This chapter discusses the climatic factors that affect drought severity at the global scale and also current drought variability and trends under climate change conditions. Quantifying drought is a difficult task in a changing climate, mainly as a consequence of the uncertain contribution of water input (precipitation) and the atmospheric water demand on drought severity. The contribution of the atmospheric water demand is poorly known since to have an accurate assessment, it is necessary to consider both radiative and aerodynamic components of evapotranspiration processes, for which a high amount of data is required. The evolution of drought severity at the global scale suggests an intensification of droughts associated with enhanced atmospheric evaporative demand. Nevertheless, data uncertainty is important, and it prevents drawing definitive conclusions about this issue. The best and less uncertain alternative to quantify trends in drought severity is to focus on the recent impacts observed in water-limited natural systems. There are many evidences suggesting that the decrease of water resources, desertification processes, forest dieback and tree mortality, and lower crop yields are enhanced by recent drought trends, for which temperature rise and increased vapor pressure deficit are playing an important role.

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