PerspectiveEnvironmental Science

Water in the Balance

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Science  14 Jun 2013:
Vol. 340, Issue 6138, pp. 1300-1301
DOI: 10.1126/science.1236460

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Earth's climate is changing, and so is its hydrologic cycle. Recent decades have witnessed rising rates of global precipitation, evaporation, and freshwater discharge (1). Extreme flooding is occurring with greater intensity and frequency in some regions; in others, extreme drought is becoming more common (2). Most climate models indicate that by the end of this century, the dry regions of the world will become drier, whereas the wet areas will become wetter (3). Meanwhile, groundwater reserves, the traditional backup for water supplies during extended periods of drought, are in decline globally (46). GRACE (the Gravity Recovery and Climate Experiment, a joint U.S.-German satellite mission) monitors these variations on monthly to decadal time scales, providing detailed data on the water cycle that are an essential prerequisite for contemporary water management.