Water covers about two-thirds of the Earth's surface, but precious little can be used by human beings. Only 2.5 percent -- a veritable drop -- of the world's water is not salty, and two-thirds of that is frozen in the ice caps and glaciers. Of the remaining third, 20 percent is located in remote places, and much of the rest is delivered via floods and monsoons, which means it is often unusable or wasted. Do the math and the results are grim: We can use less than 0.08 percent of the water on the planet.

At the same time, the demand for water is expanding explosively. Water use is expected to increase 40 percent over the next two decades. About 3 billion people will face water shortages by then. There are several reasons for the surge. Populations are rising, but more important is the rising level of affluence worldwide. As people acquire more wealth, their environmental footprint grows larger. Quite simply, the modern lifestyle demands more inputs and puts a greater strain on natural resources. While world population tripled in the last century, water use increased six times.

The main culprit is agriculture. Seventy percent of the water that is used today is used for agriculture. Moreover, changing tastes -- and the modern agricultural techniques required to feed them -- use more water. Another important contributor is pollution. Development not only increases demand for water but also renders unusable other supplies. Waste, the byproduct of modern industry, diminishes the amount of water available for more basic needs.