The Earth’s Environment has become a major concern of humanity. We realize in this new millennium that our activities are not without effect on the whole planet. As geologist Paul Crutzen (Nobel Prize in Chemistry, 1995) said: “man has become a geological factor, we are entering the Anthropocene”. The corollary of this observation is the question of sustainable development: how are we going to grow and develop on this planet, while maintaining rising living standards and social progress, but without the “natural” environment deteriorating to a point that condemns our own future? The problem of climate change provides us with an example, among others, of the global influence and the potential dangers of human activities on the evolution of Earth. Many other examples will arise in the future, such as water resources, flood forecasting, soil protection, chemical contamination, and food security.

The challenge confronting the scientific community is the need to provide solutions to manage our coexistence with Earth. In their report on the future of geosciences in October 2001, the US National Research Council Committee stressed that our disciplines are at a crossroad and how much more than ever, the multidisciplinary approach and auscultation to a long-term Earth is needed.

It particularly emphasized the importance of better understanding the “Critical Zone of the Earth”, the area between the top of the canopy and the base of the soil that is in contact with the lithospheric parent rocks. This area is in effect “Critical” for the future of humanity.

It presides over the formation of soils and evolution of the landscape, regulation of atmospheric acids, development of biomass (both visible and invisible), to the formation of water resources, the acquisition of the structure and soil chemistry (in a broad sense), the fertility of ecosystems and agricultural systems, and fate of toxins in the soil and biosphere.

This area, a true bio-physico-chemical reactor, is characterized by mechanisms with time constants ranging from the second, with regard to such bacterial activity or to adsorption, to millions of years e.g. neutralization of atmospheric CO2 by silicate weathering and the formation of new mineral phases. Following from this, is the difficulty in studying the Critical Zone, but also the challenge of understanding how quickly and by what feedback mechanisms it responds to forcings of meteorological, anthropogenic, climate or tectonic orders, and how the time scales fit together.