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In this study, we project the global terrestrial changes in Aridity Index (AI), as defined by the ratio of annual precipitation to potential evapotranspiration (PET), at 2 and 4 degrees C levels of global warming above the preindustrial conditions based on the outputs from 21 Coupled Model Intercomparison Project Phase 5 models under the Representative Concentration Pathways 8.5 (RCP8.5) scenario. Compared to the preindustrial period, the global terrestrial annual precipitation will increase by an average of 0.02% (3%), the annual PET will increase by 6% (15%) and AI will decrease by 0.1 (0.2) at the 2 degrees C (4 degrees C) warming level. More remarkable decreases in the annual AI are seen in the northern high latitudes, where will also experience greater increases in PET and precipitation. In general, the seasonal changes in AI display a similar spatial pattern to the annual changes and are characterized by the greatest decrease in December-January-February and the smallest in June-July-August. The largest expansion of drylands occurs in semi-arid regions, followed by the expansion of arid regions, with area changes of 4% (7%) and 3% (4%) under the 2 degrees C (4 degrees C) scenario, respectively. Further analyses indicate that the AI changes are determined by PET in the northern high latitudes, and the contribution of PET to AI increases from the 2 to 4 degrees C warming levels. Furthermore, changes in thermodynamic factors contribute most to the PET, and at a higher level of global warming, the contribution of thermodynamic factors increases and that of dynamic factors decreases.