Earth is often called the “Blue Planet” because oceans cover more than 70 percent of its surface. However, this vast amount of water is mostly saline, containing high concentrations of dissolved salts. Freshwater, defined as water with low salt concentrations suitable for supporting terrestrial life, is a surprisingly limited resource globally. The availability of this non-saline water for human use is extremely limited.
Setting the Stage: Total Water vs. Freshwater
The overwhelming majority of the Earth’s water is locked away in the oceans, seas, and saline groundwater, accounting for approximately 97 percent of the total global volume. This saline water is unusable for drinking, agriculture, or most industrial processes without expensive desalination. The remaining portion, freshwater, represents a mere three percent of the total water supply. This small percentage is unevenly distributed across various reservoirs, and its location dictates its accessibility for humans and ecosystems. The bulk of non-saline water is stored in just two primary locations: ice and underground.
The Dominant Repository: Glaciers and Ice Sheets
The largest single reservoir of freshwater is found in the Earth’s polar ice masses, glaciers, and permanent snow cover. Approximately 68.7 percent of all freshwater is immobilized in this frozen state. This massive volume of ice represents the greatest natural storage of non-saline water on the planet. The vast majority of this ice is concentrated in the two great ice sheets covering Antarctica and Greenland. The Antarctic Ice Sheet alone holds about 70 percent of the world’s total freshwater supply. Due to the remote location and frozen state, this resource is largely inaccessible for practical human use. The sheer scale of water locked up in these permanent ice formations means the majority of the planet’s freshwater is effectively removed from the active, liquid water cycle.
The Vast Subsurface Supply: Groundwater
The second largest freshwater reservoir is groundwater, accounting for about 30.1 percent of the total non-saline supply. This water is stored beneath the Earth’s surface in saturated zones, permeating the pore spaces and fractures within rock and soil formations; these areas are known as aquifers. Groundwater is the largest source of liquid freshwater on Earth, accessed through wells for agricultural, municipal, and industrial use globally. A distinction exists between shallow, renewable groundwater, which is regularly recharged by precipitation, and deeper, or “fossil,” groundwater. The deeper reserves accumulated thousands of years ago, and their recharge rates are extremely slow or non-existent, making them a non-renewable resource for the population that relies on them for drinking water.
The Highly Accessible Fraction: Surface Water and Atmosphere
The most visible and heavily utilized components of the freshwater supply—surface water and the atmosphere—make up the smallest fraction of the total. All surface water sources, including lakes, rivers, swamps, and soil moisture, collectively represent less than one percent of the world’s freshwater. This tiny percentage is important because it is the most readily accessible and renewable source for human consumption and ecological systems. Within this small fraction, lakes hold the most water, containing about 87 percent of the liquid surface freshwater. Rivers, which are the most common source of water for cities, represent an even smaller volume, but their rapid movement makes them a constantly replenished supply. Although the atmosphere holds a negligible amount of water at any one time, its role as the transport mechanism drives the entire water cycle, delivering precipitation to all other reservoirs.