Located over 12 000 kilometers from the Alps, the Kerguelen Islands are home to the largest French glacier, the Cook ice cap (which had an area of around 500 km2 in 1963). By combining historical information with recent satellite data, the glaciologists at the Laboratory for Space Studies in Geophysics and Oceanography (Université Paul Sabatier / CNRS / CNES / IRD) have observed increasingly rapid shrinkage of the ice.
Over the last 40 years, the Cook ice cap has thinned by around 1.5 meters per year, its area has decreased by 20%, and retreat has been twice as rapid since 1991. Their work has been just published in the Journal of Geophysical Research.
The Kerguelen Islands are located in the southern Indian Ocean, and numerous glaciers cover the highest areas of the islands. The first studies carried out in this exceptional natural laboratory for French research showed an initially slow retreat of the Ampère glacier (one of the outlet glaciers of the Cook ice cap) between 1800 and 1965, subsequently becoming much faster. Since 1974, in situ monitoring of the Cook ice cap has no longer been carried out. However, observations made from space between 1991 and 2006 have enabled scientists to collect data from this relatively inaccessible area.
Glaciologists from the Laboratory for Space Studies in Geophysics and Oceanography (LEGOS - Université Toulouse 3/CNRS/IRD/CNES) began their work by compiling a complete inventory of the glaciers on the Kerguelen Islands from an Institut Géographique National (IGN) map published in 1967. At that time, these glaciers covered over 700 km2, including 500 km2 for the Cook ice cap alone.
The scientists then used Spot and Landsat satellite images to update this inventory for the years 1991, 2001 and 2003, and to quantify glacial retreat. By 1991, the Cook ice cap covered a mere 448 km2, and by 2003 this had fallen to 403 km2. It has thus lost 20% of its area in 40 years, and it has been retreating twice as fast since 1991. In addition, the researchers estimated the volume loss (or mass balance) of the Cook ice cap over the last 40 years.
This mass balance accurately characterizes the response of the glacier to climatic variation (temperature, precipitation), and can be used to compare glacier response in various areas of the world. For instance, the Cook ice cap has thinned by as much as 300 to 400 meters in glacier tongues at low altitude, whereas variations in thickness appear to be smaller in high regions. On average, for the whole ice cap since 1963, thinning reached 1.5 metres per year, which is a very high value when compared to other glaciers in the world. In the last 40 years, the ice cap has lost about 22% of its volume. This thinning also appears to have been accelerating over the recent period.
The glaciers in the Kerguelen islands were already retreating in the 1960s, and their decline over the past 40 years cannot be attributed only to recent warming partly due to human activities. Part of this retreat can in fact be explained by a delayed response of these glaciers to the natural warming that followed the Little Ice Age (a cold period that ended between 1850 and 1900). However, the recent acceleration of ice wastage is doubtless connected to high temperatures and low precipitation since the beginning of the 1980s.
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