacier mass with respect to this variation using ICESat and DEM satellites.
Length changes measured for more than 100 glaciers in HKKH suggest that most Himalayan glaciers have been retreating since the mid-19th century. The SRTM DEM and ICESat data sets provide geographically distinguishable data of glacier thickness. Through these satellite instruments, the researchers were able to distinguish between glacier clean ice, glacier debris cover, glacier firn and snow, open water, and off-glacier. These data were used to map trends of climatological and glaciological patterns between five major subregions of HKKH: the Hindu-Kush south of the Wakhan Corridor (HK), the Karakoram (KK), Jammu-Kashmir (JK), Himachal Pradesh, Uttarakhand and West Napa (HP), and East Nepal and Bhutan (NB). ICESat data showed a +0.14 + 0.06 m yr –1 thickening in the northern and eastern parts of KK. However, ICESat data indicated that HKKH glaciers thinned on average of  –0.21 + 0.06 m yr –1. This thinning is significantly less than the estimated global average from glaciers and ice caps. This difference is mainly attributed to the glacier mass thickening in the Karakoram. ­­
Contrary to previous expectations, the average thinning rates under debris-mantled ice were similar to those of clean ice despite the insulation of the debris covers. This finding suggests that the insulating effect of debris layers with thicknesses exceeding a few centimeters acts on local scales, but not on the larger scale of entire glacial tongues.  The study applied elevation difference trends to the total ice-covered area by using a glacier mask based on a ratio between visible and short-wave infrared Landsat bands and glacier inventories.  The researchers found that there was a mass change of -12.8 + 3.5 gigatonnes per year and a sea level rise contribution of 0.035 + 0.009 mm yr-1for HKKH glaciers, accounting for 3% to 4% of the total contribution from global glaciers and ice caps.
This study suggests using satellite gravimetry to better detect large scale sub-surface mass changes such as from hydrology or tectonics. In addition, this method could better quantify errors in detecting mass changes by relying on studies such as this that estimate glacier thickness changes. Bolch et al. (2012) pushes for more developed and refined remote-sensing methods to estimate glacial changes, draws attention to the climatic and hydrological station network gap, and strongly recommends increasing the number of active research stations on glaciers. They also recommend establishing new programs to cover more climate zones and glacier types. It is important to understand the hydrological impacts of the Himalayan glaciers, and glaciers in general, because many populations rely heavily on glacial river run-off. A negative annual mass budget, or a decrease in glacial mass, yields a surplus of runoff from glacier ice. Conversely, a positive annual budget yields a deficit of runoff because ice has gone into storage on the glacier. The authors hope that their study will improve estimates of groundwater depletion in northern India, which thus far has been difficult to discriminate from glacier loss in satellite gravity observations. Furthermore, glacier melting could have catastrophic consequences on lake or dam areas that cannot accommodate for an influx of water. All in all, the researchers look to redefine and reassess the functionality of debris-covered ice and impact on river run-off. A better understanding of glacier ablation and accumulation gives a more holistic understanding to glacial regulation and preservation. 

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