Mountains are important sources of freshwater for the adjacent lowlands. In view of increasingly scarce freshwater resources, this contribution should be clarified. While earlier studies focused on selected river systems in different climate zones, we attempt here a first spatially explicit, global typology of the so-called “water towers” at the 0.5° × 0.5° resolution in order to identify critical regions where disproportionality of mountain runoff as compared to lowlands is maximum. Then, an Earth systems perspective is considered with incorporation of lowland climates, distinguishing four different types of water towers. We show that more than 50% of mountain areas have an essential or supportive role for downstream regions. Finally, the potential significance of water resources in mountains is illustrated by including the actual population in the adjacent lowlands and its water needs: 7% of global mountain area provides essential water resources, while another 37% delivers important supportive supply, especially in arid and semiarid regions where vulnerability for seasonal and regional water shortage is high.