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Whatever the human consequences of the crisis, it has an even greater effect on many other living things. Fish, birds and countless creatures are crowded out, marooned or poisoned as industry, agriculture and municipalities reroute rivers, dry up wetlands, dump waste and otherwise disrupt the normal functioning of delicate ecosystems. The world is learning that there are limits to mankind's ability to move water from one place to another without seriously upsetting the balance of nature.
The idea of a global shortage seems incredible when 70% of the earth's surface is covered by H2O. But 98% of that water is salty, making it unusable for drinking or agriculture. Desalinization is technically feasible, but it is far too expensive to use anywhere except in an ultra-rich, sparsely populated country like Saudi Arabia. Other options, like towing icebergs from the poles, are also beyond the means of poor nations.
The scarcity of fresh water for agriculture makes famines more likely every year. The world consumes more food than it produces, and yet there are few places to turn for additional cropland. Only by drawing on international stockpiles of grain have poorer countries averted widespread starvation. But those supplies are being depleted. From 1987 to 1989, the world's stock of grain fell from a 101-day surplus to a 54-day one. A drought in the U.S. breadbasket could rapidly lead to a global food calamity.
Even if rainfall stays at normal levels, current world food production will be difficult to maintain, much less increase. The food supply has kept pace with population growth only because the amount of land under irrigation has doubled in the past three decades. Now, however, agriculture is losing millions of hectares of this land to the effects of improper watering.
Without adequate drainage, continuous irrigation gradually destroys a piece of land -- and any streams or rivers near it -- through a process called salinization. As the heat of the sun evaporates irrigation water, salts are left behind. The water also flushes additional salts out of soils with high concentrations of minerals, leaving them to dry on the surface into a cakelike residue or to dissolve in groundwater and poison plant roots.
History shows that such environmental destruction can have far-reaching consequences. The salinization of irrigated land led to the fall of Mesopotamia and Babylon, and perhaps even the Mayan civilization of Central America. Similar pressures are at work today. Sandra Postel of Worldwatch Institute estimates that 60 million hectares (nearly 150 million acres) of irrigated land worldwide have been damaged by salt buildup.
Human activities have also disrupted the delicate natural systems that maintain water supplies. To obtain wood and clear land for homes and farms, mankind is chopping down forests at an unprecedented rate. But vegetation traps water, reducing runoff and replenishing groundwater supplies. Throughout the world, tree cutting has led to floods, mud slides and soil erosion during rainy seasons and acute water shortages during dry periods.