The Global Water Cycle

The global hydrological cycle describes the continuous movement of water between the atmosphere, land, and oceans. It is driven primarily by solar energy and gravity. At the planetary scale, the hydrological cycle is effectively a closed system — the total volume of water on Earth has remained broadly constant at approximately 1.386 billion km³ (Shiklomanov, 1993) for hundreds of millions of years. However, the distribution of water between stores changes over time, with profound implications for climate, landscapes, and human societies.

---

Global Distribution of Water

The vast majority of Earth's water is saline and stored in the oceans. Freshwater constitutes only about 2.5% of the total, and most of that is locked in ice caps and glaciers.