The Earth is often called the “Blue Planet” because water covers approximately 71% of its surface, yet the vast majority of this water is saline. Freshwater, defined as water with a low concentration of dissolved salts, represents a surprisingly small fraction of the global water supply. This limited resource, which is necessary for almost all terrestrial life, makes up only about 2.5% to 3% of all the water on the planet.
Global Freshwater Inventory and Distribution
The total volume of freshwater is stored primarily in two forms: ice and groundwater. Nearly 99% of all freshwater exists in these two reservoirs. Ice caps and glaciers account for the largest percentage, locking up approximately 68.7% of the total freshwater supply. The next largest component is groundwater, which constitutes about 30.1% of the world’s freshwater. All other sources combined, including lakes, rivers, soil moisture, and atmospheric vapor, represent a mere 1.2% of the global freshwater inventory.
The Dominant Frozen Reserves
The greatest accumulation of freshwater on Earth is found in its frozen state, overwhelmingly concentrated in the polar ice sheets. The Antarctic Ice Sheet alone holds the immense majority of this frozen water, representing the largest single reservoir of freshwater globally. Its complete melting would raise global sea levels by approximately 57.9 meters. The Greenland Ice Sheet is the second largest store, containing enough ice to raise sea levels by about 7.42 meters if it were to melt entirely. Together, these two continental ice sheets account for over 99% of the planet’s land ice.
Hidden Supply: Underground Water
Groundwater is the second largest freshwater reservoir and the most significant non-frozen source, stored beneath the surface in geological formations called aquifers. The capacity of an aquifer to hold water is determined by its porosity (storage space) and permeability (ease of movement). Groundwater is often categorized into shallow, renewable water and deep, non-renewable, or fossil water. Shallow groundwater is relatively accessible and can be recharged by rainfall and snowmelt over short timescales. Deep groundwater is not replenished on a human timescale, making it a finite resource when extracted, leading to depletion in many arid regions.
Accessible Surface Water and the Atmosphere
The surface water that sustains most human and ecological systems makes up less than one-tenth of one percent of the world’s total water. Rivers, lakes, and wetlands, though small in volume, are the most dynamic parts of the hydrologic cycle, experiencing rapid turnover. Lake Baikal in Siberia, Russia, is the largest single body of liquid freshwater by volume. This is due to its formation as a deep rift valley lake, making it the deepest lake in the world. The atmosphere holds the smallest fraction of freshwater, existing as water vapor and clouds, but this percentage is responsible for transporting moisture and generating the precipitation that replenishes all other freshwater sources.