The Global Water Cycle

The sustainability of life on the Earth depends critically on the availability of fresh water. Any threat to the reliability and sustainability of the fresh water supply clearly deserves focused and urgent attention. The demand for fresh water is increasing in direct response to increasing human population. The world's current human population of approximately 6 billion has more than doubled since 1950 and it is likely to increase by an additional 3 billion people by 2050. Meanwhile, the supply of fresh water is decreasing due to pollution and other stresses. Some projections suggest that rapid increases in demand, coupled with limited supplies, will lead to the development of a global water crisis in a matter of decades, with the precise timing of this crisis uncertain due to a limited knowledge of the world's water resources. While these problems manifest themselves at the local or regional level, the reservoirs and their catchment areas which supply water to the local or regional level are sometimes distributed across province/state or national boundaries. The amounts and frequencies of precipitation which supplies water to these reservoirs are influenced by global-scale, sub-continental-scale, or ocean basin-scale processes. Therefore, addressing such problems scientifically requires a global view of fresh water in the Earth-atmosphere system.

Major evaporation regions are at subtropical latitudes in the Northern and Southern Hemispheres due to sinking and drying atmospheric circulations and warm ocean surface temperatures in these regions. Major precipitation regions are in tropical and midlatitude regions. Water is supplied for precipitation in these regions by atmospheric moisture transport from the major evaporation regions and by local evaporation. Fresh water is then transported by ocean currents to the major evaporation regions. This set of atmosphere-ocean processes constitutes the global water cycle.

Evaporation and precipitation over the oceans play very important roles in the global water cycle. Approximately 70-80% of the total global evaporation and precipitation occurs over oceans. Moreover, latent heat release into the atmosphere over the oceans is the major heat source driving global atmospheric circulations, with the moisture transported by these circulations from oceanic to continental regions being the major source of water precipitating over land. In the conventional representation of the terrestrial water cycle, the assumed role of the oceans is to be near-infinite reservoirs of water with the main drivers of the water cycle being land-atmosphere interactions in which excess precipitation over evaporation is returned to the oceans as surface runoff and baseflow. While this perspective is valid for short time and space scales -- fundamental principles, available observed estimates, and results from models indicate that the oceans play a far more important role in the large-scale water cycle at seasonal and longer timescales.

There are large-scale components of the global water cycle associated with the annual cycle of solar radiation; and the El Niño-Southern Oscillation, North Pacific and North Atlantic Oscillations, Asian-Australian Monsoons, Indo-Pacific Warm Pool,, and the Gulf of Mexico-Eastern US phenomenons. CRCES scientists are heavily engaged in research on these large-scale water cycles, with observations from in situ and satellite-based instruments, and Earth System models.