The hydrological cycle is called a closed system because the amount of water on the Earth does not fluctuate. While water changes forms as it flows through the hydrological cycle, there is no significant input or output of water from the planet. Rain, snow and ice that rains down onto the planet eventually melt and evaporate, thereby replacing the water that precipitated from the atmosphere.
The vast majority of water on planet Earth has been here for billions of years. While small amounts of water are formed or broken down in various chemical processes, the processes tend to balance. For example, aerobic respiration transforms oxygen and sugar into carbon dioxide and water as it releases energy. Conversely, plants break down water in the process of photosynthesis to make oxygen and sugar.
While water never leaves the planet in any significant quantities, sometimes water stops flowing through the hydrological cycle for a period of time. For example, the water that makes up the polar ice caps has been frozen for millions of years, and very little of it has flowed in an ocean or formed a cloud in the atmosphere since the time that it froze. Scientists speculate that Earth’s water may have originated in the form of comets crashing into the planet several billion years ago.