The total amount of water on Earth, whether liquid, ice, or vapor, remains essentially constant over human timescales. This stability is governed by the principle of conservation of mass, meaning matter cannot be created or destroyed, only changed in form or location. While water is constantly moving and changing state, the global inventory of \(\text{H}_2\text{O}\) remains fixed, endlessly cycling between the planet’s surface, atmosphere, and subsurface. The perception of water “disappearing” is simply a reflection of this massive global recycling system, known as the hydrologic cycle.
The Earth’s Fixed Water Inventory
The planet’s total water budget is immense, estimated to be around \(1.386\) billion cubic kilometers. This volume defines the fixed inventory that participates in the global water cycle. Approximately \(97\%\) of this water is saline and contained within the oceans and seas.
The remaining \(3\%\) is freshwater, but this small fraction is largely inaccessible for immediate human use. Nearly \(69\%\) of all freshwater is locked away in glaciers, ice caps, and permanent snow. Roughly \(30\%\) is stored as groundwater beneath the surface. The amount of readily available surface freshwater in rivers and lakes is only a minute fraction of the overall fixed inventory.
Water Movement: The Annual Hydrologic Cycle
The hydrologic cycle constantly moves water between the major reservoirs, driven primarily by solar energy and gravity. This massive turnover ensures the Earth’s water supply is continuously renewed.
Each year, approximately \(505,000\) cubic kilometers of water moves through the atmosphere via evaporation and subsequent precipitation. The ocean is the engine of this cycle, as it is the source for about \(86\%\) of all global evaporation. Evaporation transforms liquid water into vapor, which is then transported globally by atmospheric circulation.
This evaporated water returns to the surface as precipitation, with roughly \(78\%\) falling back directly onto the ocean. The remaining precipitation falls onto land, where it soaks into the ground, becomes soil moisture, or runs off into rivers, completing the loop back to the sea. This enormous annual transfer represents a closed loop where the total volume evaporated closely equals the total volume precipitated, resulting in zero net loss from the active surface system.
Quantifying Permanent Loss and Sequestration
While the hydrologic cycle is a closed loop, there are two distinct, quantifiable processes that remove water from the active surface cycle, one permanently and one for geological timescales.
Permanent Loss: Atmospheric Escape
The only true permanent loss of water from Earth occurs through atmospheric escape into space. This process involves the photodissociation of water vapor high in the atmosphere. Ultraviolet radiation from the sun breaks down water molecules into their constituent hydrogen and oxygen atoms at high altitudes. The extremely light hydrogen atoms can then attain the necessary escape velocity to drift away from Earth’s gravitational pull. The rate of hydrogen loss is estimated to be about three kilograms per second, which is an infinitesimally small fraction of the total water inventory.
Geological Sequestration
A much larger volume of water is removed from the active cycle through geological sequestration within the Earth’s mantle. This happens primarily at subduction zones, where oceanic plates slide beneath continental plates, carrying water-logged minerals deep into the interior. Water is chemically bound into the crystal structure of high-pressure minerals, such as ringwoodite and wadsleyite, in the mantle transition zone, which sits between \(410\) and \(660\) kilometers deep. Estimates suggest that the net movement of water into the deep interior, accounting for the water that returns through volcanic activity, is in the range of hundreds of billions of kilograms each year. This sequestered water is effectively removed from the surface and atmospheric cycles that support life for vast stretches of geological time.