First results of the new French Alps glaciers inventory

The only existing glacial inventory from the French Alps, conducted by Robert Vivian and published within the context of the World Glacier Inventory, dates back to the late 60s/early 70s. Glacial withdrawal over the previous decades largely justifies an update of the status of glacial coverage in the French Alps. This work is part of the activities conducted within the RC 512 (France) of the GLIMS program. A diachronic delineation of glacial contour has been conducted using different sources: 1) 1/25.000 topographical maps of the French Geographical National Institute: IGN (~1970, with varying dates according to the massifs and the updates of the maps); 2) 1985-86, Landsat 5 TM images (30 m resolution); 3) 2003, Landsat 5 TM and Landsat 7 ETM+ images (30 and 15 m resolution); and 4) 2006-2009, aerial photographs of the IGN (50 cm resolution). For the topographical maps and the aerial photographs, glacial contour has been manually delineated. For the Landsat images, an automatic delineation has been achieved using the common NDSI method applied to the 1985-86 images. Then, all glaciers have been individually checked to adjust the delineation of the contour line, specifically for debris-covered and shadowed areas, using a 542 spectral band combination. Resulting glacial contour for 1985-86 has been finally overlaid to the 2003 images, and glacial contour has been manually adjusted for each glacier individually. The overall glacial coverage of the French Alps was about 365 km2 in the late 60s/early 70s (WGI). In 1985-86, in spite of a short advancing period in the late 70s/early 80s, glacial coverage has decreased to a value close to 340 km2. Glacial withdrawal has strongly increased during the last 25 years, and glacial coverage has been reduced to about 270 km2 in the late 2000s, which represents an average loss of 26% over the last 40 years. At the scale of the different French Alps massifs, glacial withdrawal over the last two decades is more important for the southern ones (i.e. Ecrins and Belledonne massifs). In the Belledonne Massif, in relation with the lowest elevation of this mountain range (< 3.000 m a.s.l.), glaciers have almost completly disapeared. In the Ecrins Massif (44°50' N), glacial retreat is more than three times stronger than in the Mont Blanc Massif (45°55' N). This difference is likely a consequence of both a meridional effect (warmer and drier) and a lower elevation of the overall Ecrins Massif.