[en] In the face of ever-growing global deforestation, numerous forest protection and restoration projects have been deployed at the international scale. The goal of this thesis is to provide adaptation planning in the vulnerable tropical semi-arid regions with scientific material about reforestation project constraints and opportunities at the global scale, in the context of climate change. The literature review (chapter 1) confirms that reforestation projects aimed at warming mitigation hold a better chance of success under tropical latitudes. Indeed, both biochemical and biophysical effects of the vegetation on climate converge toward a global cooling effect. As reforestation in tropical semi-arid regions aims at satisfying various ecosystemic services, it holds beneficial promises at both the global and the local scale. However, due to scarce water resources, implementing a tree cover in semi-arid conditions could turn out unsustainable in the long run. A bio-climatological is applied, in chapter 2, to a multi-model ensemble of projections in order to draw the evolution of global tropical semi-arid territory under several climate change scenarios (RCP). The present tropical semi-arid territory is expected to remain mostly so in future conditions. However, up to 25% of the this territory on average will evolve towards a rider conditions, and up to 11% towards wetter conditions. Nevertheless, the tropical semi-arid territory will increase by the end of the 21. century, by up to 13% on average (RCP 8.5). This increase results from a migration outside of the tropical belt, consistent with the Hadley circulation widening hypothesis under climate change. Chapter 3 proposes a methodology aimed at analysing the implications of this evolution for the climatic potential of tree cover sustainability. The global vegetation model (ORCHIDEE, developed at IPSL), used to simulate this potential, accounts mechanistically for all the climatic factors of the plant's growth. A typology of result profiles from the simulation experiments partitions the territory into subregions characterized by a specific relation between the tree development and the tree cover density: five types range from the least (Type 1) to the most (Type 4) favourable ones. A reference experiment is performed using observational climate data (from the Climatic Research Unit). Covering almost half of the territory, Type 1 is characterized by the impossibility to maintain a tree cover for the highest cover densities. The second type in order of surface occupation is Type 4 (28% of the territory). More favourable, it is characterized by high tree development for any tree cover density. The 'tree cover potential' of each type is characterized by its optimum: the tree cover density that realises the best compromise between tree development and total productivity. In Chapter 4, the same methodology is applied to future climate projections for RCP 8.5. The ORCHIDEE model is thus forced with global climate model outputs, for the beginning and the end of the 21. century. By the end of the century, Type 1 represents no more than 25% of the tropical semi-arid territory on average, while Type 4 becomes the dominant one (49% of the territory). Because of the stability of the tree cover potential within each type, the evolution toward a more or less favourable type can be directly interpreted in terms of an increasing or a decreasing potential. The results show that the tree cover potential in the tropical semi-arid territory does not systematically suffer from the general decrease that could be expected from increasing aridity. A complementary experiment suggests that the main reason for this result lies is the atmospheric CO₂ fertilization effect. Interpreting these results for reforestation strategy recommendations, suggests that, for the long term, areas of the tropical semi-arid territory where reforestation would be advised against are overall relatively small. (author)

[en] Highly populated, water-limited and warm drylands are challenging areas for development and are expected to expand overall under several scenarios of climate change. Here, we adopt a bioclimatic approach based on the Koppen classification to focus on the evolution of warm semi-arid regions over the projected twenty-first century, following three socio-economic scenarios and 12 global climate models from the last IPCC exercise (CMIP5). We show that a global expansion of this climatic domain has already started according to climate observations in the twentieth century (about + 13% of surface increase, i.e. from 6 to 7% of the global land surface). Models project that this expansion will continue throughout the twenty-first century, whatever the scenario: for the most dramatic one (RCP 8.5), the share of the total land surface occupied by warm semi-arid surfaces is about 38% higher in 2100 compared to the present (from 7 to 9% of the global land surface). This expansion will essentially take place outside of the tropical belt, showing a poleward migration as large as 11(ay) of latitude in the Northern Hemisphere. This expansion is linearly correlated with the projected future global warming (about 853 millions km² per degree of warming for RCP 8.5). Different types of climate class transitions and their associated mechanisms are discussed. (authors)

[en] After having outlined the necessity of a development of climate services with users, this publication highlights the strengthening of existing and emerging climate services (the Drias portal, the interdisciplinary group of national contributions, the Extremoscope project). Then, it briefly presents the development of five innovative services: the impact of climate change on coastal regions and the assessment of submersion risks, the evolution of aquifers with climate change, impacts for agriculture in western Africa, the evolution of sea ecosystems in the Mediterranean Sea and the definition of marine protected areas, and a climate service for cities. It briefly addresses associated needs in terms of basic and professional training, and in terms of communication with experts, undertakers, citizen, and so on.