India, 28 March 2007: The Himalaya, the source of the seven largest rivers of Asia are, like other mountain chains, suffering the effects of global warming. To assess the extent of melting of its 33 000 km2 of glaciers, scientists have been using a process they have been pioneering for some years.
DGPS positioning of an ablation marker on the tongue of Chhota Shigri Glacier at 4400 m. Satellite-imagery derived glacier surface topographies obtained at intervals of a few years were adjusted and compared. Calculations indicated that 915 km2 of Himalayan glaciers of the test region, Spiti/Lahaul (Himachal Pradesh, India) thinned by an annual average of 0.85 m between 1999 and 2004. The technique is still experimental, but it has been validated in the Alps and could prove highly effective for watching over all the Himalayan glacier systems. However, the procedure for achieving a reliable estimate must overcome a number of sources of error and approximation inherent in satellite-based observations.
The researchers started by retrieving satellite data for two periods, 2000 and 2004. A digital field model was extracted for each of them, representing the topography of a ground reference point in digital form and therefore usable in computerized processing. The earliest topography of the area studied was provided by NASA which observed 80% of the Earth’s surface during the Shuttle Radar Topographic Mission of February 2000. Then, in November 2004, two 2.5 m resolution images of the same area taken at two different angles were acquired especially by the French satellite Spot5 in the framework of an ISIS (CNES) project.
Comparison of these two images has helped build a Digital Elevation Model (DEM), by stereoscopic photogrammetric techniques . The DEM model reveals that NASA radar data underestimate values at high altitudes and overestimate them at lower altitudes. And the Spot satellite produces an uncertainty of +/- 25 m in the horizontal positioning of images.
The results show clear regression of the large glaciers whose terminal tongues reach the lowest levels (about 4000 m) with a thinning of 8 to 10 m below 4400 m. Such loss is 4 to 7 m between 4400 and 5000 m, passing to 2 m above 5000 m. The satellite image evaluation yields an average mass balance of –0.7 to –0.85 m/a water equivalent for the 915 km2 of glaciers surveyed, a total mass loss of 3.9 km3 of water in 5 years.
