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A comparison of observations with 20 climate model simulations from the Coupled Model Intercomparison Project, Phase 5 (CMIP5) revealed that observed dryland expansion amounted to 2.61x10(6) km(2) during the 58 years from 1948 to 2005, which was four times higher than that in the simulations (0.55x10(6) km(2)). Dryland expansion was accompanied by a decline in aridity index (AI) (drying trend) as a result of decreased precipitation and increased potential evapotranspiration across all dryland subtype areas in the observations, especially in the semi-arid and dry subhumid regions. However, the CMIP5 multi-model ensemble (MME) average performed poorly with regard to the decreasing trends of AI and precipitation. By analyzing the factors controlling AI, we found that the overall bias of AI in the simulations, compared with observations, was largely due to limitations in the simulation of precipitation. The simulated precipitation over global drylands was substantially overestimated compared with observations across all subtype areas, and the spatial distribution of precipitation in the MME was largely inconsistent in the African Sahel, East Asia, and eastern Australia, where the semi-arid and dry subhumid regions were mainly located.