In this extremely rich context, one might think that the glacial archives from those very distant polar regions are of only marginal, almost anecdotal interest. This isn’t true. Although it is true that reconstructing the temperatures in Greenland or in Antarctica adds only a few more sites, the possibility of looking in detail at the evolution of the climate through the years, and over very long periods, is unequaled. Above all, glacial archives are unique because of their ability to trap the atmosphere of the past and, thanks to their extreme purity, to retain traces of the slightest effect, whether of continental, oceanic, volcanic, or extraterrestrial origin, or of that linked to human activity. Before describing the methods used by glaciologists to get a piece of ice to unveil all its secrets, we will briefly tell the long story of a snowflake from the moment it is formed to when it disappears in the ocean.
The life expectancy of snowflakes that feed the surface of glaciers and ice sheets depends largely on climatic conditions. In cold areas without summer melting, such as the central regions of the ice sheets and the very highaltitude glaciers, they have a very long life, punctuated by several stages. Their youth varies from a few dozen to several thousand years. During this period the snow crystal rapidly loses its splendid needles, rounds out, and becomes granular, then, from the effect of more layers of snow, the grains pile up and are joined together. Nature has put into play on the surface of the white planet, well before the inventive genius of humans and on a gigantic scale, the process that powder metallurgists have perfected to create matter with satisfactory properties of cohesion and rigidity. The adolescence of a snowflake is thus the phase of transformation, called névé, which leads to its adult state: