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A l'origine considérée comme un problème de développement local, la lutte contre la désertification apparaît peu à peu sur la scène internationale comme un phénomène d'environnement mondial. La lutte contre la désertification peut-elle être envisagée comme un bien public mondial? Le dossier présente tout d'abord de manière succincte et illustrée l'ampleur du problème et, à titre d'exemples, des moyens disponibles pour l'enrayer. Mais en s'appuyant sur l'analyse des relations entre désertification et transformations des sociétés, et en particulier des aspects liés aux migrations, à la pauvreté et à la régulation foncière, l'élargissement du champ traditionnel de la lutte contre la désertification apparaît nécessaire. La résolution des problèmes de désertification passe en effet par la mise en oeuvre d'un ensemble cohérent et interdépendant d'actions conçues à différents niveaux d'organisation, du local au global, et adossées à autant de formes institutionnelles spécifiques. Ces éléments alimentent un débat permettant de décider si la lutte contre la désertification constitue ou non un bien public, en quoi il interpelle ou non la dimension et la communauté internationales et s'il y a lieu ou non de mettre en oeuvre de nouveaux mécanismes de lutte.

Originally considered as a local development problem, combating desertification is viewed more and more as a global environmental issue at the international level. May combating desertification be considered a global public good? The document shortly describes the desertification problem, its extent and examples of available solutions for stopping it. However, the analysis of the links between desertification and social changes - particularly the ones that relate to migration, poverty and land tenure -, demonstrates the interest of broadening the traditional approach of combating desertification. Such a focus highlights the need for implementing a consistent set of interdependent actions to solve desertification problems. These actions should be designed in each situation at various levels, from local to global, and supported by as many specific institutional arrangements. The discussion brings into debate elements for deciding whether combating desertification should or should not be recognized as being a global public good and for identifying which new fighting mechanisms should be implemented at international levels.

In the Sahel, the question of interactions between rangeland herding practices and the environment lies at the hert of the debate concerning sustainable development. There is disagreement about the question of Sahelian herd mobility, which remains necessary for some while for others it is outmoded, and which is judged in terms of its environmental impact. There are also many questions regarding the influence of socio-economic policies on herder behaviour. [...] We performed two modelling experiment on the theme of mobility as a means of adaptation. The first experiment studies the relation between movement strategies and the dynamics of resources in space, while the second studies the social relations maintained by herders in order to botain access to rangelands and water. To this end, we used the multi-agent modelling method. This method is based on the representation of entites which interact directly with each other or via the environment. In this article, we present the methods and describe the two modelling experiments. We conclude with a discussion on the use of such models.