How could people run short of something so abundant? The reason is that 97.5 percent of the world’s water is saltwater—undrinkable, corrosive, even toxic. More than two-thirds of the remainder is locked into polar ice caps and glaciers, the great majority of it in Antarctica. The rest—all of the planet’s lakes, rivers, swamps, and groundwater—is less than 1 percent of the total. That is the theoretically available freshwater supply. Put together, it would form a sphere about 170 miles in diameter. In fact, though, this is a wild overestimate, because more than nine-tenths of that water is
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