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Glacier Changes in Asia

For almost a decade mass changes in the High Mountain Asian glaciers have been under dispute. An observation based mass budget for all the glaciers in the region resulted from an analysis of satellite data archives.

High Mountain Asia is the region of the Earth that comprises the highest mountain peaks and the largest concentrations of glaciers outside the poles. In this region the glaciers are essential for water supply. The meltwater of these glaciers are of paramount importance, providing water for irrigation, the production of hydropower, as well as other industrial uses. The water from these glaciers has been estimated to satisfy the basic needs of 136 million people (Pritchard, 2017). The future evolution of the glaciers of the region is of great concern against this background. Since the Intergovernmental Panel on Climate Change (Cruz et al., 2107) claimed, erroneously, that the likelihood of [Himalayan glaciers] disappearing by the year 2035 and perhaps sooner is very high, glaciologists around the globe have tried to provide a better basis upon which such claims are built, as fast as possible. Brun et al. (Brun et al., 2017) have presented in Nature Geoscience a milestone in this endeavour: a geographically complete estimate that is measurement-based of the mass change of every glacier in the region during the past 1.5 decades. They have found that mass loss of regional glaciers has been less pronounced than has been suggested by previous estimates, and provide essential constrains for the variability of loss on a subregional scale.

So far, evidence of glacier mass changes for High Mountain Asia were derived either by regional extrapolation of the small number of direct observations that were available (Cogley, 2011), or by the interpretation of various types of satellite data that have specific limitations. E.g., laser altimetry (Kaab et al., 2012) has very low spatial sampling, which spaceborne gravimetery (Gardner et al., 2013) is affected by spatial resolution that is very coarse.

These difficulties were overcome by Brun et al. (Brun et al., 2013). Surface elevation changes in a set of more than 50,000 digital elevation models that had been derived from optical imagery of the advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) were automatically generated and analysed. As the data are optical and not radar images they additionally avoid difficulties with penetration of the signal, an issue that can distort results that are based on radar (Gardelle et al., 2013), as radar penetration is variable in time and is difficult to constrain. They are enabled by this approach to compute individual mass changes for 92 % of the area that was covered by glaciers in High Mountain Asia, which is an unprecedented coverage and resolution.

For the whole of High Mountain Asia the rate of mass loss from glaciers during the period 2006-2016, was 16.3 3.5 Gt/year, according to Brun et al. This is almost 3 times smaller than previous estimates based on large-scale model studies (Kaab et al., 2012; Marzelon et al., 2015). Central Asia and the southern Himalayan arch show the most pronounced differences. Brun et al. attributed the data basis that was comparatively weak that these earlier studies relied on.

Marked differences between individual regions were highlighted by Brun et al. which provides some unexpected results: e.g. they have confirmed the existence of a region where glaciers are slightly gaining mass, a phenomenon that is known as the Karakoram anomaly (Hewitt, 2005), though they did detect the centre of the region to be over the western Kunlun Mountains, instead of over the Karakorum Range itself. In the easternmost parts of High Mountain Asia, however, very strong mass losses are pointed out. The Kunlun Mountains are where the most positive changes are found, where glaciers have been thickening by close to 15 cm/year (+0.14 0.08 m water equivalent per year), whereas the most negative changes are detected in the Nyianqentanglha Region, where there were reductions in the thickness of the ice that were as large as -60 cm/year (-0.62 0.23 m water equivalent per year).

According to Brun et al. this study is one of a wave of investigations that are releasing the full potential of the existing archives for the quantification of responses to climate change on a large scale. It delivers the basis for the provision of insights into the mechanisms governing the heterogeneous glacier response, though it does not provide insights into these mechanisms; the modelling community can make use of these observations.

A unique inventory of high-resolution glacier mass changes in the highest mountain range in the world is presented by Brun et al. (Brun et al., 2017).  The results of this study will be essential for calibrating models investigating cause-response mechanisms of the evolution of glaciers, and this in turn can be expected to increase the accuracy of projections addressing the availability of meltwater in the future. According to Farinotti this will be crucial for planning sustainable responses, in light of the changes to come, in a region that is understood to be particularly vulnerable.

Sources & Further reading

Farinotti, D. (2017). "Cryospheric science: Asia's glacier changes." Nature Geosci 10(9): 621-622.


Author: M. H. Monroe
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