The way in which water moves around the Earth - on, below and above its surface.
The water cycle is essential to life on Earth. As a result of complex interactions, the water cycle acts as an integrator within the Earth/climate system, controlling climate variability and maintaining a suitable climate for life. The water cycle manifests itself through many processes and phenomena, such as clouds and precipitation; ocean-atmosphere, cryosphere-atmosphere, and land-atmosphere interactions; mountain snow packs; groundwater; and extreme events such as droughts and floods.
1. What are the mechanisms and processes responsible for the maintenance and variability of the water cycle; are the characteristics of the cycle changing and, if so, to what extent are human activities responsible for those changes?
2. How do feedback processes control the interactions between the global water cycle and other parts of the climate system (e.g., carbon cycle, energy), and how are these feedbacks changing over time?
3. What are the key uncertainties in seasonal-to-interannual predictions and long-term projections of water cycle variables, and what improvements are needed in global and regional models to reduce these uncertainties?
4. What are the consequences over a range of space and time scales of water cycle variability and change for human societies and ecosystems, and how do they interact with the Earth system to affect sediment transport and nutrient and biogeochemical cycles?
5. How can global water cycle information be used to inform decision processes in the context of changing water resource conditions and policies?
Source: Climate Change Science Program, and The Subcommittee Global Change Research. "Our Changing Planet: The U.S. Climate Change Science Program for Fiscal Year 2006." Washington: Climate Change Science Program Office, 2006.