Tropical semi-arid regions expanding over temperate latitudes under climate change

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∘ 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.