Freshwater Availability and Us

While 70 percent of the earth’s surface is water, only three percent of it is fresh water—and almost all of that three percent is inaccessible for human use (Lean & Hinrichsen, 1994; Lefort, 1996). About three-quarters of all fresh water on earth is locked away in the form of ice caps and glaciers located in polar areas far from most human habitation. In all, only about 0.01 percent of the world’s total water supply is considered available for human use on a regular basis. If the world’s freshwater supply amounted to the contents of a bathtub, the amount easily accessible to humanity would fill a thimble.

Nevertheless, even this thimble full of water is, in theory, enough to sustain an estimated 20 billion people. But in reality, only a small amount of the freshwater supply is reliable enough to be considered accessible year after year. Globally, 505,000 cubic kilometers of renewable fresh water shifts from the sea to the land every year as rain or snow via the hydrological cycle; but only 47,000 cubic kilometers per year can be considered accessible for human use (Gleick, 2000).