Snow line altitude evolution in the Franco-Italian Alps over the 1984-2010 period reconstructed using optical remote sensing images

For mid-latitude mountain glaciers, the snow line altitude (SLA) at the end of the hydrological year is a good indicator of the equilibrium line altitude and thus of the annual mass balance. This enables SLA evolution to be reconstructed for long time periods from remote sensing data, as the snow line is generally easy to identify using aerial photographs and satellite images. Consequently, this enables researchers to study the climate-glacier relationship at the massif scale, particularly in remote areas where no direct measurements are available. In this study, we present series of reconstructed SLA for approximately 50 glaciers in the Franco-Italian Alps over the period 1984-2010. More than 130 images have been used, registred by the following satellites: Landsat 4TM, 5TM, 7 ETM+, SPOT 1 to 5 and ASTER, with spatial resolutions ranging between 2.5 and 30 m. To facilitate the identification of the snow line on the Landsat and SPOT-ASTER images, we used a combination of the spectral bands 542 and 431, respectively, with radiometric thresholds for bands 2, 4 and 1, 3. On each image, the snow line has been manually delineated. Its average altitude has been calculated for only the central part of the glaciers to avoid border effects on the glacier banks, which could generate snow line position dependence on local conditions (shadows from surrounding slopes, additional snow input by avalanches or due to wind drift). A comparison between SLA time-series and in-situ measurements of glacier mass balance was set for three French glaciers where direct mass balance measurements are available over the whole time-period (Argentière, Saint Sorlin and Gébroulaz glaciers). Results show high correlation (0.74 < r2 < 0.98, p < 0.01, n = 27), underlying the quality of the SLA dataset and strenghtening the interest of the SLA as a key indicator of the climate-glacier relationship. Considering the complete time period, the average SLA of the studied glaciers has increased by about 160 m (ranging between 0 and 380 m depending of the glacier). However, this increase is in the range of the interannual variability (±112 m). Considering each glacier individually, a meridional effect appears; the SLA is about 100 m higher for the glaciers located in the southern sector of the study area (i.e. the Ecrins Massif at 44°50' N), than for the those located in the nothern sector (i.e. the Mont Blanc Massif at 45°55' N). Considering summer temperatures and winter accumulation, this observation can be attributed to warmer and drier conditions in the southern Alps. However, this meridional effect does not affect the evolution of SLA over the study period. Exposure appears to be the most important factor controlling the SLA evolution, with glaciers exposed to the East experiencing the most important SLA increase. Finally, we present a preliminary study regarding the sensitivity of the SLA to both winter accumulation and summer temperatures.