WATCH: current knowledge of the terrestrial global water cycle

Water-related impacts are among the most important consequences of increasing greenhouse gas concentrations. Changes in the global water cycle will also impact the carbon and nutrient cycles and vegetation patterns. There is already some evidence of increasing severity of floods and droughts and increasing water scarcity linked to increasing greenhouse gases. So far, however, the most important impacts on water resources are the direct interventions by humans, such as dams,water extractions, and river channelmodifications. TheWater and Global Change (WATCH) project is a major international initiative to bring together climate and water scientists to better understand the current and future water cycle. This paper summarizes the underlying motivation for theWATCHproject and themajor results froma series of papers published or soon to be published in the Journal of HydrometeorologyWATCH special collection. At its core is theWater Model Intercomparison Project (WaterMIP),which brings together a wide range of global hydrological and land surfacemodels runwith consistent driving data. It is clear that we still have considerable uncertainties in the future climate drivers and in how the river systems will respond to these changes. There is a grand challenge to the hydrological and climate communities to both reduce these uncertainties and communicate them to a wider society.