The total amount of water on Earth, known as the hydrosphere, involves both immense constancy and subtle change. The hydrosphere includes all water—solid, liquid, and gas—found on, under, and above the planet’s surface, totaling an estimated volume of about 1.386 billion cubic kilometers. On timescales relevant to human civilization, the total inventory of water molecules remains remarkably stable. However, over vast geological time, the absolute number of water molecules fluctuates slightly due to opposing mechanisms of gain and loss. While the total volume is stable, the distribution of water constantly shifts, impacting human and ecological systems.
The Global Water Cycle: A Closed System
The apparent constancy of Earth’s water supply is primarily governed by the global water cycle, a continuous process that moves and recycles water within the Earth system. This cycle functions as a near-perfect closed loop, where water perpetually changes its physical state and location but is neither created nor destroyed. The conservation of mass dictates that the total quantity of water molecules remains the same as they transition between liquid, solid, and gas.
Solar energy drives the initial phase of this cycle, causing liquid water to become water vapor through evaporation from oceans, lakes, and soil. This gas rises into the atmosphere, where cooler temperatures cause condensation, forming clouds of liquid droplets or ice crystals. Atmospheric circulation transports this moisture globally before it falls back to the surface as precipitation, such as rain, snow, or hail.
Once precipitation reaches the ground, it can flow across the land as runoff, be absorbed by plants, or seep into the ground to become groundwater. This movement ensures water molecules are constantly exchanged between the atmosphere, the surface, and subsurface reservoirs. The turnover of water in the atmosphere is rapid, with an average residence time of only about nine to ten days. This continuous movement through the cycle keeps the planet’s total water volume stable over short to moderate time frames.
Processes That Slightly Alter Earth’s Total Water Inventory
While the water cycle maintains stability, two opposing geological and atmospheric processes cause a minute alteration to the planet’s total water inventory over millions of years. Earth experiences a slow net loss of water molecules to space, counterbalanced by an influx of new water from the deep interior and space. Water molecules that reach the upper atmosphere can be broken apart by high-energy ultraviolet radiation in a process called photodissociation.
This process splits the water molecule (\(\text{H}_2\text{O}\)) into hydrogen (\(\text{H}\)) and oxygen (\(\text{O}\)). Since hydrogen is the lightest element, some atoms attain enough energy to overcome Earth’s gravitational pull and escape into outer space. This slow atmospheric escape represents the only mechanism by which Earth loses water from its system. Although the loss is minimal, it is continuous.
The planet simultaneously gains new water from two sources outside the surface water cycle. Water is constantly released from the Earth’s interior through volcanic outgassing, where superheated steam escapes from magma and deep crustal rock. Additionally, small amounts of water are delivered via the continuous bombardment of icy comets and water-rich meteorites from space. While these gains and losses are negligible compared to the total volume of the oceans, they demonstrate that Earth is not a perfectly closed system for water, with the balance shifting over eons.
The Dynamic Nature of Water Distribution and Availability
Despite the near-constant total volume, the perception that the amount of water is changing stems from the highly variable distribution of water across its major storage reservoirs. Approximately 97 percent of Earth’s water is saline water stored in the oceans. The remaining three percent is freshwater, but this small fraction is largely inaccessible.
Over two-thirds of all freshwater is locked away in the form of ice caps, glaciers, and permanent snow cover. The next largest storage is groundwater, accounting for nearly all of the remaining third. This leaves a minuscule fraction, estimated to be less than one percent of all freshwater, as easily accessible surface water in rivers, lakes, and the atmosphere.
Natural climate cycles and human-induced climate change cause massive, rapid shifts between these reservoirs, affecting availability. During past ice ages, water shifted from the oceans to the ice sheets, causing global sea levels to drop significantly. Conversely, modern global warming is causing ice sheets and glaciers to melt, shifting frozen water back into the liquid ocean reservoir and causing sea levels to rise.
These dramatic internal transfers do not change the total number of \(\text{H}_2\text{O}\) molecules on the planet, but they profoundly change the accessibility of water for human and ecological use. Changes in temperature increase the rate of evaporation, which intensifies the movement of water vapor and leads to more extreme precipitation events or deeper droughts. The variability in where water is stored and how quickly it moves is the primary reason the planet’s water resources feel so dynamic.