Multitemporal glacier inventory of the French Alps from the late 1960s to the late 2000s

Highlights

• We realize the first multitemporal inventory of the glaciers in the French Alps.

• We show that glacier extent decrease by 25% between 1967–71 and 2006–09.

• Glacier shrinkage greatly increases during the period 1985–86 to 2006–09.

• Shrinkage of the French Alpine glaciers is in agreement with other Alpine glaciers.

• Shrinkage of the French Alpine glaciers is linked with air temperature increase.

Abstract

The most recent and complete French glacier inventory was previously the Vivian database, dating from the end of the 1960s but incorporated in the World Glacier Inventory database at the end of the 1990s. Because of the important changes in glacier extent over recent decades an update of the inventory of glaciers of the French Alps was made in a digital vector format (with the associated database) for several dates covering the last 40 years. Such a multitemporal glacier inventory matches a key demand of the Global Terrestrial Network for Glaciers and the Global Land Ice Measurements from Space initiative (GLIMS). Topographical maps, aerial photographs and satellite images were used to map the extent of glaciers using both manual and automatic methods; and the database was generated considering the design of the GLIMS database. Glaciers in the French Alps covered 369 km² in 1967/71, 340 km² in 1985/86, 300 km² in 2003, and 275 km² in 2006/09. This represents a decrease in surface area of about 25% over the entire study period. Acceleration in glacier shrinkage during the study period was revealed, probably linked to the increase in average air temperature in the 20th century, which has been particularly pronounced since the 1970s. The behaviour of glaciers of the French Alps is in agreement with that of glaciers observed by other studies across the European Alps. We also report the distribution of the morpho-topographic variables (aspect, elevation, etc.) of glaciers of the French Alps for the period 2006/09, and analyse changes of these variables in the last four decades.

Introduction

As in most mountain ranges worldwide (e.g. Berthier et al., 2010, Rabatel et al., 2013a, Vincent et al., 2013), the Alps have seen general glacier shrinkage over the last decades (Abermann et al., 2009, Paul et al., 2011, Thibert et al., 2013). In this context, the accurate quantification of glacier changes is needed to better understand relationships with climate changes; and the glacier area survey is necessary to estimate at regional/global scales the ice volume, through area/volume relationships, and therefore the impacts of glacier changes on water resources or on sea level (Huss et al., 2010, Kaser et al., 2010, Bahr and Radić, 2012). Since the mid-1970s, international inventories like the World Glacier Inventory (WGI), services like the World Glacier Monitoring Service (WGMS), initiatives like GLIMS (Global Land Ice Measurements from Space), and projects like GlobGlacier have been established to address the question of glacier changes in mountainous areas by gathering measured data worldwide.

An inventory of the glacierized areas of the Alps was made by Paul et al. (2011) in the framework of GlobGlacier; it used automated techniques to identify glacier outlines on Landsat TM images acquired in 2003, with manual corrections. Previously, glacier inventories in the Alps had only been conducted at country or regional scales (Table 1). For the French Alps, two inventories were made by Mougin (1925) using topographical maps dating from the mid-19th century, and Vivian (1975) at the end of the 1960s based on topographical maps and aerial photographs; few subsequent updates were conducted at local scale for different dates (Table 1). The Vivian database was not originally in digital vector format; it was incorporated in the WGI database at the end of the 1990s as a table, and is available in digital format where each glacier or group of glacierets is represented as a single point. The lack of cartographic representation of the outlines of the glaciers and the fragmentation of glaciers since the 1970s make it difficult to compare the WGI database with the other local datasets. In addition, the exact date when the data for each glacier were collected is missing.

The recent glacier shrinkage largely justifies updating the extent of glacier cover in the French Alps, while creating repeat inventories at a time scale of a few decades is also a key demand of the Global Terrestrial Network for Glaciers (GTN-G). It is also worth emphasizing that the current work is made within the framework of the GLIMS Regional Centre #33 (French Alps) and the French observatory of glaciers: GLACIOCLIM which manages the glaciological measurements conducted on the French glaciers.

We first describe the data (topographical maps, aerial photographs and satellite images) and methods (including uncertainties quantification) used to map the glacier extent. Then, we present the results for 1967/71, 1985/86, 2003, and 2006/09, the changes the glaciers have undergone in the last four decades, and their relationship with topographical variables are presented. Finally, we discuss the timing and patterns of glacier changes, and compare our results with other Alpine regions.