How will the global water cycle evolve in response to global warming?

No element of the climate system has as much impact on society as the water cycle, yet we remain ignorant of the largest part of the water cycle, that over the oceans. The oceans are the main reservoir of free water on the planet, the source of nearly 90% of global evaporation and the site of ~80% of global precipitation. A mere 1% of Atlantic ocean precipitation matches the discharge of the Mississippi River. Water evaporates more readily from a warmer ocean, so an intensification of the water cycle is expected with anthropogenic warming. The signature of the water cycle within the oceans is in the distribution of salinity, which must be monitored in the future if we are to understand how the water cycle is changing. In addition, salinity influences ocean mixing and circulation and thus the ability of the ocean to absorb, store and transport heat and CO2. Can we initiate an observing system for upper ocean salinity that will help us to understand and predict the future evolution of the global water cycle? Can we develop a better understanding of the smaller terrestrial water cycle, where plants and drainage basins are responding to rising warmth and CO2? Can we understand the interactions between ocean, atmosphere and the high latitude ice sheets that are leading to increased melting and discharge to the ocean? The global water cycle is truly a central unifying problem for climate change, and of fundamental importance to society.