The
variations in the annual volumes of Alpine glaciers, known as mass
balances, are a direct reflection of the climate at high altitudes.
The winter balance, after accumulation, depends on winter precipitation,
and the summer balance, after ablation, results from the variations
in energy fluxes at the surface, itself correlated to fluctuations
in summer temperatures. A study of variations in the mass balance
of glaciers gives a more accurate picture of climate evolution at
high altitudes than studies of the fluctuations of glacier fronts,
since these cannot be interpreted directly in terms of climatic variations.
Since
2000, the "Observatoire des sciences de l'Univers de Grenoble"
(Observatory of Sciences of the Universe of Grenoble) has been funding
systematic observations of mass balances. The CNRS "Laboratoire
de glaciologie et de géophysique de l'environnement de Grenoble"
(Laboratory of Glaciology and Environmental Geophysics of Grenoble)
is taking biennial measurements on a set of four glaciers of the French
Alps. Relatively precise topographical maps drawn up at the beginning
of the twentieth century, direct measurements that began in the mid-twentieth
century and current observations have enabled scientists to reconstitute
the history of fluctuations in glacial volumes and thus the climatic
variations over the past century above an altitude of 2,500 meters
in the French Alps. Variations in volume are measured either directly,
using core drilling and stakes measurements, or by comparing detailed
topographical maps.
The
twentieth century is marked by two periods of significant shrinkage:
the 1940s and 1982 to the present day. Both periods were preceded
by periods of little loss in volume, or even some gain. Before the
twentieth century, observations of glacier fronts provided the only
indications of the state of glaciers. Although imperfect, they show
that glaciers have shrunk significantly since the end of the Little
Ice Age, and that this is a global phenomenon. The shrinkage of the
late 1940s was a result of both low winter precipitation and sunny
summers, whereas the marked shrinkage of the past twenty years results
only from warmer summers.
