The Atacama Desert forms one of the major hyperarid deserts of the world. Previous studies suggest that desertification commenced at 14 Ma during global climate desiccation. Sedimentologic data from middle Miocene to upper Pliocene successions in the modern Atacama Desert indicate that a semiarid climate persisted from 8 to 3 Ma, punctuated by a phase of increased aridity at ca. 6 Ma. As such, hyperaridity did not commence until the late Pliocene. Implications are (1) that the rain shadow generated by the Andean Cordillera has had a minor influence on climate change, and (2) that the upwelling, north- flowing, cold Humboldt Current, although important in establishing the generally arid climate of western South America, did not control the shift to hyperaridity. The formation of the hyperarid Atacama Desert in the late Pliocene accompanied the development of the current phase of aridification in the Sahara and Namib Deserts and is attributed to a phase of global climate cooling.