Les grands mouvements climatiques

The great climatic movements

Abstract

One of the great scientific contributions of our era will no doubt remain the decoding of planetary "archives" relative to climate which have enabled successive cold and hot, humid and dry periods to be restituted over more than a million years. Quite fine quantified descriptions of the last cycles covering a period of 150 000 years are now available. The explanation of these phenomena of the past - as regards both their rythm as well as their amplitude - remains an open question just as does the impact of human activities for the future: understanding (and simulation) of past fluctuations is not just a question of curiosity, they will enable questions about the future to be answered with more precision. The corresponding research effort is a vital part of the Geosphere-Biosphere programme which will cover the decade 1990 and from which much is to be expected in order to have a more responsable management of planetary resources. The relatively stable climatic conditions that we have been experiencing since the historic era only account for a moment in an otherwise active timescale. Drought phenomena and desertification must be interpreted by putting them back into this context depending on factors of change relative to very diverse timescales, the long term being equal to about 20 000 years (the last climatic cycle) and the short term about one century, (on the scale of anthropic factors).