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Global mean evapotranspiration (ET) has been increasing in recent decades under climate warming. Yet the magnitude and spatial distribution of ET variation remain highly uncertain. ET changes in different regions are still poorly understood due to limitations in observation records, especially in semiarid regions with undeveloped economic systems and sparse observations. Based on the Priestley-Taylor Jet Propulsion Laboratory model, ET was estimated over global typical semiarid regions for 1984-2013. All of these regions show a decreasing ET trend, which is opposite to the trend in global mean ET. In particular, North Africa has the fastest decreasing trend, 8.6 mm/year, while South Africa has the slowest decreasing trend, 0.7 mm/year. North America, South America, northern Africa, and Australia have declining trends in ET during both warm and cold seasons, while the Loess Plateau, East Asia, central Asia, and South Africa have declining trends in ET only during warm seasons. Accounting for basic factors controlling ET, three important results are identified: First, atmospheric demand is increasing over all semiarid regions due to climate warming; second, the effect of atmospheric composition and cloud weakening radiation is strengthening over all semiarid regions; and finally, annual precipitation is decreasing over all semiarid regions except for South Africa. Factorial experiments indicate that the remarkable declining trend in relative air humidity forces the decreasing trend in ET over all semiarid regions. These results imply a slowing water cycle in global semiarid regions.